Research And Application Of Acidizing Technology For Low Permeability Coal Reservoir In M Block

M block coal reservoir is low permeability coal reservoir.In the process of drilling,completion and CBM exploitation,natural and artificial fracturing in coal reservoir are blocked to varying degrees,resulting in low production or shutdown of CBM wells in this block.The main manifestations are lower permeability of coal reservoir,loss of bottom hole pressure,narrow range of pressure drop,decrease of water production and gas production of gas wells.In order to effectively develop the coalbed methane in this block,these problems need to be solved urgently.In this paper,the M block coal-rock reservoir is taken as the research object,and the geological characteristics of the coal reservoir in the study area are described.The coal sample is dominated by bright coal,the primary bedding is visible,micro-cracks are developed,or mineral particles are filled.The coal sample has a moderate degree of coalification,with lean coal,and anthracite,and the vitrinite reflectance is about 2.0%.Coal is a medium-high coal rank coal and contains clay minerals and carbonate minerals such as kaolinite,illite and chlorite.The non-uniform pore-fracture system,the pores are mostly micropores and small pores,it is difficult to meet the seepage channel conditions required for the increase of coalbed methane production,and the damage mechanism of coal reservoirs is analyzed,and the method of acidification transformation of coal reservoirs is proposed.An acid formulation of a coal reservoir is preferred.Experiments were carried out on separate acid and mixed acid solution coal rock powder samples.According to the dissolution effect of coal samples,combined with the actual situation of the site and the history of coalbed gas wells,the acid formula was preferred:8%HCl+2%CH3COOH+2%HF+3%H2O2.Hydrochloric acid(HCl)can effectively remove carbonate iron and calcium blockage in coal rock;hydrofluoric acid(HF)can effectively remove silicous plugging in coal rock;organic acid(CH3COOH)is weak acid,releasing hydrogen ions slowly.The range of acidification operation can be expanded;the oxidant can block the organic matter in the coal rock,destroy the macromolecular structure and cross-linking by oxidative dissolution,and release the inorganic scale which cannot be directly reacted by the corresponding acid.Through the test of anti-swelling performance of clay stabilizer,evaluation of iron ion stabilizer and evaluation of corrosion inhibitor performance of acid system,0.5%polyquaternary amine clay stabilizer+0.5%NTA iron ion stabilizer+0.5%XT-105 acidification inhibitor was selected as acid additive.The acidification effect of acid-treated coal rock was evaluated.The acid core soaking coal core experiment was carried out to test the permeability and porosity of the coal core before and after acidification.The experiment showed that the coal core permeability increased to 1.9?15.3 times and the porosity increased by 0.25%?3.56%.The on-site acidification construction parameters and processes were optimized and field tests were carried out,and a coalbed methane acidification scheme was summarized.After the acidification experiment was carried out on the H3-2-043 well in the M block coal reservoir,the bottomhole flow pressure of the gas well increased,the gas production increased significantly,and the acidification achieved a certain yield increase effect.It shows that the damage mechanism of M block coal reservoir analyzed in this paper,the formulation of acidizing fluid system and acidizing construction technology proposed in this paper are feasible to solve the problem of low production and plugging of M block low permeability coal reservoir,that is,acidizing plugging removal and reforming M block coal reservoir to improve its productivity have good application prospects.