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Article

Advancing PetroChina’s Development Strategies for Low-Permeability Oil Reservoirs

by
Jun Cao
1,2,
Mingqiang Hao
1,
Yujia Chen
3,
Baozhu Li
1,
Zhuo Liu
1,
Yang Liu
1 and
Jinze Xu
4,5,*
1
Research Institute of Petroleum Exploration and Development, Beijing 100083, China
2
Research Institute of Petroleum Exploration and Development of PetroChina Changqing Oilfield Company, Xi’an 710018, China
3
PetroChina Yumen Oilfield Company, Jiuquan 735008, China
4
Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada
5
College of Petroleum Engineering, Xi’an Shiyou University, Xi’an 710065, China
*
Author to whom correspondence should be addressed.
Processes 2024, 12(2), 351; https://doi.org/10.3390/pr12020351
Submission received: 5 December 2023 / Revised: 17 January 2024 / Accepted: 24 January 2024 / Published: 7 February 2024
(This article belongs to the Special Issue Advanced Technologies of Deep Mining)
Browse Figures
Versions Notes

Abstract

:
Based on PetroChina’s status and situation of low-permeability oil reservoir development, this paper analyzes the key common issues in the production capacity construction of new oilfields, the stable production of old oilfields, and enhanced oil recovery, and, in connection with the progress made in major development technologies and the results of major development tests for low-permeability oil reservoirs in recent years, puts forward the technical countermeasures and development directions. For optimizing the development of low-grade reserves, a comprehensive life-cycle development plan is essential, alongside experimenting with gas injection and energy supplementation in new fields. Addressing challenges in reservoir classification, multidisciplinary sweet spot prediction, and displacement–imbibition processes can significantly boost well productivity. In fine water flooding reservoirs, the focus should shift to resolving key technological challenges like dynamic heterogeneity characterization, and functional and nano-intelligent water flooding. For EOR, accelerating the application of carbon capture, utilization, and storage (CCUS) advancements, along with air injection thermal miscible flooding, and middle-phase microemulsion flooding, is crucial. This approach aims to substantially enhance recovery and establish a new model of integrated secondary and tertiary recovery methods.

1. Introduction

Low-permeability reservoirs refer to reservoirs with air permeability less than 50 × 103 μm2, which are generally subdivided into conventional low-permeability (50–10 × 10−3 μm2) reservoirs, extra-low-permeability (10–1 × 10−3 μm2) reservoirs, and ultra-low-permeability (1–0.1 × 10−3 μm2) reservoirs. China is rich in low-permeability reservoirs, with a total resource volume of about 537 × 108 t. They are widely developed in the eastern, central, and western regions, such as the Songliao Basin, Ordos Basin, and Junggar Basin [1]. Low-permeability reservoirs are mainly composed of sandstone, followed by conglomerate, metamorphic rock, and limestone. The majority of these low-permeability reservoirs are located at medium to deep burial depths, with those at depths ranging from 1000 to 3200 m accounting for 86.5% of the total. The crude oil in these reservoirs has good properties, with underground viscosity generally less than 10 mP·s. Since the 1980s, PetroChina has gradually explored and developed a number of low-permeability/extra-low-permeability reservoirs represented by Ansai Oilfield and Jing’an Oilfield in Changqing, Xinli Oilfield, Xinmin Oilfield in Jilin, and Huoshaoshan Oilfield and Beisantai Oilfield in Xinjiang. In the new century, it has tackled problems in the water injection development of ultra-low-permeability reservoirs represented by Xifeng Oilfield, Jiyuan Oilfield, and Huaqing Oilfield in Changqing. Following over 40 years of practice, the theory and technology of water flooding development of low-permeability reservoirs have gradually improved, and such effective development technologies as moderately advanced water injection, fine layered water injection, unstable water injection, infill adjustment of fracture network matching, and volume fracturing of horizontal wells have been developed, which leads to a continuous increase in output. PetroChina’s oil output accounted for 37.9% of the total output in 2022. Low-permeability oilfields have become the main prop of the oil industry and also the mainstream and inevitable development trend of the future oil and gas industry in China [2,3].
At present, PetroChina’s low-permeability oil reservoirs are in the high-water cut stage. After long-term water flooding development, new issues have been exposed. For instance, resource quality deterioration and oilfield aging are aggravated, the technical path to greatly enhance oil recovery upon water flooding is unclear, and development contradictions of low single-well output, low oil production rate, and low oil recovery are increasingly prominent. This paper systematically analyzes PetroChina’s status and situation of low-permeability oil reservoir development, as well as the problems and potentials, and puts forward the development route and direction of EOR technology in connection with the new progress made in major development technologies such as fine water flooding and CO2 flooding, offering a technical guarantee for the high-quality sustainable development of low-permeability oilfields.

2. Development Situation and Major Issues

2.1. Basic Development Situation

By the end of 2022, the developed geological reserves of low-permeability oil reservoirs have accounted for 44% of PetroChina’s total developed geological reserves. Since the beginning of the 12th Five-Year Plan period, PetroChina has increased an average of 420 million tons of developed geological reserves of low-permeability oil reservoirs per year, which has become its main battlefield for increasing oil reserves and production. It has established five production bases in Changqing Oilfield in the Ordos Basin, Daqing Peripheral Oilfield and Jilin Oilfield in the Songliao Basin, Xinjiang Oilfield in the Junggar Basin, and Liaohe Oilfield and Huabei Oilfield in the Bohai Bay Basin. The annual production of oil continues to increase at a high rate, with the proportion of output rising from 34.8% in 2011 to 37.9% in 2022.

