ExxonMobil has undertaken a multi-disciplinary approach to develop and integrate the required technologies for design, implementation, and evaluation of acid treatments in thick heterogeneous carbonate reservoirs.

Acid placement and diversion are critical to achieving effective stimulation in heterogeneous carbonate reservoirs. While permeability is a major factor in the distribution of acid along a completion for thin reservoirs, other parameters such as pre-stimulation skin, variability in acid-rock wormholing characteristics, distance between zones, differential reservoir depletion, and wellbore effects can dominate acid placement in thick reservoirs.

For an integrated approach to optimizing matrix acidizing, the following elements should be considered:

  1. determine the stimulation requirements given the well/reservoir objectives

  2. characterize the various rock types present in the formation

  3. conduct appropriate laboratory tests

  4. develop an integrated perforation/stimulation strategy

  5. model the stimulation process

  6. develop field procedures and implement the treatments as per design

  7. evaluate stimulation effectiveness

  8. optimize treatments based on post-stimulation performance and operational constraints

A proprietary stimulation design model based on the physics of acid-carbonate interactions, laboratory results, and field data has been developed to assist in the design and optimization of carbonate acid treatments for a wide range of reservoir and well characteristics. To further optimize the formation stimulation, ExxonMobil recently developed and patented a new stimulation technique, called "Annular Coiled Tubing Stimulation", which enables high-rate stimulation of multiple target zones in a single deployment of downhole equipment, allows for tailored acid placement, and provides positive isolation between the zones to ensure proper stimulation of each zone.

This paper features some of the technologies that have been developed and describes the methodologies used to effectively stimulate thick carbonate reservoirs.

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