Quantifying the impact of oil-based mud contamination on representative condensate reservoir fluid characterisation

Abstract

As hydrocarbon resources become more marginal, reducing data uncertainty associated with exploration and appraisal drilling becomes more important. For gas condensate fields where a great deal of the value is stored within the recovered liquids, variations in the condensate gas ratio (CGRs), or gas expansion factors (Bg) for example can have a considerable impact on the development strategy. Understanding the fluid in place is often key to the decision as to whether a development should go ahead or not. A considerable proportion of wells are drilled with oil-based mud. When fluid sampling is executed, the in-place reservoir fluid is often contaminated to various levels with this mud. The intention of this extended abstract is to gain an understanding of how much contamination can be accurately corrected. Several case studies are undertaken incorporating fluids from the same fields obtained in different fashions. Contaminated samples sourced from small volume probes on open-hole formations, for example an MDT or RCI tool, are compared with assumed clean samples obtained downhole during drill stem tests (DST). A suitable equation of state (EOS) is then selected and tuned to match experimentally derived results from both data sets. The small probe sample is then numerically decontaminated for mud, producing a clean EOS for comparison. By quantifying the differences in simulated depletion experiments, a judgement can then be made as to how much contamination can accurately be compensated.