Enhanced deformation of limestone and sandstone in the presence of high fluids

Abstract

[1] Geological repositories subject to the injection of large amounts of anthropogenic carbon dioxide will undergo chemical and mechanical instabilities for which there are currently little experimental data. This study reports on experiments where low and high P co 2 (8 MPa) aqueous fluids were injected into natural rock samples. The experiments were performed in flow-through triaxial cells, where the vertical and confining stresses, temperature, and pressure and composition of the fluid were separately controlled and monitored. The axial vertical strains of two limestones and one sandstone were continuously measured during separate experiments for several months, with a strain rate resolution of 10 À11 s À1. Fluids exiting the triaxial cells were continuously collected and their compositions analyzed. The high P co 2 fluids induced an increase in strain rates of the limestones by up to a factor of 5, compared to the low P co 2 fluids. Injection of high P co 2 fluids into the sandstone resulted in deformation rates one order of magnitude smaller than the limestones. The creep accelerating effect of high P co 2 fluids with respect to the limestones was mainly due to the acidification of the injected fluids, resulting in a significant increase in solubility and reaction kinetics of calcite. Compared to the limestones, the much weaker response of the sandstone was due to the much lower solubility and reactivity of quartz in high P co 2 fluids. In general, all samples showed a positive correlation between fluid flow rate and strain rate. X-ray tomography results revealed significant increases in porosity at the inlet portion of each core; the porosity increases were dependent on the original lithological structure and composition. The overall deformation of the samples is interpreted in terms of simultaneous dissolution reactions in pore spaces and intergranular pressure solution creep.

The Geomechanics of CO2 Storage in Deep Sedimentary Formations

“…It is well known from CO 2 flooding related to EOR that dissolution of carbonates in rock-forming minerals may enhance porosity and permeability, leading to worm holes and fingering (Ross et al 1982). Moreover, enhanced mineral dissolution may also lead to mechanical weakening as well as enhanced compaction creep and subsidence (Nguyen et al 2011;Spiers et al 2010;Le Guen et al 2007;Espinoza et al 2011). A number of laboratory studies have investigated the effects of supercritical CO 2 on rock samples, showing the importance of intergranular pressure solution, especially in limestone with rock-forming carbonates, but also in sandstones with cementing carbonates.…”

“…A number of laboratory studies have investigated the effects of supercritical CO 2 on rock samples, showing the importance of intergranular pressure solution, especially in limestone with rock-forming carbonates, but also in sandstones with cementing carbonates. In general, experiments have shown that CO 2 exposure leads to acceleration of creep by up to several orders of magnitude in carbonate-rich rocks (Le Guen et al 2007;Spiers et al 2010). In a field setting, such increased compaction strain will not only impact the stress evolution within the reservoir, but (through stress transfer) the surrounding rock and overburden as well, and lead to some irreversible subsidence.…”

“…Very few laboratory data exist and no field studies related to CO 2 injection have been focused on the potential for chemically mediated compaction so it is difficult to quantify this effect. Le Guen et al (2007) observed about 1% compaction strain over a year of exposure to CO 2 -rich fluid and to estimate long term effects such data could perhaps be extrapolated to tens or hundreds of years using mechanical creep models.…”

Preliminary simulations of planned experiments to study the impact of trace gases on the capacity of the Weyburn-Midale field to store carbon dioxide

“…Had the experiments run for a longer period of time, we would expect a much larger change in the porosity. In much longer experiments from 25 days to 6 months, Le Guen et al (2007) measured 2 to 3 % changes in porosity where verical and axial stress was independently controlled. The researchers used a slow flow rate, larger core, and CO 2 -rich and Ca-poor solutions.…”

“…Similarly Le Guen et al (2007) used x-ray micro-tomography and geochemistry to show that porosity significantly increases when reacted with pure CO 2 . While both of these studies nicely illustrate the relationship between reaction kinetics, thermodynamics, and porosity changes using x-ray micro-tomography, actual changes in a reservoir may be significantly lower because the input brines used in these studies were significantly dilute and below mineral carbonate saturation.…”