Research On The Division Method Of Longitudinally Superimposed Multi-thin Coal Seams Fracture Group

As a great potential unconventional natural gas resource,CBM exploration and development can not only alleviate the energy shortage in China,but also help to reduce the coal mine gas accident,which has great social and economic significance.The natural productivity of CBM is low,so it is necessary to take hydraulic fracturing and other stimulation measures to obtain industrial productivity,and the selection and division of fracturing formation is an important prerequisite for hydraulic fracturing.Coalbed methane reserves in eastern Yunnan and Western Guizhou rank fourth in China.The coal seams in this area overlap vertically.Fluid energy and permeability differ greatly among different layers,which are characterized by multi-layers,thin layers and small layers.Reservoir development faces two geological problems:compatibility between gas-bearing systems and sharing within the system.The traditional fracturing well selection method is not applicable.Through the screening and analysis of reservoir geological parameters,the criteria for reservoir geological division are established.Fracturing process is simulated by establishing fracture pressure prediction model and "T" fracture model of vertical fracture and horizontal fracture combination.Combining with the applicable conditions of fracturing plugging mode,the engineering division criterion of fracturing stratification is established.Combining with the criteria of geological and engineering division,the fracturing division method of vertical superimposed thin coal seam is established,and the corresponding software is developed on this basis.The main conclusions of this paper are as follows:(1)The pressure gradient and permeability of coal seam are selected as geological indexes from the geological parameters of the target block,and the geological division method of vertical overlapping multi-thin coal seam fracturing layer is established.The two geological problems are solved by using geological parameters.(2)The breakdown pressure calculation model of CBM vertical wells is established,which explains the stress concentration effect caused by casing and cement sheath,solves the problem that the radius of wellbore is unknown in the traditional model,and corrects the calculation results of stress field around wellbore.With the increase of radial distance of perforation,the influence of near wellbore stress decreases,and the breakdown pressure increases and tends to be constant.The fracture pressure is lower at medium trend and dip Angle,higher at high trend and dip Angle.Increasing in-situ stress difference is beneficial to shear fracture.(3)The "T" fracture model of coal seam is established,and the effects of flow rate,in-situ stress difference,vertical and minimum horizontal in-situ stress difference on fracture extension are analyzed.The simulation results show that the length and width of the fracture are larger,the height is lower,the height is asymmetrical and the width is not uniform,which reflects the complexity of in-situ stress.Under the same liquid volume,the plan with large flow rate has shorter construction time,less filtration and higher fracturing fluid efficiency.The greater the minimum horizontal stress difference between the caprock and the coal seam is,the longer the fracture length is,the smaller the width and height are.The high stress of caprock has the function of shielding,restraining the extension of fracture height and indirectly promoting the extension of fracture length.The bigger the difference between vertical and minimum horizontal in-situ stress at weak structural plane is,the bigger the length and width of vertical part of T-shaped fracture is,the smaller the length and width of horizontal part of fracture is,the stronger the restraint effect on horizontal fracture is,which is beneficial to effective fracturing construction.(4)The decision-making software has been developed,which has the characteristics of beautiful interface,convenient application,simple operation,stable and reliable operation,and can effectively guide the construction on site.Taking a well in the target block as an example,the feasibility of the fracturing formation division method for longitudinal overlapping thin coal seams is verified.