Fracture Acidizing In Carbonate Rock
Authors: Knox J.A. , Ripley H.E. 
Published: March 2010

Fracture acidizing has an application in carbonate formations to improve the productivity of low-permeability reservoirs and for to overcome deep formation damage. If the acid solubility is less than 65% a viscous fingering technique is usually recommended.

An ideal fracture would cover the productive interval vertically have a length equal to the drainage radius and have adequate/low capacity. Several/actors influence the geometry and development 0/fractures generated with acid. The effects of density, acid penetration fracture flow capacity, temperature acid concentration velocity or injection rate, viscosity and fluid loss are discussed.

Computer-designed acid treatments and how they help select treatment volumes and predict production improvements are reviewed, The well data and the results of fracture acidizing seven Western Canadian wells are presented.

Introduction

Most acid pumped iota carbonate rock moves through fractures, either natural or created. If natural fractures do not exist, then by injecting acid at a high enough rate to develop the pressure required, one or more fractures can be created in the formation.

In the late thirties and early forties, Clason(l) suggested that, based on Muskat's work, it would be impossible to get the production increases which were observed by merely increasing the size of the wellbore or through radial penetration of acid. According to Clason, crevices must be present and only through crevice enlargement and/ or removal of drilling fluid or other deposits from crevices or fractures could the large increases in productivity be explained. He was a little ahead of his time.

About 10 years later, it was discovered that fractures or crevices actually could be created and fracturing theories began to be developed. Since that time, great strides have been made in fracturing both with and without propping agents.

The use of acid to create and etch fractures has gone through several stages of development in the last 25 years(2), but remain something of a black art until Nierode and Williams (3, 4, 5) presented a kinetic model for the reaction of hydrochloric acid with limestones. They used this to predict acid reactions during fracturing operations and to design fracture acidizing treatments. This work has carried forward mainly by Roberts and Guin(6, 7, 8).

Fracture Acidizing

Initially, fractures were considered to be in a horizontal plane rather than most being vertical or inclined, as present theory suggests. Naturally fractured zones do exist and the' treating fluid will leak off into these natural fractures, as they represent the primary permeability.

Laboratory tests show that when fractures are created they will propagate through naturally occurring fractures without regard to their orientation and are perpendicular to the least principale stress(9).

Fracture acidizing has application in carbonate rock: limestone, dolomite and mixtures. If the formation has less than about 60% solubility in hydrochloric acid, then the possibility of a successful fracture acidizing job is greatly reduced unless the soluble portion is lining the fractures or the viscous fingering technique is used.

Two primary reasons for fracture acidizing are: (1) low formation permeability, which prevents the well from producing by radial flow at an economic rate;