2.2. Overall Development Situation

PetroChina’s low-permeability oil reservoirs currently have a comprehensive water cut of 68.7%, a percentage recovery of 8.7%, an oil production rate of 0.37%, and an oil production rate of 4.09% for the remaining recoverable reserves, which are generally in the high water cut and medium recovery stage. The development indexes are quite different in various oilfields due to differences in reservoir types and development stages.
The developed geological reserves of low-permeability oil reservoirs in the Ordos Basin account for 52.8% of the total developed geological reserves of PetroChina, the output accounts for 54.8%, the comprehensive water cut is 61.7%, the percentage recovery is 7.8%, and the oil production rate is 0.42%. On the whole, they are in the high water cut and medium recovery stage with stable production, and the contradiction of “low permeability, low pressure and low abundance” leading to “multiple wells but low output” is prominent. Among them, conventional low-permeability reservoirs are dominated by Jurassic reservoirs with edge or bottom water. Water flooding development has achieved a favorable effect, resulting in an oil recovery rate of over 25%. Currently, the majority of reservoirs have entered the stage of high water cut and high recovery. The challenges in the wellbore, such as casing damage, have intensified. Injected water and edge/bottom water interact in complex ways, resulting in intricate seepage characteristics and scattered remaining oil distribution.
Extra-/ultra-low-permeability reservoirs have entered the medium-high water cut stage. Affected by fractures and heterogeneity, there are both unidirectional advances and multi-directional breakthroughs. In addition, the laws of water flooding are complex, and some ultra-low-permeability reservoirs have poor physical properties. Therefore, it is difficult to establish an effective displacement pressure system for water flooding. The prominent development contradictions include large differences in injection–production pressure, main lateral pressure, and low-pressure maintenance levels despite high injection–production ratios. As a result, the situation of “low single-well output, low oil production rate and low oil recovery” occurs.
The developed geological reserves of the low-permeability oil reservoirs in the Songliao Basin account for 17.8% of that of PetroChina’s low-permeability oil reservoirs, and the output accounts for 15.7%. The comprehensive water cut, percentage recovery, and oil production rate of these reservoirs are 75.2%, 10.5%, and 0.36%, respectively. Most of them have entered the high water cut and high recovery decline stage. They are mainly thin and interbedded oil reservoirs, with large sand body variations and strong heterogeneity. In addition, water channeling and flooding are severe along fractures, the difference between main and lateral water flooding response is large, and the well pattern control and developing degree are low. Moreover, the contradictions of “short circulation, great difference and weak displacement” are concurrent, and the success rate of oil–water well development is only 67.8%, which further exacerbates the inadaptability of the injection–production system in the high water cut stage.
The developed geological reserves of the low-permeability oil reservoirs in the Junggar Basin account for 20.2% of that of PetroChina’s low-permeability oil reservoirs, and the output accounts for 15.9%. The comprehensive water cut, percentage recovery, and oil production rate of these reservoirs are 71.8%, 6.1%, and 0.32%, respectively. They generally have entered the high water cut and medium recovery increase stage. Low-permeability conglomerate reservoirs are affected by complex modes of and multi-modal pore structures, with severely ineffective water circulation. Extra/ultra-low-permeability conglomerate reservoirs are highly water-sensitive, making it difficult to establish an effective water flooding system. The pressure maintenance level and oil recovery are low.
The developed geological reserves of the low-permeability oil reservoirs in the Bohai Bay Basin account for 7.1% of that of PetroChina’s low-permeability oil reservoirs, and the output accounts for 6.3%. The comprehensive water cut, percentage recovery, and oil production rate of these reservoirs are 80.0%, 14.0%, and 0.36%, respectively. They generally have entered the high water cut and high recovery decline stage. With fault development, they have many thin layers, with small-scale sand bodies. In total, 73% of the reserves have been in the “high water cut and high recovery” or “low recovery and low oil production rate” stage. It is urgent to change the development mode and explore replacement technologies after water flooding.

2.3. Major Common Issues

2.3.1. Deterioration of Resource Quality

With the deepening of exploration and development, the newly proven reserves are mainly ultra-low-permeability oil reservoirs with poorer physical properties. Since the beginning of the 13th Five-Year Plan period, the calibrated deterioration of resource quality oil recovery of newly added developed reserves of low-permeability reservoirs is only 7.7%. Affected by the quality of resources, it is harder to achieve benefit-driven production in new oilfields.
The Jurassic reservoir of Changqing Oilfield, being mainly a conventional low-permeability reservoir, poses exploration challenges due to its “reduced structural amplitude, diminished reservoir scale, and lower filling degree”. The large-scale production areas of extra-/ultra-low-permeability Chang 6 and Chang 8 reservoirs shift from the main reservoir zone to the edge in the plane, and from contiguous thick layers to multiple thin layers in the longitudinal direction. With type III reservoirs with permeability lower than 0.3 × 103 μm2 dominant, the type III benefit reserves account for 80.7% of the proven undeveloped reserves. The undeveloped reserves of Daqingzi Oilfield in Jilin are mainly concentrated in the oil–water transition zone near the source area, thin and poor layers in the front area, and the outer front area. The oil–water transition zone in the near-source main layer has low oil saturation, and the non-main layer is thin. The oil layers at the front and outer front are single, with thin reservoirs and poor physical properties. The daily oil output by conventional water injection development is only 1.2–1.5 t, and it is hard to realize benefit-driven development.

