Tight carbonate reservoirs have been successfully developed in various major plays around the world through the application of bespoke hydraulic fracturing and stimulation techniques performed in long horizontal sections drilled across these reservoirs. The main purpose of a typical stimulation program is to achieve the maximum and most efficient connectivity of the wellbore to the reservoir with the ultimate objective of long term productivity and recovery. Completion equipment design, efficient stimulation, and subsequent well performance have therefore gained attention starting from the early stages of design and planning.
Matrix acidizing of carbonate rock creates conductive flow channels also known as wormholes that possess much higher conductivity compared to the reservoir rock. These wormhole conductive channels transport reservoir fluids from within the formation matrix directly into the wellbore overcoming both low permeability and near wellbore damage. The treatment composition and more importantly the injection technique play significant roles in maximizing the number and depth-of-penetration of these wormholes and are among the most predominant design criteria for successful carbonate matrix acidizing, particularly in high pressure and high temperature environments.
One of the most successful technologies which has opened access to several major carbonate reservoirs and was recently tested is the limited entry matrix acidizing (LEMA) assembly. The assembly includes multiple jets in one selected interval flanked by isolation packers in an open-hole multi-stage completion designed to enhance controlled acid jetting (CAJ) into target sections. In principle, matrix stimulation is performed by placing acid treatments at the maximum possible rate, but at a pressure that is less than the formation fracture pressure. Diversion is achieved via limited entry placement of the acid treatment through multiple injection ports distributed along the stage length.
This paper presents an overview of the various methodologies used to stimulate moderate to high flow capacity carbonate reservoirs and the results obtained. An example of a purpose-built completion for more aggressive acid stimulation is presented and the results are discussed. The technique uses a system that is designed to run as part of the production liner to provide mechanical diversion at specified intervals, thereby allowing matrix acidizing treatments to be effectively placed in the target zones.