Improved Sweep Efficiency By Selective Plugging of Highly Watered Out Zones By Alcohol Induced Precipitation
Authors: Zhu T. , Tiab D. 
Published: March 2010

A new selective plugging process has been developed for improving volumetric sweep efficiency and thus, the enhancement of recovery from oil reservoirs. The new process is based on the theory of "salting out" in which a non-electrolyte is added to water to reduce the solubility of electrolytic substances. In this new process, following a concentrated brine preflush, one or more slugs of a water soluble alcohol (such as ethanol) are injected into a reservoir. The mixing of alcohol with brine will cause salt precipitation. Alcohol and concentrated brine prefer to flow through water flooded zones because of the high relative permeability to these fluids. The formation of solid precipitate can partially or completely block the high-permeability zones and divert subsequent fluid flow into lower permeability zones where oil saturations are greater. A larger volume of the reservoir is thus contacted by the injected fluid, improving the overall volumetric sweep efficiency and the oil recovery. In homogeneous sandpack flow tests, a permeability reduction of up to 70% is obtained. In heterogeneous sandpack flow tests, the permeability is reduced to 50% of original brine permeability. The experimental results show that the new selective plugging process can recover an additional 15% of original oil in place (OOIP).

There are advantages to this new method over other selective permeability reduction techniques. The process can be applied to deep and shallow oil reservoirs. In addition, the plugging of more permeable zones is easy to remove by injecting brine with low salt concentration, if required.

Introduction

A large portion (as much as 65% average) of the original oil in place in oil reservoirs is not recoverable with conventional waterflooding technology(1). This low oil recovery is due to both low displacement efficiency and low volumetric sweep efficiency. The oil industry has been seeking enhanced oil recovery methods to improve displacement efficiency and volumetric sweep efficiency.

In oil reservoirs, almost all formations are heterogeneous. Some regions have low permeability while others have high permeability. All injected fluids have the tendency to flow through the least resistant regions (having the highest permeability) bypassing the lower permeability zones. In a waterflooded reservoir, after a substantial amount of oil is recovered from high permeability areas, the recovery process may become uneconomical because the water-oil ratio is high. Therefore, if a selective permeability plugging method could be developed to block off highly water-invaded regions any subsequently injected fluids would have to pass through regions not previously invaded by water. As a result selective permeability plugging would improve volumetric sweep efficiency and increase ultimate oil recovery.

Interest in plugging highly water-invaded zones in oil reservoirs for improved sweep efficiency, has increased over the past several decades. Numerous methods have been studied to plug highly water-invaded regions including foams, polymers, surfactants, and microbes(2–8). However, each of these methods has limited application because of some inherent problems. For example, polymers are degraded by shear forces as they are propagated through reservoir rock. In surfactant processes, adsorption of surfactants on minerals is a major cause of surfactant degradation and uneconomic performance.