Abstract

The first observation and description of hydraulic fracturing, by Grebe and Stoesser (1935), involved injecting acid to stimulate oil production from a carbonate formation. With the advent of hydraulic propped fracturing of sandstones with oil and sand in the late 1940s, fracture acidizing has been generally confined to carbonate formations; its advancement did not match that of propped fracturing. By the 1970s, propped fracturing of carbonates (in addition to sandstones) gained popularity through the greater understanding and ease of modeling fracture stimulation with non-reactive (non-acid) fluids. After the 1970s, however, advancements occurred in modeling fracture acidizing and in fracture acidizing stimulation theory. Thus, entering the 1980s and into the 1990s, fracture acidizing in carbonates increased, with development of a variety of fluid systems and multi-step procedures that are still in use today. Nevertheless, fracture acidizing continues as the less-preferred alternative to propped hydraulic fracturing in carbonates - and it has never been seriously considered as a stimulation method for sandstones.

With approximately 70% of worldwide hydrocarbon reserves in carbonate formations, and the need to simplify sandstone stimulation treatments in general, the merits of fracture acidizing and its greater possibilities - for both carbonates and sandstones - must be considered.

This paper endeavors to briefly review the historical milestones leading to fracture acidizing, and their bearing on present methods and on the imposed rules. The paper touches on the types, purposes, benefits and limitations of present technologies and methods with a focus on the propped fracturing versus fracturing acidizing decision - and with a view to future possibilities and opportunities.

Introduction

Intentional fracture acidizing treatments have been mostly confined to carbonate formations - to serve one or both of two purposes in chalk, limestone, and dolomites:

  1. Bypassing formation damage

  2. Stimulating undamaged formation

Ever since the introduction of propped fracturing to carbonate formations (after its original limitation to sandstones), fracture acidizing has been viewed largely as the an alternative to propped fracturing - and the less preferred alternative. This is understandable, given that the treatment objectives and processes have been fundamentally similar - the creation of a long conductive fracture channel extending from the wellbore into the formation. In both cases, fracture height is principally controlled by the stress contrasts in bounding rock layers; and fracture length depends upon the height containment and the leak-off properties of the fracturing fluid. The preference for propped fracturing further stands to reason, because propped fracturing is more easily modeled (non-reactive fluids; stable leak-off) and by definition, does not utilize fluids (acids) that are still widely feared, or at least avoided. Also, the effect of basic rock mechanical properties on longer term acid-etched conductivity when compared to propped fracture conductivity in the same formation having the same treatment objective in mind, and considering only the volumes and types of acids historically employed, have typically supported propped fracturing1.

With propped fracturing, fracture conductivity is maintained by propping open the created fracture with a solid material, such as sand, bauxite, ceramic, and certain lighter weight materials. With acid fracturing, non-uniform acid etching (or differential etching) of the fracture face creates lasting conductivity. This is true as long as stable points of support (asperities) along the etched fracture remain. These hold the channel open and connected to the wellbore following fracture closure.

In fracture acidizing, acid is injected into a fracture created by a viscous fluid (pad) or is itself used to create the fracture. As acid travels down the fracture, acid is transported to the fracture walls, resulting in dissolution etching. If etching is non-uniform (differential), then the fracture may close with conductivity retained, as depicted in Figure 1.

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