1. Introduction
Shale oil is considered a critical unconventional source to meet future energy demands [
1]. For the development of a shale oil reservoir, multistage fracturing in the horizontal wells (MFHW) has been broadly utilized to produce the trapped hydrocarbon resources [
2]. Through fracturing treatment, fractures with high conductivity are created in the formation rock, which is crucial for the development of unconventional reservoirs [
3,
4]. Field evidence shows that, after hydraulic fracturing, temporary well shut-ins can improve well performance and enhance ultimate recovery in low-permeability unconventional reservoirs. Due to recent fluctuations in the oil price due to the COVID pandemic, a considerable number of these wells around the world were temporarily shut-in over the past few years [
5,
6], and this validates our conclusion. Therefore, the well shut-in has become a hot topic in the development of shale and tight oil reservoirs, especially in China. In the Changqing oil field in the northwest of China, well shut-in has been utilized extensively during the development of its shale oil reservoirs. Since 2017, there have been hundreds of new horizontal wells fractured within the Chang-7 Member in the upper Triassic Yanchang formation. All those production wells underwent a shut-in before flowback and formal production were resumed. Even though temporary well shut-in has been widely used in this region, the mechanism behind this process is still unclear to the operators.
Currently, imbibition, including dynamic and spontaneous imbibition, is considered one of the major causes and can explain the mechanism behind the well shut-in [
7]. Therefore, understanding how to take an advantage of the imbibition effect to enhance well performance and achieve better long-term productivity could be substantial to the development of shale oil reservoirs [
8]. In terms of the studies concerning imbibition, nuclear magnetic resonance (NMR) imaging techniques [
9] are widely used in the experiments. Generally speaking, in those experiments, researchers saturated core samples with oil and then immersed the core samples into water to see the imbibition. During the experiment process, NMR testing is used to image the oil–water displacement [
10,
11,
12,
13,
14,
15]. Karimi et al. [
16] leveraged centrifugation and NMR techniques at the same time to study the oil–water displacement pattern, focusing on the effect of capillary force. They found that, due to the imbibition effect, water enters into the tiny pores much more easily than oil in a water-wet environment. Tu and Sheng [
17] studied the effect of pressure on imbibition in a shale oil reservoir, utilizing experimental and numerical methods. Based on the NMR imaging results, Cheng et al. [
18] claimed that submicropores contribute more to the ultimate recovery of spontaneous and dynamic imbibition, despite the nanopores having stronger capillary forces. In addition to experimental studies on imbibition, there has been some progress made in the theoretical and numerical study of this phenomenon [
19]. Schmid et al. [
20,
21] presented a semianalytical solution to describe spontaneous imbibition behavior. In their study, they also provided a numerical solution for this problem. Their simulation efforts considered two wettability cases and three viscosity ratio cases [
21]. Besides, Khan et al. [
22] simulated the oil–water displacement based on a fully implicit black oil simulator, considering strongly water-wet, weakly-wet, and mixed wet cases, respectively.
In terms of the effect of shut-in on well performance in unconventional reservoirs, studies are still relatively limited. Wang [
23] studied the characteristic of reservoir energy balance and energy storage after a well shut-in. It can be concluded that a temporary well shut-in after fracturing could accelerate the diffusion and pressure propagation and slow down the reservoir energy depletion. Zhang et al. [
24] evaluated the potential for oil recovery enhancement when considering imbibition and the corresponding time delay in a shale oil reservoir for a shut-in. In their study, the effects of the pore geometry and clay content were focused on. Liu [
25] used NMR to study the effect of well shut-in on the imbibition rate and recovery of tuffite, shale, tight sandstone, and clastic volcanic rock. Many studies looked at the effect of well shut-in on aqueous phase trapping (APT) caused by formation damage due to drilling, completion, etc. Based on the experimental results, they found that APT could be “auto-removed” after temporary well shut-in [
26,
27,
28] and the optimal postfrac shut-in time could be determined [
29]. Wang et al. [
30] presented a pressure drop model for postfracturing shut-in simulation, considering multiple effects, including fluid imbibition and oil replacements. Their research revealed that the pressure drop during shut-in can be divided into eight sequential stages, and their results can be used for the interpretation of fracture and reservoir parameters. Eltahan et al. [
31] studied the impact of well shut-in after hydraulic-fracture treatments on the productivity and recovery of tight oil reservoirs. In their study, the embedded discrete fracture model (EDFM) [
32] was used for fracture representation, and their results indicated that the well shut-in could improve the recovery by as much as 5%. Jia et al. [
33] investigated the shut-in effect on production performance, considering stress sensitivity. They found that reservoir permeability and capillary force have the most obvious effects on shut-in performance.
