Laboratory Evaluation of Single-Phase Retarded Acid System Performance in Limestone and Dolomite Rock - Part 1: Pore Volume to Breakthrough Experiments in Limestone

Abstract

The objective of this work is to conduct systematic core acidization testing on commercial limestone and dolomite core plugs to compare different commercial acid systems and identify the most cost-effective recipe(s), including optimum dosage and pumping rate. The work involves state-of-the-art equipment for the acidizing experiment at reservoir conditions and makes use of advanced high-resolution imaging to visualize the results of the pore-volume-to-breakthrough acidization experiments. Over the past few years, service companies have brought new acid stimulation products to market. These so-called retarded or delayed acid systems aim to slow down the kinetics of the acid-rock reaction to allow acid to penetrate deeper into the rock matrix and thereby increase reservoir contact. Prior to any deployment at scale, these new chemical products must be qualified by an independent lab facility rather than relying solely on performance data supplied by vendors. In addition to acid compatibility with the reservoir fluid, the ability to create deep penetration with the lowest possible acid volume needs to be quantified. A key experiment is called the pore-volume-to-breakthrough (PVBT) test conducted on cylinder-shaped reservoir rock samples called core plugs. Within a certain range of acid velocities, the chemical rock dissolution reaction yields an optimum penetration depth with a minimum acid volume. The lower the volume, the more cost-efficient the acid recipe. Nine single-phase retarded acid (SPRA) systems were evaluated, and all showed better performance than standard 15% HCl (in limestone) or 28% HCl (in dolomite) cores. Some SPRAs were marginally better than the standard HCl systems while others showed significantly improved performance. This paper will report a ranking of each of the SPRA systems evaluated from best-to-worst. Of particular note is that SPRA systems containing lower retarder dosages showed similar performance to SPRA systems that contain significantly higher retarder dosages. The lowest PVBT, which would represent a single, thin, straight cylindrical wormhole in a linear coreflood may not be optimum in a 3D radial flow pattern. We present a discussion on the need for some degree of branching of the dominant wormhole to enhance the flow into an actual wellbore, depending on the completion type. This work represents the first systematic comparison of multiple single-phase retarded acid systems at the same test conditions by an independent laboratory.