The Effect of Mutual Solvents on Adsorption In Sandstone Acidizing
Authors: Hall B.E. 
Published: December 2007

The role of mutual solvents in sandstone acidizing is investigated. An experimental technique for evaluating various solvents was used to examine various classes of solvents to relate chemical structure to mutual solvent properties. Glycol ethers were the most effective class of chemicals examined.

Introduction
Acid solutions containing hydrofluoric acid have been used extensively to stimulate sandstone reservoirs. Normally, these solutions are restricted to treating shallow clay damage due to the extremely fast spending rate of HF on clay.
In acidizing sandstone formations, the acid is pumped below fracturing pressure, and the large surface area of the formation matrix is contacted and reacted upon by the acid.
Stimulation fluids usually contain various surfactant additives in an effort to help control emulsions, asphaltenic sludge deposits, surface tension, and corrosion of steel. When acid solutions containing surfactants contact the rock surface, significant amounts of additives can be adsorbed onto the rock. These adsorbed additives are then no longer present in the fluids to perform their designed function. Consequently, conditions the surfactants were supposed to help control may occur during or after the treatment, resulting in less-than-satisfactory stimulation results.
In 1971, Gidley reported on the use of the mutual solvent ethylene glycol monobutyl ether (EGMBE) in conjunction with sandstone acidizing treatments. Significant improvements were observed when the mutual solvent was present in either the acid solution or the oil after-flush. Gidley also pointed out the importance of proper treating technique. The mutual solvent was proper treating technique. The mutual solvent was shown to decrease the stability of acid/oil emulsions in the presence of formation fines.
The compatibility of some acid additives with formation fluids and mineral surfaces was investigated by Sutton and Lasater. Their findings supported those of Gidley that EGMBE does improve the separation of acid solution/crude oil emulsions. Sutton and Lasater attributed this to decreased adsorption of surfactant on silica surfaces in the presence of EGMBE. They also found that acid corrosion inhibitors exhibited reduced adsorption on silica when EGMBE was present in the acid.
The present investigation included two phases of work:a study of various classes of solvents to relate chemical structure to mutual solvent properties, anda further study of the influence of mutual solvents on surfactant adsorption.

Experimental Procedure
Adsorption effects were observed by measuring solution properties before and after flowing acid solutions properties before and after flowing acid solutions through columns packed with either silica sand or a mixture of silica sand and clay. The procedure for preparing the columns and performing the flow tests is preparing the columns and performing the flow tests is outlined in the Appendix. Solution properties measured were surface tension, emulsification, and corrosiveness.
Surface tensions, reported in dynes per centimeter, were measured using the du Nouy ring method.
Emulsion tests were conducted by mixing equal volumes of acid solutions and a crude oil from Southern Louisiana at 15,000 rpm for 1 minute with an Eberbach stirrer equipped with an homogenizer head. Separation of acid and oil was noted after 10 minutes and was reported as percent acid separated.
JPT
P. 1439