Advanced NMR methodologies in rock core analysis

<p>¹H NMR techniques have gained extensive acceptance in petrophysics for the evaluation of fluid-saturating reservoir rocks. This thesis presents the development of new NMR methods regarding the reserves (determination of pore length scales and surface relaxivities), productivity (estimates of permeability) and recovery of fluids (resolves of saturation evolution) in rocks. Traditionally, pore lengths are evaluated from the ground relaxation eigenmodes of spin-bearing molecules in pore space. This evaluation is not straightforward since it is affected by surface relaxivity. Here, we use an approach to determine pore length from detecting the high relaxation eigenmodes, in which way the eigenvalue spectrum directly scales to the pore size distribution. Based on this, we extend this approach for the use with low-field NMR spectrometers and 2D NMR eigenmode correlation methods. Surface relaxivity can be further extracted from these 2D correlation maps, which is in agreement with an independent NMR measurement. Permeability is generally estimated from surface relaxation via empirical pore-network models. However, for heterogeneous rocks a single (or averaged) permeability value may not be adequate. Therefore, we measure surface relaxation in conjunction with MRI techniques. Permeability profiles can then be obtained from spatially resolved relaxation maps yielding local connectedness between adjacent slices. The results are confirmed by the comparison of brine-permeability measurements. MRI experiments of fluids in rocks at reservoir-like conditions may yield optimized recovery strategies of reservoir fluids. In this context we combine MRI with diffusion-relaxation correlation measurements during flooding intervals. The results provide substantial information, such as flooding front and saturation profiles of immiscible fluids discriminated by fluid type.</p>

Advanced NMR methodologies in rock core analysis

<p>¹H NMR techniques have gained extensive acceptance in petrophysics for the evaluation of fluid-saturating reservoir rocks. This thesis presents the development of new NMR methods regarding the reserves (determination of pore length scales and surface relaxivities), productivity (estimates of permeability) and recovery of fluids (resolves of saturation evolution) in rocks. Traditionally, pore lengths are evaluated from the ground relaxation eigenmodes of spin-bearing molecules in pore space. This evaluation is not straightforward since it is affected by surface relaxivity. Here, we use an approach to determine pore length from detecting the high relaxation eigenmodes, in which way the eigenvalue spectrum directly scales to the pore size distribution. Based on this, we extend this approach for the use with low-field NMR spectrometers and 2D NMR eigenmode correlation methods. Surface relaxivity can be further extracted from these 2D correlation maps, which is in agreement with an independent NMR measurement. Permeability is generally estimated from surface relaxation via empirical pore-network models. However, for heterogeneous rocks a single (or averaged) permeability value may not be adequate. Therefore, we measure surface relaxation in conjunction with MRI techniques. Permeability profiles can then be obtained from spatially resolved relaxation maps yielding local connectedness between adjacent slices. The results are confirmed by the comparison of brine-permeability measurements. MRI experiments of fluids in rocks at reservoir-like conditions may yield optimized recovery strategies of reservoir fluids. In this context we combine MRI with diffusion-relaxation correlation measurements during flooding intervals. The results provide substantial information, such as flooding front and saturation profiles of immiscible fluids discriminated by fluid type.</p>

Thermal Diffusion Characteristics in Permafrost during the Exploitation of Gas Hydrate

Methane hydrate is the vast potential resources of natural gas in the permafrost and marine areas. Due to the occurrence of phase transition, the gas hydrate is dissociated into gas and water and absorbs lots of heat. The incomprehensive knowledge of endothermic reaction in permafrost sediments still restricted the production efficiency of hydrate commercial development. This endothermic reaction leads to a complex thermal diffusion in permafrost, which directly influences the phase transition in turn. In this research, the heat during the exploitation is transferred in two forms (specific heat and latent heat). Besides, the melting point is not constant but depends on the pore size of the reservoir rock. According to these features, a thermal diffusion model with phase transition is established. To calculate the governing equation, the pore size distribution is obtained by using the nuclear magnetic resonance (NMR) method. The heating tests are conducted and simulated to calibrate the coefficient (i.e., transverse surface relaxivity) of NMR. Then, the temperature field evolution of the hydrate reservoir during the exploitation is simulated by using the calibrated values. The results show that the temperature curves have a typical plateau related to the pore size distribution, which is effective to obtain the surface relaxivity. The heat transfer is remarkably limited by the endothermic effect of the phase transition. The hydrate recovery efficiency may depend largely on the heating capacity of the engineering operation and the rate of gas production. Compared to the conventional petroleum industry, it is significant to control the maximum temperature and temperature distribution in engineering operations during hydrate development. This research on the temperature behavior during onshore permafrost hydrate production could provide the theoretical support to control heat behavior of offshore hydrate production.

Estimating the NMR surface relaxivity of Eocene sandstones: a comparison of different approaches

Nuclear magnetic resonance (NMR) relaxometry typically involves the analysis of a relaxation time distribution. The surface relaxivity ( ρ) is the key parameter that relates the relaxation time to the pore radius. Only a good estimate of the surface relaxivity enables a reliable determination of the pore radius distribution in a rock or sediment sample. A wide variety of approaches for the estimation of ρ has been proposed; however, the accuracy of ρ determination approaches has rarely been checked. We have compared different approaches of ρ determination for a set of Eocene sandstone samples. Most approaches based on a weighted logarithmic mean of relaxation times or the peak relaxation time result in significant underestimation of ρ. However, the correct weighting of the set of relaxation times proved to be a crucial approach in ρ determination. The consequent application of geometric rules suggests the application of the weighted harmonic mean ( Thm). The specific surface area per unit pore volume ( Spor), which results from the gas adsorption method, is another crucial parameter in most approaches for ρ estimation. The quantities Thm and Spor depend on the resolution of the used method. Applying the fractal theory, we adopt an approach that performs an upscaling of Spor to the resolution of the NMR relaxometry. Using equal resolution for Thm and Spor, we obtain more reliable ρ estimates. The resulting ρ values are comparable with the ones determined by using the median relaxation time from NMR and the median pore-throat radius from the mercury injection capillary pressure method.