Acid fracturing has been a widely used stimulation technique for carbonate reservoirs. While the etching action of the acid would imply an open fracture (with infinite conductivity), the strength of the reservoir rock leads to a significant reduction of the apparent conductivity after closure. In addition, acid fractures are invariably shorter than propped fractures as a result of acid spending patterns and the creation of "wormholes". Consequently, acid fractures are not automatically indicated for all carbonate formations. Instead, a comparison of the production performance of both acid and propped fractures is always necessary.
This paper presents an outline of the propagation mechanism of acid fractures including reaction kinetics and fluid leakoff, and the resulting penetration. Upon closure, the impact of the effective stress upon fracture conductivity is shown using a compilation of published data. The time dependency of the effective stress around the fracture is quantified along with its influence on the production performance using a poroelastic model. Dimensionless cumulative production type curves are given for a range of fracture in-situ conductivities (for propped fractures), and the formation effective stress (for acidized fractures). A comparison of this performance leads to a correlation between propped fracture conductivity and formation effective stress to produce a desired cumulative production within the same time period.
For a given carbonate formation and a given effective stress, a comparison of expected performance leads to a relationship between fracture conductivity and the ratio of the lengths of a propped and an acid fracture. This allows the choice of the propped and an acid fracture. This allows the choice of the appropriate stimulation treatment. Additional effects, such as conductivity variation along the fracture resulting from nonhomogeneous etching profiles and wormhole development, are examined.
Carbonate formations such as limestones (CaCO3), dolomites (CaCO3, MgCO3) and chalks (soft, higher Porosity CaCO3) react with acids (usually HCI) to form water-soluble salts, water and carbon dioxide. This reactivity has been used in the development of acid fracturing as a stimulation treatment. The treatment is conceptually simple. Acid is injected at fracturing pressures, the formation is parted, and acid etches the walls of the created fracture. When the pumping stops, the pressure falls off to the original pressure. If the reaction patterns were uniformly even, then the pressure. If the reaction patterns were uniformly even, then the fracture faces would collapse and no residual conductivity would be accomplished. However, uneven etching patterns (asperities), that are the result of formation compositional heterogeneities and fracture roughness, keep a residual etched width that provides the fracture conductivity.
Acid fracturing has certain advantages over propped fracturing.
* There is no risk of a screenout which is a major concern during the execution of a propped fracture (i.e., no limitations other than economics prevent the injection of larger acid volumes).
* There are no cleanout problems in acid fracturing in contrast to the gelled residues that may be present in propped fractures, and which are one of the two major causes for fracture conductivity impairment (the other one being, proppant crushing).
* Conceptually, the conductivity of an acid fracture and its associated open channels should be very large, approaching infinite values.
These advantages are dampened by several problems that are affecting the extent of the acidizing process and subsequent well performance: performance: P. 133