The Effectiveness of Acid Fractures and Their Production Behavior

Abstract

ABSTRACT 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 appropriate stimulation treatment. Additional effects, such as conductivity variation along the fracture resulting from nonhomogeneous etching profiles and wormhole development, are examined.