低渗气藏多级压裂水平井非稳态流动裂缝设计

Fracture design for multi-fractured horizontal well with transient flow in low permeability gas reservoir

对于低渗气藏,采用分段压裂水平井技术可以达到经济效益最大化。基于点源函数法和能量守恒方程,建立非稳态气藏渗流与井筒压降耦合模型,刻画气藏介质—裂缝—井筒的流动特征,采用非线性迭代法求解耦合系统,融入平均压缩因数迭代法修正结果。对于某气藏,应用耦合模型设计最优压裂方案,优化单裂缝参数,以单裂缝最优几何形态为基础,优化双裂缝和多裂缝设计方案,包括裂缝位置、条数等。结果表明:相同体积、不同几何形态裂缝的产量差别很大,垂向—水平渗透率比越大,越应减小裂缝的垂向尺寸;固定裂缝几何形态,裂缝位于气藏中部时产量最高;流动早期,裂缝数量的增加使流量提高,随开采进行裂缝数量增加优势逐渐不明显;结合压裂成本、油价等因素,确定多裂缝系统最优裂缝数为3条。耦合模型与数值模拟结果误差在15%以内,验证模型的可靠性,系统化裂缝设计方法为低渗气藏压裂水平井提供指导。

For low-permeability gas reservoir,we adopted multi-fractured horizontal well for economic development.Our research method is to establish a transient flow coupling model for reservoir inflow and wellbore pressure drop for reasonably describing flow mechanism,adopting the potential-superposition-based source function method to establish the reservoir inflow model,selecting appropriate source function,generating analytical solution using Newman product.We derived the basic energy balance equation to establish the wellbore pressure drop model,and got the specific function for horizontal well by adopting the nonlinear iterative method to solve the coupling system.We got the inflow rate from each fracture and the pressure along wellbore,and revised results based on iterative average compressibility factor.Fracture parameters including fracture geometry,location,number are optimized based on optimum single fracture geometry,and the optimum solution for multi-fracture system is obtained.Results show that wellbore inflow is negligible compared with fracture inflow;wellbore pressure drop is non-negligible and has great influence on system production;fractures with same volume but different geometry have great difference on production.The larger the vertical-horizontal permeability ratio,the smaller the fracture height should be controlled.For fracture of fixed geometry,production is highest when locating it in certain range in the central part of the reservoir;production rate is proportional to the number of fractures in early stage,but becomes unobvious as development forwards;for a real gas reservoir,the optimum number of fractures is three.The error between this coupling model and simulation is within 15%,and production curve trends are consistent,demonstrating its reliability.The meaning of this research is the systematic fracture design method,and fast optimum solution for real fields.