Field Evaluation of Gelled Acid for Carbonate Formations
Authors: Church D.C. , Quisenberry J.L. , Fox K.B. 
Published: December 2007

Summary
A new gelled acid was evaluated in the west Texas, southeast New Mexico, and Oklahoma areas. The purpose of this evaluation was to determine how purpose of this evaluation was to determine how successful a gelled acid, prepared from xanthan polymer, would be in several carbonate formations. polymer, would be in several carbonate formations. Several types of acidizing techniques were employed. These treatments vary from one to nine stages, with and without diverting agents. More than 20 treatments are summarized. Production figures for the wells treated are discussed, as well as pertinent related information.

Introduction
To obtain optimum results from a fracture-acidizing treatment, live acid must penetrate as far into the reservoir as possible. 1 Treatment procedure's currently employed attempt to accomplish this through one, or a combination, of these methods:acid retardation,increased fracture width to decrease the area/volume ratio,improved leakoff control, andincreased acid strength.
The "fracturing pad and acid technique" and the use of alternate stages of a viscous nonreactive pad fluid and acids were developed to attain deeper penetration. penetration.These methods incorporate nonreactive gelled pad fluids to control acid leakoff rates and to reduce the area of formation exposed to a given volume of acid by increasing the width of the fracture. Ideally, the same advantages could be realized by thickening the acid itself. This could provide the added benefit of reducing or eliminating the nonreactive, unproductive gelled pad fluid. However, economical acid gelling agents that would not degrade in typical acid concentrations either at ambient temperatures or, more importantly, under downhole treating conditions of elevated temperatures and acid spending were not available previously.
Recently, a gelled acid, with a xanthan polymer as the basic gelling agent, was introduced. This system develops a relatively high viscosity and is stable under most conditions. This thickener, tested in numerous field applications and discussed in this paper, has demonstrated considerable overall potential for use as an acid gelling agent.

Characteristics and Advantages
Increasing the viscosity of a fluid, whether nonreactive or reactive, reduces the fluid loss to the rock matrix. The same is true for a gelled acid. If leakoff is controlled during acidizing, fluid efficiency is improved and more acid remains in the primary fracture, allowing deeper penetration. The addition of 60 lbm of acid thickener per 1,000 gal of 15% HCl produces a fluid with a viscosity of 25 cp (511 produces a fluid with a viscosity of 25 cp (511 seconds-1) at 100 degrees F. The gelled acid retains viscosity during spending and exhibits this stability up to 220 degrees F, thus maintaining its fluid efficiency.
Acid that is gelled will have a reduced reaction rate. Fifteen percent HCl gelled with 60 lbm of xanthan polymer per 1,000 gal of acid yields a 3.6 fold increase in spending time. This is a 69% reduction in reaction rate compared with ungelled acid. There are two mechanisms at work to accomplish the retardation in a gelled acid system. First, a viscous fluid creates a wider fracture, thus decreasing the area/volume ratio and reducing the mass-transfer rate to the fracture wall. Second, because of this increased fracture width, the gelled acid will remain in laminar flow at higher pump rates. This permits live acid to travel a greater distance into the formation than if it were in a turbulent flow regime.
Since the gelled acid has stable viscosity and does not break upon spending, the fluid properties noted previously are maintained throughout the entire previously are maintained throughout the entire treatment. An added benefit with this stable viscosity has been improved cleanup of insoluble silts and fines released by the acid. The acid retains enough viscosity to suspend and return these fines following the treatment.
JPT
P. 2471