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.

Results of Laboratory Studies of Acid Treatment of Low-Permeability Rock Cores

“…Acid treatment is carried out by injecting acid solutions into the formation to dissolve rock minerals in order to expand existing and create new fractures, increase the overall permeability, and therefore the productivity of the well. Several types and combinations of acids can be used for acidizing, but the most commonly used acids are hydrochloric acid (HCl) and hydrofluoric acid (HF) [9][10][11][12]. HCl reacts with carbonate rocks such as dolomite, calcite and siderite.…”

A Calibrated Computational Fluid Dynamics Model for Simulating the Rotating Disk Apparatus

“…The theory of an RD was first applied to study the electrochemical processes using an RDE (Daguenet 1968;Ellison et al 1971;Chin and Litt 1972;Nanis and Klein 1972;Vahdat and Newman 1973;Bruckenstein and Miller 1977;Mohr and Newman 1976;Albery and Bruckenstein 1983;Dong et al 2008), and was later adopted to analyze the chemical dissolution reactions using RDA (Boomer et al 1972;Lund et al 1973, Lund et al 1975Fredd and Fogler 1998;Taylor et al 2004;Alkhaldi et al 2010;Rabie et al 2014;Khalid et al 2015;Aldakkan et al 2018;Hall-Thompson et al 2020;Sayed et al 2020;Kotb et al 2021).…”

Development of Acid Nanocapsules with Tailored Breaking Reservoir Temperature for the Removal of Formation Damage by Fines Migration

“…First, a poor design of acid stimulation can lead to fast and uncontrolled acid/mineral dissolution, which could generate a low penetration radius and the loss of compression forces near the well. 4 Second, well integrity issues can be related to corrosion through tubing and injection equipment due to undesirable acid−metal interactions. 5 Different investigations have shown that the failure rate of acid jobs around the world is about 30%, mainly due to poor candidate selection, lack of mineralogical information, improper chemical selection (main acid, additives, strength, and volume), improper fluid placement, and shutting-in acid treatment for too long.…”