How to Obtain Reservoir Fluid Properties from an Oil Sample Contaminated with Synthetic Drilling Mud
Authors: MacMillan D.J. , Ginley G.M. , Dembicki H. 
Published: December 2007

Abstract
Given the increased utilization of synthetic muds in offshore drilling applications, it has become necessary to predict reservoir fluid properties using contaminated field samples. An accurate characterization of phase behavior is needed to quickly determine the overall project economics. It is also needed in reservoir simulation studies and to aid in reservoir development.
A unique method is presented that can predict reservoir phase behavior from properties of contaminated fluid samples. The technique uses both laboratory measurement and equation-of-state (EOS) modeling. The method can be applied to both exploration and development wells. The novel fluid characterization method presented here is especially valuable since it can also predict wax cloud point temperature which is necessary for facilities design.
The procedure was substantiated using artificially contaminated samples. Three field applications of the method are presented. All three field samples were highly contaminated by the synthetic drilling mud. Contamination levels ranged from 10 to 25 volume percent. Accurate predictions of uncontaminated reservoir fluid phase behavior were obtained. For example, all predicted saturation pressures were within 4 percent of the respective measured values.

Introduction
Synthetic oil base drilling muds have recently gained wide acceptance in offshore drilling applications. The benefits of using synthetic oil base muds are many, including substantial cost savings, increased rate of penetration, improved hole cleaning, decreased circulating time, fewer instances of stuck pipe, reduced total drilling days, and environmental acceptability.
Despite the obvious benefits of synthetic muds, their presence has an adverse effect on well logs, geochemical interpretation and the determination of fluid properties. Synthetic drilling fluid contaminants adversely affect certain key fluid properties such as cloud point, saturation pressure, gas-oil-ratio (GOR), formation volume factor and API gravity. Accurate fluid properties are needed to quickly determine the overall project economics. Facilities design requires accurate evaluation of phase behavior, wax cloud point temperature, and asphaltene deposition potential. Reservoir simulation studies also need accurate phase behavior to aid in reservoir development. While measures can be taken to minimize contamination while sampling, no effective method of analyzing contaminated samples has yet been published.
This paper focuses on the determination of accurate fluid properties given a sample contaminated with synthetic drilling mud. The effects of contamination are nonlinear. If a polynomial fit is used for each individual fluid property, the combined results will not provide a consistent fluid description. Hence, the method presented here is based on a combination of laboratory measurement and EOS modeling. In addition to the typical fluid phase behavior (PVT) properties, the method also provides accurate predictions of wax cloud point temperature.

Description of Drilling Fluids
The predominant synthetic oil base drilling fluids used by Marathon are NovadrilTM and NovaplusTM. P. 113^