Experimental Verification of CO2 Dissolution Rate Due to Diffusion Induced Convection

An experimental setup has been designed for measuring the dissolution rate of buoyant CO2 into the water phase below. Experiments were performed in a high-pressure cell, where the water phase was stabilized by a porous medium to mimic the situation of a gas cap in a storage reservoir. As many previous tests have been performed in 2D cells, this setup allows for 3D measurements of diffusion-induced convection. The tests are performed at high pressure where CO2 is at high density, similar to a real storage situation and the dissolved CO2 is measured by metering the pump that is automatically maintaining constant pressure. This allows rate measurements in a 3D environment. The basic interest was to determine the dissolution rate in the convective regime, but also the diffusion coefficient of CO2 in water was determined by this experimental setup. In addition, the onset time of convection was estimated. The result show that the dissolution rate measured during convection was one to two order of magnitude faster than predicted by semi-empirical correlations obtained by numerical simulations. The estimated onset time of convection was shorter than theoretical prediction. The overall results suggest that diffusion induced convection plays a more important role than previously assumed

A review on the provenance of the Voltaian Basin, Ghana: Implications for hydrocarbon prospectivity

The Voltaian Basin of Ghana has taken significance as a potential oil and gas basin. Since Ghana became a petroleum-producing country, every attempt is being made to understand the sedimentary basins within the country's territorial boundaries, particularly, the Voltaian Basin. This review paper investigates studies on the Voltaian Basin drawing on a comprehensive literature review based on database searches from Scopus, Web of Science, and Science Direct, among others. Several branches of geology such as mineralogy, geochemistry, igneous and metamorphic petrology, geochronology, and sedimentary geology among others were integrated to reconstruct the source of the Voltaian Basin sedimentary rocks. Application of compositional analyses to determine provenance using petrographic, mineralogical, and geochemical techniques are also discussed. Results based on articles retrieved from the comprehensive literature review summarize the findings on provenance studies; stating the sediment source and history of the rock types in the Voltaian Basin to be most likely from the Pan-African orogenic rocks having a felsic source with some inputs from metasedimentary source rocks. Findings from provenance studies further point to the depositional environment being of shallow marine source and having a fluvial to deltaic environmental features, suggesting that the depositional environment is suitable for hydrocarbon source rocks as well as reservoir rocks formation. With regards to petroleum exploration, a lot of research work needs to be done to identify the type of sedimentary organic matter present in the shales and the limestones of the Voltaian Basin. Furthermore, the rock properties that define petroleum reservoirs such as porosity, permeability, pore type, and rock compressibility as well as electrical properties of some important sandstones need to be thoroughly investigated

Velocity-saturation relation in partially saturated rocks: Modelling the effect of injection rate changes

The relationship between P-wave velocity and fluid saturation in a porous medium is of importance for reservoir rock characterization. Forced imbibition experiments in the laboratory reveal rather complicated velocity–saturation relations, including rollover-like patterns induced by injection rate changes. Poroelasticity theory-based patchy saturation models using a constant fluid patch size are not able to describe these velocity–saturation relations. Therefore, we incorporate a saturation-dependent patch size function into two models for patchy saturation. This recipe allows us to model observed velocity–saturation relations obtained for different and variable injection rates. The results reveal an increase in patch size with fluid saturation and show a reduction in the patch size for decreasing injection rate. This indicates that there can exist a distinct relation between patch size and injection rate. We assess the relative importance of capillarity on velocity–saturation relations and find that capillarity stiffening impairs the impact of patch size changes. Capillarity stiffening appears to be a plausible explanation when a decrease in the injection rate is expected to boost the importance of capillarity

Forced imbibition into a limestone: measuring P-wave velocity and water saturation dependence on injection rate

Quantitative interpretation of time-lapse seismic data requires knowledge of the relationship between elastic wave velocities and fluid saturation. This relationship is not unique but depends on the spatial distribution of the fluid in the pore-space of the rock. In turn, the fluid distribution depends on the injection rate. To study this dependency, forced imbibition experiments with variable injection rates have been performed on an air-dry limestone sample. Water was injected into a cylindrical sample and was monitored by X-Ray Computed Tomography and ultrasonic time-of-flight measurements across the sample. The measurements show that the P-wave velocity decreases well before the saturation front approaches the ultrasonic raypath. This decrease is followed by an increase as the saturation front crosses the raypath. The observed patterns of the acoustic response and water saturation as functions of the injection rate are consistent with previous observations on sandstone. The results confirm that the injection rate has significant influence on fluid distribution and the corresponding acoustic response. The complexity of the acoustic response - that is not monotonic with changes in saturation, and which at the same saturation varies between hydrostatic conditions and states of dynamic fluid flow – may have implications for the interpretation of time-lapse seismic responses