2.3.2. Aging of Oil Field

Petroleum reservoirs accounting for 60% of the developed reserves of low-permeability reservoirs have entered the high water cut stage, with the dominant channel of water flooding formed, plane and section conflicts intensified, the water consumption rate and water flooding index greatly increased (Figure 1), and water flooding efficiency decreased. In addition, there are many wells with long shutdown, low production, and casing damage, and oil–water wells have a low rate of well opening. These factors further exacerbate the imbalance of an injection–production system, resulting in a low daily oil output (1.1 t), a low oil production rate (0.37%), and low recovery (19.4%) for a single well.
Low-permeability oil reservoirs in the Zhundong zone, represented by Huoshaoshan Oilfield, have generally entered the high-water cut stage. The water breakthrough and response are affected by the current maximum principal stress direction, natural fractures, and the distribution of sedimentary sand bodies. The injection–production contradiction is increasingly prominent in the plane and profile, the effectiveness of water injection continues to decline, and the rate of oil production is only 0.19%. It becomes more difficult to control water cuts and stabilize oil production by conventional water injection adjustment, and benefit-driven development and EOR are facing challenges. The extra-low-permeability oil reservoirs in the Songliao Basin, represented by Xinmin Oilfield, are currently in the high water cut and high recovery stage. The opening rate of producing wells and water injection wells are only 67.6% and 66.4%, respectively. As to water injection wells, multi-interval commingled injection and mixed injection are mainly adopted. The contradictions of “short circulation, great difference and weak displacement” coexist, with an oil production rate of 0.36% and an average single-well output of 0.59 t. The full cost has been at a high level of USD 60/barrel since the beginning of the “13th Five-Year Plan” period.

2.3.3. Tertiary Recovery Has Not Yet Been Applied in a Large-Scale and Profitable Manner

Low-permeability reservoirs are characterized by dense lithology, thin pore throat, low-pressure coefficient, and low potential for increased fluid flow. Some oil reservoirs have natural fractures and high-formation water salinity. The development of tertiary recovery technology suitable for such oil reservoirs is still a worldwide problem. Effective injection, microscopic sweep expansion, crude oil enablement and migration, etc., in low-permeability reservoirs still need to be explored, and there are great challenges in the development and application of oil displacement technologies and systems.
Breakthroughs have been made in field tests such as development mode change in Huaqing Oilfield and carbon capture, utilization, and storage (CCUS) in Daqingzi Oilfield in recent years, which, however, have not been popularized on a large scale. For instance, the crude oil output of Changqing Oilfield accounts for 23% of PetroChina’s total output, but the output by tertiary recovery technology only accounts for 4.3%. The coverage scale of tertiary recovery does not match the production scale of low-permeability reservoirs. Oil recovery by water flooding is low in low-permeability reservoirs. According to the evaluation of the recoverable reserves of PetroChina’s 353 low-permeability oil reservoirs, the dynamic recovery of reservoirs accounting for 57.8% of developed geological reserves is only 17.3% (Table 1). It is urgent to speed up the tackling of key problems in EOR technologies and the popularization of EOR technologies and reserve economical, applicable, green, and safe EOR technologies suitable for different types of oil reservoirs in advance.

3. Major Development Technologies and Development Potential

3.1. Solid Resource Base

According to the “14th Five-Year Plan for Modern Energy System” jointly issued by the National Development and Reform Commission and the National Energy Administration, it is required to enhance domestic oil and gas supply capacity, accelerate the use of reserves, do a good job of “controlling decline” and “enhancing recovery” of developed oil fields, promote the stable production of old oil and gas fields, increase capacity construction in new areas, and ensure continuous stable production and increase production. In recent years, the focus of domestic oil and gas development has been to increase the exploration and development of unconventional resources, and China’s deep, ultra-deep, and unconventional natural gas resources have great potential, but the degree of exploration and development is low, which is the main direction of the current and future oil and gas industry to increase exploration and development efforts and new growth poles of natural gas storage and production. Through unremitting efforts, China’s unconventional natural gas (shale gas, coalbed methane) production accounted for less than 7% of the country’s total natural gas production to about 14%. Continental shale oil has gradually become an important field in oil development. Ordos Basin proved geological reserves of more than one billion tons of shale oil integrated in a large oil field.
The low permeability reservoirs involved in this paper are also an important part of China’s unconventional reservoirs. Through innovative technologies to greatly improve recovery efficiency and control the decline rate of old oil fields, strengthening clean and efficient drilling and completion and pressure flooding technologies, rapid drilling and completion technology of large-platform ultra-long horizontal wells, and low-cost development technology, the output of low permeability reservoirs has continued to increase, accounting for more than 1/3.
The low-permeability oil reservoirs in the main oil-gas-bearing basins in China account for 60% of the total remaining oil resources, the reserves of low-permeability reservoirs in the Songliao, Ordos, Qaidam, and Junggar Basins account for more than 85% [4], and the low-permeability oil and gas reservoirs are predominant in the total remaining oil and gas resources. With the deepening of geological understanding, progress in exploration and development technologies, and improvement in evaluation methods, the field and scope of oil and gas exploration are expanding, and more and more low-permeability reservoirs are being discovered. There will be more prospective low-permeability oil and gas resources in China. China is rich in low-permeability oil and gas resources, which has laid a good foundation for oil and gas output increase and sustainable development in the future, and it also has long-term development potential [5].
Since the beginning of the “12th Five-Year Plan” period, the reserves-to-production ratio of PetroChina’s low-permeability reservoirs has increased year by year, which is kept at above 20, and the reserve replacement ratio is above 1, with a good overall resource base.

