The THM Coupling Mechanics Test Of Coal Under The Condition Of Stress Disturbance And Numerical Simulation Of Borehole Deformation In Horizontal Wells

China's high-quality energy productivity is seriously insufficient,such as oil and gas.While the academician of Chinese Academy of Engineering Yuan Liang pointed out that in 2030 China's CBM yield is expected to reach ninety billion m3.The exploitation and utilization of CBM resources can not only effectively containment the coal mine gas disaster,reduce the greenhouse effect of methane gas on the atmospheric environment,but also optimize the energy structure of our country and alleviate the energy shortage.However,the development progress of CBM in China is still slow.Mainly because of the low mechanical strength and the strong stress sensitivity in coal,which makes the coal from the reservoir is easy to break.In addition,the coal around borehole of the CBM well is in a dynamic environment coupling of heat fluid solid(THM)in the drilling process,it is easy to cause sticking,burying drilling and other complex downhole accidents.Therefore,the following studies have been done in this paper.1.The basic mechanical parameters such as compressive strength,cohesion,internal friction angle,Poisson's ratio and elastic modulus of gas bearing coal were obtained through the conventional three axis mechanics test.2.The strength characteristics,deformation properties and failure characteristics of gas bearing coal under the condition of temperature field,seepage field and in-suit stress redistribution were revealed by THM coupling experiment;According to the distribution of in-suit stress in drilling process,based on elastic plastic mechanics,seepage mechanics and thermodynamics,and combined with the elastic plastic characteristics of coal,taken porosity of coal as bridge and taken the influence of the new fracture on the permeability of the coal during the plastic deformation into consideration,a THM coupling model of gas bearing coal was established.The model was imported into COMSOL Multiphysics software,and the correctness of the model was verified by the experimental results.The study of THM Coupling of CBM has been deepended.3.The CBM reservoir of Taiyuan formation in Ningwu basin was taken as background,the stress and deformation of the surrounding rock of the horizontal well under the condition of unbalanced and near balanced drilling were simulated and analyzed by using the THM coupled mathematical model,the simulation results were in good agreement with the actual situation.The results will provide guidance and suggestions for the improvement of the design scheme in building up section and the prevention measures of wellbore instability.The results of this paper deepen the understanding of the mechanism of CBM horizontal well borehole stability under drilling disturbance,the THM mathematical model reveals the relationship among formation temperature,fluid pressure and in-suit stress of gas bearing coal in the drilling process,and provides a theoretical basis for the study of borehole stability in CBM wells.The numerical simulation of THM coupling will make reasonable technical measures for drilling in CBM wells,and it can be used as reference for maintaining wellbore stability.