A Robust Method for Calculating Formation Mobility With a Formation Tester

Abstract

SummaryDetermining formation mobility (permeability/viscosity) with a formation tester dates back to the introduction of the tool in the mid-1950s. Many methods have been developed over the years to calculate a formation mobility from a pressure test. Here, a rigorous method is presented that has several convenient features. It is based on a model that assumes the presence of both tool-storage and formation compressibility and so is appropriate for a fully unsteady flow induced in the formation, but it does not require the determination of either compressibility to determine the mobility. The full impact of the wellbore geometry is included (i.e., it does not assume the "spherical-flow model"). The mobility is determined by a simple, direct calculation of the measured time-dependent pressure, as opposed to methods based upon data regression or solving an inverse problem. On the other hand, this new method shares several of the restrictions common to existing techniques: It assumes that the formation contains a single phase of slightly compressible fluid. The calculated mobility characterizes the formation in the vicinity of the wellbore wall, in which formation damage is most likely to occur.This publication is dedicated to Gérard Catala, a colleague with tremendous insight.