A high quality formation fluid sample is essential to identify the composition of formation fluids which is required in designing expensive production facilities. Formation fluid samples are also needed to assess hydrocarbon reserves and reservoir recovery efficiency.
Minimal contaminant is desired in formation fluid samples for proper composition and PVT analyses. The contaminant is the drilling fluid filtrate that invaded near wellbore formations during and after the drilling process. The filtrate in the sample compromises sample quality by reducing usable sample volume and altering its composition and PVT properties. To sample clean enough formation fluid, a certain volume of fluid must be pumped out prior to sampling. Although initially a mixture of invading filtrate and formation fluid is produced, generally clean enough formation fluid is sampled after a certain fluid volume is pumped out.
The objective of this study is to identify the controlling parameters in determining the optimum pump out volume. After a certain pump out, a minute increase in sample quality requires long pump out times. Therefore, pre-job planning to obtain pump out volume and duration of pump out required, based on the sample quality, is important for the tool and hole safety.
Finite element simulations with realistic wellbore, formation geometry and mud cake characteristics are run to study the effects of pumping rate, invaded zone thickness, damaged zone permeability, formation anisotropy, and mud cake quality. The simulation results indicate that pump out volume, not pumping rate, controls sample quality. Formation damage and anisotropy improve sample quality by limiting the production from the invaded zone. Poor mud cake quality and deeper invaded zones have adverse effects on sample quality and require more careful planning.