Abstract

Acid corrosion inhibitors provide protection by adsorbing on metal surfaces, thus forming a barrieragainst acid attack. Upon entering the formation, the inhibitor is strongly adsorbed on clays and other mineral s present within the matrix of the rock. In some cases, inhibitors contain acid-insoluble residues which can cause surface plugging of the formation face. Inhibitor adsorption and formation plugging by inhibitor residues can have a major influence oncleanup rate and the ultimate effective permeability of the treated zone. Consequently, acid corrosion inhibitors should be evaluated both for their ability to protect metal and for their potentially damaging effect on stimulation results.

Laboratory tests, using a computerized core test perrneameter. demonstrate the strong influence of act d inhibitors upon acid injectivity, cleanup rate and stabilized long-term effective permeability. The adsorption characteristics of the inhibitor were shownto strongly influence test results. The beneficial effect of mutual solvent upon injection rate and, insome cases, well cleanup was also demonstrated.

Data are presented showing the influence of temperature, differential pressure and mutual solvent upon injectivity. cleanup rate and apparent stabilized oilpermeability. Various types of inhibitors were evaluated under a variety of test conditions. In general, it was found that damage resulting from the corrosion inhibitors was more pronounced at the lower temperatures and that mutual solvents reduced the magnitude of the problem.

Introduction

Corrosion inhibitors are routinely used in acidizing to prevent acid attack on tubing and other downhole metal equipment. These inhibitors tenaciously adsorb on metallic surfaces, thus forming a protective film between the add and metal. A.cid inhibitors are normally evaluated l only for their ability to control acid corrosion under simulated treating conditions. Formation damage resulting from insoluble materials in the inhibitor and inhibitor adsorption on clays and other minerals present in the matrix of the rock is seldom considered. This is unfortunate since most of the inhibitor used is eventually adsorbed within the formation rather than on the metal surfaces which it is designed to protect. Adsorbed inhibitor can produce altered wettabi1ity or result in severe emulsion problems. This can result in slow or incomplete cleanupfollowing the treatment.

Acid corrosion inhibitors are usually rather complex in nature and commonly consist of such materials as amines, quaternary amines, various nitrogen heterocyclics. thioureas, Mannich reaction products, sulfoniumsalts and acetylenic alcohols. They usually contain surfactants to disperse the inhibitor in the acidand solvents necessary to formulate the product. The various nitrogen compounds, being cationic in nature, strongly adsorb on clay surfaces. The surfactants also adsorb on the clays. These adsorbed materials can have a major influence on both cleanup rate and, in some cases. the long-term capacity of the well.

LABORATORY TEST METHOD

In order to study the effect of immihibitor adsorption on acid injectivity and cleanup, laboratory tests, designed to stimulate well treatments, were performed.

Equipment used in these tests is described in Figure 1. Basically, the test sequence consisted of first determining the apparent permeability to oil of a previously brine-saturated Berea Sandstone core.

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