Further Investigation of CO2 Quasi-Dry Fracturing in Shale Reservoirs—An Experimental Study
Abstract
:1. Introduction
2. Methodology
- Shale damage test: This aspect aimed to assess the permeability reduction induced by water-based fracturing fluid and CO2 quasi-dry fracturing fluid. In this part, the PDP instrument was used to determine the shale permeability. By subjecting cores to both types of fluids, we evaluated the extent of damage inflicted on the cores. This evaluation provides insights into the comparative performance of the two types of fracturing fluids in terms of core preservation and permeability retention.
- Rheological experiment: This involved studying the rheological behavior of CO2 quasi-dry fracturing fluid under varying water–carbon ratios and temperatures.
- True triaxial fracturing experiment: Quasi-dry CO2 fracturing fluid and slickwater were applied to conduct the fracturing experiment. By subjecting shale rock samples to different hydraulic fracturing, the fracture characteristics were further evaluated.
2.1. Rock Sample Preparation
2.2. Experimental Setup and Procedures
2.2.1. Rheological Test of CO2 Fracturing Fluid
- Maximum torque: 200 mN·m;
- Volume: 400 mL;
- Minimum rotation speed CR: 10 rpm;
- Maximum rotation speed: 1500 rpm;
- Temperature range: high-temperature electric heating system: 20 degrees to 400 degrees; able to withstand pressure of 40 MPa;
- Maximum lateral loading capacity: 6 Kn.
- Base fluid preparation: The experimental water was prepared with NaCl at a salinity of 600 ppm. The base fluid was then configured with various water–carbon ratios. For instance, if the ratio was 30/70 (water/CO2), 132 mL of water and 268 mL of CO2 were combined. The thickeners, VIC-6 and VIC-2, were added in appropriate proportions. Specifically, the VIC-6 volume was 1.2% of the water volume (1.584 mL), while the VIC-2 volume was 1.5% of the CO2 volume (4.02 mL). After adding the thickeners, the mixture was stirred thoroughly to ensure uniform distribution before transferring it to the miscible device.
- CO2 fracturing fluid configuration: CO2 was introduced into the miscible device using a gas booster pump to increase the pressure inside the device. Once the pressure stabilized, mechanical stirring was employed to disperse and mix the miscible system. The stirring was performed in both the forward and reverse directions for 15 min at a speed of 2500 rpm to ensure thorough dispersion and homogeneity of the fracturing fluid.
- Rheological testing: The prepared CO2 fracturing fluid is then subjected to rheological testing using a Harkar rheometer (Thermo Fisher, Waltham, MA, USA) to evaluate its rheological characteristics at different temperatures.
2.2.2. Shale Permeability Damage Test
- The column samples were dried, and the permeability of the untreated shale was measured to establish a baseline.
- The core sample comprising both the core plug and fragments was placed in the high-temperature and high-pressure reactor. The reactor was then evacuated and subsequently immersed in a water bath device set to a constant temperature of 60 °C.
- CO2-based fracturing fluid was pumped into the high-temperature and high-pressure reactor until the pressure within the reactor increased by 10 MPa. The core sample was allowed to react with the fracturing fluid for designated durations of 3, 5, and 7 days, respectively.
- After the specified reaction periods, the core plug and fragments were removed from the reactor. The core plug was dried and subjected to permeability testing to assess any changes in permeability resulting from the interaction with the fracturing fluid. Meanwhile, the fragments underwent further treatment to meet the requirement of the XRD test, and the alterations in mineral composition were evaluated.
2.2.3. True Triaxial Fracturing Experiment
- A stainless steel shaft was drilled into the center of the rock sample, and the well was cemented using high-strength cement to ensure stability.
- The prepared sample was placed in the triaxial loading chamber, ensuring the device’s airtightness. CO2 fracturing fluid was prepared in an intermediate container, and the pipeline was connected. Triaxial stress is gradually applied until it reaches stability. Fluid is injected into the sample at a set flow rate, and fracturing tests are conducted. Pressure changes are recorded during the test, and injection ceases when a significant pressure drop occurs, indicating completion of fracturing.
