How Stress and Temperature Conditions Affect Rock-Fluid Chemistry and Mechanical Deformation

Abstract

We report the results from a series of chalk flow-through-compaction experiments performed at three effective stresses (0.5, 3.5, and 12.3 MPa) and two temperatures (92 and 130 • C). The results show that both stress and temperature are important to both chemical alteration and mechanical deformation. The experiments were conducted on cores drilled from the same block of outcrop chalks from the Obourg quarry within the Saint Vast formation (Mons, Belgium). The pore pressure was kept at 0.7 MPa for all experiments with a continuous flow of 0.219 M MgCl 2 brine at a constant flow rate; 1 original pore volume (PV) per day. The experiments have been performed in tri-axial cells with independent control of the external stress (hydraulic pressure in the confining oil), pore pressure, temperature, and the injected flow rate. Each experiment consists of two phases; a loading phase where stress-strain dependencies are investigated (approximately 2 days), and a creep phase that lasts for 150-160 days. During creep, the axial deformation was logged, and the effluent samples were collected for ion chromatography analyses. Any difference between the injected and produced water chemistry gives insight into the rock-fluid interactions that occur during flow through the core. The observed effluent concentration shows a reduction in Mg 2+ , while the Ca 2+ concentration is increased. This, together with SEM-EDS analysis, indicates that magnesium-bearing mineral phases are precipitated leading to dissolution of calcite. This is in-line with other flow-through experiments reported earlier. The observed dissolution and precipitation are sensitive to the effective stress and test temperature. Higher stress and temperature lead to increased Mg 2+ and Ca 2+ concentration changes. The observed strain can be partitioned additively into a mechanical and chemical driven component.

Porosity Evolution during Chemo-Mechanical Compaction

“…As such, for unconsolidated sands and calcitic mudstone, in which α > 0:9, this fraction is significant and may be responsible for additional calcite dissolution [10]. As has been shown previously (in e.g., [18,22] and also before that), the contact area ratio is linked to the Biot stress coefficient (α).…”

“…It is likely to assume that based on the accelerated strain presented in [10] minute changes to the solid volume increase the rate of pore collapse (also seen in [4]). …”

“…(20)- (23) in [10]). In this model, overall volumetric strain is additively partitioned into a pore and solid volume component in which the pore volume equals, V p ¼ ϕV b .…”

“…Pressure solution can occur in closed systems, in which the overall mass and density remain fixed. For high Biot coefficients, the local stress at particle contacts may become significant (see Figure 7 in [10]), and thus, a stressdependent production of Ca-ions is observed where more Ca-production for high stress than low stress (see Figure 6 acquired from [10]). …”

Incorporating electrostatic effects into the effective stress relation — Insights from chalk experiments

“…During hydrostatic stress buildup, the axial ε ax strains were measured from the change in length divided by the initial length, ε ax ¼ −ΔL∕L 0 . Typically, at hydrostatic stress conditions, the sample does not keep its cylindrical geometry during loading even when the material is nearly isotropic (Nermoen et al, 2015), an effect most likely originating from the boundary conditions imposed by the steel end pieces on both ends of the sample (Nermoen et al, 2016). This leads to a larger radial strain at the center of the sample than at the ends, where negligible radial strain is seen.…”