Value of In-Situ Seismic Waves - Regain Lost Reserves, Increase Oil Cut

Abstract

When Occidental of Elk Hills (Oxy) opted to explore a new enhanced-oil-recovery (EOR) method—in-situ seismic stimulation—to increase oil recovery in declining reservoirs, liberating residual oil was but one outcome. Oxy’s seismic-stimulation project offered improved oil recovery and increased flow, positive impact on relative phase permeability, and an understanding of the magnitude and extent of the stimulated area. This technology and project results are based on and can be correlated to oil response resulting from natural earthquakes. It is the first departure from variants on conventional stimulation methods (thermal, polymer, chemical, microbial) to be introduced commercially since the 1970s. While efforts were made in the 1980s to duplicate earthquake effects by use of vibrators above a targeted zone, these attempts had disadvantages that limited commercial viability. Hydro-Impact stimulation services developed by Applied Seismic Research Corp., currently in use in the U.S. in California and Texas, involve the placement of tools to generate subsurface shockwaves that mimic primary waves generated by earthquakes. These shockwaves, generated every 10 seconds, produce power ranging from ≈1 to 10 million watts with pressure at the wave front that can be in excess of 4,000 psi. The waves, traveling at approximately 1½ miles/sec, generally are noticeably felt on the surface only in close proximity to the wellbore, but they have a significant measured seismic signal at distances of ¾ mile from the source (Fig. 1). Historically, effects from sonic stimulation could reach only 20 ft. The waves affect oil-bearing strata and increase the potential for oil flow by inducing coalescence of residual oil. The effect of the seismic shock: oil droplets are dislodged from the pore wall and coalesce into larger droplets that become mobile and move into flow streams that then begin to move into any existing flow system such as fractures. In some cases, reservoirs exhibit a permanent increase in oil cut as a result of a permanent fieldwide improvement in injectivity when deployed on injectors, but this is unpredictable. Other fields return to the original decline soon after seismic stimulation is stopped and require stimulation on a continuous basis. Because the seismic wave passes through the wellbore casing and cement within microseconds, it does not damage the wellbore or formation. The wave, propagating in all directions to eventually become an elastic wave, also can be used simultaneously with ongoing injection processes and considerably increases injectivity both in the injection well in which the tool is installed and other injection wells within the affected area.