Adverse effects of poor mudcake quality: A supercharging and fluid sampling study

Wireline formation testers are routinely used at discrete depths of a well to collect reservoir fluid samples and to estimate undisturbed reservoir pr essures, near-wellbore formation permeabilities, fluid compressibilities, a nd saturation pressures. A pressure profile in the vertical direction yield s fluid densities and fluid contacts (gas/oil and water/oil contacts) in th e reservoir. Reliable results are obtained when the mudcake isolates the we llbore from the formation. When the mudcake cannot provide isolation, mud f iltrate invasion continues and supercharging occurs. The issue of sample qu ality becomes critical when using oil-based muds because the filtrate is al so oil and is difficult to separate from the formation oil, a pure sample o f which is needed for fluid characterization studies. This study investigat ed the effects of poor mudcake seal on sample quality and formation test da ta and its analysis when oil-based muds are used. Modeling studies were con ducted using a finite-element simulator. The results of the study indicate that mudcake permeabilities must be less than 1 mu d and mudcake-to-formation permeability ratios must be less than 10(-4) to achieve sample qualities higher than 90%. Such conditions as high pumpout rates, low overbalance pressures, and shallow filtrate invasion de pths improve sample quality. The presence of a permeability-damaged zone ar ound the mudcake improves the sample quality but reduces the sampling press ure. The formation rate analysis (FRA(SM))(dagger) technique estimates form ation permeability accurately in the presence or absence of supercharging. The formation pressure estimated using the buildup data is the pressure at the mudcake-formation interface. The supercharged pressure must be subtract ed from the apparent formation pressure to obtain the true formation pressu re. A simple procedure is developed for estimating the mudcake permeability and the supercharged pressure. Supercharged pressure is shown to be a prod uct of the apparent overbalance pressure, mudcake-to-formation permeability ratio, and an invasion factor representing the distance up to which superc harging extends.