Acoustic Downhole Oil/Water/Fines Separation (ADOWFS)

Abstract

Summary The oil and gas industry's global need for downhole-oil/water-separation (DOWS) systems is increasing with reservoir maturation and rising water cuts in producing wells in an effort to reduce the costs and environmental impacts associated with managing large volumes of produced water. Currently, between 3 and 10 barrels of formation water are produced to the surface for every produced barrel of oil. In general, existing and emerging DOWS technologies use hydrocyclones, gravitational-separation, or membrane-separation techniques to separate oil from water downhole. These methods suffer from high cost and limited capacity, in addition to numerous systemic problems and mechanical-parts failure. They are hardly tunable to changes in flow rate or water cut, unless special valves and pumping systems are used, and they are not capable of separating fine solids from the fluid-stream downhole. Acoustic-separation techniques can provide a viable solution to these limitations and can add numerous advantageous functions. In this paper, we introduce the concept of the Acoustic Downhole Oil-Water-Fines Separation (ADOWFS) and its field implementation. ADOWFS is a three-phase separation technique that uses a sequence of acoustic standing-wave patterns along a production tube with a decreasing number of loops in the direction of flow. It is capable of simultaneously separating oil droplets and solid fines from the water stream in the production tube. Oil droplets are directed and concentrated in the center of the production tube so it can be lifted to the surface while solid fines are directed and concentrated near the tube's walls. Water can then be injected downhole into a nonproducing formation after removing the acoustically separated hydrocarbon residues and fines. ADOWFS, thus, not only increases the oil-to-water ratios in producing wells, but also reduces the risk of pore clogging and formation contamination if water is reinjected back in the formation. Moreover, ADOWFS allows instantaneous tuning by means of surface controls to meet changes in flow rates and water cuts and can be integrated into existing downhole-separation infrastructures (e.g., to replace the hydrocyclones). A set of proof-of-concept experiments that used direct microscopic visualization in a microfluidic channel demonstrated the instantaneous separation of micron-sized oil droplets and solid fines from water by exciting the proper acoustic standing-wave fields in the flow channel. The oil droplets and solid fines were separated by a distance of λ/4 (λ is the wavelength of the acoustic wave), which is the expected separation between the pressure nodal and antinodal planes of the standing wave.