Acid-Etched Fracture Conductivity with In Situ-Generated Acid in Ultra-Deep, High-Temperature Carbonate Reservoirs
Abstract
:1. Introduction
2. Acid-Etched Fracture Conductivity Experiment
2.1. Specimen Preparation
2.2. Experimental Equipment and Procedures
2.3. Experimental Schemes
- (1)
- Fracture types: Due to the natural fracture development in the ultra-deep reservoirs, two types of fractures are expected to form during acid fracturing: (i) primary acid-etched fractures, which extend from the wellbore and have a larger aperture and (ii) secondary acid-etched fractures, which are natural fractures with a small aperture and etched by the leak-off acid during the propagation of the primary fracture. This study simulates these fractures by varying the experimental fracture width and acid concentration at the fracture entrance.
- (2)
- Fracture width in the experiment: The fracture widths of the primary fracture and the secondary fracture are obtained through numerical simulation using a fracture propagation model. The input geological parameters and engineering parameters used in the simulation are the typical values of the Shunbei oil field in the Tarim basin, China [29]. Based on the numerical simulation results of complex fractures, the typical widths of primary and secondary fractures were set at 5 mm and 0.5 mm, respectively.
- (3)
- Experiment temperature: The experimental temperature was set to the typical temperature of 160 °C in the Yijianfang formation. After the experiment temperature reached the target value, it was maintained for 3 h to ensure thorough heating of the acid fluid and rock slabs.
- (4)
- Acid concentration at the secondary fracture entrance: Given the strong uncertainty and randomness in the distribution of natural fractures, the concentration of acid at the entrance to the secondary fractures (intersection of primary and secondary fractures) is difficult to predict. In the experiment, the acid concentration at the entrance to the secondary fractures was set to half of that at the entrance to the primary fractures.
- (5)
- Acid injection rate: The injection rate is determined according to the flow regime similarity [19]. The Reynold’s number of the acid flowing in the fracture in the field condition can be calculated through Equation (2):
- (6)
- Acid–rock contact time (injection time) and injection rate: Based on typical acid–rock contact times during field operations, the acid–rock contact times in the experiment were set to range from 20 to 90 min.
3. Experimental Results and Analysis
3.1. Variation Pattern of Acid-Etched Fracture Conductivity in Primary Fractures with In Situ-Generated Acid
3.1.1. Acid-Etched Surface Morphology and Dissolution Volume
3.1.2. Acid-Etched Fracture Conductivity
3.2. Variation Pattern of Acid-Etched Fracture Conductivity in Secondary Fractures with In Situ-Generated Acid
3.2.1. Acid-Etched Surface Morphology and Dissolution Volume
3.2.2. Acid-Etched Fracture Conductivity
3.3. Comparison of Fracture Conductivity between In Situ-Generated Acid and Conventional Acid Systems
4. Empirical Model of Acid-Etched Fracture Conductivity with In Situ-Generated Acid
5. Conclusions
- (1)
- After in situ-generated acid etching, the surface morphologies of the primary and secondary fractures showed significant differences. The primary fracture surface presented a dense particulate pattern, while the secondary fracture surface exhibited distinct grooves. Due to grooves formed on the secondary fracture surfaces, the secondary fractures exhibited higher conductivity than the primary fractures.
- (2)
- The conductivity of acid-etched fractures increased with the acid–rock contact time. However, when the acid–rock contact time exceeded approximately 70 min, the rate of increase in conductivity slowed down, which suggests that a minimum contact time of 70 min should be achieved during field operation.
- (3)
- The conductivity of both primary and secondary fractures treated with in situ-generated acid was lower than that treated with gelled acid but higher than with crosslinked acid. A combination of gelled acid and in situ-generated acid is conducive to a deep penetration distance and high fracture conductivity.
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
References
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Sample Size (mm) | Permeability (mD) | Density (g/cm3) | Mineralogy (%) | |||
---|---|---|---|---|---|---|
Calcite | Dolomite | Quartz | Feldspar | |||
178 × 38 × 20 | 0.01 | 2.710 | 95.3 | 1.6 | 2.5 | 0.6 |
No. | Acid Type | Injection Time (min) | Acid Concentration (%) | Fracture Type |
---|---|---|---|---|
1# | in situ-generated acid | 20 | 20 | primary fractures |
2# | 30 | |||
3# | 40 | |||
4# | 50 | |||
5# | 60 | |||
6# | 70 | |||
7# | 80 | |||
8# | 90 | |||
9# | 20 | 10 | secondary fractures | |
10# | 40 | |||
11# | 60 | |||
12# | 80 | |||
13# | gelled acid | 60 | 20 | primary fractures |
14# | 10 | secondary fractures | ||
15# | crosslinked acid | 20 | primary fractures | |
16# | 10 | secondary fractures |
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Jia, H.; Pu, H.; Li, J.; Wang, J.; Chen, X.; Mou, J.; Gao, B. Acid-Etched Fracture Conductivity with In Situ-Generated Acid in Ultra-Deep, High-Temperature Carbonate Reservoirs. Processes 2024, 12, 1792. https://doi.org/10.3390/pr12091792
Jia H, Pu H, Li J, Wang J, Chen X, Mou J, Gao B. Acid-Etched Fracture Conductivity with In Situ-Generated Acid in Ultra-Deep, High-Temperature Carbonate Reservoirs. Processes. 2024; 12(9):1792. https://doi.org/10.3390/pr12091792
Chicago/Turabian StyleJia, Haizheng, Hongyuan Pu, Jianmin Li, Junchao Wang, Xi Chen, Jianye Mou, and Budong Gao. 2024. "Acid-Etched Fracture Conductivity with In Situ-Generated Acid in Ultra-Deep, High-Temperature Carbonate Reservoirs" Processes 12, no. 9: 1792. https://doi.org/10.3390/pr12091792