A Differentiated Acidizing and Plugging Removal System for Stability Control of Unconsolidated Sandstone Reservoirs and Its Application

An oilfield in the Middle East has a major sandstone reservoir that is composed of mainly sandstone and argillaceous siltstone, the cement of which is mainly feldspar. The reservoir rocks are unconsolidated and the pores therein are easy to be blocked by sand production. Conventional acid job takes effect slowly and may result in dispersion of the loose sandstone, thereby exacerbating sand production. In this paper the mechanism of pore blocking by sand production is analyzed, and a technology of blocking removal through compound acidizing for unconsolidated sandstone reservoirs is presented. Using a clue of “first stabilizing the borehole wall, and then removing the blocking”, this technology is able to mitigate formation damage by pore blocking and to maintain the stability of the reservoir formation. Through laboratory experiment a sandstone stabilizer was developed for high flushing rates. At a flow rate of 1,800 mL/h, the rate of sand production can be steadily controlled below 0.01%. Through the synergism among different working fluids, taking “removing more scale and dissolving less sandstone” as a goal, an acid fluid for blocking removal through differential erosion was developed. Using this acid fluid, the rate of scale dissolution is ≥95% and the rate of sand dissolution is <25%. The rate of sandstone dissolution can be brought under control with this acid, thereby maintaining the stability of the rock structure. Field test results show that this technology can improve the average production rate and effective production life of oil wells, and it provides a theoretic basis and practical guidance for stimulating unconsolidated sandstone reservoirs through blocking removal with acidizing treatment.