Improved water saturation estimation in shaly sandstone through variable cementation factor

Estimation of water saturation, Sw, in shaly sandstone is an intricate process. The surface conduction of clay minerals adds up to the electrolyte conduction in the pore spaces, thus generating high formation conductivity that overshadows the hydrocarbon effect. In each resistivity-based water saturation model, the key parameter is formation factor, F, which is typically derived from Archie’s Law. Referring to a log–log plot between formation factor and porosity, cementation factor reflects the slope of the straight line abiding Archie’s Law. In the case of shaly sandstone, derivation based on Archie’s Law in combination with Waxman–Smits equation leads to higher cementation factor, m*. In the shaly parts of the reservoir, high m* is counterbalanced by clay conductivity. Nonetheless, high m* used in clean parts increases Sw estimation. In this study, the variable cementation factor equation is introduced into the standard correlation of Sw versus Resistivity Index, RI, to develop a water saturation model with shaly sandstone parameters. Data retrieved from two fields that yielded mean arctangent absolute percentage error (MAAPE) were analysed to determine the difference between calculated and measured data within the 0.01–0.15 range for variable cementation factor method. The conventional method yielded maximum MAAPE at 0.46.

Fractal Characterization and Petrophysical Analysis of 3D Dynamic Digital Rocks of Sandstone

It is a crucial issue to comprehensively study the relations between microstructure and seepage capacity of porous media. Several physical-based parameters of fractal geometry can analyze the pore structure of rocks, while permeability and electrical conductivity are used to study seepage capacity. In this paper, we first created 3D dynamic digital models of nine different sandstones with varying clay content, cements, and intragranular pores in feldspar. These nine models were divided into three groups. Then, fractal dimension, lacunarity, and succolarity, permeability, and electrical properties of the models were calculated, and their relationships were investigated. We used fractal parameters to interpret the correlation between fluid flow and pore structure as one of the main petrophysical properties of a rock. Results showed that the coefficient of determination for cementation exponent m and fractal dimension is 0.869, while between m and porosity, and succolarity, it is 0.784 and 0.781, respectively. This indicates that the fractal dimension and cementation exponent describe the complexity of pores. The coefficient of determination between permeability and succolarity is 0.975, which is higher than that between permeability and the fractal dimension or porosity. The coefficient of determination between formation factor and succolarity is 0.957, which is higher than that between formation factor and the fractal dimension or porosity. Overall, a stronger relationship between petrophysical parameters, permeability in particular, and succolarity allows this lesser-used fractal parameter to be a good measure for characterizing the connectivity of pore space and pore network.

Rock Typing Based on Wetting-Phase Relative Permeability Data and Critical Pore Sizes

Summary Rock typing based on mineralogical, hydraulic, or petrophysical similarities is important to reservoir characterization and simulation. In the literature, classifying rocks using single-phase data has been widely studied. Most methods use porosity and permeability measurements to identify rocks with similar characteristic pore sizes. In this study, we invoke concepts from critical-path analysis (CPA) and propose a new rock-typing method on the basis of two-phase flow data, such as water relative permeability krw. We classify rocks based on their similarities in the critical pore radius rc at the same effective water saturation Se. For this purpose, we first convert the Sw−krw plots to Se−rc curves and then apply a curve clustering method to identify similar rocks. To evaluate the proposed approach, we simulated flow in pore networks with many different pore-scale properties. By varying the pore-throat size distribution, contact angle, pore coordination number, pore-shape distribution, and clay content, we generated a wide range of pore networks. Overall, two-phase flow in 240 pore networks were simulated. In addition to synthetic pore networks, pore networks were generated based on properties of Berea, Mt. Simon, and Fontainebleau sandstones. By analyzing the single-phase simulation results, we identified 8 and 15 rock types using the porosity-formation factor and reciprocal formation factor-permeability data, respectively. However, using the two-phase data, we detect 12 rock groups.

Pathogenic variants of meiotic double strand break (DSB) formation genes PRDM9 and ANKRD31 in premature ovarian insufficiency

Purpose The etiology of premature ovarian insufficiency (POI) is heterogeneous, and genetic factors account for 20–25% of the patients. The primordial follicle pool is determined by the meiosis process, which is initiated by programmed DNA double strand breaks (DSB) and homologous recombination. The objective of the study is to explore the role of DSB formation genes in POI pathogenesis. Methods Variants in DSB formation genes were analyzed from a database of exome sequencing in 1,030 patients with POI. The pathogenic effects of the potentially causative variants were verified by further functional studies. Results Three pathogenic heterozygous variants in PRDM9 and two in ANKRD31 were identified in seven patients. Functional studies showed the variants in PRDM9 impaired its methyltransferase activity, and the ANKRD31 variations disturbed its interaction with another DSB formation factor REC114 by haploinsufficiency effect, indicating the pathogenic effects of the two genes on ovarian function were dosage dependent. Conclusion Our study identified pathogenic variants of PRDM9 and ANKRD31 in POI patients, shedding new light on the contribution of meiotic DSB formation genes in ovarian development, further expanding the genetic architecture of POI.

Imaging the Structure and the Saltwater Intrusion Extent of the Luy River Coastal Aquifer (Binh Thuan, Vietnam) Using Electrical Resistivity Tomography

With the growing population and the adverse effects of climate change, the pressure on coastal aquifers is increasing, leading to a larger risk of saltwater intrusion (SI). SI is often complex and difficult to characterize from well data only. In this context, electrical resistivity tomography (ERT) can provide high-resolution qualitative information on the lateral and vertical distribution of salinity. However, the quantitative interpretation of ERT remains difficult because of the uncertainty of petrophysical relationships, the limitations of inversion, and the heterogeneity of aquifers. In this contribution, we propose a methodology for the semiquantitative interpretation of ERT when colocated well data are not available. We first use existing wells to identify freshwater zones and characterize the resistivity response of clayey deposits. Then, we approximate the formation factor from water samples collected in the vicinity of ERT data to derive a resistivity threshold to interpret the saline boundary. We applied the methodology in the shallow aquifers of the Luy River in the Binh Thuan province, Vietnam, where water resources are under pressure due to agricultural, aquacultural, and industrial production. Twenty-one ERT profiles were collected and revealed a much larger intrusion zone, compared to the previous study. Saltwater is present in lowland areas of the left bank over almost the whole thickness of the aquifer, while the right bank is constituted of sand dunes that are filled with freshwater. At a larger distance from the sea, a complex distribution between fresh and saltwater is observed. Our methodology could be applied to other heterogeneous aquifers in the absence of a dense monitoring network.