Summary
Wireline formation testers are routinely used at discrete depths of a well to collect reservoir fluid samples and to estimate undisturbed reservoir pressures, near-wellbore formation permeabilities, fluid compressibilities, and saturation pressures. A pressure profile in the vertical direction yields fluid densities and fluid contacts (gas/oil and water/oil contacts) in the reservoir. Reliable results are obtained when the mudcake isolates the wellbore from the formation. When the mudcake cannot provide isolation, mud filtrate invasion continues and supercharging occurs. The issue of sample quality becomes critical when using oil-based muds because the filtrate is also oil and is difficult to separate from the formation oil, a pure sample of which is needed for fluid characterization studies. This study investigated the effects of poor mudcake seal on sample quality and formation test data and its analysis when oil-based muds are used. Modeling studies were conducted using a finite-element simulator.
The results of the study indicate that mudcake permeabilities must be less than 1 µd 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 depths improve sample quality. The presence of a permeability-damaged zone around the mudcake improves the sample quality but reduces the sampling pressure. The formation rate analysis (FRASM)*** technique estimates formation 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 subtracted from the apparent formation pressure to obtain the true formation pressure. A simple procedure is developed for estimating the mudcake permeability and the supercharged pressure. Supercharged pressure is shown to be a product of the apparent overbalance pressure, mudcake-to-formation permeability ratio, and an invasion factor representing the distance up to which supercharging extends.
Introduction
Drilling typically alters formations in such a way that a mudcake, a fines-invaded zone, and a filtrate-invaded zone are created between the wellbore and the native formation (Fig. 1, top). Zone properties such as thickness, permeability, porosity, and fluid saturation depend upon the mud and formation properties, hole size, and overbalance pressure, which is the difference between the wellbore and the formation pressure. Mudcake is an external (outside the formation) layer created by the fines-migration mechanisms of size exclusion and bridging.1 The fines-invaded zone is created by smooth deposition and bridging. The fines involved are generated by the processes of drilling, sudden salinity changes in porous media, and high viscous forces. The zone permeability may be an order of magnitude less than that of the formation. The filtrate-invaded zone usually extends beyond the fines-invaded zone. Poor quality mudcakes with low thicknesses and high permeabilities are commonly formed on surfaces of low permeability formations because the rate of filtrate flow through the formation is low. The filtrate invasion continues and the pressures in the near-wellbore area are higher than the native formation pressure. This phenomenon is called supercharging (Fig. 1, bottom).
Use of oil-based muds has increased recently because of advantages such as faster penetration, good wellbore stability, better lubrication that is especially important in deviated wellbores, and less solid and filtrate invasion into the formation. Lee and Kasap2 used a three-dimensional, single-phase, two-component, isothermal finite-element simulator to study the quality of samples (fraction of formation oil in the sample) received from a wireline formation tester (WFT) when oil-based muds were used. The simulator models wellbore geometry and formation-tool connections realistically; wellbore radius, mudcake thickness, permeability, and porosity are simulated functionally. Effects of viscous and dispersive forces are considered but not those of gravitational forces. For a sealing-type of mudcake, the results indicated that the sample quality reached 90% for a filtrate invasion distance of 10 cm. The rate of increase in sample quality with further pumpout was too low. The pumpout rate and formation permeability were insensitive parameters. The pumpout time required to obtain high-quality samples increased exponentially with the depth of filtrate invasion. The presence of a permeability-damaged zone around the wellbore improved sample quality because the angular inflow of filtrate from the invaded zone decreased. Higher formation anisotropy (horizontal-to-vertical permeability ratio) also improved sample quality because the vertical flow from the filtrate-invaded zone decreased.
Effects of leaking mudcakes have previously been studied to a limited extent.2,3 This study investigates the effects of mudcake quality on fluid sampling and supercharging when oil-based muds are used. The results of the study indicate that both the mudcake permeability and the mudcake-to-formation permeability ratio must be low to achieve high-quality samples. Conditions including high pumpout rates, low overbalance pressures, and shallow filtrate invasion depths improve sample quality. The presence of a permeability-damaged zone around the mudcake improves sample quality but reduces the sampling pressure. A simple procedure is developed for estimating the supercharged pressure that must be subtracted from the apparent reservoir pressure to obtain the true formation pressure.
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