3.2. Increasingly Improved Fine Water Flooding Technology

Sweep efficiency refers to the ratio of the volume of the oil formation affected by a water-driven ripple to the total volume of the oil formation, which is an important parameter describing the interaction between oil and water phases of fluid in the water-driven process. It is an important parameter to describe the interaction between oil and water in the process of water drive. In oilfield development, it is of great significance to study and master the water drive wave coefficient to improve the recovery rate and reduce risk. Low-permeability reservoirs, due to fracture development and strong reservoir non-homogeneity, are prone to cause injection water to surge and form dominant channels, thus reducing the water-driven ripple coefficient.
The percentage recovery of low-permeability reservoirs is only 8.7%. The sweep efficiency of water flooding is small due to poor reservoir physical properties and fractures. Further expanding the sweep range through the improvement in water flooding technologies is of great potential. For example, extra-low-permeability reservoirs such as Wuliwan Oilfield and Wangyao Oilfield have been developed by water flooding for over 20 years, with a percentage recovery of more than 20%. Well inspections show that one-third of oil layers in the longitudinal direction are slightly or not flushed by water (Figure 2), with the remaining oil being enriched.
There is large room for improving oil recovery by water flooding in low-permeability reservoirs. Based on a water flooding efficiency of 40–50% and a sweep efficiency of 70%, theoretical oil recovery should be 30–35%, and the calibrated oil recovery of low-permeability reservoirs is 19.1%. With current development methods, oil recovery can be increased by at least 10%.
Through continuous research, fine water flooding technologies such as low-permeability oil reservoir description, moderately advanced water injection, well pattern infill adjustment, fine layered water injection, intelligent layered water injection, and nano-microsphere profile control are becoming mature. Compared with conventional water flooding, fine water flooding technologies are expected to improve the oil recovery of low-permeability reservoirs by more than 5% [6]. Since the beginning of the “13th Five-Year Plan” period, Changqing Oilfield has vigorously promoted the application of fine water flooding technologies such as single sand body characterization, unstable water injection, fine layered water injection, microsphere profile control and flooding, and comprehensive reservoir management. As a result, remarkable results have been achieved. The control and utilization degrees of water flooding reserves in low-permeability reservoirs have increased year by year, and the nature decline ratio has decreased from 14.3% in 2016 to 11.2% in 2022.

3.3. Increasingly Abundant EOR Means

Through years of technical research and mineral practice, the EOR means and technologies for low-permeability oil reservoirs have been continuously improved, and the technical directions of EOR technologies in different types of oil reservoirs are basically clear. For conventional low-permeability reservoirs with a certain injection–production capacity, the oil displacement efficiency can be improved by tertiary recovery technologies such as polymer flooding, polymer–surfactant binary flooding, and microbial flooding. Permeability is further reduced in extra-low-permeability reservoirs, which are hardly developed by conventional polymer flooding or water flooding, so the injection pressure should be reduced. Tertiary recovery technologies such as surfactant flooding, foam flooding, and nano-material flooding can be employed to reduce the pressure, increase the injection, and increase the swept volume. As to dense ultra-low-permeability oil reservoirs, large-scale volume fracturing combined with imbibition can be adopted for oil production, and effective energy supplement and efficient displacement can be achieved through CO2 flooding, N2 flooding, and air flooding. However, CCUS technology is currently facing challenges such as high costs, unclear business models, and insufficient policy support, and it is necessary to reduce costs, explore business models, and strengthen policy support through a variety of ways to promote its commercialization and application.
With the support of PetroChina major oil development tests, remarkable results have been achieved in CCUS, and development mode change, a new development mode of “secondary-tertiary combination”, has been explored for low-permeability reservoirs, and oil recovery has been improved by 10–25%, laying a foundation for large-scale promotion and a great improvement in oil recovery [7].

3.3.1. CCUS-EOR Technologies

CCUS (carbon capture, utilization, and storage) is a technology that allows carbon dioxide emissions from production processes to be sequestered or purified and recycled into new production processes. CO2 flooding has multiple benefits of oil displacement and emission reduction, which is in line with the national strategies of “peak carbon dioxide emissions” and “carbon neutrality” [8]. Hence, its application prospect is broad in low-permeability reservoirs, especially in ultra-low-permeability reservoirs where water-based displacement agents are difficult to inject [9]. After more than ten years of research and practice, an integrated demonstration zone of CO2 capture, displacement, and storage with high efficiency and low consumption has been built in Jilin Oilfield, the whole process of carbon capture, pipeline transportation, gathering and transportation, and circular injection has been opened up, various supporting technologies have been optimized and developed, and a series of CO2 oil displacement and storage technologies for low-permeability reservoirs with continental deposits have been initially developed [10]. The accumulated oil increment is 32 × 104 t, and the CO2 storage capacity is 250 × 104 t. The industrial application of CCUS-EOR has been strongly supported [11].
To accelerate the life cycle of CO2 flooding and quickly evaluate the EOR effect of CO2 flooding in low-permeability reservoirs with a high water cut, a pilot test of CO2 flooding with small well spacing was carried out in Hei79 North Block of Daqingzi Oilfield in 2011 [12]. In the test site, the geological reserves were 40 × 104 t, the permeability was 4.5 × 10−3 μm2, and the burial depth of the oil reservoirs was 2250 m [13]. As to the test scale, there were 10 injection wells and 27 producing wells. In addition, an inverted seven-spot well pattern (80 m × 240 m) was used, with an injection–production well spacing of about 144 m on average, and the well array distance was shortened to half of the original well pattern for water flooding. At present, the cumulative gas injection is 36.5 × 104 t [equivalent to 1.13 times of hydrocarbon pore volume (HCPV)] [7,14]. This pilot test is a unique CO2 flooding test project in China that has practiced the whole process from the early stage to the middle and late stages [15,16]. In contrast with that by water flooding, the daily oil output by CO2 flooding is increased by about five times (Figure 3), which is elevated by about six times in the core evaluation area, the cumulative oil increment is 2.67 × 104 t, the percentage recovery is increased by 23.3% in stages, and the oil recovery is predicted to be increased by 25% (Figure 4). The results achieved in the test site fully affirmed the potential and technical directions of CO2 flooding technologies in low-permeability reservoirs to greatly improve oil recovery [17,18].

