Wireline formation sample contamination and quality control depend on formation parameters and mud characteristics. The invasion process is a complex multi-phase problem particularly when mixed miscible and immiscible fluids are present. The scenario is complex when either oil-based mud (OBM) or water-based mud (WBM) invades a reservoir with initial oil-water concentration. Either oil or water may be the principal contaminating fluid depending on mud type. This paper presents the results of analyses based on the first wellbore invasion simulator that models the complete problem. The governing differential equations for miscible and immiscible fluids, with fluid compressibility, rock heterogeneity and anisotropy, and gravity are developed from first principles and implemented in a finite difference code. In addition, general boundary conditions allow dynamically coupled mudcake growth, which do not assume that mudcake completely controls invasion into the formation. The simulator developed also allows bed boundary modeling as well as assessment of multi-probed pumping.

Previous invasion simulations in the literature are based on two-phase immiscible fluids where WBM invades an oil zone. Of greatest interest, however, in sampling quality is the OBM invasion problem. OBM contains a mixture of oil and water, where the oil is the continuous phase and encapsulates the water. The continuous mud filtrate phase dominates the invasion and, according to molecular diffusion dynamics, mixes miscibly with formation fluids. The effects of anisotropy, mudcake growth, gravity, and two-phase flow properties on this problem can thus be assessed.

While invasion has a strong influence on potential sample quality, the pumpout wireline formation tester (PWFT) sampling procedures can be designed to have an equal influence on sample quality. The new simulator can evaluate the concentration of the fluid phases and types during the sampling process.

Although it is a fact that sampling rate as well as the placement of a probe within a zone can have an impact on sample quality, in practice the oil's bubble point pressure limits the sampling pumpout rate. However, if a tester is equipped with more than one probe, as the new tester is, pumping through both probes can increase sampling rates. Additionally, if the sampling probe is placed near a permeability barrier, a further improvement on sample quality can be expected. All of these options can potentially improve sample quality and reduce pumpout time.

To study these effects a sensitivity analysis has been performed using the simulator to determine the impact sampling procedures have on sample quality. PWFT log examples are used to compare with the simulated results.

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