Experimental Study On Acidization And Permeability Enhancement Of Low And Medium Rank Coal Reservoirs In Xinjiang By H₂S Solution

Coalbed methane(CBM)wells with abnormally high H₂S contents present higher CBM productivity in the coal development areas of Xinjiang,China.It is worth studying whether the abnormal high concentration of H₂S can improve the porosity and permeability characteristics of coal reservoir and affect the production of CMB.Therefore,acidization experiments using a H₂S solution on low and medium rank coal of the Wucaiwan coalfield,Shaerhu coalfield,Baicheng coalfield,Aiweiergou coalfield were conducted.The pore structure characteristics and permeability at the horizontal and vertical stratifications were compared before and after the acidization experiments using various methods including porosity and permeability test,high pressure mercury injection porosimetry(HPMIP),nuclear magnetic resonance test(NMR),X-ray diffraction(XRD)and field emission scanning electron microscope(FESEM).The results show that:(1)The porosity of low rank coal is obviously higher than that of medium rank coal before and after acidization by H₂S solution.Due to acidization,the porosity increase of low rank coal is obviously higher than that of medium rank coal.After H₂S solution acidization,the permeability of coal increases significantly,and the permeability increase of low rank coal is significantly higher than that of medium rank coal.The permeability increase in parallel bedding direction is obviously higher than that in vertical bedding direction.(2)Before and after the H₂S solution acidization,the total pore volume of coal decreases with the increase of coal rank.Due to acidization,the pore volume increase of low rank coal is obviously higher than that of medium rank coal.Before and after the H₂S solution acidization,the specific surface area of total pores showes a trend of"increasing-decreasing-increasing"with the increase of coal rank.The effect of acidization on the specific surface area increase of total pores is not obvious.Before the H₂S solution acidization,the open pore volume decreases with the increase of coal rank,and the movable fluid saturation increases first and then decreases with the increase of coal rank.After the acidization,the open pore volume and the movable fluid saturation decrease with the increase of coal rank.The acidization makes the open pore volume and the movable fluid saturation of low rank coal increase,while them of medium rank coal decrease.This shows that acidification is more conducive to enhance the flow of CMB in low rank coal.(3)The improvement mechanism of the H₂S solution on coal pore and fracture structure is mainly derived from the following two aspects:on the one hand,acidizing causes the dissolution of carbonate minerals with acid solution,which results in the dissolution of calcite and other carbonate minerals filled in the pores and fractures,thus increasing the positive effect of increasing void volume and enhancing connectivity;on the other hand,acidification causes clay minerals in coal to expand and disperse fine particles,and precipitate formed by pyrite reaction to block pores,resulting in the negative effect of reduced void volume and connectivity.The low rank coal is dominated by positive effect,while the medium rank coal is dominated by negative effect.Through the above research,the purpose is to reveal the reasons for the high production of CMB in the area of abnormal high concentration of H₂S in Xinjiang.The research results are expected to enrich the theory of CMB reservoir transformation and guide the prediction of favorable areas of CMB.