Numerical simulation and parameter optimization of hydraulic fracturing for hot dry rock reservoirs based on cohesive zone method
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摘要: 水力压裂技术是实现低渗油气及地热储层的高效开发利用的关键技术手段，为了研究干热岩型地热储层水力压裂过程中水力裂缝的扩展规律，本文使用粘结单元法（Cohesive Zone Method， CZM）研究了压裂液排量、压裂液粘度以及水平地应力差对水力裂缝形态的影响，并利用正交试验对上述压裂工艺参数的组合进行优化。结果表明：压裂液排量对水力裂缝的长度具有重要影响，而压裂液的粘度对水力裂缝的宽度具有显著影响；压裂液的排量和粘度的增加，促进了分支裂缝的萌生和扩展；水平地应力差为1 MPa时，本文所建立的模型在压裂液排量和粘度分别取0.004 m3/s和0.07 Pa·s条件下，可获得最佳的压裂改造效果；随着压裂液的排量和粘度的持续增加，当压裂液的排量和粘度分别超过0.004 m3/s和0.07 Pa·s后，继续增加压裂液的排量和粘度将导致水力裂缝的长度和宽度的减小，可见在实际压裂过程中不能盲目通过提高压裂液的排量和粘度的方式实现对压裂效果的持续改进。本文丰富了干热岩储层改造的数值模拟手段，相关研究成果有望为干热岩型地热资源开采过程中裂缝扩展行为预测和压裂工艺参数的优化提供技术支撑。Abstract: Hydraulic fracturing technology plays a key role in the efficiency exploitation of low-permeability oil-gas and geothermal reservoirs. In order to study the extension pattern of hydraulic fractures within the hot dry rock, the effect of flow rate and viscosity of fracturing fluid and horizontal geo-stress difference on the morphology of hydraulic fractures were investigated by the cohesive zone method (CZM), and the combination of the above fracturing process parameters was optimized using orthogonal tests. The results show that the flow rate of fracturing fluid has a significant effect on the length of the hydraulic fracture, whereas the viscosity of the fracturing fluid has a significant effect on the width of the hydraulic fracture. The increase in the flow rate and viscosity of the fracturing fluid promotes the emergence and extension of branching fractures. When the horizontal geo-stress difference is 1MPa, the model established in this paper can obtain the best fracturing modification effect under the conditions that the fracturing fluid flow rate is 0.004m3/s and the viscosity is 0.07Pa·s; As the flow rate and viscosity of fracturing fluid exceed 0.004m3/s and 0.07Pa·s respectively and continuously increase, a decrease in the length and width of the hydraulic fracture will occur. Therefore, it is considered that the continuous improvement of fracturing effect cannot be achieved blindly by increasing the flow rate and viscosity of the fracturing fluid during the actual fracturing process. The findings of this study are expected to provide significant support in predicting the fracture extension behavior and the optimizing the fracturing parameters during the exploitation of hot dry rock geothermal resources.
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