Numerical Investigation of Oil-Base-Mud Contamination for Condensates: From Cleanup to Sample Quality
Authors: Malik Mayank , Dindoruk Birol , Elshahawi Hani , Torres-Verdin Carlos 
Published: October 2009

Abstract
Formation Testers are widely used to determine pore pressure, estimate formation permeability, and detect reservoir connectivity through pressure transient testing after the onset of invasion. Mud filtrate invasion takes place in reservoirs penetrated by a well that is hydraulically overbalanced by mud circulation, or due to capillary forces. In water-base muds (WBM), the invading mud is immiscible with respect to the formation hydrocarbons. Therefore, water can be physically separated from the in-situ hydrocarbons leading to the best estimates of the in-situ PVT properties and thereby formation properties.
Oil-base muds (OBM) are partially to completely miscible with the reservoir hydrocarbons, and so OBM contamination causes alteration of fluid properties which becomes even more critical for condensates when changes in fluid viscosity, density, and relative permeability occur. Due to the complexity of partial miscibility with gases and gas condensates, limited work has been done to simulate invasion by OBM. The goals of our work were to:Determine conditions to obtain better samples.Quantify the errors in numerical cleaning methods necessary for obtaining in-situ fluid compositions and properties.Investigate the physics of the clean-up process.Investigate the feasibility of tracers for monitoring contamination.
Our results show that for condensates and lean gases, quantifying OBM contamination in terms of the live/bulk fluid alone can be misleading. For such fluids, contamination in the stock tank oil is just as critical as that of the bulk fluid because only the former predicts the errors in saturation pressures and CGR numbers observed in laboratory analyses. Lean fluids can take extremely long times to completely clean up during a formation test or even during a well test. For those fluids, it is more essential than ever to clearly define the primary objectives of the sampling program and to decide which answers are most critical.

Introduction
Wireline formation testers (WFT) are widely used to measure formation pressure, estimate permeability, perform downhole fluid analysis (DFA), and collect representative fluid samples. The presence of drilling mud in the wellbore at pressures usually higher than formation pressure causes the mud-filtrate to invade the near-wellbore region. When Oil based muds (OBM's) are used, sampling without adequate cleanup will yield unrepresentative fluid samples for PVT or flow assurance analyses. As OBM is miscible to some degree with all reservoir hydrocarbons, OBM contamination causes alteration of fluid properties. Such alteration becomes even more critical in condensates due to partial/unknown miscibility. Prolonged cleanup times will improve sample quality, but this comes at the expense of increased time, cost, and/or risk. Therefore, as with any optimization problem, the objective is to find the optimal combination of operating parameters that maximizes the value of the obtained formation fluid properties subject to the operating constraints at hand.