July 2018 /// Vol 239 No. 7)
Research foci to improve EOR
Current recovery rates, especially in unconventional reservoirs, have improved significantly over the past decade, but there is still room for improvement. Below are several nascent technologies from various sources that have the potential to enhance oil recovery, particularly in unconventional reservoirs, through additional R&D.
Improved, real-time subsurface imaging. Despite significant advances in subsurface imaging over the past decade, real-time depictions of subsurface conditions and changes remain elusive. To fully optimize recovery from unconventional reservoirs, it will be necessary to develop real-time subsurface imaging on a micro scale, to both determine the effects of EOR and to assess subsurface conditions and reactions to treatment.
One area for possible progress is the enhancement of multi-physical imaging. Multi-physical imaging is of growing importance in geophysics, because the uncertainties in the inverted models can be reduced by integrating multi-physical measurements, which include surface seismic, microseismic, gravity, radiometric, electromagnetic, magneto telluric, hyperspectral, and wellbore-production data. For instance, interactions between fluid and solid minerals of subsurface reservoirs can be sensed by electromagnetic and seismic signals that can be jointly inverted to image the subsurface distribution of petrophysical properties.
Improved understanding of geological chemistry. As with subsurface imaging, significant strides have been made in the past decade to better understand subsurface chemistry in oil and gas operations. This is essential, as poorly understood, or misunderstood, chemical reactions in the subsurface can clog reservoirs, inhibit flow, and create malevolent chemical compounds. Work should progress on subsurface chemical analytical devices to provide data and basic analysis that will lead to a fundamental understanding of the chemical and physical properties of rocks at the micro-pore level, and the transformation of rocks and minerals in the presence of CO2, methane, and other fluids injected into the subsurface.
Seismic, electric, and electromagnetic heating. The electromagnetic heating of oil wells and reservoirs refers to thermal processes for the enhanced production of oil from underground reservoirs. The source of the heat, generated either in wells or the reservoir, is the electrical energy supplied from the surface. The main effect is the reduction of the viscosity of heavy and extra heavy crudes and bitumen, with the corresponding increase in production. Focus is centered on systems (and models that describe their effects) that have been used for electromagnetic heating in the production of extra heavy petroleum and bitumen.
Another method to introduce heat to the reservoir, in a controlled manner, is microwave heating. Different experiences have shown that the successful production by microwave heating strongly depends on the initial conditions of the reservoir, such as initial water saturation and water salinity. Microwaves produce heat more efficiently, in case of absorption inside the material, but crude is not a good absorber of microwaves. To enhance the absorption power of crude oil, microwave receptors (activated carbon, iron oxide and methanol) are used to stimulate the heating process. Microwave heating is affected by the design of the microwave source and the dielectric properties.
Seismic waves also may be used to heat reservoir fluids to improve viscosity and flow characteristics.
Microbial (MEOR) and enzymes (EEOR) processes. One special EOR technique uses microorganisms, such as bacteria and archaea, to dislodge the micro-trapped or absorbed oil from the rock. The goal of this technique, microbial enhanced oil recovery (MEOR), is to increase recovery of the original subsurface hydrocarbons, using bacteria rather than costlier chemical recovery processes. These biological processes typically use microorganisms to achieve results similar to chemical methods, reducing IFT and the mobility ratio of the water drive fluid to oil.
The major mechanisms that microbes are believed to operate by are: 1) alteration of the permeability of the subterranean formation by producing low molecular weight acids from biodegradation of hydrocarbons, which cause rock dissolution; 2) production of biosurfactants that can decrease IFT and form micelles of oil in water; 3) mediation of changes in wetability of the oil droplet by growing on the droplet and changing the surface of the oil to less hydrophobic; 4) production of bio-polymers that improve the mobility ratio of water to petroleum; 5) production of lower molecular weight hydrocarbons by enzymatically cleaving the large hydrocarbons into smaller molecules, and 6) generation of gases (predominantly CO2 and nitrogen) that increase formation pressure.
Of all the EOR processes, MEOR is considered the lowest-cost approach, but is generally the least often used, because it is not always successful or predictable. Furthermore, bacteria in oil wells, pipes and tanks are known to cause problems. Therefore, many petroleum engineers see bacteria as a problem, not a solution. In fact, the growth of bacteria could degrade the oil or increase the hydrogen sulfide concentration in the reservoir.
Enzymes have been reported as effective EOR agents. Both laboratory and field tests have demonstrated significant increases in ultimate oil production due to: wettability improvement of the rock surface; formation of the emulsions; reduction of oil viscosity; and removal of high molecular weight paraffins. However, enzyme EOR is not fully understood, per its chemical and physical reactions, as is the case of much of the technology discussed above, and further investigation should take place.