Abstract
Acid stimulation in high-temperature sandstone reservoirs with high clay content can lead to undesired results due to secondary and tertiary reactions between treatment fluids and reservoir clays. Although there have been significant advancements in treating clastic formations over the years, high bottomhole temperature (BHT) coupled with high clay content of up to 35% and subhydrostatic conditions still presents a major challenge. A stimulation workflow to address these challenges was adapted to treat and successfully enhance well production in sandstone reservoirs in southern Pakistan.
Candidate wells were selected for acidizing treatments based on declining production trend and identification of significant damage skin. X-ray diffraction tests on core samples indicated presence of acid-sensitive clays and calcite. Due to the risk of precipitation from secondary and tertiary reactions, conventional hydrochloric and hydrofluoric acid treatments were not viable options. Core flow testing was conducted to assess the efficiency of alternative acid systems at the reservoir conditions with BHT above 320°F, validating the selection of a high-performance sandstone acid system that was designed to handle undissolved clays in the critical matrix by helping to bind the clays to the pore surfaces, thus preventing them from migrating and plugging the pore throat during flowback.
The matrix stimulation campaign included vertical and deviated dry gas wells, completed with 3 1/2-in. to 4 1/2-in. production tubing and 7-in. liner, with perforated intervals averaging 20 ft. Prior to the main acid treatment, high-pressure rotary jetting across the target intervals was conducted by pumping organic acid via coiled tubing. This wellbore conditioning technique allowed maximizing the acid performance by delivering 360° high-energy fluid to clear the perforations of scale and improve injectivity. The main treatment consisted of an organic acid preflush and a high-performance sandstone acid system as the main fluid, followed by a brine post-flush. Throughout the treatment, nitrogen was added to all fluids to facilitate fluid flowback under subhydrostatic conditions. The wells treated using this matrix stimulation engineered workflow yielded sustained production gains from 3 MMscf/D to 3.5 MMscf/D, exceeding expectations by more than 50% and achieving payback periods less than 20 days. The success of the treatment was largely due to the carefully designed stimulation workflow and its flawless execution.
Acidizing high-temperature sandstone reservoirs with 30 to 35% clay content is uncommon. The experience gained in southern Pakistan validates the high-performance sandstone acid system as a reliable option for matrix acidizing in hot, acid-sensitive sandstone reservoirs. It also provides a detailed engineering workflow for candidate selection, treatment design, and job execution and evaluation, which can easily be adapted to regions facing similar challenges.