Experimental Simulation Of Competitive Mechanism Of Crack Propagation In Shale Induced By High Voltage Electric Pulse Water Pressure

Shale gas,as a kind of clean energy,has abundant reserves in the world.Developed countries such as the United States and Canada have developed some quite mature shale gas development technologies.The amount of our proved shale gas resources is（28～100）×10¹²m³.However,the shale gas in our country is characterized by deep burial,tight reservoir,low permeability and strong heterogeneity,which makes it difficult to exploit shale gas.Shale gas is usually exploited by hydraulic fracturing technology abroad,but is subject to the complex geological conditions in our country such as lack of water resources.Many experts and scholars have applied the high voltage electric pulse hydraulic fracturing technology used in oil plugging removal technology to shale gas exploitation,and found that the technology has the characteristics of safe and controllable,sufficient cracking,repeatable discharge and so on.However,in the process of fracturing,multiple cracks propagate synchronously and compete with each other.At present,the research on the mechanism and law of multi-crack competitive propagation is not in-depth.In this paper,the distribution of stress field in each stage of hydraulic fracturing is modeled according to the hydraulic fracturing process.According to the hydraulic fracturing construction process,the primary stress field of the reservoir before fracturing,the stress field around the wellbore after drilling,the stress field induced by hydraulic fracturing,the stress field induced by hydraulic augmentation of the reservoir pores after hydraulic fracturing and the stress field around artificial fractures generated by fracturing operations are analyzed respectively.According to the principle of stress superposition and considering all factors comprehensively,The formula of stress field distribution around wellbore after hydraulic fracturing is deduced.Compared with previous studies,the established model fully considers the influence of many influencing factors on the multi-crack stress field,so it is more consistent with the actual characteristics of hydraulic fracturing.Then,different proportions of 40-80 mesh quartz sand,cement,water and early-strength agent were used to obtain materials with similar physical and mechanical parameters to shale.The prefabricated crack device was used to prefabricate specimens with prefabricated cracks of different angles and lengths.The high-voltage electric pulse hydraulic fracturing test was conducted through the true triaxial high-voltage electric pulse hydraulic fracturing platform and the test results were analyzed.Finally,extended finite element numerical simulation is used to further study the test results.The main conclusions are as follows:（1）In the high-voltage electric pulse hydraulic fracturing test,during the process of double crack propagation with different prefabricated angles and lengths,it is found that under the condition of the same length,the larger the Angle between cracks,the smaller the deflection Angle of crack propagation,that is,the smaller the interference caused by competitive propagation;Under the same Angle,the total deflection Angle of double crack with prefabricated length of 20mm is 32%less than that of double crack with prefabricated length of 10mm.（2）In the numerical simulation,the competitive propagation degree between the two cracks is positively correlated with the included Angle between the prefabricated cracks and the length of the prefabricated cracks.The longer the prefabricated length is,the longer the propagation length is.When the included crack Angle is 60°,the total propagation length of the double cracks in the 20mm-10mm group increases by 7.68%compared with that in the 10mm-10mm group.Under the same prefabricated length,the crack propagation length increases with the increase of Angle.Compared with 60°,the crack propagation length at 90°increased by 8.77%.The crack propagation length increased by 28.87% at 120° angle.（3）The experimental and simulation results show that when the three cracks are propagated synchronously,the intermediate crack is affected by the stress interference generated by the crack propagation process on both sides,and the intermediate crack basically propagates along the original prefabricated direction,and the propagation effect is weaker than that of the left and right lateral cracks.（4）Through the analysis of the stress nephogram,it can be found that the stress concentration is mainly distributed in the injection point at the edge of the water injection hole,the crack watershed and the crack tip.The crack is affected by the concentrated stress field,and the crack propagation direction is deflected and the crack extension effect is enhanced.（5）When the horizontal geostress difference is 0MPa and the horizontal geostress increases from 6.5MPa to 8.5MPa,the total deflection Angle of 10-20-10 three cracks decreases by 3°（7.3%）,and the total crack growth length decreases from 22.14cm to 13.3cm（66.47%）.The total area of crack growth is reduced by 47.1%,that is,with the increase of horizontal ground stress,the crack growth effect weakens sharply,but its effect on the direction of crack growth is negligible.（6）When there is horizontal in-situ stress difference（σ₃≠σ₁）,that is,σ₃ stress remains unchanged,σ₁ stress increases from 6.5MPa to 8.5MPa,10-20-10 left and right cracks extend in the direction ofσ₁ stress,intermediate cracks also extend in the direction ofσ₁ stress,and the total deflection Angle increases by 107.7%.Whenσ₁ stress is constant,σ₃ stress increases,the crack propagates in the direction ofσ₃ stress,and with the continuous increase of horizontal in-situ stress difference,the cracks change from a trend away from each other to a trend closer to each other.In other words,the competition between cracks and horizontal in-situ stress difference simultaneously affect the direction of crack propagation.