How stress and temperature conditions affect rock-fluid chemistry and mechanical deformation

We report the results from a series of chalk flow-through-compaction experiments performed at three effective stresses (0.5, 3.5, and 12.3MPa) 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 0.7MPa for all experiments with a continuous flow of 0.219M MgCl2 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 Mg2+, while the Ca2+ 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 Mg2+ and Ca2+ concentration changes. The observed strain can be partitioned additively into a mechanical and chemical driven component.publishedVersio

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

This is an author accepted manuscript of an article from the journal Geophysics, published by the Society of Exploration Geophysicists (SEG). Reuse is subject to SEG terms of use and conditions.Which forces are responsible for holding highly porous chalks together? We use the effective stress to quantify the electrostatic effects around particle contacts originating from the adsorption of ions onto charged mineral surfaces. The induration of chalk indicates that it is held together by contact cement,where planar crystal contacts allowthe action of short-ranged adhesive Van der Waals forces. At particle distances exceeding a few nanometers, recent studies have indicated electrostatic repulsion between water embedded adjacent particles. The magnitude of the repelling force depends, among other parameters, upon temperature and brine composition. Our premise is that by perturbing the electrostatic forces at the particle level, we can control themechanical behavior of chalk samples tested in triaxial cells.We report the results of an experimental series, investigating howthemechanical strength and stiffness varied among samples saturated with four different brines, tested at two temperatures, and tested directly or after aging for three weeks at high temperature. We associate stiffness with bulk modulus and strengthwith the stress at yield. Systematic softening and weakening is observed, especially when the pore fluid is sulfate bearing, as well as for some high-temperature experiments and for aged samples. However, softening and weakening are not totally correlated, and neither brine composition, temperature, nor aging can alone dictate the mechanical behavior of the chalk — a combination is required to predict the chalk stiffness and strength. To obtain a coherent description of our experimental results, we estimated the electrostatic stress arising from ion adsorption and found it unnecessary for these experiments to postulate significant dissolution or precipitation-related changes to the rock frame.acceptedVersio

Mineralogical alterations in calcite powder flooded with MgCl2 to study Enhanced Oil Recovery (EOR) mechanisms at pore scale

Seawater injection into chalk-reservoirs on the Norwegian Continental Shelf has increased the oil recovery and reduced seabed subsidence, but not eliminated it. Therefore, understanding rock–fluid interactions is paramount to optimize water injection, predict and control water-induced compaction. Laboratory experiments on onshore and reservoir chalks have shown the need to simplify the aqueous chemistry of the brine, and also the importance of studying the effect of primary mineralogy of chalk to understand which ions interact with the minerals present. In this study, the mineralogy of the samples tested, are simplified. These experiments are carried out on pure calcite powder (99.95%), compressed to cylinders, flooded with MgCl2, at 130 °C and 0.5 MPa effective stress, for 27 and 289 days. The tested material was analysed by scanning and transmission electron microscopy, along with whole-rock geochemistry. The results show dissolution of calcite followed by precipitation of magnesite. The occurrence and shape of new-grown crystals depend on flooding time and distance from the flooding inlet of the cylinder. Crystals vary in shape and size, from a few nanometres up to 2 μm after 27 days, and to over 10 μm after 289 days of flooding and may crystallize as a single grain or in clusters. The population and distribution of new-grown minerals are found to be controlled by nucleation- and growth-rates along with advection of the injected fluid through the cores. Our findings are compared with in-house experiments on chalks, and allow for insight of where, when, and how crystals preferentially grow

A laboratory scale approach to wettability restoration in chalk core samples

Wettability in chalk has been studied comprehensively to understand fluid flow mechanisms impacting coreflooding experiments. Wettability becomes paramount in understanding the parameters influencing chalk-fluid interactions. The main objective of this work is to evaluate as to which degree the wettability in chalk core samples can be controlled in the laboratory. Kansas chalk samples saturated with brine (1.1 M/64284 ppm NaCl) and an oil mixture (60% - 40% by volume of Heidrun oil and heptane) were aged at a constant temperature of 90oC with aging time as the laboratory control variable. A multimodal method incorporating contact angle measurements, wettability index via USBM test, and SEM-MLA analysis was applied in evaluating wettability. A systematic approach was applied with the three different methods to quantify the degree of uncertainty linked to a) wettability estimation and b) the aging procedure to control wettability alteration of Kansas chalk. With a comprehensive suite of samples, we were successfully able to alter the wettability of chalk cores

