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

Influence of temperature cycling and pore fluid on tensile strength of chalk

Calcite has a highly anisotropic thermal expansion coefficient, and repeated heating and cooling cycles can potentially destabilize chalks by breaking cement bonds between neighboring particles. Based on tensile strength measurements, we investigated how temperature cycles induce weakening of chalk. Tensile strength tests were performed on chalk specimens sampled from Kansas (USA) and Mons (Belgium), each with differing amounts of contact cement. Samples of the two chalk types were tested in dry and water-saturated states, and then exposed to 0, 15, and 30 temperature cycles in order to find out under what circumstances thermally induced tensile strength reduction occurs. The testing results show that the dry samples were not influenced by temperature cycling in either of the chalk types. However, in the water-saturated state, tensile strength is increasingly reduced with progressive numbers of temperature cycles for both chalk samples, especially for the more cemented Kansas chalk. The Kansas chalk demonstrated higher initial tensile strength compared to the less cemented Mons chalk, but the strength of both chalks was reduced by the same relative proportion when undergoing thermal cycles in the water-saturated state. Keywords: Tensile strength, Weakening by heating and cooling cycles, Anisotropic thermal expansio

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

Beta cell function, hepatic insulin clearance, and insulin sensitivity in South Asian and Nordic women after gestational diabetes mellitus

South Asians have higher risk of type 2 diabetes after gestational diabetes mellitus (GDM) than Nordic women; however the mechanisms behind this difference remain unclear. We investigated insulin sensitivity, beta cell function, and hepatic insulin clearance, in 179 South Asian and 108 Nordic women ∼17 months after GDM (mean age 35.3 years and BMI 29.1 kg/m2), via an oral glucose tolerance test using deconvolution of C-peptide kinetics. 31% of South Asian and 53% of Nordic participants were normoglycemic at the time of measurement. South Asian women had higher area under the curve (AUC) for glucose, pre-hepatic insulin, peripheral insulin, and lower levels of insulin sensitivity, disposition index, and fasting hepatic insulin clearance compared with Nordic women. In the group with prediabetes or diabetes, South Asian women displayed similar AUC for glucose and pre-hepatic insulin, but higher AUC for peripheral insulin, and lower levels of disposition index, and fasting hepatic insulin clearance compared with Nordic women. The waist-to-height ratio mediated ∼25-40% of the ethnic differences in insulin sensitivity in normoglycemic women. Overall, our novel data showed that normoglycemic South Asian women after GDM displayed lower insulin secretion for a given insulin resistance, and lower hepatic insulin clearance compared with Nordic women. South Asian women are at high risk of developing type 2 diabetes after GDM, and preventive efforts should be prioritized

High prevalence and significant ethnic differences in actionable HbA 1C after gestational diabetes mellitus in women living in Norway

Background: The type 2 diabetes risk after gestational diabetes mellitus (GDM) is twice as high in South Asian compared to European women. Current guidelines differ regarding which test to use as a screening-tool post-GDM. We aimed to identify ethnic differences in the prevalence rates and early predictors for actionable HbA1c (defined as prediabetes and diabetes) short time after GDM. Methods: This cross-sectional study, enrolling South Asian and Nordic women 1–3 years after a diagnosis of GDM, was undertaken at three hospitals in Norway. We performed a clinical and laboratory evaluation including an oral glucose tolerance test (OGTT). Medical records were used to retrieve data during pregnancy. Prediabetes was classified with HbA1c alone or combined with OGTT glucose measurements according to the WHO, WHO-IEC, and ADA criteria (fasting plasma glucose (FPG) 6.1–6.9 mmol/L, FPG 6.1–6.9 mmol/L and/or HbA1c 42-47 mmol/mol (6.0-6.4%), and FPG 5.6–6.9 mmol/L and/or HbA1c 39-47 mmol/mol (5.7-6.4%)). Ethnic differences in prevalence and predictors of glucose deterioration were assed by χ2 (Pearson) tests and logistic regression models. Results: We included 163 South Asian and 108 Nordic women. Actionable HbA1c levels were highly prevalent and more so among South Asian than Nordic women (WHO-IEC-HbA1c: 25.8% vs. 6.5% (p ≤ 0.001), ADA-HbA1c: 58.3% vs. 22.2% (p ≤ 0.001)). Although adding OGTT-data gave higher combined prevalence rates of prediabetes and diabetes (WHO: 65.6% vs. 47.2% (p ≤ 0.05), WHO-IEC: 70.6% vs. 47.2% (p ≤ 0.001), ADA: 87.8% vs. 65.7% (p ≤ 0.001)), the excess risk in the South Asian women was best captured by the HbA1c. Important predictors for glucose deterioration after GDM were: South Asian ethnicity, GDM before the index pregnancy, use of glucose-lowering drugs in pregnancy, higher age, and higher in-pregnancy fasting glucose levels. Conclusions: In women with GDM 1–3 year previously, we found high prevalence and significant ethnic differences in actionable ADA-HbA1c levels, with South Asian ethnicity, GDM before the index pregnancy, and the use of glucose-lowering drugs in pregnancy as the most important risk factors. This study reinforces the importance of annual screening—preferably with HbA1c measurements—to facilitate early intervention after GDM

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