Competitive Tendering In The Netherlands: Central Planning Or Functional Specifications?

Institute of Transport and Logistics Studies. Faculty of Economics and Business. The University of Sydne

SPWLA 57th Annual Logging Symposium

The conventional interpretation of nuclear magnetic resonance (NMR) measurements on fluid saturated reservoir rocks assumes that the T2 distribution and the pore size distribution are directly related. However, this relationship breaks down in many multi-scale porosity systems due to diffusion coupling between different pores. This case is more common in unconventional and carbonate rocks both because of smaller pore sizes and consequently shorter distances to be traveled by moving spins, in addition to significantly higher diffusion coefficients present in the case of gas reservoirs. A particular case not generally considered is that of inter-bed rather than intra- or inter-pore diffusion coupling. In such cases, traditional methods to estimate permeability and movable fluid fractions like the T2,cutoff method may give erroneous results.This paper aims to illustrate through simulations and experiments the impact of inter-bed diffusion coupling on NMR responses of laminated porous media exhibiting different mean pore sizes. Two 3D structures have been modeled based on a Boolean particle process, providing a range of structural to diffusion length ratios to explore the relationship between pore geometry, surface magnetic properties, and NMR transverse relaxation time. Moreover, synthetic laminated systems of two different grain diameters with varied layer thickness were made and high-resolution micro-CT images obtained. Low field NMR experiments were carried out at 100% water saturation.The magnetization decay affected by diffusion coupling was acquired numerically by running two different simulation sets, the first when coupling was allowed and the second when coupling was prohibited. This information was then used along with prior knowledge about responses from the individual layers in a careful treatment to identify the coupling strength (ξ) to improve the apparent pore size distributions. A coupling strength (ξ) is introduced as the difference in T2lm between the cases of coupling and non-coupling NMR responses of the individual thinbeds over the measurement time. The value of ξ represents the degree of diffusion coupling on a scale from 0 to 1 where 1 means total coupling between the thin layers. We calculate the vertical and horizontal permeability via the lattice Boltzmann method as references to study the improvement in NMR permeability estimation after completing the decoupling process.The influence of surface relaxivity and diffusion coefficient on T2 relaxation responses have been tested to study the diffusion coupling strength. An escalation in pore coupling was observed with decreasing bed thickness as well as increasing the diffusion coefficient and decreasing the surface relaxivity. When pore coupling was strong, the T2 distribution clearly misrepresents the underlying bimodal distribution of the different morphologies. Consequently, the bimodal relaxation time becomes merged and the relative amplitude of the peaks fails to reflect the true morphologies of the models. The decoupling process by using the value of ξ was applied, rock-typing was successfully achieved, and horizontal and vertical permeabilities derived with good accuracy

A Pore-Scale Upscaling Approach for Laminated Sandstones using Minkowski Maps and Hydraulic Attributes

Digital core analysis offers unprecedented insight into fluid transport mechanisms at the pore scale, yet the integration of this knowledge into standard workflows requires the consideration of larger sample volumes. For hierarchical materials, this can be achieved by appropriate classification schemes and associated upscaling of pore-scale transport properties to continuum measures. This work introduces a robust upscaling approach based on regional Minkowski measures or Minkowski maps with or without an additional hydraulic attribute. Data clustering of this feature vector field is achieved with a Gaussian mixture model leading to spatially compact rock types capturing the layering of the analyzed laminated sandstone. Other regional attribute fields like pore size, throat size, and volume/surface ratio are analyzed using these partitions. This is followed by a characterization of their transport properties, namely, electrical conductivity and permeability, for which an efficient sampling scheme utilizing spectral methods is introduced. We compare the upscaled transport properties on the basis of homogenization to full-scale direct pore-scale simulations for a thinly laminated sandstone and demonstrate very good agreement and large computational speed-up

