Acid Stimulation in EOR – A Novel Operating Philosophy: Case Study from Mukhaizna Oman
Authors: Al-Aulaqi Talal , Al Battashi Sultan , Al Bulushi Hussain , Al Hashmi Hashim , Al Amri Sultan , Al Habsi Ali , Al Hakmani Salim , Al Qabtan Al Husain , Al Rashdi Ahmed , Al Manhali Mohamed , Al Wardi Jamal , Al Mufarraji Bader , Al Mamari Omar 
Published: October 2022

Over the last 50 years, Acid Stimulation has been growing as an effective method for enhancing Permeability, improving inflow, and reducing wellbore skin. In mature EOR projects, maintaining high inflow to offtake producers are of the main challenges to safeguard reserves and meeting long-term production expectations. In this paper, a novel workflow is presented to discuss best practices into acid stimulation campaign during 2020-2022, resulting incremental oil gain which quick payback period during cyclic oil price environment. This is a thermal EOR operation in deep reservoirs (&gt; 2,000 ft) with extremely high viscosity (&gt;10,000 cp) with temperatures exceeding 500°F. The area is a mature thermal area with 15 years of continuous steam flood operations resulting in different type of scale deposit including Carbonate, Sulfate and Sulfide scales that impact the well deliverability over the last 15 years. The novel workflow, based on a combination of chemical analysis, root cause analysis for Artificial lift failures, zonal treatment systems, was developed in-house and deployed in &gt; 50 wells with a great success rate.

The pilot area consists of multiple reservoir zones that have undergone vertical steam injection since 2005 and horizontal producing at dedicated reservoir zone. The skin is induced either by Rock/fluid interaction causing scale drop out and resulting in Artificial lift failures, which holds a larger amount of the remaining oil. The Subsurface and Well Engineering teams collaborated to design a novel well deployment methods treating up to 3000 ft horizontal lateral and operation using coiled tubing units with high rate techniques.

The well and reservoir surveillance included gathering data for injectivity/productivity assessment, vertical injection logging, temperature profiles, production in offset producers, and well testing for determining water cut. The low inflow wells manage to increase their pump fillage to highest level in the last 5 years, post flow back shows a short spike of hardness as a result of successful stimulation. In addition, wells with high H2S remain a challenge for stimulation as a result for iron sulfide scale limited dissolving with available chemical in the industry. The final oil production tripled over a period of 3 months, which paid back the cost of the pilot.

To our knowledge, based on an SPE literature search, this is the first comprehensive Sandstone Acid Stimulation in thermal EOR operation conducted with the following combination of technologies: 1) skin characterization techniques either wellbore or deep reservoir, and 2) Using downhole sensors with rigless operation of coiled tubing units at harsh conditions. The outcomes open a new frontier for well enhancement in matured thermal EOR development in multi-stack reservoirs, offering better offtake management, safeguard reserve over the field life cycle. The cost of the stimulation per well project was paid off in the first 4 weeks, and chemicals used were developed in an eco-friendly system with much less CO2 emission compared to commodity chemical which allow better management for CO2 emission.