Experimental evidence has shown that water-saturated chalk specimens are weaker than oil-saturated samples. The extent of water-induced weakening does not rely only on the stress state and degree of saturation but also on temperature, brine composition, and mineralogy. Our goal is to assess chalk weakening induced by chemical interactions by performing 1-D compaction simulations under three different waterflooding scenarios in the Dan field. While scenario 2 assumes Na2SO4 brine injection, scenarios 1 and 3 simulate MgCl2 brine injected in pure and impure chalk. These scenarios are compared to a base case scenario that is representative of true reservoir compaction. In this purpose, we collect published experimental data on the yield strength of outcrop and reservoir samples. Deemed complementary with laboratory works, these modelling scenarios consider simplistic fluid compositions to assess the impact of single ions on rock compaction at the reservoir scale. Sulfate adsorption on calcite surface is the most influential parameter that could increase the total vertical strain by a factor of three. The dissolution of Opal-CT by MgCl2 also enhances compaction by promoting the plastic deformation of low-porosity rocks. Sensitivity studies at the reservoir scale of the chemical effects on chalk compaction observed in laboratory represent an important step in developing advanced chemo-mechanical models.
Most hydrocarbon reservoirs still shelter considerable amounts of oil after primary/secondary recovery. To mobilize the residual oil, different enhanced oil recovery (EOR) techniques can be applied. Considering the numerous existing experimental studies, modified salinity waterflooding (MSW) (Austad et al., 2012; Fathi et al., 2011; Puntervold et al., 2015; Strand et al., 2008) and CO2-EOR (Fernø et al., 2015; Forooghi et al., 2009; Suicmez, 2019) have been envisioned as the most suitable and promising methods for the reservoir units in the Upper Cretaceous chalk formation. Moreover, depleted chalk fields may also be repurposed as CO2 storage sites. However, considering the observed compaction in the Ekofisk field (Nagel, 2001), the mechanical behavior of chalk becomes a reasonable concern when assessing the feasibility of any EOR/storage activities. The mechanical properties of chalk can be easily shuffled not only by the nature of the hosted fluid (e.g, exchanging oil by water) (Risnes et al., 2003) but also by the specific fluid composition (e.g., presence of certain ions in an aqueous solution) (Heggheim et al., 2005). Numerous publications focused on determining, especially, the impact of SO42− and Mg2+ on the mechanical behavior of chalk, given the observed positive impact of these ions on the oil recovery from core flooding and imbibition experiments (Fathi et al., 2010; Zhang et al., 2006, 2007). Korsnes et al. 2008 showed that chalk saturated with sulfate-containing solutions is weaker than samples exposed to synthetic seawater without SO42− and that the degree of weakening becomes more prominent with increasing temperature. They explained that sulfate promotes the replacement of calcium by magnesium at intergranular contacts, being this substitution the eventual cause of the weakening. To better understand the role of sulfate on the weakening, Megawati et al. 2013 performed flooding and mechanical tests with single Na2SO4 electrolytes at several ionic strengths. In agreement with Korsnes et al. 2008, they also observed a decrease in both the yield stress and bulk modulus with increasing temperature and sulfate concentration. In this case, the weakening was observed despite the absence of Mg2+ ions and at sulfate concentrations that, theoretically, should not trigger any mineral (e.g., anhydrite) precipitation. The observed behavior was attributed to the increase in the negative charge at the calcite surface caused by sulfate adsorption resulting in greater intergranular disjoining pressure and causing the weakening. Analogously, a higher surface charge caused by the adsorption of positive ions at the calcite surface, e.g., Mg2+ (Katika et al., 2018), will also increase the disjoining pressure. This weakening mechanism triggered by increased surface charges was also supported in the work of Nermoen et al. 2018 where they suggest defining an explicit contribution of the electrostatic effects arising from the surface charge development on the effective stress. Madland et al. 2011 found that the presence of sulfate is not a pre-requisite for the weakening. High-temperature mechanical tests showed that chalk samples exposed to MgCl2 suffered increased strain rates compared to those exposed to NaCl or deionized water. The analysis of the chemical composition of the effluents during these tests revealed increased Ca2+ production and Mg2+ retention in the core. Although the production of Ca2+ in presence of Mg2+ had been previously explained as Ca-Mg ion exchange or substitution, both geochemical modelling and rock sample analysis after the flooding proved the precipitation of magnesium-bearing carbonates. The loss of carbonate triggered by the precipitation leads to the dissolution of calcite minerals. Successive precipitation-dissolution episodes are thus responsible for the observed increased weakening. In a later study, Andersen et al. 2018 showed that the concentration evolution of key ions in the effluents during flooding experiments with MgCl2 and NaCl varies considerably depending on the content of chemical impurities contained in the sample. However, no correlation between the deformation rate and the impurities content can be a priori defined as the fluid chemistry governs the amplitude of the interactions with the different minerals. Chalk containing more quartz deformed less in presence of NaCl solution but exhibited the opposite trend when exposed to MgCl2 (Andersen et al., 2018); Alam et al. 2014 showed that chalk samples from the Tor formation were less affected by exposure to carbonated water compared to Ekofisk samples that have higher quartz content. Another factor that may complicate the prediction of the chemical effects on the chalk geomechanical behavior is the initial wetting state of the rock sample. A recent study has shown that although oil-wet chalk samples are stronger and stiffer than water-wet samples, the response to creep was similar (Sachdeva et al., 2019).