Society of Core Analysts 2017 Annual Symposium

The asphaltene fraction of crude oils is one of the main factors defining wettability conditions and ultimate oil recovery. At reservoir scale asphaltenes may cause reservoir compartmentalization and at pore scale govern wettability phenomena. To reproduce reservoir conditions, aging in oil is a common step in laboratory core analysis. Oils relevant to the origin of the plugs are the apparent choice for aging, while for outcrop rocks various hydrophobic chemicals and anti-wetting agents or arbitrary oils are often used. We evaluate alteration properties of synthetic oils represented by various proportions of commercially available bitumen, aromatics and alkane for studies requiring wettability alteration.Low-field NMR relaxation measurements have been applied in the past to characterise the wettability of rocks by introducing an NMR wettability index. However, the latter requires multiple reference measurements at end-point saturation states. NMR correlation techniques have a higher prediction capacity, e.g. T2-store-T2 (REXSY) experiment is naturally sensitive to spatial variation of physical properties by detecting diffusion exchange between different environments. It has been applied to study the connectivity of the pore space in aqueous systems such as gypsum, cement pastes, soils, etc. We applied REXSY to study effect of asphaltenes deposition on wettability of siliceous systems. The change of wettability over aging time in different synthetic oils was tracked using T2 relaxation measurements, providing estimates of aging dynamics useful in designing wettability-related experiments. Quantitative information about fraction of altered surface area and deposition pattern was inferred from combination of T2 experimental and numerically simulated responses and from T2-store-T2 experiments.Results show that the wettability alteration process is strongly sensitive to both chemical composition of synthetic oils and asphaltenes origin (light or heavy oil). It can be performed in controlled manner to set variety of heterogeneous wetting conditions. Elements of resulting deposition pattern and wetting state of the core were identified using low-field NMR relaxation and relaxation exchange techniques

International Symposium of the Society of Core Analysts

The integration of numerical simulation and physical measurements, e.g. digital and conventional core analysis, requires the consideration of significant sample sizes when heterogeneous core samples are considered. In such case a hierarchical upscaling of properties may be achieved through a workflow of partitioning the sample into homogeneous regions followed by characterization of these homogeneous regions and upscaling of properties. Examples of such heterogeneities are e.g. fine laminations in core samples or different micro-porosity types as consequence of source rock components and diagenesis. In this work we utilize regional measures based on the Minkowski functionals as well as local saturation information derived through a morphological capillary drainage transform as a basis for such a classification/partitioning. An important consideration is the size of the measurement elements utilized, which could be considerable in the case of larger heterogeneities; in such case the calculation of the regional measures can be computationally very expensive. Here we introduce an FFT approach to calculate these measures locally, utilizing their additivity. The algorithms are compared against direct summation techniques and shift-overlap approaches for a selection of different averaging supports to illustrate their speed and practical applicability. We consider a range of artificial Boolean models to illustrate the effect of including hydraulic information on the resulting classifications scheme. This allows the determination of bias, since for these model systems local classes are known ab-initio. The classification framework is tested by comparing to the known initial micro-structure distribution and relative bias quantified in terms of choice of averaging elements (size and shape). Importantly, depending on the actual morphological transition between micro-type partitions, partitions including hydraulic attributes differ from pure morphological partitions with applications to electrofacies and hydraulic unit definitions

Characterization of reactive flow-induced evolution of carbonate rocks using digital core analysis- part 1: Assessment of pore-scale mineral dissolution and deposition

The application of X-ray micro-computed tomography (μ-CT) for quantitatively characterizing reactive-flow induced pore structure evolution including local particle detachment, displacement and deposition in carbonate rocks is investigated. In the studies conducted in this field of research, the experimental procedure has involved alternating steps of imaging and ex-situ core sample alteration. Practically, it is impossible to return the sample, with micron precision, to the same position and orientation. Furthermore, successive images of a sample in pre- and post-alteration states are usually taken at different conditions such as different scales, resolutions and signal-to-noise ratios. These conditions accompanying with subresolution features in the images make voxel-by-voxel comparisons of successive images problematic. In this paper, we first address the respective challenges in voxel-wise interpretation of successive images of carbonate rocks subject to reactive flow. Reactive coreflood in two carbonate cores with different rock types are considered. For the first rock, we used the experimental and imaging results published by Qajar et al. (2013) which showed a quasi-uniform dissolution regime. A similar reactive core flood was conducted in the second rock which resulted in wormhole-like dissolution regime. We particularly examine the major image processing operations such as transformation of images to the same grey-scale, noise filtering and segmentation thresholding and propose quantitative methods to evaluate the effectiveness of these operations in voxel-wise analysis of successive images of a sample. In the second part, we generalize the methodology based on the three-phase segmentation of normalized images, microporosity assignment and 2D histogram of image intensities to estimate grey-scale changes of individual image voxels for a general case where the greyscale images are segmented into arbitrary number of phases. The results show that local (voxel-based) porosity changes can be decomposed into local mineral dissolution and deposition. Moreover, it is found that the microporosity evolutions are differently distributed in the samples after the reactive coreflood experiments. In the last part of the paper, for the case of quasi-uniform dissolution, we combine the tomographic images with numerical calculations of permeability along the core to characterize the relationship between changes in permeability and the fractions of the mineral dissolved and deposited. A consistency is found between the calculated longitudinal permeability changes and the quantified distribution of mineral dissolved and deposited along the sample

