Evaluation of the Effect of Oil Viscosity on the Disproportionate Permeability Reduction of a Polymeric Gel Used for Controlling Excess Water Production

Abstract

The property by which polymeric gels reduce water permeability in larger proportion than oil permeability is known as disproportionate permeability reduction (DPR). This phenomenon is believed to be the result of preferential blockage of water pore channels combined with the exclusive capacity of oil to deform the gel. This property represents an opportunity to successfully control water production, regardless the cause of the excess water inflow or the type of well completion. For this reason, many investigations intended to understand and improve DPR have been carried out. The relationship between DPR and parameters such as rock permeability, wettability and flow rate has been widely described. Nevertheless, no efforts have been made yet to evaluate the influence of oil viscosity. Considering the importance of heavy oil production for Venezuelan oil industry, as well as the growing impact of excess water production in these reservoirs, an experimental study was carried out to evaluate how DPR is affected by oil viscosity. Four coreflood experiments were carried out in high-permeability Berea cores to measure oil and water permeability before and after gel placement. In the first three tests, DPR was determined using oil with viscosities of 1.8; 67 and 140 cp. The last test consisted on sequentially measure the permeability of each of the oils when flowing through the same fully gel-saturated Berea core. Experimental results indicate that DPR improves as oil viscosity increases because 1) oil viscosity affects gel placement favouring selective plugging of water channels; and 2) the more viscous the oil, the larger the deformation of the gel, thus the easier for the oil to open a channel through it. According to this, a large potential of controlling excess water production in viscous oil reservoirs through gel technology application was established. Introduction In most of cases, as reservoirs reach maturity a steady increase in water production occurs. Production costs relative to lifting, treatment and disposal increases accordingly. However, the largest impact of excess water production is the accelerated decline in oil production caused by oil bypassing, which shortens productive lifetime of wells and affects the ultimate recovery of the reservoir. This situation becomes more severe as the oil-to-water viscosity ratio increases, because of the implied reduction in displacement efficiency. Nowadays, the water cut of those viscous oil reservoirs with larger production history in Venezuela can be as high as 65% and approximately 500 wells from these fields are currently shut-in because of high water production. Many of these wells are horizontal, therefore zonal isolation interventions are limited by the complexity and risk involved in such well completions; instead sidetracks and new wells are drilled.

Water Control with Gels Based on Synthetic Polymers under Extreme Conditions in Oil Wells

“…2). Oil can penetrate the gel, but only through a narrow pathway in the center of pores by fingering through gel particles 24–26. In this way, oil is selectively produced relative to water in production wells.…”