Investigating the wettability effect on gas hydrate bearing sediments
Ghazanfer Raza Abbasi
Date of Award
Edith Cowan University
Doctor of Philosophy
School of Engineering
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. Natural Gas Hydrate (NGH) occurs in sediments under certain pressure and temperature conditions and has the potential to meet the increasing global energy demand. However, an efficient exploitation of NGH requires a precise characterization and understanding of the hydrate formation, accumulation and dissociation mechanisms. In this work, a comprehensive study has been conducted to augment the understanding of NGH and their wettability behaviour in hydrate bearing sediments. Systematic experiments were conducted to address this by developing various approaches presented in this work. The approaches adopted to achieve the study objectives were: i) review of previous research on NGH, ii) the effect of wettability on gas hydrate in sediment, iii) the influence of rock wettability on the electrical resistivity of hydrate bearing sediments (HBS), (iv) wettability effect on measured velocity properties of HBS.
In this context, the micro-structural characterization of gas hydrate is essential and requires the use of specialized methods and equipment. While traditional imaging and characterization tools offer fundamental microstructure analysis, X-ray micro-computed tomography (μCT) has gained recent attention to produce high resolution three dimensional (3D) images of pore structure and pore habits of hydrate-bearing sediments and provide spatial distribution and morphology of gas hydrate. Micro-CT (μCT) offers direct visualization of hydrate structure and growth habits at high resolution ranging from macro-metric to micro-metric scale, therefore, it is extensively used in natural gas hydrate characterization. This review, therefore, summarizes the theoretical basis of μCT imaging spanning the setup of the experimental apparatus and visualization techniques. The applications of μCT in natural gas hydrate reservoirs characterization – hydrate types and constituent, physical and chemical properties of gas hydrates as well as their occurrence and accumulation are presented. Hydrate characterization using μCT imaging including the general understanding of hydrate pore habit prediction, hydrate saturation and percolation behaviour, hydrate seepage and permeability and the influence of hydrate saturation on the mechanical properties of hydrate-bearing sediments (HBSs) are explicitly discussed. Lastly, conclusions and recommendations for future research are provided. Thus, this review offers a reference for understanding in the application of micro-CT to evaluate gas hydrates – which in turn contributes towards the exploitation of these energy resources.
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 sediment 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 (THF) hydrate during formation and dissociation in water- and oil-wet Bentheimer sandstone. We also measured the NMR porosities and hydrate saturations at different temperatures during hydrate formation/dissociation for both water-wet and oil-wet sandstone. Significantly higher hydrate saturation was observed in water-wet sandstone (when compared to 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 sandstone. However, water-wet sandstone generally had a T₂ relaxation range due to the higher water affinity to 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 behaviour 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.
Classically, rock wettability is one of the key factors in predicting fluid flow behavior, fluid distribution, reserves and productivities. However, the effect of wettability on electric resistivity of hydrate formations is only poorly understood. Thus, to evaluate the influence of rock wettability on the electrical resistivity (note that resistivity logging is a key well logging tool) of hydrate-bearing sandstone, Nuclear Magnetic Resonance experiments were conducted. Clearly, the effective porosity and liquid saturation increased with increasing temperature, due to hydrate dissociation. Furthermore, resistivity index, rock resistivity (Rt), and formation factor all decreased with increasing liquid saturation, and the formation factor demonstrated a positive correlation with hydrate saturation, though formation factor for oil-wet (OW) sandstone was higher than that of the water-wet (WW) sandstone. This work will thus significantly improve the fundamental understanding of the petrophysical properties of gas hydrate reservoirs, so that energy production can be optimized, geo-hazards can be avoided, and the hydrate gun hypothesis can be better assessed.
Furthermore, the effect of wettability on velocity properties of hydrate formations is poorly understood. Thus, to evaluate the key information about wettability effect on measured velocity properties of hydrate bearing sandstone, we conducted several experiments followed by NMR measurements. The P-wave velocities of water wet, and oil wet sandstones were obtained before and after hydrate formation. Our results demonstrate that the percentage of velocity in water wet sample is higher than oil wet sample which indicates high hydrate saturation in water wet sandstone. This is also confirmed by the NMR results which also showed that the hydrate saturation in water wet sandstone is higher than that in oil wet sandstone. This study can be used to quantify the hydrate occurrence in different wetting conditions. This study provides insights on acoustic velocity measurements for hydrate formation in the gas production pipelines.
Access to this thesis has been embargoed until 18th April 2028.
Abbasi, G. R. (2023). Investigating the wettability effect on gas hydrate bearing sediments. Edith Cowan University. http://dx.doi.org/10.25958/z7sv-ye32