Comparison of Wireline Formation-Tester Sampling with Focused and Conventional Probes in the Presence of Oil-Base Mud-Filtrate Invasion

摘要

In the course of fluid sampling, varying concentrations of oil base mud (OBM) will lead to variations of fluid properties such as viscosity, density, and gas-oil ratio (GOR). A focused probe can be useful in reducing OBM contamination by diverting flow into different channels without compromising fluid pumpout time. However, it is important to properly quantify the relative performance of focused and conventional probes for a wide range of field conditions. The objective of this paper is to appraise the performance of different probes under the same simulated field conditions and for a comprehensive set of petrophysical and fluid properties. Results indicate that sample quality generally improves when the flow is split between the guard and sample probes, but the specific amount of improvement depends on probe geometry, fluid composition, and formation properties. Permeability anisotropy, presence of a flow boundary, and lack of mud-filtrate invasion can help to improve sample quality. In addition, fluid cleanup can be accelerated by altering both the probe design and the flow-rate ratio between the sample and guard fluid streams, thereby leading to increased pressure differential between the sample and guard areas and enhancing the "coning" of the mud-filtrate invasion front.