A laboratory scale approach to wettability restoration in chalk core samples

Wettability in chalk has been studied comprehensively to understand fluid flow mechanisms impacting coreflooding experiments. Wettability becomes paramount in understanding the parameters influencing chalk-fluid interactions. The main objective of this work is to evaluate as to which degree the wettability in chalk core samples can be controlled in the laboratory. Kansas chalk samples saturated with brine (1.1 M/64284 ppm NaCl) and an oil mixture (60% - 40% by volume of Heidrun oil and heptane) were aged at a constant temperature of 90oC with aging time as the laboratory control variable. A multimodal method incorporating contact angle measurements, wettability index via USBM test, and SEM-MLA analysis was applied in evaluating wettability. A systematic approach was applied with the three different methods to quantify the degree of uncertainty linked to a) wettability estimation and b) the aging procedure to control wettability alteration of Kansas chalk. With a comprehensive suite of samples, we were successfully able to alter the wettability of chalk cores

Temperature effects on rock engineering properties and rock-fluid chemistry in opal-CT-bearing chalk

In this study, eight tri-axial tests on Cretaceous age outcrop chalk from Aalborg have been performed systematically by injecting MgCl2 for the first time at different temperatures (25, 60, 92, 110 and 130 °C) and for comparison, NaCl at 130 °C. Whole-rock geochemistry, stable isotope measurements, pycnometry, Field Emission Gun Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy, X-Ray Diffraction (XRD) and measurements of Specific Surface Area (Brunauer-Emmett-Teller theory (N2)) were applied to analyse unflooded and flooded cores. Based on analyses of changes in brine composition, mineralogy, specific surface area, solid density, porosity and permeability some conclusions can be drawn on temperature effects on rock engineering properties and rock-fluid chemistry. The MgCl2 flooded cores show systematically higher creep rates at higher temperature and the cores tested at 25 and 60 °C show similar creep rates as the two NaCl flooded cores at 130 °C. All fluid-rock interactions were more pronounced at higher temperature. After flooding with MgCl2 at 110 and 130 °C newly formed magnesite is observed. In the cores tested at 25, 60 and 92 °C magnesite crystals have not been positively identified, but minute increases in MgO in whole-rock geochemistry analyses are seen. Si4+ originating from the dissolution of silica bearing phases (mainly diagenetic opal-CT), has taken part in the re-precipitation of Si-Mg-bearing minerals during MgCl2 injection from 25 to 130 °C, leading to an increase of the specific surface area. This is partly balanced by opal-CT dissolution, which should lower the specific surface area. The flaky Si-Mg-bearing minerals, covering significant portions of the pore surfaces and throats are the main drivers to reduce permeability in cores flooded at high temperatures. Additionally, in NaCl flooded cores where mineralogical changes are minute, the dissolution of parts of the existing opal-CT has lowered the SSA. At high temperatures, the following chemical changes must be carefully acknowledged when porosity reduction is calculated: calcite and opal-CT dissolution and precipitation of new minerals, particularly Mg-bearing minerals. The presence of opal-CT strongly influences the mineralogical alterations in flooded cores, hence the geo-mechanical evolution.Fil: Minde, Mona W.. University of Stavanger; NoruegaFil: Wang, Wenxia. University of Stavanger; NoruegaFil: Madland, Merete V.. University of Stavanger; NoruegaFil: Zimmermann, Udo. University of Stavanger; NoruegaFil: Korsnes, Reidar I.. University of Stavanger; NoruegaFil: Bertolino, Silvana Raquel Alina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; ArgentinaFil: Andersen, Pål Ø.. University of Stavanger; Norueg