| Hydraulic fracturing has been widely used in the field of traffic engineering,water conservancy project and energy engineering.It is the theoretical basis of oil and gas exploitation,grouting reinforcement in surrounding rock and water inrush prevention and control technology,and also the core method of oil and gas production and splitting grouting.Under traditional hydraulic fracturing,hydraulic fractures propagate in main fracture manner,and the fracture network distribution between hydraulic fractures and natural fractures is small.Under pulse hydraulic fracturing,micro fractures can fully extend,and rock mass can generate multiple fractures,which is conducive to the formation of fracture network system.There are different characteristics of hydraulic fracture propagation under static hydraulic fracturing and pulse hydraulic fracturing,the combination hydraulic fracturing methods of the two hydraulic fracturing need to be further explored.The interaction mechanism of hydraulic fractures and preexisting fractures and the formation of fracture network under pulse and combined hydraulic fracturing have not been studied.It is an urgent problem to understand the interaction mechanism between hydraulic fractures and preexisting fractures under different hydraulic fracturing methods and master the fracture network evolution laws and fracture network fracturing effects,which can obtain good grouting reinforcement effects and improve the effect of oil and gas production.To solve the above problems,laboratory experiments,theoretical analysis and numerical simulation are used to study the interaction mechanism of hydraulic fractures and preexisting fractures and the fracture network formation laws under static hydraulic fracturing,pulse hydraulic fracturing and combined hydraulic fracturing.The rock fracture mechanics model under pulse hydraulic fracturing is established,the hydraulic fracture propagation mechanism and the criteria for hydraulic fractures penetrating preexisting fractures are obtained under pulse and prepulse combined hydraulic fracturing.The fracture propagation calculation module for pulse and combined hydraulic fracturing is developed and the optimal combined hydraulic fracturing method is obtained.(1)The pulse hydraulic fracturing tests under different pulse loading stress ratio is carried out,and the fatigue tensile strength and tensile strength decay law of rock like materials are studied.With the increasing the ratio of pulse loading times,the damage tensile strength of the specimen decreases gradually.The propagation criterion of hydraulic fracture under pulse hydraulic fracturing is obtained,the fracture damage decay model of rocks under pulse hydraulic fracturing is established,and the formation law of hydraulic fracture network under pulse fracturing is revealed.(2)The criterion of hydraulic fractures penetrating preexisting fractures under pulse fracturing is proposed,and hydraulic fractures under pulse hydraulic fracturing are more difficult to penetrate the preexisting fractures than traditional hydraulic fracturing.The stress field of hydraulic fracture tips and the rock strength variation laws under different pulse hydraulic loading parameters are analyzed.The mechanical model of hydraulic fractures extending to preexisting fractures under pulse hydraulic loading is established.The propagation behavior of hydraulic fractures extending to preexisting fractures under pulse hydraulic fracturing is mainly affected by approaching angle,pulse loading parameters and rock fatigue tensile strength.(3)The interaction mechanism between hydraulic fractures and preexisting fractures under different pulse loading parameters is explored.The induction effect of existing fractures on hydraulic fracture propagation path and fracture network fracturing effect under different pulse loading parameters are obtained.The propagation direction of hydraulic fracture under pulse hydraulic fracturing is easier to propagate to the existing fracture direction than that under traditional hydraulic fracturing.With the increase of pulse loading frequency and stress ratio,the hydraulic fracture propagation mode tends to be consistent with that under traditional hydraulic fracturing.(4)According to the characteristics of hydraulic fracture and fracture network hydraulic fracturing effects under static hydraulic fracturing and pulse hydraulic fracturing method,a combined hydraulic fracturing method of pulse hydraulic fracturing and static hydraulic fracturing is proposed.The propagation behavior and fracture network formation of hydraulic fractures and preexisting fractures under different hydraulic fracturing methods are analyzed,and the criterion of hydraulic fractures penetrating preexisting fractures under combined hydraulic fracturing is established.Under combined hydraulic fracturing,the hydraulic fractures can better communicate with the preexisting fractures than static hydraulic fracturing and pulse hydraulic fracturing.The more dense fracture networks and deeper growth of hydraulic fractures under combined hydraulic fracturng can improve the hydraulic fracturing effect and shorten the hydraulic fracturing time.(5)The interaction mechanism between hydraulic fractures and preexisting fractures and the fracture network formation laws under different combined hydraulic fracturing parameters are studied.The propagation pattern of hydraulic fractures and preexisting fractures and the fracture network formation under combined hydraulic fracturing are affected by combined loading parameters.When the pre-pulse loading stress ratio is small,with the increase of pre-pulse loading times,more hydraulic fractures can generate and extend to the preexisting fracture surface,forming a complex fracture network.We summarizes the fracture network formation laws under combined hydraulic fracturing,and obtains the optimal combined hydraulic loading modes,which improves the rock hydraulic fracturing effects.(6)Based on the continuous discontinuous numerical simulation method(CDEM),the calculation modules of pulse and combined hydraulic fracturing are established.By introducing the fracture damage decay model,the interaction between hydraulic fractures and preexisting fractures under different hydraulic fracturing methods is simulated.The propagation mechanism of hydraulic fractures encountering preexisting fractures and the fracture network formation mechanism under different hydraulic fracturing methods has been revealed. |
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