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Abstract

The Big Sinking Field is a mature waterflood in the Appalachian Basin of Eastern Kentucky. The 100 plus million barrel (15.9 million m³) field is primarily a shallow tight oil reservoir at 1,150 feet (350 m). A primary constraint to production is the ability to inject fluid into the reservoir. This project was an effort to increase injectivity by injecting an alkali-surfactant solution to reduce the residual oil saturation with a resultant increase in effective permeability to water. Oil is mobilized away from the injection well bore where a substantial portion of the injection pressure is lost by reducing capillary forces that trap oil. A new well was drilled and cored, and a laboratory program performed prior to field implementation. A variety of mixtures of alkali and surfactant reduced the interfacial tension between oil and water to 0.001 dyne/cm from 23.6 dyne/cm. Injection of a Na₂CO₃ plus ORS-62HF alkaline-surfactant solution increased the relative water permeability by 425% and a NaOH plus AX-210-6 alkaline-surfactant solution increased the effective water permeability by 310%. Subsequent fresh water injection resulted in a loss of effective permeability to water to 155% times the original waterflood base for the Na₂CO₃ plus ORS-62HF solution. The NaOH plus AX-210-6 coreflood maintained injectivity at 325% with subsequent water injection. In September 2003, injectivity testing began with injection of fresh water into a newly completed well to establish initial injection rate. A 1,500 barrel (238 m³) injection of NaOH plus ORS-162HF alkaline-surfactant solution followed. Return of fresh water injection showed an increase in fresh water injection from 41 to 75 barrels per day (6.5 to 11.9 m³).

Introduction

Bretagne operates of a 3,500 acre (14,163 Mm²) water flood in the Big Sinking Oil Field that has produced over 17 million barrels (2.7 million m ³) of secondary recovery reserves. The primary limiting factor in this waterflood, as well as many other mature waterfloods, is the injectivity. Previous operators have increased injection rate by converting additional wells to injection, showing immediate increase in production. Injection history has shown that as injection rate changed, production rate changed along with it. With the ending ratios of 30 bbls (4.8 m³) of water injected per bbl of oil produced, additional mobile oil remains as well as additional recovery of mobile oil from crossflooding.^1,2

In order to maximize the flooding efficiency, other EOR processes are being evaluated. However, most of these process options also are dependent upon injectivity. While closer spacing can be used to compensate for the injectivity constraint, the amount of oil produced per pattern reaches diminishing returns. Stimulation options and maximum injection pressure restrictions are limited with the reservoir at an average depth of 1,150 feet (350m).

Injectivity can be increased by decreasing oil saturation which, in turn, increases the effective permeability to water due to changes in relative permeability.³ Oil saturation reduction can be achieved by injecting chemicals, such as alkali and surfactant that lower the interfacial tension between oil and water, reducing the capillary forces that trap oil. When attempting to increase injection rate, an interfacial tension reducing solution is injected in a volume sufficient to affect the reservoir near the injection well where a substantial portion of the injection pressure occurs.

Objectives and Expected Significance of Research

The objectives of the research are:

• Demonstrate that a low interfacial tension solution can be developed for the Big Sinking field reservoir that will reduce oil saturation and increase effective water permeability and inject into a low permeability Corniferous formation core

• Demonstrate that water injectivity can be improved as a result of injection of a low interfacial tension solution. This will increase the reservoir throughput rate and determine optimum spacing for future development.

Statement of Theory

Two problems exist in old, waterflooded reservoirs like Big Sinking. 1.) Water cuts are high as the oil reservoir approaches the end of its productive life. 2.) Water injection rate is restricted which limits the rate of fluid produced and, therefore, the oil rate. Both problems can be addressed by changing the effective permeability to water.