Research On Reservoir Types And Their Adaptive Fracturing Methods Of Coal Measure Gas In Carboniferous-Permian Period Of Shanxi Province

As the vanguard of the energy revolution,improving the recovery rate of coal measure gas is an important work in Shanxi Province during the"Fourteenth Five-Year Plan"and a longer period in the future.Coal measure gas reservoir has the characteristics of thin layer,multi lithologic interbedding and strong plasticity,and forms different combination types in different regions and layers,which brings great difficulties for efficient fracturing and improving pumping.Existing work has mostly used hydraulic fracturing to fracturing a single reservoir,causing problems such as low resource utilization and unsatisfactory combined production of coal measure gas.Adopting adaptive fracturing methods for different coal-measure gas reservoir types will be the development trend of coal measure gas combined pressure and co-production.Through the methods of field investigation,data statistics,theoretical analysis and numerical simulation,the coal measure gas reservoirs and reservoir types,the reservoir physical properties and high-quality production intervals,the reservoir mechanical properties and their response characteristics to different fracturing methods,the influence of different fracture initiation positions on the fracture morphology and height of coal measure gas reservoir were systematically carried out.The structural characteristics of coal measure gas reservoirs were clarified,high-quality mining intervals were determined,and the reservoir fracturing methods and initiation layers were optimized,which provided a theoretical basis for the efficient transformation of coal measure gas reservoirs in Carboniferous-Permian period of Shanxi Province.The main work and achievements are as follows:（1）The types of coal measures gas reservoir and structural characteristics in Carboniferous-Permian period of Shanxi Province.Take the coal measure gas reservoirs of the main gas producing coal fields such as Qinshui Coalfield,Hedong Coalfield,Huoxi Coalfield and Xishan Coalfield in Carboniferous-Permian period of Shanxi Province as the research object.Based on the statistics of typical borehole logging data,gas logging data and other geological data,this paper analyzes the structure of coal measure strata,identifies the caprock of coal measure gas bearing system,defines the independent gas bearing system of coal measure,and divides the structural types of coal measure gas reservoir in Carboniferous-Permian period of Shanxi Province.The results show that the coal measure gas reservoirs in Carboniferous-Permian period of Shanxi Province can be divided into four types:independent coalbed methane,independent sandstone gas,coalbed methane-shale gas,coalbed methane-sandstone gas.There are all types of coal-measure gas reservoirs in the Qinshui coalfield;there are two types of coalbed methane-shale gas and coalbed methane-sandstone gas in the Hedong coalfield;there are three types of independent coalbed methane,coalbed methane-shale gas,and coalbed methane-sandstone gas in the Huoxi coalfield;there are three types of independent coalbed methane,coalbed methane-shale gas,and coalbed methane-sandstone gas in the Xishan coalfield.There are 9 types of coal measure gas reservoirs,including single coal seam,single sandstone,roof sandstone-coal seam,roof mudstone-coal seam,coal-mudstone-coal,roof limestone-coal seam,roof mudstone-coal-floor mudstone,coal seam-floor sandstone and coal seam-floor mudstone.There are six types of single coal seam,single sandstone layer,roof sandstone-coal seam,roof mudstone-coal seam,coal-mudstone-coal and coal seam-floor mudstone in Qinshui coalfield;there are three types of roof sandstone-coal seam,roof mudstone-coal-floor mudstone and roof limestone-coal seam in Hedong coalfield;there are four types of single coal seam,roof sandstone-coal seam,roof mudstone-coal seam and coal-mudstone-coal in Huoxi coalfield;there are three types of single coal seam,coal-mudstone-coal and coal-floor sandstone.（2）The physical property of reservoir and the evaluation of high-quality production interval.Taking reservoir physical parameters as the research object,the physical properties of each reservoir and their interrelationships were analyzed.Seven reservoir physical property parameters,including depth,thickness,vitrinite content,clay mineral content,gas content,permeability and porosity,were selected as evaluation indicators for coal measure gas quality production intervals.The thickness weighted average method was used to calculate the evaluation index values of the composite reservoirs,the range transformation method was used to standardize the evaluation index values of the composite reservoirs,and the entropy method was used to calculate the objective weight of each evaluation index.Based on the grey correlation analysis method,the evaluation model of high-quality production intervals of coal measure gas was established to determine the high quality production intervals.The results show that the gas content of coal reservoir is positively correlated with vitrinite content and burial depth,and negatively correlated with inorganic mineral content.Pore volume and specific surface area are mainly provided by micropores,and there is a good positive correlation between pore volume,specific surface area and porosity.The gas content of rock reservoirs is positively correlated with buried depth,pore volume is dominated by macropores and mesopores,and there is a good positive correlation between pore volume and permeability.The importance of evaluation indicators for high quality mining intervals from high to low are thickness,permeability,burial depth,gas content,porosity,clay mineral content,and vitrinite content.From high to low,Qinshui coalfield reservoir is ranked as No.3 coal seam group,No.15coal seam group and siltstone of Taiyuan Formation（1435m）;Hedong coalfield is ranked as No.8+9 coal seam group and No.4+5 coal seam group;Huoxi coalfield is ranked as No.11coal seam group,No.10 coal seam group and No.2 coal seam group;Xishan coalfield is ranked as No.2 coal seam group,No.9 coal seam group and No.8 coal seam.（3）Optimization of reservoir fracturing methods.Considering the characteristics of pressure rise rate and pressure peak,hydraulic fracturing,liquid CO₂ phase transition fracturing and explosive fracturing are typical fracturing methods.Taking coal measure gas reservoir as the research object,considering different fracturing methods,the numerical model of fracture of single coal series gas reservoir was established.The response law of fracture propagation to typical fracturing methods was analyzed under the condition of rock brittle index,fracture toughness and compressive strength.The evaluation index of adaptive fracturing method of coal measure gas reservoir considering rock mechanical properties was formed,and the interval value of corresponding fracturing method evaluation index was given.Based on the calculation of the evaluation index value of the adaptive fracturing method for the coal measure gas reservoirs in Carboniferous-Permian period of Shanxi Province,the adaptive fracturing method is optimized for the coal measure gas reservoirs in Shanxi Province.The results show that:with the increase of rock brittleness index,the fracture height increases logarithmically under the action of three fracturing methods,and the hydraulic fracturing method increases the most.With the increase of fracture toughness and compressive strength of rock,the fracture height of reservoir decreases in a negative exponential law under the action of three kinds of fracturing methods,and the explosive fracturing method decreases the most.The adaptive fracturing method of coal reservoir is explosive fracturing method,the adaptive fracturing method of carbon mudstone and sandy mudstone reservoir is liquid CO₂ phase transition fracturing method,and the adaptive fracturing method of siltstone,sandstone and limestone reservoir is hydraulic fracturing method.（4）The optimal initiation zone of the reservoir was determined.Taking coal measure gas composite reservoirs in Carboniferous-Permian period of Shanxi Province as the research object,considering different fracturing methods,a numerical model of coal measure gas composite reservoir fracturing had been established.The influence of different initiation horizons on the morphology and height of fracture penetration in the reservoir was investigated,and the optimal initiation horizon of the reservoir was determined.The results show that the best initiation position for roof sandstone-coal type reservoir is roof sandstone reservoir;roof mudstone-coal type reservoir is roof mudstone reservoir;roof limestone-coal type reservoir is roof limestone reservoir;coal-mudstone-coal type reservoir is the middle mudstone reservoir;coal-floor mudstone type reservoir is floor mudstone reservoir.