Abstract

Core flow tests evaluating HF acid systems designed to remove formation damage in sandstone formations, are traditionally conducted with undamaged cores often not taken from the actual formation of interest. However, HF acid treatments considered for field use should be evaluated in actual formation cores damaged prior to acidization, because sandstone acidizing success is dependent on both the formation sensitivities to HF acid and the effective removal of near-wellbore formation and perforation damage. Such damage may include drilling mud filtrate effects, plugging by fluid loss material, permeability reduction due to compaction and formation debris generated by perforating, and formation flow restriction caused by migratory or injected fines.

This paper presents a systematic approach to core flow testing, in which procedures employed are designed to first damage cores in a manner representing expected damage mechanisms, then remove damage with appropriately selected HF treatment systems and nonacid pre-acidizing treatment systems, if necessary. Degree of damage removal and stimulation is measured following each treatment step. Acid systems are selected on the basis of formation characteristics, including mineralogical make-up. The most cost-effective damage removal and acid treatment solution is then selected, based on comparative test results.

Introduction

Core flow testing has long been an accepted method utilized to screen products, determine fluid sensitivities, and acidization effects. Frequently, these tests use "standard" core material, such as Berea core, rather than formation core material. In addition, test procedures often use cores which are in an undamaged condition1,2. Studies with undamaged cores are not without purpose - they can be very meaningful in assessing acid sensitivities of the rock itself, for example, including acid/formation reaction re-precipitation products and acid system compatibilities with a particular formation. However, with respect to evaluating comparative effectiveness of specific HF acid mixtures and systems in removing damage and limiting formation acid sensitivities, testing in undamaged cores only is not sufficient. Certain studies have reported effects of preacidization damage, but these studies are generally restricted to Berea core, with the damage resulting from "perforating" the core material3.

The main purpose of sandstone matrix acidizing is removal of siliceous particles such as formation fines, including clay, feldspar and quartz fines that are restricting near wellbore permeability. Such plugging particles may be naturally occurring or may have been introduced into the formation during well operations. The resulting formation damage can occur during drilling, perforation, completion, workover and production operations. Removal of damage is accomplished with injection of acid formulations containing HF (hydrofluoric) acid, as HF is a common acid that dissolves siliceous particles sufficiently. HF acid reaction with siliceous material is a function of mineral surface area. Therefore HF acid reacts more rapidly with high surface area minerals such as clays and zeolites, and more slowly with lower surface area minerals such as quartz. Stimulation of sandstone formations with HF acid is challenging because it involves complex chemical and physical interactions of HF with the formation. Some of these interactions may result in formation damage, if not controlled or moderated.

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