Evaluation of The Effectiveness of Mud Acid in Well Stimulation

Abstract

Production enhancement from oil and gas fields is of key importance to operators. In order to achieve enhanced production, well stimulation techniques are often deployed to maximize recovery from oil and gas wells. Matrix acidizing is the most prominent technique deployed among other well stimulation techniques; considering its relatively lower cost, compared to hydraulic fracturing. Also of importance is the suitability of matrix acidizing to generate extra production and restore original productivity in wells that are damaged. Matrix acidization involves injection of an acid solution into the reservoir formation, at a pressure below the fracture pressure to dissolve some of the minerals within the rock with the key objective of removing damage near the wellbore, subsequently restoring the natural permeability and improving the well productivity. The standard acid treatments used in this work were HCl-HF (mud acid) formulations to dissolve the plugging minerals, mainly silicates (clays and feldspars). Experiments were carried out on sandstone samples that had been immersed in drilling fluid to allow cake formation and mud cake samples to evaluate the effectiveness of the formulated acids in well stimulation. Three standard mud acid concentrations were prepared, (13% HCl-3% HF, 17% HCl-5% HF and 24% HCl-6% HF).Results derived from the experiments indicated that the highest concentration of acid used (24% HCl-6% HF), gave a good result (8.05% of the original mass was dissolved). For the experiments involving the mud cake, the highest concentration of acid used (24% HCl-6% HF) gave a good result (94.86% of the original mass was dissolved). This showed that the higher concentration mud acid was a good candidate for skin removal in sandstone reservoirs, and wellbores that have been damaged by drilling fluid invasion.