An Evaluation of Acid Fluid Loss Additives Retarded Acids, and Acidized Fracture Conductivity
Authors: Nierode D.E. , Kruk K.F. 
Published: December 2007

This paper was prepared for the 48th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Las Vegas, Nev., Sept. 30-Oct. 3, 1973. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgement of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.
Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
An evaluation of acid additives and retarded acid systems indicates that the stimulation resulting from acid fracturing can be increased when effective fluid loss additives used in HC1, or when the acid viscosity is increased significantly. Acid emulsions were found to have a low fluid loss rate and to be retarded, whereas oil wetting surfactants gave no retardation at typical field injection rates. Conductivity studies show that, in general, the fracture flow capacity resulting from acid reaction is very high, except when rock embedment strength and/or rock solubility is low, or the closure stress is high.

Introduction
In an acid fracturing treatment, either acid alone is injected into the formation at a high rate, or the acid is preceeded by a viscous fluid (the pad fluid) to form a long, wide fracture. When acid is used without a pad fluid, the fracture will generally be short and narrow since the rate of fluid loss for acid is high. If a viscous pad fluid is used, the long, wide fracture that is formed will begin to close as acid is injected, and will approach the geometry expected if acid alone had been used. This decrease in fracture volume occurs because the acid wormholes through the region invaded by the viscous fluid and thereby increases the rate of fluid loss.
The stimulation obtained in an acid fracturing treatment is controlled by the length of the fracture that is effectively acidized, not by the induced fracture length. The distance reactive acid moves along the fracture (the acid penetration distance) is governed by the acid penetration distance) is governed by the acid flow rate along the fracture, the rate of acid transfer to the fracture wall, and the reaction rate at the rock surface. It has been shown that under most circumstances, the reaction rate between acid and rock is very fast, and that the rate of mass transfer to the rock face controls the overall acid reaction rate.
A design procedure that combines the previously discussed fracturing aspects with previously discussed fracturing aspects with the reaction behavior of acid has been recently developed and compared with field treatment results. The design method considers the bounds on the acid penetration distance (the fluid loss limit and the reaction rate limit) shown in Fig. 1. The fluid loss limit is estimated assuming the benefits of the pad fluid are lost instantaneously through the formation of wormhole channels and is identical to the penetration calculated if no pad fluid is used. The reaction rate limit is the theoretical maximum acid penetration distance.