Horizontal well pressure transient analysis in anisotropic composite reservoirs—A three dimensional semi-analytical approach

摘要

A composite reservoir is one with multiple compartments, each having distinct rock properties separated by geologic discontinuities; these discontinuities regulate the degree of compartment distinctiveness depending on the amount of leakage involved. Geologically, composite reservoirs separated by leaky faults are rife but literature containing detailed three dimensional analyses of such anisotropic systems is few. For the first time using transform methods, a general customizable three dimensional point source semi-analytical solution for the drawdown response in composite clastic systems separated by a leaky fault is developed. This solution is converted to line source for horizontal wells by integrating along the producing well length while treating the leaky fault as a thin finite conductivity fracture separating two reservoirs with different properties. An arbitrary well orientation to principal permeability direction is adopted to obtain a general solution in the anisotropic system and this semi-analytical solution is achieved using a simple computer program which generates type curves in a computationally cheap and effective manner. Sensitivity analysis conducted on the horizontal well drawdown signature with respect to areal permeability anisotropy, angle of orientation, well position, reservoir dimensions, dimensionless fault conductivity and mobility ratios in composite clastic reservoir systems reveals that the drawdown response is a convoluted function of reservoir properties from both compartments and the separating semi-permeable medium. Field data from the Niger Delta is used to validate this model and direct synthesis of reservoir parameters from the resulting equations is examined. The result of this work is attractive where formation juxtapositions separated by leaky faults in the reservoir are suspected to allow flow from adjacent fields and where vertical permeability plays a major role in depletion.