COMPARISON OF WIRELINE FORMATION-TESTER SAMPLING WITH FOCUSED AND CONVENTIONAL PROBES IN THE PRESENCE OF OIL-BASE MUD-FILTRATE INVASION

摘要

Acquisition of fluid samples is challenging in the presence of oil-base mud (OBM) filtrate invasion due to its partial or full miscibility with reservoir hydrocarbons. In the course of fluid sampling, varying concentrations of OBM will lead to variations of fluid properties such as viscosity, density, and gas-oil ratio (GOR). Contamination of OBM filtrate in the sampled fluid can drastically affect sample quality and lead to non-representative fluid properties. Fluid pumpout time can be extended to reduce filtrate contamination, although at the expense of increasing overall rig cost. 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. We simulate the process of fluid sampling with a focused probe in a vertical well and compare its performance to that of a one-inch diameter conventional probe in the presence of mud-filtrate invasion. A commercial adaptive-implicit compositional reservoir simulator is used to model both invasion and filtrate-cleanup processes. Comparison between fluid sampling with focused and conventional probes identifies cases in which focused fluid sampling leads to improved sample quality in a shorter period of time. 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. In addition, because the focused probe withdraws fluids through a larger cross-sectional area, the pressure drop caused by the focused probe is smaller than the pressure drop associated with a conventional probe. Permeability anisotropy, presence of a shale 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. Performance of the focused probe improves with increasing fluidfront dispersion because focusing enhances the “coning” of the mud-filtrate invasion front. Lastly, we perform inversion jointly from transient measurements of GOR and probe pressures to estimate formation permeability, anisotropy, and rate of mud-filtrate invasion. It is shown that, in general, focused fluid sampling provides enhanced degrees of freedom in the pressure and GOR transient measurements to improve both accuracy and reliability of the estimated formation properties.