Acid Stimulation-Best Practices for Design, Selection and Testing of Acid Recipes in Low Permeability Carbonate Reservoirs

Abstract

Acid stimulation in low permeability carbonates is challenging and the performance depends on a good choice of acid recipe and comprehensive testing that supports the extent of wormholes or fracture patterns. In this paper, a comprehensive testing proposal that involves initial enhancements to reaction kinetic for organic, inorganic and composite recipes and later supported by core tests to quantify optimized pore-volume to breakthrough (PVBT) times for optimum worm-hole propagation is discussed. A protocol is demonstrated that outlines important tests, analysis and recommended practices to select efficient acid recipes for enhanced stimulation performance in low permeability carbonate reservoir. This protocol demonstrates that the design of efficient acid recipes should take into consideration: rock characteristics, lithological distribution, mineralogy, petrophysical qualities and petrographic information. A summary of tests, procedures and success criteria demonstrates the benefits of an integrated testing approach prior to deploying acid stimulation. Outlined in this work are important tests, analysis and recommended practices to select efficient acid recipes for enhanced stimulation performance specifically high rate matrix acidization (HRMA) in low permeability carbonate reservoir. These recommendations demonstrate the benefits of an integrated procedure for achieving superior well productivity, costs savings, operational flexibility and long term applicability that addresses subsurface and operational requirements for acid stimulation. This system is considered in offshore environments where logistics and cost are important in designing a stimulation strategy and is also applicable to onshore environments. Some key findings are that under simulated reservoir conditions and similar rock properties chemical or/and physical retarded acid provided deeper wormhole results from PVBT point of view than straight acid. Viscoelastic aicd (VES) diverter acid on the other hand showed deeper penetration by lower PVBT which will demonstrate efficiency based on the viscosity. The increase in viscosity could increase bottomhole (BH) pressure limiting matrix regime applications. Three Single Phase Retarded Acids (SPRA) were tested and all demonstrated excellent wormhole penetrations and improved benefits in terms of offshore logistics due to their reduced volumes, reduced viscosity and environmental benefits.