Field Results For A New Retarded Sandstone Acidizing System
Authors: Hall Bobby E. , Anderson B.W. 
Published: December 2007

Abstract
A new retarded clay-dissolving system has been developed for stimulating fluid production from clay damaged sandstone formations. This system utilizes the ion-exchange properties of clay minerals to generate in-situ hydrofluoric arid on the clay mineral surfaces.
Fluid properties and job designs eve discussed for this process, which has been used in over 200 treatments. In addition, production data are presented for selected treatments to show common presented for selected treatments to show common applications.

Introduction
Matrix acidizing treatments in sandstone reservoirs with solutions containing hydrofluoric acid are routinely performed to help remove clay damage and, thereby, improve production. This technique is especially useful in treating loosely consolidated sandstone formations where hydraulic fracturing may not be feasible.
Matrix acidizing involves pumping acid, at pressures less than the formation fracturing pressure, pressures less than the formation fracturing pressure, into the porosity of the rock. This acid contacts a large surface area of rock and thereby is spent very rapidly. Studies have shown that the depth of penetration of unreacted hydrofluoric acid is penetration of unreacted hydrofluoric acid is primarily dependent on the clay content of the primarily dependent on the clay content of the formation, although only limited penetration can be achieved even in the absence of clay. For a typical sandstone formation containing 10-percent clay, the theoretical penetration radius of unreacted 3-percent hydrofluoric acid is about 6 in. Consequently, clay minerals at depths greater than 6 in. from the wellbore are difficult to remove with hydrofluoric acid.
Sequential injection of a hydrochloric-acid solution followed by a solution containing fluoride ion is a new method for generating in-sits hydrofluoric acid. This will be referred to hereafter as SHF. This process will react with and dissolve clays deeper than is presently possible with conventional hydrofluoric acid. This paper will discuss the mechanism of this process as well as job design and field results.
SEQUENTIAL HF PROCESS (SHF)
The SHF process utilizes the ion-exchange properties of clay minerals to generate hydrofluoric properties of clay minerals to generate hydrofluoric acid on the clay particle in-situ. In practice, a solution of hydrochloric acid containing no fluoride ion is pumped into the formation. This acid solution will contact the clays in the rock and exchange protons (H+) for the cations natural to the clay protons (H+) for the cations natural to the clay minerals. The clay is now converted to an acidic clay particle as shown in Fig. 1.
Next, a neutral or slightly basic solution of fluoride ion is pumped into the formation. This solution will contact acidic clay particles and combine with protons previously adsorbed to generate hydrofluoric acid on the clay minerals. In addition, some anion exchange will occur whereby fluoride ions (F-) will substitute for the anions natural to the clays. The hydrofluoric acid that is generated rapidly reacts with and dissolves a portion of the clay as shown in Fig. 1.
The sequential injection of a hydrochloric-acid solution followed by a fluoride-ion solution is repeated as many times as necessary to obtain the desired penetration of hydrofluoric acid. Since hydrofluoric acid is not formed until both solutions have contacted the clay minerals, the depth of penetration is mainly dependent on the volume and penetration is mainly dependent on the volume and strength of the acid and fluoride-ion solutions.
Clays are the primary minerals present in a sandstone formation which exhibit ion-exchange properties, and the SHF process will react primarily properties, and the SHF process will react primarily with these materials. In a clay-damaged formation, this is ideal since little of the hydrofluoric acid is wasted by dissolving sand.