Multiple Factors That Influence Wireline Formation Tester Pressure Measurements And Fluid Contacts Estimates

Abstract

Abstract Pressure measurements using conventional wireline formation testers (WFTs) can be influenced by near-wellbore effects such as mudcake buildup and filtrate invasion, which are coupled physical events. These combined effects can change the interpretation of the pressure data, shifting the gradient curves and the fluid contact depths. Recent papers have discussed corrections for wettability and for capillary pressure effects on formation tester measurements, the next steps needed for improved predictive measurements.1,2,3 Suggestions by different investigators have stimulated interest in several research directions. Because the foregoing effects are the result of mud filtrate invasion, which normally involves multiphase flow, it is necessary to quantify all competing results using a numerical simulator that models all effects rigorously. The work described in this paper models the complete process quantitatively with an invasion simulator that includes the dynamically coupled effects of mudcake growth and multiphase, immiscible filtrate invasion. The simulator is used to model the invasion profile, that is, the changing oil-water ratio while pumping. It includes wettability and capillary pressure effects. Many of the parameters predicted in the simulation can be crosschecked from log data, including mobility estimates from WFTs and fluid saturations from NMR and array induction logs. Modern WFTs such as the Reservoir Description Tool (RDT) can monitor the fluid properties while pumping.4 This information can, in turn, be used to determine the volume fractions of the produced fluids. The same simulator used for invasion is used to match pump-out results and determine supercharge (i.e., elevated pressures near the wellbore) and capillary pressure effects. Several field examples are used to illustrate how these methods are combined to correct pressure measurements, gradients, and fluid contacts.