Improved Oil Recovery by Seismic Vibration: A Preliminary Assessment of Possible Mechanisms

Abstract

ABSTRACT Use of seismic vibration has been proposed as a low-cost IOR method, with reports of some successful field results. A key difficulty with the method is that its mechanism for oil recovery is not yet known, and accordingly, process performance prediction and reliable project design is so far not possible. In this study, a critical review of literature on the field and laboratory results and theoretical modeling work is first made. Employing a simple pore-level model and the well-established capillary number correlation, mobilization of waterflood residual oil by seismic vibration is shown to be unlikely, confirming earlier coreflood data. A more likely mechanism for IOR is that vibration squeezes out the oil bypassed by earlier recovery methods, due to reservoir heterogeneity. While it is quite difficult to bring the injection fluids to the low-permeability zones in a highly heterogeneous reservoir, vibrational wave can transmit effectively through both high- and low-permeability zones. When a seismic vibration propagates through a heterogeneous reservoir, the pore pressure response in different permeability layers becomes different, generating a transient pressure difference and crossflow between the layers, potentially squeezing out oil from the low-permeability zones. The rock deformation and fluid flow equations are coupled to describe the effects of low-frequency oscillatory poroelastic deformation on crossflow. Improved oil recovery due to vibration-induced crossflow is approximately estimated. Our assessment that seismic vibration recovers only the oil bypassed due to reservoir heterogeneity may explain why the method's success is rather erratic, since accurate advance characterization of reservoir heterogeneity and the bypassed oil is inherently difficult. On positive side, seismic vibration may improve access of IOR chemical to low-permeability zones such as matrix portion of naturally fractured reservoirs, for better oil extraction from hard-to-access tight matrices. Another important question that needs to be addressed is how to deliver vibrational energy effectively to a large volume of reservoir where the oil is. Fluid pressure oscillation damps down quickly in porous media and even the rock deformation wave attenuates significantly. One possible way is to transmit rock displacement wave continually so that its accumulation can form resonance in the reservoir. The resonance generation with the reservoir length scale as the resonance wavelength requires a very slow wave, and recent literature on the subject is discussed. INTRODUCTION Increase in oil production from mechanical vibration has been initially observed in Russian oil fields when earthquake occurred nearby (Beresnev and Johnson 1991; Nikolaevskiy et al. 1996). While some of the observations of production change were obviously due to the large-scale upheaval in reservoir structure from the earthquake's high energy input, the production changes have also been observed when the distance between the epicenter and the reservoir was quite far, so that the reservoir has been subjected to only low-frequency, low-amplitude vibrations. Prompted by these natural occurrences, vibration has been deliberately applied to affect a reservoir's oil production response. All manners of vibration were applied:periodic "thumping" of the surface with a large weight to generate seismic waves (vibroseis);injection of water in pulses from well to generate pressure waves in a water-hammer fashion;sonic or ultrasonic generation in the fluid phase at the wellbore; andvarious modes of chemical/nuclear explosions either downhole or at or near the ground surface.