Complementary Fishbones Jet Drilling And Production Optimization In Coalbed Methane Reservoirs

Fishbones drilling is an efficient way of exploiting the coalbed methane(CBM)wells.However,we are still facing numerous problems such as high operation costs,hole collapse,low recovery and economic inefficient during the development of CBM reservoirs.To solve these issues,this paper proposes an innovative drilling technique called "slotted liner sheathing coiled tubing(SLSCT)" to synchronize the jet drilling and liner running in one trip.The major advantages of this technique are cost effective and borehole stable.Besides,in order to enhance the network interconnections and improve the ultimate gas recovery,reliable production performance estimation methods are needed to optimize the fishbones patterns and drilling directions.To achieve these purposes,this paper presents an integrated workflow which complementarily utilizes the fishbones jet drilling and production optimization analysis to design an efficient approach to develop CBM reservoirs.First of all,we performed a series of laboratory experiments to design the critical downhole tools,opimize the tool parameters and verify the feasibility of this technique.These tests include tubular friction performance,straight-swirlling high pressure water jet rock breaking,cuttings transport efficiency with the dual pipes and integrity tests of plastic slotted liners.Second,the "node-analysis combined with discrete fracture method " is used to model the gas and water flow in multi-scaled fracture networks.The connected large-scale natural fractures with hydraulic fractures are modeled explicitly as discretized panels,and the disconnected small-scale natural fractures are treated implicitly as “enhanced matrix permeability”.In addition,we incorporate the critical gas flow mechanisms such as gas adsorption/desorption and molecular diffusion,and stress sensitivity of the fracture network in the model.The two-phase flow is incorporated by iteratively correcting the relative permeability to gas/water for each fracture segment and capillary pressure at each node with the fracture depletion.Then,fishbones are embedded into the proposed two-phase flow model.We treat each fishbone lateral as a fracture with equivalent flowing properties.Sensitivity analysis and optimization design of the fisbone patterns are made based on the field cases on sounthern part of Qinduan block,Qinshui basin in Shanxi,China.Simultaneously,various lateral geometries including curving,zigzag,undulating,varying length and varing radius are simulated to illustrate the effects of non-planar fishbone laterals on gas/water production.Furthermore,the model is also used to study the influence of connectivity between fishbone laterals with cleat system from simple to complex on gas/water production behavoir.Hence,the optimum drilling direction of the fishbones can be obtained.Based on the experimemtal and simulation works,multi-objective optimization(MOO)models are established by integrating the drilling/completion operation with production optimization.The non-dominated sorting genetic algorithm II(NSGA-II)is implemented to solve the model.Finally,a selection and operation guide for the "SLSCT" fishbone drilling is provided,which can be used in the further field tests for drilling and completion engineers.The key findings of this work are expected to provide the initial aproximations and first-step explorations for the newly proposed fishbones drilling technique.Finally,the proposed “cross-disciplinary” evaluation method is expected to provide a fast and valuable optimization approach for completion engineers.