3.3.2. Technologies for Changing Water Flooding Mode in Ultra-Low-Permeability Reservoirs

Energy-increase fracturing technology is a new type of low-permeability reservoir development technology based on the concept of unconventional reservoir volume development, which mainly includes supplementary energy before fracturing, increased energy during fracturing and energy diffusion after fracturing. Before fracturing, the formation energy is replenished by injecting into the extraction wells instead of the neighboring wells, and more than 8000 square meters of water is injected into the horizontal wells, so that the formation pressure reaches more than 100%. The increase in energy in fracturing is to increase the amount of fracturing fluid into the ground to more than 30,000 square meters, and the construction displacement is increased to more than eight square meters, so that the formation energy is increased rapidly, thus improving the scale of fracturing. The wells are shut down for two months after fracturing so as to promote the pressure diffusion and fully increase the elastic energy through dialysis replacement.
Ultra-low-permeability reservoirs are increasingly characterized by large covered reserves, dense reservoirs, fine pore throats, developed fractures, the difficult effective displacement of a matrix, serious water flooding and channeling, high proportion of ineffective water injection, a low oil production rate, and low oil recovery. To tackle these difficulties, a major test of water flooding mode change was carried out in Yuan284 Block of Huaqing Oilfield in 2016, and a new development concept of “breaking the boundaries between oil and water wells, combining the energy-increase refracturing with mutual flooding between wells, displacement between fractures, and imbibition and energy supplement of wells” was put forward, and key technologies such as horizontal well volume refracturing, directional well volume fracturing, directional well huff and puff, and injection–production between fractures in horizontal wells were developed. The daily oil output of horizontal wells in the test site was increased from 1.8 t to 15.3 t (control of flowing production in the initial stage), the oil production rate was increased from 0.20% to 1.15%, and the oil recovery was increased from 5.2% to 17.3% [19]. A novel mode of “reconstruction of seepage field, integration of pressure and flooding, multi-way energy supplement and all-round displacement” has been developed for increasing output and efficiency. This mode has entered the industrial test stage in Changqing Oilfield (Figure 5 and Figure 6), enabling the efficient utilization of 120 million tons of low-grade reserves. It has been applied in Daqing Oilfield, Xinjiang Oilfield, and other oilfields, with a development prospect of five million tons at the end of the 14th Five-Year Plan period.

3.3.3. “Secondary-Tertiary Combination” Technologies

“Secondary-tertiary combination” refers to the coordinated and optimized deployment of fine water flooding in secondary recovery and bed set well pattern in tertiary recovery, fine potential tapping based on the current water flooding mode, and the conversion to tertiary oil production in time. It aims to pursue the optimized connection of fine water flooding and tertiary oil production, and the orderly conversion of various oil layers, to maximize overall oil recovery and optimize the economic benefits. In 2014, PetroChina clearly put forward that “secondary-tertiary combination should be the strategic choice for the sustainable development of old oilfields” [15].
“Secondary-tertiary combination” is an innovative development concept proposed for oilfields with a high water cut. It has been applied in extra-low-permeability reservoirs such as No. 8 Block of Xinjiang Oilfield and Huoshaoshan Oilfield in Zhundong, breaking the limitation of single-block single-mode adjustment. Overall oil recovery has increased by more than 20%, which is 4–5% higher than that of direct tertiary recovery. Among them, 5–7% are attributed to water flooding, proving technical advantages of “1 + 1 > 2”.
The lower Urho Formation in No. 8 Block of Xinjiang Oilfield belongs to extremely thick extra-low-permeability glutenite reservoirs with massive fractures. After 40 years of water flooding development and five infill adjustments, the corresponding system of water flooding injection–production has not been effectively established, the reservoir pressure is kept at only 62.7%, and the percentage recovery is 20.2%, which is close to the marginal state of development. Through careful reservoir re-evaluation and development mode change, a novel secondary–tertiary combination mode of “combination of gas flooding and water flooding, combination of vertical wells and horizontal wells, and combination of plane flooding and gravity flooding” has been innovatively developed. Vertical wells and horizontal wells are combined to reconstruct the series of development, gas injection at the top makes the attic oil drained downward by gravity, and gas injection at the wing keeps the gas–liquid interface moving stably. It is estimated that the output will be 1.8 million tons, and the oil recovery will be enhanced by more than 25%, thus realizing the reconstruction of 100-million-ton old oilfields.
H42 oil reservoirs in Huoshaoshan Oilfield in Zhundong have entered the “extra-high water cut and extra-high recovery” stage, with a comprehensive water cut of 90.4% and a percentage geological reserve recovery of 27%. Combined with the characteristics of large structural dip, a thick oil layer, and low crude oil viscosity (Figure 7), a secondary–tertiary model of “fine water flooding + crestal CO2 flooding” has been formed (Figure 8), which is expected to improve the oil recovery by 26%.

4. Technical Countermeasures and Development Directions

4.1. Tackling of Problems in Benefit-Driven Development Technologies for Difficult-to-Produce Reserves

Low-permeability reservoirs have a good resource base, but the resource quality deterioration is aggravated. The key to effectively developing low-grade reserves lies in continuous improvements in concepts and technologies. The focus should be on strengthening the concept of all-factor life-cycle development, changing the development mode, testing gas injection huff and puff, gas flooding energy supplement development, supporting and integrating technologies able to greatly improve the single-well output and EUR, efficiently using the resource potential, and promoting the leap from the effective utilization to benefit-driven development of low-grade reserves.

4.1.1. Design of All-Factor Life-Cycle Development Schemes

Compared with high-level development schemes of first-class companies in foreign countries (Table 2), domestic schemes have a short basic research cycle in the early stage of design and inadequate consideration of uncertainties, thus requiring frequent adjustments during implementation. Therefore, it is necessary to strengthen the basic comprehensive research work in the early stage of scheme preparation, develop geological reserve risk assessment technologies, and strengthen the basis of recoverable reserves, reducing production risk from the source. In addition, the application of new technologies in scheme design should be increased, with focus on the study of IOR/EOR technical countermeasures, controlling the development benefit and oil recovery from the source. In addition, the whole process optimization of development schemes should be strengthened, ensuring that updating and the optimization of geological models as well as analysis and control of uncertain risks are conducted throughout the whole process of oilfield development, and improving the development level in the whole life cycle.

