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.

Acid stimulation and fracturing in multiple layers formation requires an integrated approach to design and execute an effective stimulation treatment. An extensive campaign of acid stimulation on carbonate formation to increase productivity index in a deep High Pressure High Temperature (HPHT) gas bearing carbonate formation is discussed here. A high degree variation in permeability across long interval, stress barriers between target zones, and completion limitations were main challenges to design an effective stimulation treatment. The target was to achieve effective acid stimulation for multi-layers carbonate formation with different formation properties and stress barriers. Although all layers had similar initial pressures and gas compositions; but different properties. Comingled flow was planned from the formation, therefore effective stimulation for each layer was recommended to drain effectively and improve recovery. To assess the best stimulation strategy in early stage of development, complete set of diagnostics conducted across all formations before and after the acid treatment coupled with geomechanical studies on actual core samples. Various stimulation techniques were implemented from Small Matrix Treatments (SMT) with Coiled Tubing, Matrix Acidizing Treatments (MAT), High Rate Matrix Acidizing (HRMA), and Acid Fracturing Treatment (AFT). Thorough study of results as part of learning process was conducted to optimize the important parameters that affect the stimulation efficiency such as perforation strategy, completion design, diverter selection, fracturing design and implementation. The importance of modeling in optimization process, comparison between different treatment types and fluid systems is discussed supported by actual data such as temperature logs, post-treatment flowback analysis, pressure transient analysis and production logging results.