- After fracturing, the sample is placed in a high-precision CT device. Cracks in the rock are reconstructed using the CT device and post-processing software. This step allows for detailed visualization and analysis of fracture distribution within the rock sample, providing valuable insights into the fracturing process and fracture characteristics.
3. Results and Discussion
3.1. High-Pressure and High-Temperature Rheological Experiment
3.2. Mineral Composition and Permeability Variation
3.3. True Triaxial Fracturing Experiment Results
- Fractures induced by slickwater
- 2.
- Fractures induced by CO2 fracturing fluid
4. Conclusions
- Compared to conventional water-based fracturing fluids, quasi-dry CO2 fracturing fluid causes less damage to the reservoir. Under the influence of CO2 fracturing fluid, core permeability initially increases before decreasing. Short-term exposure to the fracturing fluid enhances reservoir permeability, but prolonged exposure is detrimental.
- The acidic environment of CO2 quasi-dry fracturing fluid promotes the dissolution of feldspar, illite, and chlorite minerals, which initially compensates for the decrease in permeability caused by shale expansion. However, as soaking time increases, the weakening effect of chemical reactions allows shale hydration expansion to regain dominance, leading to a decrease in shale permeability.
- The viscosity of conventional CO2 is insufficient for proppant transport into fractures, and water-based thickeners are necessary to improve CO2 viscosity. Rheological tests demonstrate that a water–carbon ratio of 3:7 yields a maximum viscosity of 104 mPa·s for CO2 fracturing fluid. Adjusting the water proportion affects viscosity, with higher proportions leading to decreased viscosity. Additionally, temperature significantly influences fracturing fluid viscosity.
- Fractures formed by conventional water-based fracturing fluids tend to be relatively singular, with small volumes but larger widths (approximately 0.27 mm). In contrast, fractures induced by CO2 fracturing fluid exhibit larger volumes, with an average width of only 0.14 mm.
- CO2 fracturing fluid exhibits a high rock-breaking ability. Particularly, in cores with numerous weak surfaces, the fluid tends to rapidly open along these weak surfaces, leading to the formation of a complex fracture network.
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
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Water–Carbon Ratio | Temperature, °C | ||
---|---|---|---|
50 | 70 | 90 | |
30:70 | 104.25 | 93.37 | 78.70 |
35:65 | 63.50 | 45.63 | 22.56 |
40:60 | 19.43 | 16.77 | 15.62 |
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Zheng, B.; Tang, W.; Wang, Y.; Li, Y.; Shen, B.; Wang, Y.; Hu, L.; Deng, Y.; Wu, M.; Xi, S.; et al. Further Investigation of CO2 Quasi-Dry Fracturing in Shale Reservoirs—An Experimental Study. Processes 2024, 12, 912. https://doi.org/10.3390/pr12050912
Zheng B, Tang W, Wang Y, Li Y, Shen B, Wang Y, Hu L, Deng Y, Wu M, Xi S, et al. Further Investigation of CO2 Quasi-Dry Fracturing in Shale Reservoirs—An Experimental Study. Processes. 2024; 12(5):912. https://doi.org/10.3390/pr12050912
Chicago/Turabian StyleZheng, Bo, Weiyu Tang, Yong Wang, Yipeng Li, Binbin Shen, Yongkang Wang, Longqiao Hu, Yougen Deng, Mingjiang Wu, Shangyong Xi, and et al. 2024. "Further Investigation of CO2 Quasi-Dry Fracturing in Shale Reservoirs—An Experimental Study" Processes 12, no. 5: 912. https://doi.org/10.3390/pr12050912
APA StyleZheng, B., Tang, W., Wang, Y., Li, Y., Shen, B., Wang, Y., Hu, L., Deng, Y., Wu, M., Xi, S., & Liu, X. (2024). Further Investigation of CO2 Quasi-Dry Fracturing in Shale Reservoirs—An Experimental Study. Processes, 12(5), 912. https://doi.org/10.3390/pr12050912