Effect of Clays, Limestone, and Gypsum on Soluble Oil Flooding

Abstract

Introduction In our investigation of the soluble oil flooding process we observed that the process is less efficient in some porous rocks than in others. There are two basic porous rocks than in others. There are two basic factors that affect the oil displacing action of the soluble oil process:composition of interstitial fluids, andcomposition of reservoir rock. The literature has much data showing that composition of interstitial water, particularly the presence of divalent ions, greatly affects the efficiency of a soluble oil flood. This study is not concerned with that aspect of the problem. problem. This study is concerned with the effect of rock minerals on soluble oil flooding. We evaluated the possibility that certain rock minerals can drastically possibility that certain rock minerals can drastically affect the efficiency of the soluble oil flood. One previous study has shown some of the possibilities in previous study has shown some of the possibilities in this regard. Experimental Procedure The flooding experiments were conducted in unconsolidated sandpacks (No. 16 sand) containing from 0 to 2 percent of either gypsum, montmorillonite clay, or calcium carbonate. The sand was packed in 3-ft long, 2-in. diameter plastic tubes to a porosity of about 40 percent and a permeability of about 4 darcies. In preparation for the flooding experiments, the systems were first saturated with brine or fresh water, permeability was determined, and then crude oil was permeability was determined, and then crude oil was injected to irreducible water. Next, the system was waterflooded to residual oil saturation, and, finally, a soluble oil flood was made. Oil saturations after waterflooding ranged from 28 to 32 percent PV. In each soluble oil flood, 0.03 PV of soluble oil was injected and followed by 1 PV of polyacrylamide solution that had a viscosity of about 32 cp. Effect of Calcium Sulfate On the Soluble Oil Process Gypsum (CASO4 2H2O) is a slightly water-soluble mineral present in some oil reservoirs. Its maximum solubility is about 2,000 ppm in fresh water, and about 7,000 ppm in NaCl brine. We ran several flood tests to determine whether this slightly soluble mineral would affect the soluble oil flooding process. The data in Fig. 1 show that 0.5 percent or more gypsum in the sand reduces oil recovery by the soluble oil-polymer flood. With no CASO4 present, oil recovery was 96 percent, while with 2 percent CASO4 in the sand, oil recovery was 55 percent. The most probable mechanism by which gypsum reduces the oil-displacing effectiveness of the soluble oil-polymer process is related to the dissolution of solid gypsum by flooding solutions and the consequent reaction of dissolved Ca++ with flooding chemicals. Dissolved Ca++ can affect the soluble oil flooding process in two ways:it can precipitate some process in two ways:it can precipitate some of the petroleum sulfonates, andit can reduce the viscosity of polyacrylamide solution. JPT P. 179