Hertel, Stefan A. (Shell International Exploration and Production Inc.) | Rydzy, Marisa (Shell International Exploration and Production Inc.) | Anger, Benjamin (Shell International Exploration and Production Inc.) | Berg, Steffen (Shell Global Solutions International B.V.) | Appel, Matthias (Shell International Exploration and Production Inc.) | de Jong, Hilko (Shell International Exploration and Production Inc.)
Digital rock technologies were developed to augment traditional core analysis and led to a much improved understanding of the microstructure of many rock core types. However, to produce an upscaled description of the reservoir, one must consolidate the measurements in scale over six orders of magnitude. Here, we show that a whole-core CT scan may serve as the natural link between the length scales of digital rocks and modern logging tools. While the CT scan contains a fingerprint of the structure of the reservoir, the digital rock models show the microscopic composition of each CT-scan voxel. For upscaling purposes, we established a quadratic correlation between the gray values in a CT scan and the porosities measured on core plugs. This correlation allowed us to generate a synthetic porosity log of millimeter resolution. After that, the length scale was increased using moving averages in the vertical direction. We investigated a thin-bed reservoir with layers of halite-filled sandstone alternating with layers free of halite at variable layer thicknesses. In this reservoir, the resulting synthetic porosity log compared well with the NMR log porosity within the uncertainty band over a total depth interval of 53.6 meters. We propose that field decisions could be accelerated if the quadratic correlation parameters can be generalized for these types of sediment. In this case, one may generate synthetic porosity logs as soon as the CT scan is available, which is typically the first step in standard core analysis.
Whether hydrate-bearing sediments are formed in nature or in the laboratory, the manner of gas hydrate generation will influence the hydrate habit and distribution, and consequently impact the bulk physical properties. In the last couple years we have begun to better understand the mechanical interactions between hydrate and sediment and how they are controlled by the hydrate formation method. We have developed a conceptual picture of hydrate formation in sediment and how it affects the elastic properties of the hydrated sample. The theory was based on a series of experiments conducted on laboratory-formed gas hydrate-bearing sediments. Ultrasonic velocities were measured in conjunction with MRI in hydrate-bearing Ottawa Sand F110 during hydrate formation and dissociation. P- and s-wave velocities were determined as a function of gas hydrate saturation. Hydrates were formed out of solution using Tetrahydrofuran (THF) and through methane injection into partially water-saturated samples. For the latter, samples with low and high initial water saturation (Swi) were tested. The recorded velocities exhibited a noticeable dependence on Swi. At low saturations (~20%) the hydrate in the sediment acted cementing and increased the ultrasonic velocities dramatically. The final velocities, however, decreased for increasing initial water saturations. Even small changes in the initial water saturation resulted in significant changes in velocities. At high initial water saturations (~80%), the velocity increased linearly with increasing hydrate content even at very low saturations. This behavior differed from the one observed for hydrate formed from out of solution. Ultrasonic velocities recorded during THF hydrate dissociation in sediment did not increase until a critical saturation of 30-35 percent was exceeded. Comparison with model data calculated using the effective medium theory indicated that hydrates formed from a free gas phase and low and high Swi act cementing and load-bearing, respectively. On the other hand, hydrates formed out of solution are pore-filling below and load-bearing above a critical hydrate saturation of about 35-40 percent. This was corroborated with micro X-ray computed tomography.