Implementing a New Foam-Acid Technology for Matrix Stimulation of Challenging Low Pressure, Naturally Fractured Carbonates Reservoirs: Case Studies, Northern Iraq

Abstract

Historically, low-pressure, highly-fractured limestone formations have shown challenges in achieving proper acid diversion. In addition to the well deviation, this challenge becomes critical for wells close to the Water-Oil-Contact (WOC), with a latent likelihood to stimulate water zones. Consequently, a pinpoint placement technique thru Coiled Tubing (CT) plus a novel stimulation fluid is required to reduce/overcome this challenge. Common foamed acid has a short foam-stability time e.g., 10 minutes under native conditions. Hence, plain nitrified acid is pumped into the formation resulting in uneven treatment. Thus, custom foaming acids with stable foam qualities increments (e.g., 60% to 80%) are required to eliminate the risk of acid segregation to water zones. Then, wellbore fluids are over displaced by nitrogen, followed by the novel foamed acid with an initial bottom-hole foam quality of 60%, allowing the subsequent foamed stages (e.g., 65% to 80%) to divert upwards from the interval. Additionally, the CT mechanical diversion enables squeezing the treatment into the planned intervals. Carbonate reservoir stimulations in the north region of Iraq are performed using conventional hydrochloric acid (HCl) treatments. The foam acid diverting technology was implemented in challenging wells with a high risk of early water breakthrough based on water cut development in offset wells. Foamed matrix stimulation treatments were carried out through CT using a highly stable acid foam with self-diversion capability in ESP producer wells, demonstrating outstanding acid distribution over the interval of interest and sidestepping acid segregation to the water conductive zones. Despite the proximity of water zones, the use of foamed acids enhanced oil production and showcased a production gain of up to 3000 BOPD without water increase. The technology also allowed decreasing the volume of injected acid per meter of net interval by 41.7%, without jeopardizing the treatment efficacy which made it a cost-effective project. Based on the results, customized foamed acid treatments were incorporated in most of the stimulation programs. This paper discusses a novel foamed-acid system and the pinpoint placement technique used to stimulate challenging carbonate formations to get even fluid distribution, reducing the fluid segregation, thus minimizing the acid contact with the offending water zones.