Study On Diversion Capacity Of Self - Supporting Fractures In Clear Water Fracturing Of Shale Gas Reservoir

Due to its wide distribution, rich resources and long exploration life, shale gas has become a hot field in the wave of global unconventional gas exploration recently. As we know, China has a wide distribution of shale gas as well as abundant resources, the large-scale development of shale gas will certainly have strategic impacts on energy supplies, consumption structure and exploration pattern. The development of shale gas must go through fracturing technology to achieve effective exploitation. Among all technologies, water fracturing is one of most widely used in the shale gas exploration currently. However, not all self-supporting cracks of shale reservoirs can provide effective conductivity after treatments due to different formation characteristics. How to evaluate the validity of local fractures after treatments is essential to the network fracturing projects and post frac productivity prediction. At present, there have few researches on this aspect at home and abroad, the understanding of influence mechanism on self-supporting fracture conductivity isn’t enough as well. Therefore, it is necessary to establish a laboratory testing method to make an intensive study of post frac self-supporting fracture conductivity.In this paper, through studying fissure flow, non-Darcy effect and slippage effect, we establish a model to calculate the conductivity of self-supporting fractures after water frac treatments which is suitable to shale gas reserviors. Results indicate that the model is consistent with actual situation and can describe gas flow in self-supporting fractures very well, which is sufficient for experiments requirements. On this basis, we programed a system to test self-supporting fracture conductivity in gas conditions. Based on the articles about self-supporting at home and abroad, we establish a complete laboratory test methods. Finally, by means of this model, we selected some shale outcrops in Longmaxi formation of Sichuan for experimental study, analyzed the influential factors of self-supporting fracture conductivity.The experimental results show that fractures do provide significant conductivity if the fracture faces are displaced; the conductivity can vary more than two orders of magnitude when the type of rock is different. Under low closure stress conditions, samples which have higher roughness can get higher conductivity, while under high closure stress condition, there has a positive relevance between Young’s modulus and conductivity. Stress is the key factor affecting self-supporting conductivity and there has an exponent relation between them; for gas flow in fractures, it’s easier to achieve non-Darcy flow under higher closure stress conditions. Longer term fracture conductivity measurements have demonstrated a 11% reduction in propped fracture conductivity in the first 25 hours, the conductivity tends to decrease slower after 25 hours and finally get a 12.3% reduction in the following 60 hours.This article provides a new laboratory testing method for self-supporting fracture conductivity mesurements, we believe that the conclusions will have a certain degree of guidance to discover the rule of self-supporting fracture conductivity and the research will help design waterfrac treatments as well as postfrac productivity calculation.