Abstract

One of the main problems in chemical treatments concerns the proper placement of the treatment fluids. Specifically, a very proper placement of the treatment fluids. Specifically, a very distorted inflow pattern may result after stimulation by matrix acidising treatments carried out without a means to distribute the injected fluids over the entire zone to be produced. A theoretical and experimental study is reported aimed at developing a better understanding of the diversion process. This has resulted in practical guidelines for the application of chemical diverters in general and, in particular, for the use of benzoic acid and its derivatives. The results of the current study also indicated some areas of uncertainties requiring further investigation.

It is concluded that benzoic acid can successfully be applied in matrix acid treatments in water injectors and gas wells.

Introduction

Matrix acidising is a frequently employed technique to increase the productivity of oil and gas wells. It is an operationally simple and economical method to remove or mitigate the effect of many types of near wellbore permeability damage. However, obtaining the maximum well response from chemical treatments presents a number of problems. Proper placement of the treatment fluids is one of the-these problems. Injection of the acid into the upper zone or those of highest permeability (which frequently need stimulation the least) will occur if a means to distribute fluids over the entire zone is not available. A disappointing productivity increase may be the result, and, probably more important, a very distorted post-treatment production probably more important, a very distorted post-treatment production profile will result. profile will result. A number of methods to divert the flow of fluids in a well are available. They range from plugging granular material (" chemical diverters"), through ballsealers (designed to completely block perforations) to the use of mechanical means, such as coiled tubing or retrievable packer arrangements, such as the newly developed through tubing Selective Placement Tool (SPT). Both operationally and economically, chemical diverters are very attractive but, historically, they have proved to be the least reliable system. Their poor record is however, to some extent, due to the lack of a proper design methodology. In our experience, their application is usually based on local experience, heavily influenced by subjective judgement, with little basis for an engineered design.

It needs to be noted here that the efficiency of mechanical forms of diversion depends on good zonal isolation having been achieved through effective cementation of the production casing. Chemical diverters, which are placed in the production casing. Chemical diverters, which are placed in the sandface and/or in the perforations and (cement free) channels, can still be effective even when zonal isolation across the production zone has not been achieved.

This study has concentrated on the influence of particle size on the engineered design of diverter treatments. Chemical diverting agents fall into two classes, determined by the permeability of the diverter cake, which in turn depends on the permeability of the diverter cake, which in turn depends on the (downhole) particle size. These materials either

  • form a thin, but very low permeability film on the sandface or

  • fill up the perforations with a (relatively) high permeable diverter cake.

Use of the first type of diversion results in high injectivity zones being effectively plugged off. The application and design of this type of diversion has been extensively studied in ref. 2. However, it carries the risk of almost total loss of injectivity if too much diverter is used.

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