Obtaining clean samples of undersaturated crude oil near its bubblepoint from undercompacted, weak, low-permeability mudstones in the presence of miscible mud filtrate invasion poses a daunting challenge.
Pressure drawdown must be tightly controlled to avoid both formation erosion and fluid phase transitioning while flow rates must support sufficient cleaning and minimize the risk of tool sticking. The borehole geometry and relatively shallow reservoir depth limit the volume of crude oil that can be allowed into the mud system, further increasing the importance of efficient cleanup.
Legacy fluid-sampling attempts in this environment failed to produce representative samples. Even the lowest achievable flow rates generated excessive pressure drawdown, causing fluid phase transitioning and formation erosion that resulted in tool plugging. The lack of conclusive data on reservoir fluid behavior limited confidence in both formation evaluation and reservoir flow simulation.
The new Ora intelligent wireline formation testing platform has integrated pumps that each precisely manage flow rates from 0.1 to 60 cm3/s. This extended low-flow-rate capability and the platform's large inlet flow area can be employed to control pressure drawdown during cleanup pumping, ensuring sustainable single-phase flow at optimized flow rates. Controlled flow can also be used after sampling to slowly decompress the fluid in the platform's sampling flowline while recording its temperature, pressure, optical density, and density to enable real-time downhole bubblepoint determination.
Controlled downhole decompression of the crude oil pumped into the flowline at a sampling station in the well enabled real-time measurement of its bubblepoint pressure to within 0.4 bar [6 psi] of that measured in the laboratory. Focused sampling and tight control on flow rates provided the first clean samples from the reservoir, finally enabling confident fluid characterization.
