Shale wettability characteristics via air/brines and air/oil contact angles and influence of controlling factors: A case study of Lower Indus Basin, Pakistan

Wettability is the fundamental parameter that influences the productivity of hydrocarbon reservoirs. The knowledge of this regarding shale formation is yet inadequate; thus, detailed analysis is essential for successful development of such reservoirs. The Early Cretaceous Sembar formations in the Lower Indus Basin, Pakistan, is considered as the key target for energy exploration; however, it exhibits large uncertainties due to the lack of data availability. Sembar shales hold significant hydrocarbon volumes rich in organic content; however, prior to this, no comprehensive research has been conducted to quantify the wetting behavior of these shales. Thus, precise information about the wetting behavior of Sembar shale formations is essential, as it is influenced by many factors. Therefore, in this study, we examined the wettability of Sembar shale samples by performing a suit of contact angle (CA) measurements. The CA measurements on shale samples were performed using different salt types (NaCl, KCl, MgCl2, and Reef Salt) and concentrations of 0.1 M and 0.5 M under ambient pressures and varying temperatures (25 - 50 °C). The CA was measured via air-brine and air-oil under prevailing pressure and temperature conditions. Subsequently, the sample morphology and surface topography were examined via field emission scanning electron microscopy and atomic force microscopy, respectively. The mineral compositions were obtained via X-ray diffraction studies. The results clearly show that the Sembar shale possesses a mixed wetting behavior. Under dry surfaces, they have large affinity to oil and deionized water in which the droplet spreads quickly on the sample surfaces. Conversely, the samples aged with n-decane and NaCl brines exhibited higher CAs than the untreated samples. Additionally, the CA measured by changing temperatures led to an increase for all brine droplets; the CA further increased as the concentrations of salts increased from 0.1 to 0.5 M. We then discussed the possible reasons for the discrepancy in CA values due to temperature changes and brine concentrations. Moreover, the CA was measured corresponding to the surface roughness from which it appears that it merely affects the wettability of these shale samples. However, the present study results lead to an improved understanding of the wettability of Sembar shale of the Lower Indus Basin in Pakistan

Influence of rock wettability on THF hydrate saturation and distribution in sandstones

Natural gas trapped in hydrate deposits is a potentially enormous source of energy which can in principle be extracted from the underground reservoir structures. These reserves can potentially also catastrophically release very large quantities of greenhouse gases to the atmosphere. One key parameter which is well known to strongly influence fluid distribution, saturation, and production is rock wettability. However, the effect of wettability on gas hydrate in sediments has not been investigated yet. We thus used nuclear magnetic resonance (NMR) spectrometry to measure relaxation times (T2 and T1) and the corresponding surface relaxivity of tetrahydrofuran hydrate during the formation and dissociation in water- and oil-wet Bentheimer sandstones. We also measured the NMR porosities and hydrate saturations at different temperatures during hydrate formation/dissociation for both water-wet and oil-wet sandstones. Significantly higher hydrate saturation was observed in the water-wet sandstone (when compared to the oil-wet sandstone) at all stages of hydrate formation and dissociation. Furthermore, the T2 spectra moved from the lower relaxation domain (before hydrate formation) to the fast relaxation domain (after hydrate formation) in both water-wet and oil-wet sandstones. However, the water-wet sandstone generally had a T2 relaxation range due to the higher water affinity to the water-wet rock and the associated faster demagnetization of the water molecules. These results demonstrate that low-field NMR can be used to quantify the rock wettability and observe hydrate behavior in geologic sediments. This fundamental information thus aids in the development of gas extraction from hydrate reservoirs and the assessment of potential greenhouse gas emissions from such reservoirs into the atmosphere

Influence of diagenetic features on petrophysical properties of fine-grained rocks of Oligocene strata in the Lower Indus Basin, Pakistan

Nari Formation is considered as one of the most important oil and gas exploration targets. These fine-grained tight sandstone reservoirs face enormous challenges due to their extremely low matrix porosity and permeability. Hence, in this regard, the study was carried out to collect the high-quality data on petrophysical properties along with mineralogy and microstructural characteristics and diagenesis. The experiments performed includes the petrographic study and scanning electron microscopy, and X-ray diffraction analyses. Besides, the measurement of petrophysical properties was carried out to assess the likely influence of the reservoir quality. The petrographic analysis shows predominantly fine- to medium-grained grey samples along with calcite, clay, lithic fragments and iron oxides. Further, the thin-section observations revealed that the quartz is a principal mineral component in all the analysed samples ranging from 52.2 to 92.9%. The bulk volume of clay minerals that range from 5.3 to 16.1% of. The porosity and permeability measured range from 5.08 to 18.56% (average 7.22%) and from 0.0152 to 377 mD (average 0.25 mD), respectively. The main diagenetic processes that affected the sandstones of Nari Formation are mechanical compaction, grain deformation, cementation and quartz dissolution and have played a significant role in influencing the quality of the reservoir rock. Overall, it appears that the primary petrophysical properties (porosity and permeability) were decreased due to the mechanical compaction, lithification, cementation, and framework grain dissolution. Based on the integrated mineralogical, microstructural analysis, and the laboratory-based petrophysical properties, the samples exhibited poor porosity, permeability, and moderate clay content, which indicate that the Nari Formation is a poor quality reservoir

Gas hydrate characterization in sediments via x-ray microcomputed tomography

Natural gas hydrates (NGHs) are efficient and promising energy resources because of their high energy density. In addition, NGH occurs in sediments under certain pressure and temperature conditions and has the potential to meet the increasing global energy demand. However, efficient exploitation of NGH requires a precise characterization and understanding of the hydrate formation, accumulation, and dissociation mechanisms. In this context, the microstructural characterization of gas hydrate is essential and requires specialized methods and equipment. While traditional imaging and characterization tools offer fundamental microstructural analysis, x-ray microcomputed tomography (μCT) has gained recent attention in producing high-resolution three-dimensional images of the pore structure and habits of hydrate-bearing sediments and providing the spatial distribution and morphology of gas hydrate. Further, μCT offers the direct visualization of the hydrate structure and growth habits at a high resolution ranging from the macro- to micro-metric scale; therefore, it is extensively used in NGH characterization. This review summarizes the theoretical basis of μCT imaging spanning the setup of the experimental apparatus and visualization techniques. The applications of μCT in NGH reservoir characterization, such as hydrate types and their constituents, physical and chemical properties, occurrence, and accumulation, are presented. Hydrate characterization using μCT imaging is explicitly discussed, including a general understanding of hydrate pore-habit prediction, saturation and percolation behavior, seepage and permeability, and the influence of hydrate saturation on the mechanical properties of hydrate-bearing sediments. Last, conclusions and recommendations for future research are provided. This review offers a reference for understanding the application of μCT to evaluate gas hydrates, which contributes to exploiting these energy resources