Study On Geomechanical Condition And Coal Reservoir Stability In Coalbed Methane Development

Geomechanical conditions of Coalbed Methane(CBM)reservoir is the key factor affecting the efficiency of reservoirs stimulation.As the most of CBM reservoirs in China experienced multi-stage tectonic movements,its geomechanical condsions are complicated.Besides,the geomechanical parameters are dynamic with the CBM development rather than keep constant,which easily lead to the reservoir damage.However,previous studies paid little attention to the geomechanical condtions of CBM reservoirs and its stability during depletion.Based on these questions exsiting in CBM development,the CBM reservoirs of Zhengzhuang block in the southern Qinshui basin was chosen as research area and a series of studies were performed by data analysis,laboratory experiment,field testing,theoretical analysis and mathematical model.These studies included the characteristics of complete stress-strain-permeability-acoustic activity of coal during loading stress,distribution of in situ stress in Zhengzhuang block,change rules of in situ stress and borewell wall stress during depletion and the stability of CBM reservoir and borehole wall.Then,the geomechanics conditions and change rules of geomechanics parameters of CBM reservoirs during depletion were revealed and the evaluation methods for their stability were established.The results are mainly as follows:(1)The influence of coal rank and stress on coal strength,permeability and AE activity and its control mechanism were studied by a series of complete stress-strain,permeability and AE experiments.The results revealed that the coal compressive strength,AE activity and permeability are governed by coal pore and crack systems.Under a constant confining pressure,the compressive strength of coal exhibits a trend of first decreasing and then increasing(an asymmetric?U?shape)with the increase in vitrinite reflection.The minimum value occurs in medium rank coal.However,as confining pressure rises,the compressive strength increases,but its growth rate decline.The change rules of AE and permeability during loading process can be divided into three stages:Compaction and Elastic Deformation Phase,Non-Elastic Deformation Phase and Rupture and Development Phase.Each stage has different features.(2)The in situ stress of CBM reservoir in Zhengzhuang block was investigated by multi-loop hydraulic fracturing tests.And an in situ stress calculation model based on hydraulic fracturing data was established,where the low strength and cracks,developed in CBM reservoirs,were considered.Then,this study revealed its distribution rules and evaluated it.The results indicated that most of the in situ stress of CBM reservoir in Zhengzhuang block belong to meidium and low level(?_h<15MPa or15<?_h<20MPa).The effective stress ratio and tectonic stress ratio are 0.7436 and 0.25,respectively.In addition,the in situ stress has obvious zoning features.As the buried depth increases,the lateral stress ratios decrease.(3)In consideration of the matrix shrinkage effect,theoretical models were built to describe the dynamical rules of in-situ stress during CBM depletion.The result shows that in drainage stage,the horizontal principal stress(?_H/h)decreases and effective horizontal stress(S_H/h)increases linearly with the reduction in pore pressure,However,in desorption stage,both of?_H/h and S_H/h decrease non-linearly with the drop in pore pressure.In addition,with the increase of Langmuir volumetric strain or initial elastic modulus,the decrease rate of?_H/hincrease and the increase rate of S_H/h drop.(4)Based on the dynamical rules of in-situ stress,the dynamical models calculating the wellbore wall stress during depletion was established.In the process of depletion,the maximum effective stress difference value(stress difference)in the vertical wellbore wall drop.However,the stress differences of horizontal well wall in the orientation of maximum and minimum horizontal principal stress increase in normal faulting stress regimme.In strike-slip faulting stress regime,they decrease and increase respectively.In reverse faulting stress status,both of them decline.During the whole depletion,the matrix shrinkage effect can enhance the decline or increase rates.(5)Considering the limitation of coal strength and frictional strength,the criteria for the stress-based failure of intact CBM reservoirs and faulted CBM reservoirs during depletion were built and their failure rules were revealed.The results indicated that for intact CBM reservoirs,in a normal faulting stress regime,a reservoir is most easy to be damaged;in a reverse faulting stress regime,reservoir failure is not apparent during depletion.For faulted CBM reservoirs the stability of the reservoir is controlled by the frictional strengths(?_i).When the ratio of maximum and minimum principal stress exceeds a value of((?_i²+1)^1/2+?_i)²,the faults will reactive.(6)Based on the dynamical models of the wellbore wall stress,the criterion for the stress-based failure of wellbore wall was established and the calculation equations of collapse pressure and breakdown pressure were built,and its failure rules during depletion were revealed.It indicated that in drainage stage,both of the collapse pressure and breakdown pressure of vertical wellbore wall decrease;the decrease rate of breakdown is higher,so the window of the drilling fluid density enlarges and its stability enhance.For the horizontal wellbore wall in the orientation of maximum and minimum horizontal principal stress,in normal faulting stress regime,their breakdown pressure have an obvious decrease,so their windows of the drilling fluid density narrow and the stability reduce;in reverse faulting stress regime,their collapse pressure and breakdown pressure decline and rise respectively,so their windows of the drilling fluid density enlarge and the reservoir stability enhance;in strike-slip faulting stress regime,there are some differenct about these two directional wells.Their breakdown pressure drop and rise respectively,and their window of the drilling fluid density narrow and enlarge,so their stability reduce and reinforce.(7)Combining the actual data,the change rules of in-situ stress and wellbore wall stress during depletion in Zhengzhuang block were revealed.The evaluation index H(the difference value between actual maximum principal stress and critical maximum principal stress)and evaluation index K(the difference value between limitation value of frictional strengths and the ratio of maximum and minimum principal stress)were adopted to evaluate the stability of CBM reservoirs of Zhengzhuang block.In addition,the collapse pressure,breakdown pressure and the window of the drilling fluid density were calcuated to evaluate the stability of wellbore wall of Zhengzhuang block during depletion.