Research On The Evolution Of Fracture Network During Coal-bearing Gas Reservior Stimulation Based On Damage Theory

The coal bearing strata is enriched with abundant unconventional natural gas.To exploit the unconventional gas can not only supplement the conventional gas resources,but also serve as an important approach to preventing and controlling the coal mine gas hazards,meanwhile reducing the methane evacuation during coal mining and reducing the greenhouse effect.Previous studies only focus on the coal-bed methane but not on the exploitability of shale gas and tight sand gas.Recent studies show that the content of the two unconventional natural gas are abundant,and the joint exploitation of these three kinds of gas has obvious technical advantages compared with the single exploitation of coal-bed methane.In the thesis,the mechanism to reconstruct the fracture network of coal gas reservoir has been addressed systematically by laboratory test,theoretical analysis,numerical simulation and field application.The main findings are as follows:(1)The automatic identification method of coal GSI quantitative characterization of rock mass structure were established and preliminarily developed the identification software,which avoids the influence of human factor and enables more accurate acquisition of the mechanical properties of non-integral coal rock.(2)The functional relationship between permeability of non-integral coal rock and GSI were build.The results of laboratory permeability test of coal and rock samples show that the permeability of fractured rock sample is much higher than that of coal sample,which indicates that compared with coal seam reservoirs,it is easier to reconstruct the fracture network of rock reservoirs and to exploit coal-bed methane after the reconstruction.The permeability of either coal or rock will decrease abruptly under the action of effective stress when the coal or rock is of cataclastic or mylonitic texture and cannot be restored by fracturing stimulation.This indicates the hydraulic fracturing cannot be performed in highly cataclastic coal rock..(3)Based on the characteristics of the mechanical response of coal rock under loading,a GSI-based coal rock elasto-brittle damage constitutive relationship has been proposed and this model has been extended to a three-dimensional stress state.The flow and pressure distribution of the fluid in the pores and the interaction between the fluid and solid media under seepage-stress coupling effects have been studied.A numerical calculation method based on elasto-brittle damage theory has been developed for coal rock under hydraulic coupling effects by the criteria of tensile and sliding failures.On the Ansys software platform,a computing program for fracture network transformation has been composed using APDL language to perform numerical simulation of the forming process of fracture network in coal rock under varying working conditions such as planar model,3D dipping reservoir,vertical wells,horizontal wells,deviated wells,etc..On the basis of verification test,the mechanical mechanism of the forming of coal rock fracture network has been further analyzed.(4)Through laboratory fracturing tests,the forming process of fracture network has been studied by means of acoustic emission monitoring,macroscopic observation,etc.The results of hydraulic fracturing experiment of hard coal under variable confining pressure conditions indicate that the change of stress conditions will result in the change of the direction of fracture propagation and extension.Therefore,changing the stress field is an effective way to achieve the transformation of the fracture network.The fracturing tests of many similar materials consisting of weak planes show that the direction of propagation will change under hydraulic effects when the fracture encounters a weak plane.The multi-stage fracturing test of similar materials shows that it is easier to generate fracture network through multi-stage fracturing as this process can create interference between short fractures.Through these simulation tests,the accuracy of theoretical model and the reliability of the program have been verified.(5)According to the geological and engineering data of Wulihou,a large geological calculation model has been built for Jingtian Carboniferous-Permian coal bearing system in Wulihou of Zuoquan County,Shanxi Province.Employing variable flow rate technology,the calculation and analysis of fracture network reconstruction of the gas bearing coalbed in this coal system have been carried out,and the expansion of fracture network has been predicted and applied in the design and construction of fractured wells.The results of field monitoring show that the fracture length,height and fracturing pressure are basically consistent with the results of numerical simulation.For two years,the daily production of the well still remains above 1000 cubic meters and the bottom hole flowing pressure has been kept at around 1 MPa,which further proves the accuracy and practicality of the theoretical model and calculation program proposed in this paper.