4.1.2. Gas Injection and Energy Supplement Development Technologies in New Fields

According to the results of gas injection tests in unconventional reservoirs in foreign countries, which showed that CO2 huff and puff can improve oil recovery by 12% [16] (Figure 9), and the development characteristics of ultra-low-permeability reservoirs in China, the passive thinking of low-energy energy re-storage is changed, and the technical research of huff-and-puff gas injection and gas flooding energy supplement in new fields is sped up. The integration technology of group fracturing combined with huff-and-puff gas injection in ultra-low-permeability reservoirs is tested, changing from production increase by fracturing to EOR. Group energy storage, interference, and deflection fracturing are promoted, synchronous and asynchronous huff and puff modes and technical policies are optimized, and the synergistic energy supplement effect of displacement and imbibition is fully played. The gas flooding development test in new fields is accelerated. Furthermore, foam-assisted gas flooding technologies are developed for reservoirs with a single oil layer and small dip angle, and for reservoirs with a large thickness, a large dip angle, and small ground saturation pressure difference, circulating gas injection and energy supplement technologies are researched.

4.1.3. Technologies Greatly Increasing Single-Well Output

Based on the law of reservoir enrichment and distribution, the key technologies for “sweet spot” prediction and evaluation in the integration of geological prospecting and well drilling with big data and high precision as the core are developed so as to strengthen the following geological target while drilling and dynamic adjustment and improve the drilling catching rate of high-quality reservoirs. The study of reservoir classification and zoning evaluation technologies is deepened, the development mode, series of development, well pattern and well type, technical policy, and so on are optimized, and the producing degree of reserves is improved. Based on the test results of development mode change in Huaqing Oilfield, the development technology of “energy storage before fracturing and displacement-imbibition combination” for ultra-low-permeability reservoirs is explored. Some machine learning approaches are also used to investigate the controlling factors of tight oil and gas productivity [20,21,22,23,24].

4.2. Development and Improvement of Water Flooding Technologies

As low-permeability reservoirs are developed, water flooding dominates. For low-permeability reservoirs where an effective displacement pressure system can be established, fine reservoir description technologies are deepened, multi-directional response injection–production systems are constructed, precise injection–production regulation and control are carried out, and problems in functional water flooding and intelligent water flooding are tackled to improve water flooding technologies, expand swept volume, and maximize water flooding potential. As to some extra-/ultra-low-permeability reservoirs where water flooding is difficult to apply, the key point is strengthening the research on seepage mechanisms and development technical boundaries, changing the development mode, and tackling the key problems in effective displacement–imbibition combination development technologies to improve the development level.

4.2.1. Fine Reservoir Description Technologies

Different from that in conventional oilfields, fine reservoir description in low-permeability oilfields focuses on reservoir space and seepage capacity. At present, fine reservoir description technologies mainly include multi-method fine prediction and characterization of single sand body, multi-information fracture characterization, research on reservoir micro-pore structure, classification of flow units in low-permeability reservoirs, and low-permeability reservoir protection technologies [25]. With the deepening of water flooding development, the inherent heterogeneity of reservoirs is aggravated by artificial fractures and dynamic fractures, making the distribution of the remaining oil more complicated. Therefore, it is necessary to focus on the research of technologies of reservoir heterogeneous dynamic characterization so as to clarify the time-varying law of reservoirs, realize the transformation from static to dynamic characterization, and improve the accuracy of geological model and remaining oil characterization, offering technical support for fine water flooding regulation.

4.2.2. Functional Water Flooding Technology

This technology mainly plays a synergistic role in greatly enhancing the oil displacement efficiency and swept volume by improving the desorption ability of crude oil from the rock surface through the adjustment of injected water ions and expanding water flooding in a deeper formation through the addition of micro-nano bubbles. Key technologies include ion-matched water flooding technology and gas–liquid micro-dispersion oil displacement technology. Field tests have been carried out in Xinmu Oilfield in Jilin, Xinghebei Oilfield in Changqing, Wuliwan Oilfield, and other oil reservoirs. Among them, pilot tests of 12 well groups were carried out in Wuliwan Oilfield in 2020; the daily fluid output of well groups rose and the water cut decreased. It is estimated that oil recovery will be increased by more than 3% (Figure 10). In the next step, the continuous tackling of problems in ion precise matching technology and improvements in the stability and shut-off capacity of micro-nano bubbles will be carried out. This technology is expected to become a green, efficient, and low-cost new technology to improve water flooding.

4.2.3. Nano Intelligent Water Flooding Technology

The design ideas of this technology include “small enough” nano-oil displacement agents to realize the full sweep of extra-/ultra-low-permeability reservoirs, “strong hydrophobicity and strong lipophilicity” with self-flooding force to realize intelligent oil detection, and “dispersed oil gathering” to capture dispersed oil, form oil wall or oil-rich zone and be displaced.
Currently, some assumptions have been realized in the laboratory. Nanoparticles modified by alkanes have enhanced hydrophobicity and can spontaneously diffuse and migrate to the oil–water interface by being attracted by the oil phase. In addition, the adsorption of nanoparticles at the oil–water interface can effectively reduce the interfacial tension and improve the carrying capacity of the water phase for the oil phase [26]. In 2019, the pilot test with “10 injection wells and 36 producing wells” was carried out in Luo I Block of Jiyuan Oilfield. The initial results were obtained, with a net oil gain of 1467 tons during the comparative phase (Figure 11). This technology is expected to become a subversive strategic replacement technology for improving water flooding in low-permeability reservoirs, with broad application prospects. However, a lot of detailed basic research and technical research tests also need to be conducted so as to finally realize the three functions envisaged by this technology.

