Study On Optimal Design Theory Method Of Well Trajectory In Cluster Horizontal Wells For Shale Gas Development

Cluster horizontal wells has dual characteristics of horizontal well and cluster wells.It is the advanced well type for exploiting shale gas fields.The optimal design of horizontal well trajectory is an indispensable part in well construction of shale gas field,which need to be further studied.The single well of cluster horizontal wells is three dimensional horizontal well,which belongs to the category of complex structure well.In the design method of trajectory to be drilled for well intersection,trajectory design in intersection process is devided into design with undesired wellbore direction and design with desired wellbore direction according to the wellbore direction of intersecting point.By use of vector algebra method and minimum curvature method,the calculating formulas for trajectory design in intersecting process are obtained when different known parameters are given.The geometrical shape of the target area is extended and the calculation formulas of trajectory design for landing on complex structures are obtained.The formulas for predicting the closest distance to the adjacent well or the landing surface are also presented combined with vector formulation of toolface angle.In solution of the system of trajectory design equations,the quasi analytic solution theory is applied in the established formulas,which avoids to solve complex nonlinear equations and makes the calculation more efficient.The design equations for deviation correction well have been established according to different target trajectory,multi-objective optimization model has been established taking trajectory length,trajectory coincidence rate and well trajectory complexity as objective functions.Case studies indicate that the maximum wellbore curvature can minimize total trajectory length and maximize trajectory coincidence rate,while the maximum horizontal displacement can minimize well trajectory complexity.In well trajectory design of sidetrack horizontal well in cluster wells,according to different trajectory types for bypassing obstacles,taking well trajectory length,well trajectory complexity and targeting accuracy as the optimized objective function and taking anti-collision with adjacent wells as constraint,then the optimal design model is established.According to learning curve of “well factory” drilling,the concept of “learning index of drilling” is proposed to quantify the reduced drilling cost versus drilling number in “well factory” drilling.The optimization model for platform location has been established by taking account of learning index of drilling.Case studies show that the drilling number of each platform would increase while the platform number would decrease with consideration of learning effect,which is contribute to applicate “well factory” drilling for reducing engineering cost.Under limited surface condition,arrange extended reach well and arrange negative displacement well at the meanwhile can enhance “well factory” efficiency and reduce well construction cost.