Among the various design and operating parameters, efficient acid stimulation in deep carbonate reservoirs depends on the placement technique, injection profile and treatment composition. Unlike acid fracturing, matrix acidizing creates several conductive flow channels with substantially higher conductivity compared to the reservoir rock. These conductive channels transport reservoir fluids from the formation matrix directly into the wellbore overcoming both low permeability and near wellbore damage. The treatment composition, and more importantly, the injection technique to maximize the number and depth of penetration of these conductive channels, are among the most predominant design criteria of successful carbonate matrix acidizing especially in a high-pressure and high temperature environment.
The Permian Khuff carbonate reservoir in the Ghawar structure of Saudi Arabia produces nonassociated gas and condensate. The reservoir is characterized by heterogeneous porosity and permeability distribution extending in both areal and vertical directions with varying in-situ stress contrast along the structure extension. Due to the reservoir complexity, each well requires individual assessment to determine the optimum completion design necessary to achieve efficient matrix or fracture acidizing treatment. Some wells may need simple matrix acid treatment, while other wells may need open hole multistage (OHMS) fracture stimulation. Results demonstrate that OHMS completion was required to successfully stimulate all net pay intervals.
This paper presents an overview of Saudi Aramco’s efforts to evaluate various stimulation methods used in the Khuff reservoir and highlights an optimal carbonate stimulation technique for certain reservoir conditions via the first trial test application of a limited entry OHMS completion for effective stimulation. The technique uses a system that is designed to run as part of the production liner and provides mechanical diversion at specified intervals, thereby allowing multiple matrix acidizing treatments to be effectively placed in the target zones. The technique was applied recently for the first time in Saudi Aramco’s gas program and the details are discussed in this paper.