4.3. Reserve Technologies Greatly Enhancing Oil Recovery

4.3.1. Gas Flooding EOR Technologies

Gas flooding is the major technical direction to improve oil recovery in low-permeability oilfields. In China, continental sedimentary reservoirs have more crude oil and strong reservoir heterogeneity compared with marine sedimentary reservoirs. As to reserves where gas flooding is applicable, the immiscible oil accounts for 70%, and the oil displacement efficiency is decreased by 15% on average. It is necessary to deepen the understanding of the difference between vertical and horizontal seepage in reservoirs, effectively use the characteristics of gas overlap, and develop key gas flooding EOR technologies able to promote miscibility and control gas channeling. According to the idea of “multi-gas simultaneous development and gas injection according to reservoirs”, the industrialization of gas injection EOR technologies with CCUS as the core and oxygen-reducing air (nitrogen) flooding as the auxiliary is accelerated, gas injection potential evaluation technologies are improved, and core technologies of gas flooding swept volume expansion and mixing promotion, key technologies of the cooperative application of hydrocarbon gas flooding and storage, gas channeling prevention and advanced control technology, CO2 flooding techniques, and key CCUS technologies are researched.

4.3.2. Middle-Phase Microemulsion Flooding Technology

As a thermodynamically stable system formed by mixing surfactants, additives, and oil–water, microemulsion can be divided into lower-phase, middle-phase, and upper-phase microemulsions according to the coexistence of the microemulsion and oil–water phase. Middle-phase microemulsion can solubilize both oil and water (Figure 12), which can sharply reduce the interfacial tension of crude oil remaining in rock pores in oil reservoirs to the order of l0−3–10−4 mN·m, making oil droplets easily deform and flow, and then gather to form an oil wall and be produced, and thus greatly improving oil displacement efficiency.
The results of the laboratory tests showed that for realizable middle-phase microemulsion flooding at a low surfactant concentration, the surfactant concentration is only 0.3%, and the oil displacement efficiency is over 90%. A middle-phase microemulsion system suitable for Jing’an Oilfield Chang 6 reservoir has been developed by optimizing the molecular structure and length. Under the condition of the 0.3% concentration and 0.25 PV, the best oil–water solubilization can be realized, a stable middle phase can be formed, and the percentage recovery can be increased by 13.2%. At present, it has entered the field test stage. In the next step, focus will be on the design and development of interface-expanding surfactants based on field test results, the determination of the best middle-phase microemulsion formula, proppant slug combination and displacement mode, the establishment of microemulsion phase state control methods, realization of the tackifying and solubilization of microemulsion in situ in reservoirs, and research into EOR technologies after water flooding.

4.3.3. Air Injection Thermal Miscible Flooding Technology

It was found in systematic experiments that a high-temperature thermal oxidation front at about 300 °C is formed by an air injection in light oil reservoirs, and a high-temperature and high-pressure thermal miscible displacement zone is formed by crude oil, light hydrocarbon gas, and flue gas, with oil displacement efficiency greater than 80% (Figure 13) [27]. A high-efficiency oxygen-consuming start-up technology of chemical catalytic accelerant has been developed, which can quickly increase the temperature to 300 °C under the condition that the initial temperature of the oil layers is 50 °C, forming a stable thermal oxidation front. This start-up technology does not require thermal recovery well completion and complex ignition equipment, which greatly reduces the start-up cost and is safe and controllable.
In Moliqing Oilfield in Jilin, the water injection pressure is high, the water flooding effect is poor, and the nature decline ratio is 48%. A pilot test of air injection thermal miscible flooding was carried out in well group Y10-3 in Moliqing Oilfield, and remarkable results were achieved: the daily oil output of response wells increased from 2 t to 7–15 t, and the oil recovery of the well group increased by 20.2%. The air injection thermal miscible flooding technology has great economic and technical advantages and application prospects in low-permeability reservoirs with a high water cut, which can elevate the oil recovery by over 40% theoretically [28].

5. Conclusions

This paper analyzes the key common issues in the production capacity construction of new oil fields, the stable production of old oilfields and enhanced oil recovery, and, in connection with the progress made in major development technologies and the results of major development tests for low-permeability oil reservoirs in recent years, puts forward technical countermeasures and development directions. (1) Low-permeability reservoirs have become the main prop of the oil industry and also the main body of oil reserves and output growth in the current period and the coming time in China in terms of resources, reserves, and output. (2) After more than 40 years of water injection development, low-permeability reservoirs have entered the high water cut stage, and new development contradictions have been exposed. (3) Low-permeability reservoirs have a good resource base, and the reserves-to-production ratio is increasing year by year, which is kept at above 20. (4) Fine water flooding technologies are increasingly perfect, which can improve oil recovery by more than 5% compared with conventional water flooding. For oil reservoirs where water flooding is applicable, emphasis should be laid on improving water flooding technologies by tackling key problems in reservoir heterogeneity characterization, functional water flooding, nano-intelligent water flooding, etc. (5) The directions of EOR technologies for different types of oil reservoirs are basically clear, and such technologies as CCUS, development mode change, and “secondary-tertiary combination” have achieved remarkable field results.

Author Contributions

J.C.: Conceptualization, methodology, formal analysis, investigation, resources, data curation, writing—original draft; M.H.: conceptualization, methodology, validation, formal analysis, and editing. Y.C. and B.L.: validation, data curation, formal analysis, investigation. Z.L. and Y.L.: software, formal analysis, investigation. J.X.: formal analysis and investigation. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by China National Petroleum Corporation project “Research on the mechanism and development technology of low permeability reservoirs to improve recovery”(2022KT0802) and Changqing Oilfield scientific research project “Research on countermeasures to improve the replacement pressure system in Jiyuan ultra-low permeability reservoirs” (2022DJ0401).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.