Temperature-Dependent Oxygen Effect on NMR D- T<inf>2</inf> Relaxation-Diffusion Correlation of n-Alkanes

Nuclear magnetic resonance (NMR) diffusion-relaxation correlation experiments (D-T2) are widely used for the petrophysical characterisation of rocks saturated with petroleum fluids both in situ and for laboratory analyses. The encoding for both diffusion and relaxation offers increased fluid typing contrast by discriminating fluids based on their self-diffusion coefficients, while relaxation times provide information about the interaction of solid and fluid phases and associated confinement geometry (if NMR responses of pure fluids at particular temperature and pressure are known). Petrophysical interpretation of D-T2 correlation maps is typically assisted by the “standard alkane line”—a relaxation-diffusion correlation valid for pure normal alkanes and their mixtures in the absence of restrictions to diffusing molecules and effects of internal gradients. This correlation assumes fluids are free from paramagnetic impurities. In situations where fluid samples cannot be maintained at air-free state the diffusion-relaxation response of fluids shift towards shorter relaxation times due to oxygen paramagnetic relaxation enhancement. Interpretation of such a response using the “standard alkane line” would be erroneous and is further complicated by the temperature-dependence of oxygen solubility for each component of the alkane mixture. We propose a diffusion-relaxation correlation suitable for interpretation of low-field NMR D-T2 responses of normal alkanes and their mixtures saturating rocks over a broad temperature range, in equilibrium with atmospheric air. We review and where necessary revise existing viscosity-relaxation correlations. Findings are applied to diffusion-relaxation dependencies taking into account the temperature dependence of oxygen solubility and solvent vapour pressure. The effect is demonstrated on a partially saturated carbonate rock

Characterization of reactive flow-induced evolution of carbonate rocks using digital core analysis - part 2: Calculation of the evolution of percolation and transport properties

© 2017 Elsevier B.V. Percolation of reactive fluids in carbonate rocks affects the rock microstructure and hence changes the rock macroscopic properties. In Part 1 paper, we examined the voxel-wise evolution of microstructure of the rock in terms of mineral dissolution/detachment, mineral deposition, and unchanged regions. In the present work, we investigate the relationships between changes in two characteristic transport properties, i.e. permeability and electrical conductivity and two critical parameters of the pore phase, i.e. the fraction of the pore space connecting the inlet and outlet faces of the core sample and the critical pore-throat diameter. We calculate the aforementioned properties on the images of the sample, wherein a homogeneous modification of pore structure occurred in order to ensure the representativeness of the calculated transport properties at the core scale. From images, the evolution of pore connectivity and the potential role of micropores on the connectivity are quantified. It is found that the changing permeability and electrical conductivity distributions along the core length are generally in good agreement with the longitudinal evolution of macro-connected macroporosity and the critical pore-throat diameter. We incorporate microporosity into critical length and permeability calculations and show how microporosity locally plays a role in permeability. It is shown that the Katz-Thompson model reasonably predicts the post-alteration permeability in terms of pre-alteration simulated parameters. This suggests that the evolution of permeability and electrical conductivity of the studied complex carbonate core are controlled by the changes in the macro-connected macroporosity as well as the smallest pore-throats between the connected macropores