Crosslinked Acid Gel
Authors: Deysarkar Asoke K. , Dawson Jeffrey C. , Sedillo Lawrence P. , Davis Susan Knoll 
Published: March 2010

Abstract
A new, stable, crosslinked acid gel has been developed jar fracture acidizing applications. This crosslinked acid can be prepared from acid strengths up to 28 per cent hydrochloric acid. Laboratory tests show the fluid possesses high viscosity and shear stability at temperatures up to 93 °C(200 °F). Viscosity of the acid gel can be reduced in a controlled manner by a chemical additive to aid well cleanup.
Rheological data were obtained in a newly designed Viscometer, the pressure rheometer. The rheometer used a bob/cup arrangement to measure the fluid rheology and the fluid was fed to the gap between the bob and cup. A desired pressure was directly applied to the test fluid without using a second pressurizing fluid such as mineral oil or nitrogen.
In addition to its viscosity characteristics, the acid reactivity of the crosslinked acid gel with limestone specimens, in a dynamic reaction setup, is drastically retarded. The crosslinked fluid is also residue free in live or spent acid minimizing formation damage. This newly developed fracture acidizing fluid possesses other important fracturing fluid properties such as low friction pressure, good proppant transport characteristics, and good fluid loss control.
Results from computer simulation of fracture acidizing treatments indicate deeper penetration is achieved by the cross- linked acid compared to unaltered acid.
Field results also show that in several carbonaceous formations, production could be substantially improved if stimulated with this crosslinked acid.

Introduction
Hydrochloric acid, ranging in strength from 3 to 28% has been used to commercially stimulate carbonate formations since 1932(1). However, the reaction of HC1 (hydrochloric acid) with carbonate rock occurs too rapidly resulting in a limited depth of penetration of live acid into the formation. In order to increase the depth of penetration, acids have been retarded by: adding a suitable gelling agent(2), adding chemical retarders(3), foaming the acid(4), emulsifying the acid(3), or by crosslinking suitable acid gelling agents(5). Each method of retardation has limited use in field application. With the exception of cross-linked acid, each of these methods generate fluids of low viscosity and a low degree of viscous stability (viscosity changing with time) with increasing reservoir temperature. If hydrochloric acid can be crosslinked to generate high viscosity and better viscous stability then not only will it be possible to retard the acid spending but it will also produce longer and wider fractures. Furthermore, crosslinked fluids normally have lower leakoff rate to the formation during pumping than uncrosslinked fluids.
This paper presents a newly formulated crosslinked acid gel developed for fracture acidizing. Rheology, acid reaction rate, fluid loss, friction pressure and sand fall data obtained from laboratory studies will be presented. Results obtained from field application of this crosslinked acid will also be presented.

The Crosslinked Acid System
Crosslinked acid was formulated to be easily mixed on the well site. The system is similar to other commercial crosslinked water-based fracturing fluids commonly used for hydraulic fracturing.