Recently the U.S. Department of Energy (DOE) has funded joint programs involving the Los Alamos National Laboratory, the Lawrence Berkeley National Laboratory, several universities, and several industrial companies. Tests in both the laboratory and field have shown that low frequency elastic waves (below 100 Hertz) can enhance the oil recovery rates by as much as 50%. These tests also show that pore pressure elastic waves as small as 70 Pa (0.01 psi) can improve the oil mobility and recovery rate.
Current research and analytical modeling are attempting to define the underlying mechanisms to explain how elastic wave stimulation can improve the fluid flow rates in abandoned oil fields. Several mechanisms have been suggested to explain why elastic wave stimulation increases the oil recovery rate. Oscillations caused by the elastic waves may induce the small, individual oil droplets to coalesce thereby increasing the oil flow. Elastic waves may increase the wettability of the formation material allowing the water flooding to be more effective. Exposure of laboratory samples to elastic waves has reduced the oil viscosity, perhaps caused by the heat generated by higher frequency stimulation. Elastic waves may increase the permeability of the oil bearing formation by “unplugging” the pores and pore throats in the formation. Elastic waves may also reduce the permeability by reducing the surface tension between the formation and oil-water mix.
When the current research is complete, the results will guide the selection of the elastic wave parameters (minimum amplitude and maximum frequency) that produce the optimum results for various geological and formation conditions and for different fluid properties. Until these studies are complete, the inherent flexibility of the HAI source can be used to generate elastic waves of different frequencies and amplitudes in order to experimentally arrive at the best elastic wave parameters for a particular oil field.
One of the standard ways that oil is taken out of the ground is through lowering the pressure in a well (via pumping), thereby inducing a pressure gradient that drives flow into the well. However, the presence of other fluids, especially water, tends to interfere with the flow of oil, and the entire system can exhibit quite complex, nonlinear, hysteretic and coupled behavior due to the presence of multiple fluid phases as well as any exchange phenomena across fluid-fluid or fluid-solid interfaces. Complex behavior and nonlinear feedbacks, from the pore scale to larger length scales, lead to oil recovery efficiencies on the order of 40%. That is, about 60% of the oil in a typical reservoir is left behind because it cannot be induced to flow into the well.
a new EOR technique using Acoustic and Seismic energy to reduce interfacial tension and promote coalescence of oil ganglia using the resonant frequency principle. The technique uses pressure and sound waves at pre-determined frequencies to set the oil molecules in motion to promote coalescence and thus reducing the fingering effects. The flood pattern can be optimized by recovering oil from previously unswept areas. It thus enhances the life of the primary flood providing valuable economic savings. The process can also be applied to various reservoirs in their tertiary recovery phase to extend the life of the project. Experiments conducted in the lab show that the frequency of oil and water may differ from reservoir to reservoir and compositional analysis of samples is required to determine the resonating frequency. The design of the downhole tools for injector wells to implement this technique are also discussed briefly.
Seismic stimulation
Reports of low-frequency, high-energy elastic waves mobilizing oil date from the early 1950s when earthquakes were shown to increase oil production by as much as 45%. The first man-made low-frequency, high-energy source used to mobilize oil was a Russian surface vibroseis in the early 1980s.
Detailed Russian field studies using surface vibroseis proved oil could be successfully mobilized and more importantly the mobilized oil came from the virgin reservoir area; thus, the application of low-frequency, high-energy elastic waves is an enhanced oil recovery method.
In 1998, Los Alamos National Laboratories (LANL) showed only a small pore pressure disturbance, less than 70 Pa (0.01 psi), could increase the permeability of a saturated porous medium and mobilize trapped fluids; hence, the process does not need elastic waves with large magnitudes to improve oil production.
In real terms, seismic stimulation has improved oil production and oil cut in wells as far away as 1.4 mile from the well in which the tool was installed.
The mechanisms describing the process of mobilizing oil under low-frequency, high-energy elastic wave fields center on two main phenomena:
1. Dislodging oil droplets from pore walls.
2. Coalescence of two or more droplets or thin oil films into one of higher mobility.
World Oil Online June 2009 Vol. 230 No. 6 Simulations reveal mechanisms of seismic waves for EOR
Simulations reveal mechanisms of seismic waves for EOR
The pore-scale effects of seismic stimulation on two-phase flow are numerically modeled in random two-dimensional grain-pack geometries.
Steven R. Pride, Lawrence Berkeley National Laboratory; Eirik G. Flekkøy and Olav Aursjø, University of Osl