Abstract

In onshore Middle East, local practices for matrix stimulation of openhole horizontal carbonate water injectors consist of spotting hydrochloric acid treatment via coiled tubing (CT) along the uncased section using a specific fluid dosage per unit length of the pay zone. Thus far, that approach has delivered inconsistent results in wells completed across tight carbonate rock, most often leading to a rapid decline in injection rates following the treatment.

An alternative workflow was implemented to take full advantage of real-time downhole measurements and the presence of fiber optics in the CT for telemetry. The approach leverages distributed temperature sensing (DTS) to evaluate the original water injection coverage across the reservoir. Results enable segmenting the open hole into intervals requiring different levels of stimulation. Each section benefits from a customized treatment that increases injectivity and improves uniformity of injection. A high-pressure jetting tool, controlled with the help of real-time downhole pressure data, is key to that workflow because it enhances penetration of acid into the targeted intervals.

Previous studies showed that energized acid is key to a successful stimulation of tight carbonates. However, the use of CT to convey and pump acid along the open hole often limits the rate at which fluid is pumped, and customized nozzles may fall short of expectations if the downhole conditions are not favorable to their proper actuation. The introduction of real-time downhole readings and DTS surveying into the stimulation workflow helped overcome those limitations and get the most out of equipment and fluids. DTS offers a visualization of high- and low-intake zones along the open hole throughout the operation, thus enabling informed decisions on design adjustments for each stimulation stage. Downhole pressure measurement is instrumental in determining whether downhole conditions are favorable for the use of the high-pressure jetting nozzle, which has a direct impact on the exact pumping sequence, with the potential addition of stimulation stages to bring the openhole in optimum conditions. Downhole pressure reading also allows optimal operation of the jetting nozzles within the designed range. The engineered workflow has been successfully implemented delivering injectivity improvements of nearly 8,000 B/D in the intervened wells, with the DTS survey confirming significant gains on injection coverage along the openhole section.

This advanced matrix stimulation workflow, brings reliability and flexibility to the acidizing of tight carbonate water injectors. Use of the full array of downhole parameters not only yields unprecedented injection coverage in complex reservoirs, it also eliminates uncertainties associated with wellbore conditions and helps in keeping injection under the fracturing gradient, thus eliminating the risk of differential sticking events.

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