Abstract

Many wells are damaged in the course of drilling and workover by the use of drilling muds, drill-in fluids, and kill-pills that contain, amongst other things, polymeric constituents. The latter may consist of a single polymer or may consist of mixtures of polymers in aqueous solution/suspension which are added for the purposes of viscosification, leak-off control, lubrication, etc. Typically, the polymers used include xanthans, starches, celluloses, guars, and derivatives of these main groups. In addition, most of these fluids contain some form of particulate to impart density and to improve fluid-loss control, commonly calcium carbonate.

It has been established that well productivity can be significantly impaired by the use of these mixtures, due to the persistence of residues. Their removal can result in substantial improvements in production. There are those who believe that such damage can be removed simply by applying sufficient drawdown but, generally, chemical methods using acids and/or oxidisers like sodium hypochlorite are used. Neither of these approaches is effective alone at removing all the damage caused by cake components. This often necessitates multistage soak treatments, with large volumes of treating fluid, in extended reach wells. Such procedures complicate the operation and cause additional expense due to the time involved.

This paper describes a novel chemical system, capable of removing all common mud polymers while, simultaneously, dissolving the calcium carbonate and accomplishing this in a single one-step treatment. These systems are safe and effective and can remove essentially 100 percent of all residues, restoring permeability to its original level.

Introduction

Wells are drilled with a variety of mud formulations. These mud systems are usually selected with a view to improving the drilling process. However, success in improving the drilling process does not always translate into improvements in well performance. Over the years, concerns have been raised about the effects of many different mud formulations on productivity. Of particular interest are the so-called "drill-in" fluids, usually used for the construction of long lateral or horizontal drainholes. These drill-in fluids are mixtures of polymers and bridging solids, generally calcium carbonate, which build a very tight, low permeability cake on the borehole.

It has been established, from numerous studies worldwide, that well productivity can be significantly impaired by the use of such mixtures of polymers and particulate materials in the drilling and completion process, due to the persistence of residues. Field case histories, backed-up by laboratory work, has confirmed that their removal can result in substantial improvements in production. Contrary to a widely held belief, simple acidising is not an efficient method of removing these systems.

Historically, removal of these materials has involved the use of soaking with strong mineral acids (e.g. HCl), or alkaline oxidising agents like sodium or lithium hypochlorite. More recently, enzymes (so-called Mudzymes) have been used to remove the polymeric constituents, although it should be noted that not all polymers are vulnerable to attack by enzymes and not all real-world mud systems respond favourably, probably due to their high drill-solids content.

Strong mineral acids (e.g. Hydrochloric Acid), are capable of hydrolysing some polymers and can dissolve calcium carbonate. However, in practice, the latter is coated with polymer residue and the removal has been shown to be non-uniform, possibly due to wormholing of the acid through the cake. When the well is put on production, in such a scenario, this results in non-optimal inflow performance, with higher drawdowns and, potentially, greater risk of early water breakthrough, fines migration and formation failure. Also, the acid is corrosive and inefficient and large volumes must be used in extended reach wells.

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