Conflicts of Interest

Authors Jun Cao, Mingqiang Hao, Baozhu Li, Zhuo Liu, and Yang Liu are employed by the company Research Institute of Petroleum Exploration and Development; Author Jun Cao is employed by the company Research Institute of Petroleum Exploration and Development of PetroChina Changqing Oilfield Company; Author Yujia Chen is employed by the company PetroChina Yumen Oilfield Company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest. The authors declare that this study received funding from China National Petroleum Corporation and Changqing Oilfield. The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

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Figure 1. Relationship between water consumption rate and water cut in Changqing extra-/ultra-low permeability reservoirs.
Figure 1. Relationship between water consumption rate and water cut in Changqing extra-/ultra-low permeability reservoirs.
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Figure 2. Water flushed conditions of different types of oil reservoirs in Changqing displayed by inspection wells.
Figure 2. Water flushed conditions of different types of oil reservoirs in Changqing displayed by inspection wells.
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Figure 3. Production curve by CO2 flooding with small well spacing in Hei79 North Block.
Figure 3. Production curve by CO2 flooding with small well spacing in Hei79 North Block.
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Figure 4. Prediction curve of oil recovery by CO2 flooding with small well spacing in the core area of Hei79 North Block.
Figure 4. Prediction curve of oil recovery by CO2 flooding with small well spacing in the core area of Hei79 North Block.
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Figure 5. Daily oil output curve of Yuan284 Block in the mode change test.
Figure 5. Daily oil output curve of Yuan284 Block in the mode change test.
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Figure 6. Deployment diagram of the mode change test.
Figure 6. Deployment diagram of the mode change test.
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Figure 7. Reservoir profil e of H4 layer of Pingdiquan Formation in Huoshaoshan Oilfield. Red: oil; blue: water.
Figure 7. Reservoir profil e of H4 layer of Pingdiquan Formation in Huoshaoshan Oilfield. Red: oil; blue: water.
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Figure 8. Schematic diagram of gas injection in Huoshaoshan Oilfield.
Figure 8. Schematic diagram of gas injection in Huoshaoshan Oilfield.
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Figure 9. Actual effect of huff-and-puff gas injection in Yingtan Oilfield.
Figure 9. Actual effect of huff-and-puff gas injection in Yingtan Oilfield.
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Figure 10. Production curve of functional water flooding well groups in Wuliwan Oilfield.
Figure 10. Production curve of functional water flooding well groups in Wuliwan Oilfield.
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Figure 11. Production curve of 26 intelligent water flooding well groups in Luo I Block of Jiyuan Oilfield.
Figure 11. Production curve of 26 intelligent water flooding well groups in Luo I Block of Jiyuan Oilfield.
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Figure 12. Diffusion solubilization and lateral sweep microscopic mechanism test of microemulsion flooding.
Figure 12. Diffusion solubilization and lateral sweep microscopic mechanism test of microemulsion flooding.
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Figure 13. Lithological test comparison of oil displacement efficiency of different flooding methods.
Figure 13. Lithological test comparison of oil displacement efficiency of different flooding methods.
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Table 1. Results of evaluation of the dynamic recovery of PetroChina’s low-permeability oil reservoirs.
Table 1. Results of evaluation of the dynamic recovery of PetroChina’s low-permeability oil reservoirs.
Development ClassificationNumber of Oil Reservoirs Evaluated Developed Geological Reserves
(10,000 Tons)		Developed Recoverable Reserves
(10,000 Tons)		Annual Oil Output
(10,000 Tons)		Proportion of
Geological Reserves
(%)		Proportion of Output (%)Comprehensive Water Cut (%)Percentage Recovery of Recoverable Reserves (%)Dynamic Recovery
(%)
Class I2118,7735061832.52.451.157.627.0
Class II2947,85311,6943376.59.971.066.624.4
Class III114244,70850,504123233.236.166.155.120.6
Class IV189426,70673,873176457.851.672.451.717.3
Total353738,039141,1323415100.0100.069.954.419.1
Table 2. Comparison of development schemes in China and foreign countries.
Table 2. Comparison of development schemes in China and foreign countries.
ItemHigh-Level Development Scheme of First-Class Companies in Foreign CountriesPresent Situation of Development Schemes in China
Basic research time in the early stage3–5 years1–3 years
Basic data situationComprehensive, detailed and completeRelatively detailed, but sometimes incomplete due to short time.
Scheme design modeWhole life cycle schemes, with a designed development period of as long as 50–80 years, and a stable production period of 20–30 years.Focus on production in the first 3–5 years, with little attention to the countermeasures for the uncertain situations that will occur after 15 years.
Parameter optimization demonstration processComparison of each parameter under more than 5 situations, and repeated argumentation with iterative correction.Comparison of each parameter under three situations mostly, and one-way circulation workflow mode without continuous correction
Scheme review and implementationRigorous process review and rigid implementationFocus on result review, with frequent adjustments during implementation
Implementation effectThe ratio of targeting is greater than 90%The ratio of targeting is 65–85%, and the contribution rate is 35–45%
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Cao, J.; Hao, M.; Chen, Y.; Li, B.; Liu, Z.; Liu, Y.; Xu, J. Advancing PetroChina’s Development Strategies for Low-Permeability Oil Reservoirs. Processes 2024, 12, 351. https://doi.org/10.3390/pr12020351

AMA Style

Cao J, Hao M, Chen Y, Li B, Liu Z, Liu Y, Xu J. Advancing PetroChina’s Development Strategies for Low-Permeability Oil Reservoirs. Processes. 2024; 12(2):351. https://doi.org/10.3390/pr12020351

Chicago/Turabian Style

Cao, Jun, Mingqiang Hao, Yujia Chen, Baozhu Li, Zhuo Liu, Yang Liu, and Jinze Xu. 2024. "Advancing PetroChina’s Development Strategies for Low-Permeability Oil Reservoirs" Processes 12, no. 2: 351. https://doi.org/10.3390/pr12020351

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