Critical Review of Solvent Usage and Placement in Matrix Acidizing

Abstract Since the first acid stimulation treatment nearly 100 years ago, techniques to improve the effectiveness of stimulating well productivity/injectivity have evolved to address challenging downhole conditions. While the emphasis in stimulation improvements has focused on optimization of the acid and diversion methods, proper design and implementation of solvent is a critical element of stimulation effectiveness. Whether used as a solvent preflush, a mutual solvent included into the acids, or a diverted solvent stage, solvents are critical for removing organic damage and residue to enable productivity enhancement alone or through the acid. Historical stimulation treatments are compared from multiple global locations to assess the impact of solvent usage in matrix stimulation. Laboratory results are first presented on historical selection and effectiveness of solvents for acid treatment, including as a preflush. Additionally, field results are shown from global stimulation treatments that used solvent span a variety of ways to select, apply, and divert solvents. The current work first reviews the empirical effectiveness of solvent flush and solvent/acid blends numerous ways. Solubility tests are routinely critical for selecting the preferred solvent, versus previous industry standard practices. However, the benefit of mutual solvents added to acids is often difficult to isolate in testing such as core testing and is optimized using best practices from experience. Case histories are presented which showcase the tangible impact of solvents during execution of an acid treatment. In this work, cases show solvent preflush that may be diverted periodically in the proposed acid pump schedule, and results show differences in effectiveness. Empirical results are presented to explain interactions between solvent and some chemical diverters, leading to a holistic assessment of different strategies for solvent/diverter interaction. The application of solvent is often treated as secondary to the matrix acid design, as solvent is considered only an enabler to acid effectiveness. But experience confirms that solvent is crucial both to remove organic damage and to render the formation/damage (fines, inorganic scales) water-wet for improved acid reaction. This review of best practices in solvent application challenges readers to carefully consider solvent selection in matrix acidizing, apply optimized QA methods, and implement best practices in execution to achieve successful productivity enhancement through the actions of both solvent and acid.