Study On Failure Of Cement Ring Hydraulic Fracturing Under Coalbed Methane Reservoir

To increase the yield of coalbed methane well,CBM fracturing simulation technique is widely used.However,some CBM well failed to meet the designed gas production after fracturing due to excess water production and low gas production.One possible explanation for this is that hydraulic fractures extend beyond the coal seam roof and floor plates and water formation is connected,another is that well cement ring is damaged during the process of hydraulic fracturing and 1ayer of water is connected.Few researches have been done on the integrity of underground well cement ring after hydraulic fracturing due to the lack of research tools.Hence,it is necessary to conduct fracturing experiments on the well cement ring to investigate what influence that hydraulic fracturing will have on the damage of cement ring and obtain a better understanding of the water production of CBM well.Therefore,it is of great significance to investigate what impact that hydraulic fracturing have on the integrity of well cement ring.In this paper,the coal rock mechanics parameters of rock and roof and floor were obtained by test,the roof and floor rocks of in-situ stress were calculated by using the logging data and differential strain core experiment method,the coal rock stress were calculated indirectly by the generalized Hooke’s law and the combination spring model,the method was formed by using experimental and theoretical model calculation of in-situ stress of coal rock.The cement ring hydraulic fracturing indoor physical simulation test which includes sample size,the size of the casing,parameters of cement loop and perforation parameters was designed to simulate and analyze the cement ring fracture damage under casing-cement ring-rock system.The experiments which were to analysis cement ring damage include core description,sample dissect,industrial CT scan and fracturing curve analysis methods aims to the coal fracturing and fracturing coal containing floor.The test results show that the hydraulic fracturing in the cement ring tensile damage zone and fracturing cracks through the area are consistent.Within a short distance of cement ring shear failure area through the fracturing cracks area,the farther the distance,the lighter the damage degree.The interface separation of cement ring is serious in the coal rock,and light in the sandstone.This paper simulated the coal rock fracturing test cement ring damage,and compared with the experimental results by using ABAQUS finite element software.Simulation results show that the injection pressure of hydraulic fracturing of coal rock can make tensile failure and shear failure of cement ring near the hole prone,but in the double hole distance,the possibility of cement ring damage reduced sharply.Comprehensive thought,it is greatest possible that coal hydraulic fracturing cracks connected the water layer.