New Chemical Treatment Provides Stimulation And Clay Control In Sandstone Formations
Authors: Thomas R.L. , Crowe C.W. 
Published: December 2007

Abstract
A new chemical treatment has been developed to stimulate problem sandstone formations. This treatment is designed to overcome a commonly observed problem in which wells initially respond to matrix acidizing but show rapid production declines following treatment. These rapid declines are generally attributed to migration of clays and other fines. The treating fluid, which slowly generates hydrofluoric acid, reacts more slowly than conventional Mud Acid and thus penetrates a greater distance into the formation before spending. The fluid also acts to stabilize clays and other fines by chemically fusing them to the sand grains. The chemistry of this system is entirely different from that previously reported in the literature.
The effectiveness of this stimulation and clay control method is demonstrated by core test studies. The control mechanism is illustrated by scanning electron microscope studies and x-ray analyses of treated clays. Preliminary field test results are also presented.

Introduction
Sandstone matrix acidizing has long been used as a means of improving production of oil and gas by removing formation damage and increasing permeability of the zone immediately around the wellbore. Laboratory and field studies have demonstrated the effectiveness of this type treatment. In spite of widespread use, however, many formations do not respond satisfactorily to conventional HCl/HF acid (Mud Acid) treatments. This is normally attributed to rapid spending of HF near the wellbore. Some wells initially show good stimulation but later experience an unusually rapid decline in production rate. Such production declines are commonly observed in wells producing from unconsolidated sands and are usually attributed to plugging by migratory clays and other fines.

Plugging by Migratory Fines
Formation plugging by migratory fines was demonstrated by Kruger et al in both laboratory and field studies. Core test results revealed that high flow rates caused dislodgment of fines with resulting loss of permeability. Kruger also reported field studies in which the production decline rate was accelerated by flowing wells above an optimum rate. As the production rate was increased, a corresponding increase in inorganic solids content of the produced oil was also observed. These observations strongly support the theory of formation damage from fines migration. Migratory fines are apparently released first by exposure to strong acid and later by mechanical forces resulting from the increased flow of produced fluids. Various treatments and production techniques have been devised in an effort to control or minimize this problem. Kruger and Templeton described treating techniques in which wells, following stimulation, were returned to production at a gradually increasing rate in order to minimize fines migration. Various clay stabilization agents have also been applied in an effort to control movement of fines. Delayed acting acidizing systems have been developed to provide deeper live acid penetration and thus remove damaging fines some distance from the wellbore.
A new retarded HF acid system (Clay Acid) has proven extremely effective for matrix acidizing sandstone formations. The Clay Acid is usually applied as an overflush to Mud Acid. Upon entering the formation, the Clay Acid slowly generates hydrofluoric acid thus providing greatly increased live acid penetration. The system requires neither the use of flammable organic esters nor multiple staging of treating fluids for HF generation. In addition to providing deep live acid penetration, the treatment also stabilizes any undissolved clays contacted. In contrast to conventional clay stabilizers, which act by ion exchange or adsorption, Clay Acid produces an actual chemical fusion of fines and clay platelets.