Geology And Engineering Cooperative Control And Prediction For The Deliverability Of CBM Straight Wells In Southern Qinshui Basin

In the paper, geologic and engineering factors for the deliverability of CBM (coalbed methane) straight wells in Fanzhuang Block are discussed on the basis of geologicfeatures and development engineering data of CBM wells in Southern Qinshui Basin.Further, cooperative control actions of geology and engineering for the deliverabilityof CBM straight wells are studied, and cooperative control mechanisms are elaborated.Typical geology-engineering model for CBM development of Southern Qinshui Basin,typical physical model of post-fracturing coal reservoir, mathematical model for coalreservoir and dynamical CBM production process, solution numerical model andcomputer model are established following the research thought as “experiment toanalysis to theory to application” and numerical simulation method as “geologicdescription (geologic model) to physical model to mathematical model to numericalmodel to computer model to model application”. Then, CBM production numericalsimulation software with proprietary intellectual property rights is developed forengineering application. The main research achievements are concluded as follows.(1) Cooperative control actions and mechanisms of geology and engineering forthe deliverability of CBM straight wells in Southern Qinshui Basin are exploredGround water fluid potential and permeability are key geologic factors of thedeliverability of CBM straight wells in research area that they control drainage anddepressurization effect of coal reservoir. Production system and hydraulic fracturingare key engineering factors of the deliverability of CBM wells. Production systeminfluences drainage and depressurization effect of coal reservoir and hydraulicfracturing improves coal permeability and enlarges gas supply area. In addition, ratioof critical desorption pressure to reservoir pressure, gas content, coal structure, coalbed burial depth, regional structure, pattern arrangement and well pattern thickening,drilling technology and cementing technology are also geologic or engineering factorsinfluenced the deliverability of CBM straight wells in research area.The relations of interaction among coal-rock mass with rich gas, pore-fracturesystem of procreative coal reservoir, CBM development projects and CBM outputprocess are geologic and engineering control mechanisms for the deliverability ofCBM wells in Southern Qinshui Basin. CBM output process is a continuous processthat constituted by adsorption/desorption, diffusion and infiltrating fluid of CBM.CBM output process is controlled by coal-rock mass with rich gas, pore-fracture system of procreative coal reservoir and CBM development projects, and reacts up onthem by the disruption of system balance of initial coal reservoir. The cooperativecontrol process of geology and engineering factors shows as matching relationshipamong in situ geology types, exploitation geology types, Optimization matching typesof CBM well productivity and CBM well development modes. Optimal matchingrelations between exploitation geology types and CBM development projects decidecooperative control types of geology and engineering for the deliverability of CBMstraight wells. Different Control types of the deliverability of CBM straight wells havedifferent exploration models and engineering choices.(2) Productivity prediction models for CBM straight wells are established on thebasis of cooperative control of geology and engineeringTypical geology-engineering model for CBM development of high-rank coalreservoir consists of initial geologic model of coal reservoir, geologic model ofpost-fracturing coal reservoir and geologic model of dynamical CBM well production.The initial geologic model and geologic model of post-fracturing coal reservoirdescribe development characteristics of macrostructure and microstructure, structuralrelationships and production fluid flow model of pore-fracture system of procreativecoal reservoir. Development degree of cleats of No.3coal bed in research area is notnotable that hydraulic fracture becomes the most important medium for fluid flow andchanges the flow pattern and demarcation of flow stage which are very different frominitial high-rank coal. Ultramicroscopic fissure, micro-fissure are importantpassageways connecting pores, pore and fracture, fracture and fracture, and veryimportant to transition of flow pattern and demarcation of flow stage in high-rank coal.Dynamic change model of coal bed permeability includes restorative dynamic changemodel and descending dynamic change model of coal bed permeability. Coal bedreservoir with high shrinkage or gently production system belongs to restorativedynamic change model and coal bed reservoir with low shrinkage or rapid productionsystem belongs to descending dynamic change model. Dynamic change of fracturingfracture permeability only has one phase in descending dynamic change model offracturing fracture permeability which is just a decline phase of permeability.On the basis of typical geology-engineering model for CBM development ofSouthern Qinshui Basin, typical physical model of post-fracturing coal reservoir isestablished by the modification of Warren-Root physical model. The typical physicalmodel can give a comprehensive interpretation of seepage mechanism in post fracturing high-rank coal reservoir. Based on the modified Warren-Root model,mathematical model for coal reservoir and dynamical CBM production process isestablished. Mathematical model is composed mainly of seepage flow differentialequations, auxiliary equations, initial conditions and boundary conditions. Gravity,viscous force and Klinkenberg effect are considered in mathematical model.Hydraulic fracture is treated as an individual medium in mathematical model thatmore in accordance with the reality of CBM development. By the discretization ofmathematical model for coal reservoir and dynamical CBM production,corresponding numerical model and computer model are established which meansforecasting process of the deliverability of CBM straight wells are achieved.(3)“CBM production numerical simulation software” with high reliability isdeveloped according to the foundation models“CBM production numerical simulation software” with proprietary intellectualproperty rights is developed on the basis of cooperative control model and controlprocess model. The software can be used for forecasting and matching deliverabilityof CBM straight well, demonstrating depressurization cone and achieving inversion ofcoal bed reservoir parameters. According to the results of instance verification ofCBM well in Southern Qinshui Bain, computation results of the software are reliableand calculation errors are slight.“CBM production numerical simulation software”has high reliability and can satisfy the needs of prediction and matching fordeliverability of CBM straight wells, which can prove that productivity predictionmethod of CBM wells that based on the cooperative control mechanisms of geologyand engineering factors has high availability.