Effective Stimulation of Very Thick, Layered Carbonate Reservoirs Without the Use of Mechanical Isolation
Authors: Shuchart Chris E. , Jackson Shalawn , Mendez-Santiago Janette , Choi Nancy Hyangsil , Montgomery John K. , Khemakhem A.S. David , Sieben Christopher John , Clancey Byron Michael , Chintaluri S Ram , Farah Ali M. , Wang Zhihua 
Published: May 2016

Effective matrix acid stimulation is one of the keys to maximizing and maintaining long-term North Field well productivity. ExxonMobil and RasGas Company Limited (RasGas) had jointly developed an integrated methodology to optimize matrix stimulation for layered Khuff reservoirs, specifically for K1-K3 and K4 completions. The integrated methodology is a continuous process which consists of five main elements to help overcome the well and reservoir challenges, including reservoir objectives, completion strategy, stimulation design, implementation, and evaluation.1

Success of K1-K3 and K4 completions led to high expectations for K1-K4 completions required for the recent development expansion. However, the much longer K1-K4 producing interval substantially increased the challenges such that existing stimulation tools and methods were no longer sufficient to achieve the aggressive stimulation targets desired for these wells.

Initially, retrievable mechanical isolation plugs were developed and qualified for use to achieve effective stimulation using already proven methods for K4 completions and K1-K3 completions. Due to the increased operational risk associated with mechanical isolation techniques, development of alternative methods and extension of existing methods were necessary. Multiple parallel paths were taken to investigate all aspects of well stimulation, including perforating techniques, diversion, number of stimulation treatments, stimulation vessel capabilities, and well / reservoir productivity. Field trials were conducted for selected technologies, and additional data were collected prior to, during, and after the stimulation treatments. Additionally, a process and associated tools to quantitatively evaluate completion and stimulation options in terms of both initial and long-term production performance were developed. Consequently, stimulation decisions could be made based on reservoir performance metrics balanced with the risks and costs associated with each option.

To evaluate well performance and optimize the stimulation strategy for future wells, an advanced post-stimulation analysis methodology incorporating stimulation predictions, sequential flow data, flowback samples, and production logs has been developed. Results of the analyses suggest stimulation performance comparable to stimulation with plugs, at a greatly reduced completion cost and substantial risk reduction and time savings. Additionally, stimulation strategy optimizations were possible such that the number of stimulation treatments could be reduced for most wells without compromising stimulation effectiveness or predicted long-term performance.

This paper discusses the development and implementation of alternative strategies and designs to effectively stimulate K1-K4 completions without the risk associated with mechanical plugs. Two case histories will be presented to illustrate application of the enhanced methodology.