The Influence Of Hydrogeoloical Conditions On Coalbed Methane Development In The Hancheng Area

As an unconventional natural gas resource,coalbed methane(CBM)has become one of the most realistic energy replacements.The exploitation of CBM is affected by many factors,such as regional geology,gas reservoir characteristics,hydrogeology,extraction technology,etc.,these factors all affect whether CBM can benefit economic development in a scale.The production of CBM is mainly achieved through drainage and pressure reduction.Hydrogeology controls the preservation and enrichment of CBM.and throughout the entire process of CBM formation,accumulation and production.Groundwater is not only a medium for the formation of CBM,but also a carrier for the driving force and output of CBM migration.Therefore,regional hydrogeological conditions not only affect the enrichment status of CBM.but also the main factors affecting the commercial exploitation of CBM.This paper takes the Hancheng block in the eastern margin of the Ordos Basin as the research object,comprehensively applying theoretical knowledge in structural geology,hydrogeology,CBM geology and exploitation engineering,starting with actual geological data,systematically studing the regional structural features,characteristics of coal reservoirs,distribution of aquifers,and characteristics of hydrodynamic and hydro-chemical fields,dividing the hydrogeological zoning of the Hancheng block,aiming at the effects of different types of hydrogeological belts on the enrichment status and exploitation of CBM,proposing corresponding measures and recommendations.The main results of this paper are as follow s:(1)Through the literature survey of the general geology of Hancheng block,the block tectonic setting,hydrogeological characteristics,hydrodynamic activity,structural features,and characteristics of coal-bearing strata are summarized.Meanwhile,combining with the history and current status of coalbed methane exploration and exploitation,systematic study the geological background of the study area(2)There are four sets of aquifers in the upper and lower of main coal seams of No.3,No.5 and No.11 in Hancheng block.The hydrodynamic field of coal reservoirs is distributed from northwest to southeast.Hydrodynamics are active in the eastern part of the block and near the fault,and the mineralization degree of coal seam water is greatly affected by the influence of fault and make-up water.The water type is mainly Na-HCO3 type(sodium bicarbonate type)and Na-Cl type(sodium chloride type).On this basis,the hydrogeological planar distribution of Hancheng block is divided into three zones:free replacing zone,obstructive replacing zone(obstructive replacing upper and lower subzone)and lagged zone.(3)By comparing the relationship between the characteristic parameters of the hydrodynamic field and the gas content of the block,the author found:The area with high reservoir pressure coefficient has a large gas content in the coal seam.The fault structure significantly affects the regularity of CBM rich accumulation.Near the fault,the coal seam has low gas content.From the gradient and distribution of the vvater level isoline map,the regional water flow direction is frol northwest to southeast.The hydrodynamics are relatively active in the eastern part of the block and near the fault,and the hydrodynamic fields in the north and southwest are relatively stable.(4)By comparing the relationship between the characteristic parameters of the hydrochemistry field and the gas content,the authors found:The minoralization degree characterized by high in the west and low in the east,it is low in the fracture zone and the shallow part,and gradually higher in the deep.With the increase of mineralization degree,the concentration of cations in the formation water ions did not change significantly,and dominated by Na+and K+.Meanwhile,the anion gradually transitions from HCO3-to Cl-.These phenomena indicate that the strata are gradually transitioning from poor sealing to well sealing.The sodium chloride coefficient and carbonate equilibrium ratio of the block are high in the shallow part and the fault area,and gradually decrease in the deep part.The mineralization degree,sodium chloride coefficient and carbonate equilibrium ratio can indicate the enrichment characteristics of CBM.(5)According to hydrodynamic field and water chemical field characteristics,the enrichment degree of the block CBM is divided into three zones:the failure zone(free replacing zone),the transition zone(obstructive replacing upper subzone)and the enrichment zone(obstructive replacing upper lower subzone and lagged zone).The hydrodynamic conditions from the southeast to the northwest gradually weaken.Accordingly,the preservation condition of CBM gradually getting better from the failure zone to the enrichment zone.(6)There is a clear corresponding relation between the CBM enrichment area identified by block hydrogeological conditions and the production status of CBM wells.High water production and poor gas production effect in the failure zone,while the enrichment area has good gas production effect.According to the results and understanding of hydrogeological research,prioritized blocks for exploration and exploitation have been identified.Proposed to carry out reservoir fracturing treatment of wells in the enrichment area and the gas production is relatively good.The research results are used for water source judgment of drainage wells,produced water treatment,dynamic change analysis,cross-well communication identification and drainage institution,etc.Proposing corresponding exploitation countermeasures for CBM wells in different areas,and carry out classification measures and achieve remarkable results.