Chemical Stimulation Improves Oil Recovery From Carbonate Reservoirs
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 89424, "Improved Oil Recovery From Carbonate Reservoirs by Chemical Stimulation," by X. Xie, SPE, and W.W. Weiss, SPE, Correlations Co., and Z. Tong, SPE, and N.R. Morrow, SPE, U. of Wyoming, prepared for the 2004 SPE/DOE Symposium on Improved Oil Recovery, Tulsa, 17-21 April.
Often, a pressure-pulsing technique is used in naturally fractured shallow-shelf carbonate reservoirs to improve oil recovery. However, in some situations, imbibition of water can be promoted by chemical stimulation to alter the reservoir wettability toward water-wetness such that oil is expelled at an economic rate from the rock matrix into fractures. In this work, promotion of imbibition was deter-mined for a cationic surfactant and a nonionic surfactant.

Introduction
Approximately 22% of the original oil in place (OOIP) in the U.S. was in shallow-shelf carbonate reservoirs. Most of these reservoirs are heterogeneous and naturally fractured, and waterflooding leads to bypassed rock matrix and early water breakthrough. Spontaneous imbibition can aid oil recovery from fractured reservoirs. However, spontaneous imbibition occurs only when the pore surfaces are effectively water-wet, such that water imbibes into the rock matrix, expelling oil into the fractures and flushing it toward the production wellbore.
The significance of spontaneous imbibition as a recovery mechanism was recognized for the naturally fractured water-wet Spraberry field in west Texas in the 1950s, and this method is still being pursued. Oil recovery by spontaneous imbibition into fractured chalk is a key production mechanism for North Sea chalk reservoirs. Also, it has been reported that pressure pulsing, intended to stimulate imbibition, increased the oil-producing rate and ultimate recovery from individual wells in Wyoming.
Laboratory studies provided direct demonstration of spontaneous imbibition as a recovery mechanism. If rocks are oil-wet or neutral-wet, imbibition does not occur. Use of surfactants to alter the rock’s surface toward water-wetness to promote imbibition of water into the matrix of fractured reservoirs has been recognized as an important mechanism for enhanced recovery.
Applying petroleum sulfonates for tertiary recovery from sandstone reservoirs has been under investigation for more than 50 years. Use of these comparatively low-cost surfactants in carbonate reservoirs is problematic because of adsorption resulting from the positive charge of the rock surface. The use of anionic surfactant in combination with sodium carbonate has been proposed. At high pH, the carbonate ion is the potential-determining ion and reverses the charge of the carbonate surface to mitigate the adsorption problem.
The full-length paper details laboratory imbibition tests on combinations of crude oil, water, and rock for three carbonate (dolomite) reservoirs. All three reservoirs have a long history of very low oil recovery. Performance often is ascribed to the combined effect of fractures and the unfavorable wetting state of the rock matrix with respect to spontaneous imbibition. The objective of this work was to assess the potential for single-well surfactant stimulation by injecting low-concentration surfactant solutions. The main focus was to study the effectiveness of nonionic and cationic surfactants.