Investigation of reservoir sensitivity evaluation and damage mechanism in deep coalbed methane reservoirs of the Daji Block in Eastern Ordos Basin

Chen Mingjun; Chen Xueni; Kang Yili; Guo zhidong; Wang Yubin; Shi Yang; You Lijun; Dai Wangru; Yan Maoling

doi:10.16186/j.cnki.1673-9787.2025030056

Abstract：Objectives To clarify the sensitivity characteristics and damage mechanisms of deep coalbed methane(CBM),a study on sensitivity evaluation and damage mechanisms is conducted.Methods The deep CBM reservoirs in the Daji Block of the Ordos Basin is investigated in this study.Petrographic thin-section analysis,X-ray diffraction(XRD),and scanning electron microscopy(SEM)are employed to characterize the deep CBM reservoirs and identify potential sensitivity-inducing factors.Laboratory experi-ments are conducted to evaluate the fluid and stress sensitivity of the deep coal samples,thereby elucidat-ing the mechanisms of damage associated with sensitivity effects.Results(1)The coal maceral composition in the study area is dominated by vitrinite,with a low inertinite content and an absence of liptinite.The in-organic components are mainly quartz and clay minerals,with quartz content at 22.60%and clay minerals at 57.05%(predominantly kaolinite).The pore structure is characterized by abundant cast membrane pores,cellular pores,and gas pores,with well-developed fractures that are mostly filled by clay minerals or calcite.The porosity and permeability are relatively low,and pore connectivity is poor.(2)The CBM res-ervoir exhibits velocity sensitivity,water,salt,alkali,and stress sensitivity.The velocity sensitivity index is 82.64%,indicating strong damage.The water,salt,and alkali sensitivity indices are 53.90%,57.00%,and 63.50%,respectively,representing moderately strong damage.The stress sensitivity coefficient ranges from 0.692 9 to 0.938 3,indicating moderately strong to strong damage.However,no acid sensitivity dam-age is observed,and acid treatment can enhance permeability.(3)The high-temperature,high-pressure,and high-stress environment of deep CBM reservoir leads to low rock mechanical strength,making them highly susceptible to stress sensitivity damage.The high content of pyrite,kaolinite,and illite in the min-eral composition tends to autogenous cementation under complex fluid conditions,causing pore and fracture blockage that hinders gas production.Furthermore,high-salinity formation water is prone to salt precipita-tion and crystallization blockages during drilling and production.Conclusions The findings improve the ra-tionale for designing working fluid systems and production regimes.This is of positive significance for en-hancing coal reservoir protection and increasing coalbed methane production.