Carbonate acidizing continues to be a vital process for improving the production of oil and gas wells. Laboratory studies and field evaluations of carbonate acidizing during the past 30 years have shown a continually improved understanding of the fundamental issues. This paper discusses the current state of the advances in carbonate stimulation. Average reactivity data for several limestones and dolomites are presented and can be used as improved default values for simulators. Wormhole development and structure during matrix acidizing are viewed as symmetry dominated processes controlled by fluid flow that obeys the native permeability contrasts within the matrix. The resulting simplification allows for rational treatment designs for matrix acidizing of carbonates. Zonal coverage of long carbonate sections, whether vertical or horizontal, remains a challenge. However, using the "75–25" rule for horizontal wells, creating a "thief zone" at the bottom or toe of the well, and utilizing the "top decade of permeability" rule can aid in achieving reasonable designs for maximizing productivity. The three fundamental issues of fracture acidizing are addressed: reactivity control, fluid loss control, and conductivity generation. Because synthetic polymers for acid gellants have made reactivity control easy, fluid loss control is usually the most dominant issue to be addressed in fracture acidizing.

Carbonate Reactivity

Several models are available to predict the spending of acid on carbonates. Some calculate the spending during fracture acidizing while some calculate the spending during matrix acidizing and wormhole generation. The earliest spending tests were simple spending time experiments in open beakers. However, it soon became clear that this was an inadequate procedure because mass transport definitely plays a role. Experiments conducted on quarried limestone and dolomite during the 1970s gave our first estimates of the temperature dependence of the reactivity of HCl on carbonates. These experiments suggested that limestones had incredibly high reactivity with acid such that one could assume a mass transport limited process.

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