Laboratory Study of Rock Softening and Means of Prevention During Steam or Hot Water Injection
Authors: Day John J. , Huitt J.L. 
Published: December 2007

Abstract
Laboratory tests were made with pure minerals and actual reservoir rock samples to study the effects of hydrothermal (steam or hot water) treatments on reservoir rock properties. These tests showed that hydrothermal treatment of many reservoir rocks can result in significant rock softening. The softening was attributed primarily to the partial destruction of dolomite and kaolinite and the synthesis of montmorillonite in the presence of excess silica. In many cases, the softening was great enough to cause considerable healing of propped fractures; therefore, a serious reduction in well productivity could result. In other tests it was found that the addition of ammonia to a hydrothermal treating fluid in a concentration as low as 0.013 lb/lb of water not only could prevent rock softening but could cause rock hardening. The results of X-ray diffraction analyses of rock samples showed that when ammonia was added to the treating fluid, ammoniummica and analcite were formed instead of montmorillonite. No significant permeability damage was observed in the sandstones that were subjected to the ammonia-hydrothermal environment; in some sandstones, permeability improvements resulted.

Introduction
As the use of steam or hot water becomes more prevalent in well stimulation methods, the need for information on the effects of such treatments on reservoir rock properties becomes increasingly apparent. Several works have been published on high-temperature changes in rock properties, but these are more applicable to in situ combustion operations than to steam or hot water injection processes. The mineralogical literature contains many publications which report on the changes occurring with various pure minerals in hydrothermal systems. A review of this literature denotes the ease with which entirely new, crystalline mineral phases can be synthesized from other minerals in hydrothermal environments at temperatures, pressures and residence times typical of those encountered in oilfield thermal recovery or stimulation processes. Discussions of many of these experiments are given by Deer et al., Grim, Roy et al., Zen and Hawkins. One of the most significant of these pure mineral studies is the hydrothermal synthesis work of Levinson and Vian in which montmorillonite was synthesized from naturally occurring minerals; i.e., kaolinite, quartz and carbonate minerals (particularly dolomite). These reactions occurred at 575F in only 2 days and at 300F in 5 days. These results may be applicable to petroleum reservoir rocks since the minerals studied were those which are commonly found in sandstones. Furthermore, the environmental conditions imposed in the studies were very similar to those involved in thermal stimulation of petroleum reservoirs, e.g., steam or hot water injection. Other studies have suggested that weak rocks could be hardened by "electrochemical induration", a process in which an electric current is applied to a clay-containing rock body. These tests can be regarded as hydrothermal treatments since they were conducted in some instances with aqueous solutions and since clay temperatures during the electrical treatment reached a maximum of about 100C. Although a number of studies of the reaction of pure minerals have been reported, very little has been published on the reactions of petroleum rock-hydrothermal systems. No work has been reported on preventing the more detrimental rock changes in rock softening which might occur during the injection of steam or hot water into reservoirs. The studies described in this paper were conducted to provide such information.

EXPERIMENTAL APPROACH
The laboratory tests included studies of the effect of hydrothermal treatment on core samples from several different reservoirs. The hydrothermal treatments (simulated steam treatments) were conducted with distilled water at 575F, for the most part, for periods of 2 to 6 days. This temperature level was selected because it represented that temperature which would probably prevail in several cases under consideration for steam injection projects. The core samples, as well as the pure mineral samples, were contained in high-pressure stainless steel, autoclave-type pressure vessels. The effects of the hydrothermal treatments were evaluated by measuring the penetrometer hardness, formation rock embedment strength and permeabilities of the core samples before and after the treatments (Fig. 1). In addition, the mineralogical changes in the specimens were studied by X-ray diffraction. Concomitant with the core sample studies, the mineralogical changes were studied in greater detail by conducting hydrothermal synthesis experiments with pure minerals at similar temperature levels and residence times.
JPT
P. 703ˆ