Role of Fluxing Agents in Thermal Alteration of Sandstones
Authors: Somerton W.H. , Gupta V.S. 
Published: December 2007

Rock may undergo great changes in physical properties when heated to high temperatures and then cooled. The temperature and intensity of reactions causing rock alterations can be controlled by introducing certain chemicals during heat treatment. Three typical outcrop sandstone samples were saturated with common salt solutions, then heated to several maximum temperatures. After cooling, it was found that permeabilities had increased much more for salt-saturated samples than for samples not saturated with salt but heated to the same temperatures. This was only true, however, for samples heated above the melting points of the particular salts. Potassium chloride was particularly effective with Banders sandstone. Samples saturated with potassium chloride solution and heated to 900C showed an 11-fold increase in permeability. Samples without potassium chloride but heated to the same temperature showed only, a 2.5 fold increase in permeability. In application, it seems possible that injecting chemicals into the formations from a wellbore followed by applying intensive borehole heating might promote reactions which would greatly improve permeability of the formations.

Introduction

Great changes in the physical properties of rocks which have been heated to temperatures in the range of 600 to 800C have been reported earlier. Permeability increases of 50 per cent, and equivalent decreases in sonic velocity and breaking strength have been observed. Although there might be some suppression of certain reactions responsible for these changes when rock samples are heated under simulated reservoir pressure conditions, recent work has shown that these are more than offset by the increased importance of other reactions at high pressures. The reactions considered responsible for alteration of rock properties by heating include differential thermal expansion, dehydration, phase changes and dissociation of mineral constituents. Ceramists have long known that temperatures at which reactions occur can be raised or lowered by the presence of certain impurities in the system. Thus the possibility exists that by saturating rocks with appropriate solutions, desired reactions might be accelerated and unwanted reactions might be suppressed when the rock is heated. Purpose of the present work was to investigate the effects of several common salts on the thermal alteration of sandstones. Types of reactions which might occur are reviewed and the findings of a number of thermochemical alteration tests are presented.

THERMOCHEMICAL ALTERATION OF SANDSTONE MINERALS

The most abundant constituent of most sandstones is quartz. Quartz can exist in at least eight different forms, but in the temperature range of immediate interest (to 900C) only two forms are of importance-alpha quartz below 573C and beta quartz above this temperature. Although the alpha-beta inversion of quartz is very important in thermal alteration of rocks, it is not particularly important here because the inversion temperature cannot be changed significantly by the presence of impurities or by the application of pressure. The transition temperature of quartz to tridymite is 867C, but the transition is very sluggish. Tridymite is considered to be a metastable transition phase between two stable phases: quartz-cristobalite. However, the transition temperature for tridymite to cristobalite is 1,470C. The quartz-tridymite-cristobalite transition can be accelerated substantially by the presence of fluxing agents. Finely divided calcium, magnesium and titanium oxides accelerate the conversion while Al 0 may retard it. A mixture of NaCl and CsCl reduces the temperature for cristobalite formation by as much as 420C.Feldspars constitute the second most important mineral in sandstones. The melting points of feldspars range between 1,000 and 1,700C depending upon the variety. No information available indicated any important role of fluxing agents in the thermal alteration of this group of minerals. Carbonate minerals are subject to dissociation at temperatures within the range of the present investigation. The dissociation of magnesite can start at a temperature of 373C, but the reaction is sluggish and might not occur until a temperature of 500C or -higher is reached. Dolomite dissociates in two stages at 500 and 890C, whereas calcite dissociation temperature is about 885C.Because CO2 is released in the dissociation of carbonates, all such reactions are somewhat dependent upon CO2 partial pressure. In the absence of CO2 in the surrounding atmosphere, the dissociation of calcite starts at 500C. However, when 1 atm of CO2 surrounds the sample, the dissociation does not start until 900C. The other carbonates are apparently much less sensitive to a change in partial pressure of CO2.The aragonite-calcite transformation can occur anywhere within the temperature range of 387 to 488C, depending upon the presence of impurities such as barium, strontium, lead and perhaps zinc. The differential thermal analysis (DTA) curves of both magnesite and dolomite vary with the presence of impurities.

JPT

P. 585ˆ