Efficient and accurate estimation of permeability and permeability anisotropy from straddle-packer formation tester measurements using the physics of two-phase immiscible flow and invasion

This paper describes the successful application of a novel methodology to estimate permeability and permeability anisotropy from transient measurements of pressure acquired with a wireline straddle-packer formation tester. Unlike standard algorithms used for the interpretation of formation tester measurements, the methodology developed in this paper rigorously incorporates the physics of two-phase immiscible flow as well as the process of mudcake buildup and invasion. The effects of supercharge, skin, and convective transport of salt are also explicitly included in the estimation algorithm. An efficient 2D (cylindrical coordinates) implicit-pressure explicit-saturation finite-difference algorithm is used to simulate both the process of invasion and the pressure measurements acquired with the straddle-packer formation tester. Initial conditions for the simulation of formation tester measurements are determined by the spatial distributions of pressure and fluid saturation resulting from mud-filtrate invasion. Inversion is performed with a rigorous Levenberg-Marquardt nonlinear minimization algorithm. Sensitivity analyses are conducted to assess non-uniqueness and the impact of assumptions on rock-fluid and mud properties such as fluid viscosity, capillary pressure, relative permeability, mudcake growth, and time of invasion. Applications of the estimation algorithm to noisy synthetic measurements include homogeneous, anisotropic, layered, and multi-layered formations, for both low- and high-porosity rocks. We also study the effect of unaccounted impermeable bed boundaries on the inversion results. For cases where a-priori information can be sufficiently constrained, our inversion methodology provides reliable and accurate estimates of permeability and permeability anisotropy.