Regarding the gaps in the current research on well shut-in methods, although a great amount of research has been presented, there still exist some problems to be solved in this area. First, most of the experiments mentioned before were carried out in a core scale, for which the conclusion may not be convincing when repeated at the field scale. Second, it is known that well shut-in operation can be effective for the long-term production of shale oil reservoirs. However, several key points still need to be addressed for optimizing future production scenarios, including determining the optimum shut-in interval, what is the difference between optimum shut-in time in different reservoirs, and the potential for multiple rounds of shut-ins. Finally, for The Chang-7 Member, the shale oil reservoir in the Changqing oil reservoir, extensive field tests of well shut-in have been carried out while there lacks specific guidance for field production, especially for the potential of multiple rounds of well shut-ins. This issue demands we present a specific investigation [
34]. In summary, the studies mentioned before still cannot fully explain the mechanism behind the well shut-in and provides guidance on the shut-in operation in the field. Further studies are still necessary in this area.
The objective of this research is to study the feasibility of the temporary shut-in method in the development of shale oil reservoirs. Besides, we also want to design reasonable production schemes and explore the potential for multiple rounds of well shut-ins. This paper is organized as follows: First, the geological background of the research area will be described. Then, a coupled mathematical model for the simulation of a well shut-in and its numerical implementation is explained in
Section 3. The numerical model is verified by comparing the results with the Buckley–Leverett equation in
Section 4.1. After that, the detailed results and discussion section are presented in
Section 5, including oil–water displacement during shut-in, determining the optimal well shut-in interval, and the potential for multiple rounds of well shut-ins. This study provides new insight into the optimal well shut-in time and the potential benefits of multiple rounds of well shut-ins. The results and conclusions from this paper can be expected to provide quantitative guidance to optimize the operation scenario in the development of shale oil reservoirs. Some suggestions derived from this study can be directly applied in the Chenghao area and for the Changqing oil field.
2. Geological Background
This study focuses on the shale oil developments in the Chenghao area, which is located in the Ordos Basin, northwest of China. In this area, production wells are mainly drilled to exploit shale oil in the Chang-7 Member in the upper Triassic Yanchang formation [
35]. The Chang-7 Member consists primarily of profundal laminated shale, occasional tuffaceous mudstone, and muddy siltstone interbeds. It is one of the most organic-rich intervals in the central and southern parts of the Ordos Basin [
36]. The Chang-7 Member in the Ordos Basin has porosities ranging from 6% to 11% and an average porosity of 8.8%. The permeability of the reservoir matrix ranges from 0.08 to 0.3 mD, and the average permeability value is 0.13 mD [
35]. According to the high-pressure mercury intrusion tests carried out for this field, the pore throat radius mainly distributes from 0.02 to 1 μm. Besides, the organic matter carbon content in the matrix ranges from 0.45% to 35.85%, with an average of 9.02%. Hydrocarbon generation potential ranges from 0.19 to 116.17 mg/g, with an average value of 34.55 mg/g [
37]. Since 2017, there are now hundreds of new horizontal wells fractured in the Chang-7 Member in the upper Triassic Yanchang formation. It is worth mentioning that all the production wells underwent shut-ins before flowback and production. Those newly developed wells are (on average) drilled with 1709 m of horizontal length and fracked with an injection of 28612 m
3 of slickwater and 3268 m
3 of proppants placed into 23 clusters with a total of 111 induced fractures [
38,
39,
40]. Geological information of the Chang-7 Member can be found in the reference [
41].
6. Conclusions
In this study, the feasibility of the temporary well shut-in in a shale oil reservoir was studied using numerical modeling. A mathematical model for the simulation of immiscible two-phase flow in a shale oil reservoir was presented based on the coupling of the phase transport in the porous media module (PTPM) and Darcy’s law. The model was validated by comparing its results with an available analytical solution. The geological background of the Chang-7 Member (Chenghao, China) is presented, and the corresponding parameters were built into the establishment of the model simulation. Based on the simulation results, a quantitative analysis was carried out, and several conclusions can be drawn.
According to the calculated saturation distribution, an oil–water displacement can be easily identified during well shut-ins, which reveals the effect of the dynamic imbibition. Based on the calculated production rates, it can be concluded that the implementation of a shut-in can decrease the initial oil rate; however, this decreases the oil decline rate as well, which is beneficial to long-term production. After 1000 days of production, the oil production rate, with 120 days of shut-ins, was 9.85 % larger than that with no shut-in operation. By judging the average daily production rates, the optimal shut-in time was determined as 60 days for our given field conditions. Besides, the potential of several rounds of shut-ins was also explored. Single and double rounds of shut-ins are equally beneficial to long-term production under the presented reservoir condition. When two rounds of shut-ins are implemented, it is recommended to perform the second shut-in round after 300 days of production. To sum up, this study reveals a workflow for feasibly studying temporary well shut-in operations in any shale oil reservoir and provides guidance for optimizing overall development scenarios.