Chemical Alterations Induced by Rock–Fluid Interactions When Injecting Brines in High Porosity Chalks

Abstract

Effect of the aqueous chemistry on the mechanical strength of chalk has extensively been studied during the last decade. At high temperatures (∼130 • C), chalk exposed to seawater is significantly weaker compared to chalk exposed to distilled water when considering the hydrostatic yield strength and the following creep phase. The explanation of these experimental results must be of a chemical nature, as the density and viscosity of the aqueous phase vary little among these different brines. We present the results from simplified aqueous chemistry using MgCl 2 brines, and compare these results with seawater. Previous studies show that different ions, e.g. Ca 2+ , Mg 2+ , SO 4 2− in the injected brine, as well as the chalk mineralogy have an impact on the stability of the rock. We performed mechanical tests on chalk cores from Liège and Stevns Klint; it was found that these two outcrop chalks exhibit an unexpected difference in their mechanical responses when comparing cores flooded with NaCl and MgCl 2 at 130 • C. The results of this study show that the effects of magnesium seem to be governed not only by the differences in mineralogy, but also a time dependency on chalk deformation is additionally observed. Independent of the chalk type tested, the chemical analyses performed show that when MgCl 2 is flooded through the core, a significant loss of magnesium and a considerable additional amount of calcium are detected in the effluent. The experimental observations fit very well with the time-dependent chemical changes gained from the mathematical model of this study that accounts for transport effects (convection and molecular diffusion) as well as chemical processes such as precipitation/dissolution. Based on the calculations and chemical analyses, we argue that the loss of magnesium and the production of calcium cannot solely be a consequence of a substitution process. The calculations rather indicate that magnesium is precipitated forming new mineral 123 680 M. V. Madland et al. phases and in this process not only calcite, but also silicates are dissolved. The amount of dissolved calcium and silicon from the rock matrix is significant and could thus cause an additional deformation to take place. Both the retention of magnesium in the chalk core and the formation of newly precipitated magnesium-bearing carbonates and/or magnesium-bearing clay-like minerals after flooding with MgCl 2 brine were demonstrated using scanning electron microscopic methods. In addition, precipitation of anhydrite as a result of flooding with seawater-like brine was proven. The water-induced strain not only depends on the ion composition of the injected brine; moreover, the presence of non-carbonate minerals will most likely also have a significant influence on the mechanical behaviour of chalk.

Mechanical and physical behavior of high-porosity chalks exposed to chemical perturbation

“…Rock mechanical testing was performed in a hydraulically operated triaxial cell similar with that of previous studies Korsnes et al, 2008;Madland et al, 2011). The aim has thus not been to capture strength variations from complete yield curves, but to select a repeatable test method in order to study the various chemical effects.…”

“…Chemical effects due to reactions between the chalk framework and the injected water are one of the major mechanisms in the so-called water weakening processes (Newman, 1983;Rhett and Teufel, 1991;Teufel and Rhett, 1992;Piau and Maury, 1994;Risnes and Flaageng, 1999;Risnes et al, 2005;Madland et al, 2011). Chemical dissolution precipitation have been investigated through various approaches such as numerical simulations (Pietruszczak et al, 2006;Lydzba et al, 2007;Hueckel and Hu, 2007), experimental studies (Hellmann et al, 2002b), as well as microscopic studies (Hellmann et al, 2002a).…”

“…In terms of chalk mechanical behavior, earlier works (Heggheim et al, 2005;Korsnes et al, 2006a,b;Madland et al, 2011) have shown that not only the aqueous chemistry of the pore fluid, but also the presence of non-carbonate minerals such as silica and clay minerals control the chalk mechanical behavior. In Madland et al (2011) injecting MgCl2 brine leads to supersaturation of new minerals which further shift the chemical equilibrium towards dissolution of calcite.…”

“…In Madland et al (2011) injecting MgCl2 brine leads to supersaturation of new minerals which further shift the chemical equilibrium towards dissolution of calcite. When injecting NaCl brine with the same ionic strength, however, only a minor change in the aqueous chemistry was demonstrated.…”

“…Simplified brines with seawater ionic strength (0.219 M MgCl2 and 0.657 M NaCl) were used. Since mechanical properties measured under isotropic compression are less scattered and highly repeatable as shown from previous studies (Risnes et al, 2003;Heggheim et al, 2005;Korsnes et al, 2006bKorsnes et al, , 2008Zangiabadi et al, 2009;Madland et al, 2011), this type of test is chosen. Microscopic studies by Scanning Electron Microscopy (SEM) are included.…”