Study On Stress Sensitive Damage Evaluation Of Low Permeability Tight Gas Reservoir In Linxing Area

A significant proportion of the natural gas reserves in low-permeability reservoirs has greatly increased in recent years.Hence,several nations such as China pays more attention to the exploitation of low permeability reservoirs.Moreover,the tight sandstone gas reservoir in Linxing area is easy to be damaged by stress sensitivity due to it has high clay content.And as the exploitation process progresses,the reservoir is often prone to stress sensitivity,which will result in a decline in the well’s productivity.Stress sensitivity evaluation is required.However,In the stress sensitivity evaluation experiment,the correction range of permeability at each measurement point is different due to the dynamic slippage effect,and the use of different stress sensitivity evaluation methods and parameters.Finally,those methods will have the difference in stress sensitivity evaluation and will affect the accuracy of stress sensitivity evaluation.Therefore,it is necessary to optimize and improve the stress sensitivity evaluation experiment of low permeability reservoirs.Taking a tight sandstone reservoir from the LX area as the research object,the reservoir characteristics and pore characteristics were analyzed.Verify the dynamic slippage effect of low permeability gas reservoir,and clarify the variation law of gas slippage effect of low permeability reservoir with effective stress.To achieve an accurate evaluation of the stress sensitivity,proposed an improved method of permeability correction using the equivalent liquid permeability method considering the dynamic slippage effect,using the experimental parameters under in-situ formation conditions,and using two stress sensitivity evaluation methods were used for experiments.One is to change the confining pressure at constant inlet/outlet pressure,and the other is to change the inlet/outlet pressure at constant confining pressure.According to the experimental results,the permeability and effective stress were fitted,and the productivity formula of the stress-sensitive tight gas well was established,and considering the elimination of the slippage effect,the research outcome provides a basis for predicting the range of production pressure difference during field operation.It was found that there are nano-scale pore-throat channels in tight sandstone reservoirs of the LX area that belongs to low porosity and low permeability reservoir.The gas slippage effect increases with the increase of effective stress.After the dynamic slippage effect correction of laboratory experimental data,the stress-sensitive damage rate obtained by the two evaluation methods increased to different degrees.In the stress sensitivity evaluation experiment of variable confining pressure,the average permeability damage rate increases by 0.9% after modification.In the stress sensitivity evaluation experiment of variable pore pressure,the average damage rate increased by12.09% after modification.The difference in stress-sensitive damage rate obtained by the two experimental methods is 17.58%.A gas well productivity equation was established and verified by fitting the exponential relationship between permeability and effective stress.Considering the stress sensitivity that eliminates the slippage effect,the productivity of the gas wells decreases by 29.28%.To avoid the influence of stress sensitivity on gas well production,the range of production pressure difference should be controlled from 1.29 MPa to 2.21 MPa.There are some differences between the two evaluation methods after dynamic slippage effect correction,but the variable pore pressure test method is closer to the field scenario.In situ formation conditions,the dynamic gas slippage effect has little influence on the test method of variable confining pressure and can be ignored.But the variable pore pressure test method is performed under a low-pressure condition,the influence of gas slippage was significant and can’t be ignored.Otherwise,the experimental results will be underestimated.Further,during natural gas exploitation,it is necessary to strictly control the range of bottom hole pressure to ensure a reasonable range of production pressure difference,thereby mitigating the decline in gas production caused by stress sensitivity.