Carbonated Water Imbibition Flooding: An Enhanced Oil Recovery Process for Fractured Reservoirs

Abstract

Water imbibition studies were conducted on core plugs at confining pressures of 2,000 psi. Water was enriched with CO2 at different carbonation pressures, ranging from 50 to 500 psi. Conventional displacement as well as Nuclear Magnetic Resonance (NMR) methods were used to study the effects of carbonated water imbibition on oil recovery rate and ultimate recovery. The major factors influencing Oil recovery by carbonated water imbibition displacement appear to be:An increase in oil mobility. An increase m core permeability in carbonate cores A reduction in the operating pressure below the bubble point pressureafter the core had been allowed to imbibe the carbonated water induced asolution gas drive effect which substantially increased recovery. Carbon dioxide dissolved into the water being imbibed by the rock was found to accelerate oil recovery rates and increase ultimate oil recovery when compared to unadulterated water imbibition. Recovery rate and ultimate recovery increased as a function of the amount of CO2 dissolved into the imbibed water. Recovery time was reduced to one third when the gas liberation process was applied cyclically. This new method holds promise to increase oil production rates and oil reserves from fractured, low matrix permeability, low gas-oil ratio, oil reservoirs. II. Introduction Naturally fractured, dual porosity oil reservoirs present a particular oil production problem. Usually oil is readily produced from the fracture portion of the system. However, the oil located in the, matrix blocks is not easily displaced because of the relative ease of the fluids to channel through the fractures and bypass the matrix pore system. The application of any external drivemechanism to displace oil will mainly affect the high permeability avenues of the fracture system. Generally, the matrix blocks remain essentially unaffected by normal secondary enhanced oil recovery oil displacement methods. Figure 1 illustrates the flow problem for this particular type of reservoir. The effects of capillary forces to spontaneously imbibe water into the matrix blocks has previously been used to displace otherwise unrecoverable oil reserves. Several tests on the Spraberrry field, and other fractured reservoirs have proven that spontaneous water imbibition can be an important oil producing mechanism. However, the process with counter current oil flow is usually very time dependent. The inclusion of CO2 in the imbibed water could not only possibly accelerate but also increase oil recovery from the low permeability matrix blocks in a fractured, dual permeability reservoir. The use of CO2 as a method to increase oil recovery has been previously studied. The most important mechanisms leading to higher oil recovery are:CO2 becomes miscible with water and most crude oils at pressures greater than 1,400 psi,CO2 might create swelling of oil,CO2 has an acidic effect on calcareous rocks, andCO2 creates a gas drive when pressure depleted below carbonation pressures. However, no one has studied the effect of replacing unadulterated water with carbonated water to accelerate and increase ultimate recovery by the water imbibition-oil displacement process. This idea would be very applicable to enhancing oil recovery from dual porosity, naturally fractured, carbonate oil reservoirs. P. 79^

A Literature Analysis of the WAG Injectivity Abnormalities in the CO2 Process

“…33,34,35,36,37 Additionally, the use of surfactants in carbonated water imbibition significantly increased oil recovery in rock samples with mixed or oil-wet conditions. 38 Injection of water with dissolved gas and injection of water above the bubblepoint may result in the most even distribution of gas throughout the reservoir where oil is otherwise trapped.…”

Carbonated water injection revisited

“…The tests revealed CWI could significantly improve oil recovery (McFarlane et al, 1952). CWI greatly improved oil recovery, also increased rock permeability (Perez et al, 1992). The cores were 1 inch in diameter and 2 inch in length.…”

Selection of Proper EOR Method for Efficient Matrix Recovery in Naturally Fractured Reservoirs

“…Injection of carbonated water [40][41] and surfactant solutions [42][43][44] are the most common examples. All these studies are limited with laboratory scale tests.…”