Geomechanics and elastic anisotropy of the Bakken Formation, Williston Basin

Many of the earth’s rocks exhibit anisotropic characteristics. Anisotropy is particularly common in many sedimentary rocks, such as shales. Anisotropy is defined as the spatial alignment of mineral grains, layers, fractures and stresses which causes elastic wave velocity and other elastic properties to vary with direction. There are two types of anisotropy: intrinsic and stress-induced. Intrinsic anisotropy is caused by beddings, microstructures or aligned fractures formed during deposition. Stress-induced anisotropy is caused by strain associated with external stresses. Intrinsic anisotropy originates in the absence of external stresses, while stress-induced anisotropy results from tectonic and overburden stresses. The style of earth material alignment causes two simplified, but convenient models of anisotropy: vertically transverse isotropy (VTI), like shale, and horizontally transverse isotropy (HTI), like vertically fractured medium. These models have been used to describe how physical properties of rock vary in a medium. Identifying the anisotropy in a formation is important in reservoir characterization seismic data processing and oil-field development. Deep shales are the most abundant yet least characterized sedimentary rocks in the Williston Basin of North Dakota. They are significant sources of hydrocarbon unconventional resources in this basin. This dissertation aims to fulfill an investigation of anisotropy in this rock type in several different facets through exploiting of field data. I seek to generate key information for better interplay of field in-situ stress and the existing natural fracture systems for the purpose of drilling, well completion, perforating, hydraulic fracturing and defining reservoir properties. In this study advanced sonic logging data has been processed and interpreted to calculate three independent shear moduli. These parameters then will be used to estimate Thomsen (1986) anisotropy parameters, elastic stiffness coefficients and principal stresses of deep shales in the Williston Basin. The parameters then will be used to generate shear radial profiles and slowness-frequency plots analyze formation anisotropy type and origin as well as reservoir quality. The next step will be to evaluate direction and magnitude of the minimum and maximum anisotropic principal horizontal stresses as the governing element in geomechanical modeling. I will analyze wellbore stability and predict wellbore behavior under stress alteration caused by drilling. Elastic anisotropy of the formation will be included in the 3 D numerical models. In addition the effects of local geological features on the mode of anisotropy both in the far-field and around the borehole to get an in-depth insight of the fractures will be studied. Finally, by generating stress polygons for the reservoir, before and after production and pressure decline, I will try to study how reservoir depletion may cause future geological natural hazards such as faulting and induced seismic events in the region

The impact of pore size distribution data presentation format on pore structure interpretation of shales

Understanding the nature of pore structures in unconventional reservoirs such as shale oil/gas can assist in evaluating the storage potentials and reveal transport mechanisms. Pore size distribution is one of the most importance pore structure parameters which needs to be evaluated accurately and presented in various formats to provide more in-depth information from the rocks. In this paper, several shale samples were collected and analyzed by using N2 adsorption and high-pressure mercury intrusion. Three different presentations of the pore size distributions: incremental pore volume versus diameter (DV), differential pore volume versus diameter (DV/Dd) and the log differential pore volume versus diameter (DV/DlogD) were calculated from these two different methods, respectively. The comparison results showed that different presentations from the same sample could demonstrate various type of important pore information. The DV curve is largely depended on the experimental data spacing while the other two presentations do not. The DV/Dd curve could amplify the role of smaller pore ranges while the DV/Dlogd would represent the importance of the larger pore ranges. The multifractal analysis showed that the heterogeneity index calculated from the DV/Dd curve is much larger than the heterogeneity index from the DV/Dlog curve. It was concluded that DV/Dd is more suitable for characterizing the pore size distribution from N2 adsorption while DV/Dlogd works better for the high-pressure mercury intrusion.Cited as: Liu, K., Ostadhassan, M. The impact of pore size distribution data presentation format on pore structure interpretation of shales. Advances in Geo-Energy Research, 2019, 3(2): 187-197, doi: 10.26804/ager.2019.02.0

Effects of pore connectivity and water saturation on matrix permeability of deep gas shale

Shale matrix permeability is an important indicator for evaluating gas transport and production. However, the effects of pore connectivity and water saturation on the matrix permeability in deep gas shales have not been adequately studied. In this study, the permeability of deep shales in the Yichang area of the Middle Yangtze was characterized using three methods. These included the determination of apparent permeability in different directions via pulse-decay, also matrix permeability obtained via the Gas Research Institute method, and the connected pore network permeability via the mercury injection capillary pressure technique. The results revealed a significant difference between the horizontal and vertical permeability of deep shales. The smaller the size of the multiple connected pore network, the larger was the effective tortuosity and the lower the permeability. Comparison of the three permeabilities and combined microscopic observations revealed that microfractures and laminae were the dominant gas transport channels. Importantly, the matrix permeability decreased exponentially with increasing water saturation, with water vapor adsorption experiments revealing that water occupation of pores and pore-throat spaces smaller than 10 nm in diameter was the main reason for this decrease in matrix permeability. Collectively, proposed method of evaluating effective permeability with an index for shale gas reservoirs is significant for sweet spot selection and production prediction of shale gas reservoirs around the globe.Cited as: Zhao, J., Sun, M., Pan, Z., Liu, B., Ostadhassan, M., Hu, Q. Effects of pore connectivity and water saturation on matrix permeability of deep gas shale. Advances in Geo-Energy Research, 2022, 6(1): 54-68. https://doi.org/10.46690/ager.2022.01.0

Geomechanical Upscaling Methods: Comparison and Verification via 3D Printing

Understanding geomechanical properties of rocks at multiple scales is critical and relevant in various disciplines including civil, mining, petroleum and geological engineering. Several upscaling frameworks were proposed to model elastic properties of common rock types from micro to macroscale, considering the heterogeneity and anisotropy in the samples. However, direct comparison of the results from different upscaling methods remains limited, which can question their accuracy in laboratory experiments. Extreme heterogeneity of natural rocks that arises from various existing components in them adds complexity to verifying the accuracy of these upscaling methods. Therefore, experimental validation of various upscaling methods is performed by creating simple component materials, which is, in this study, examining the predicted macroscale geomechanical properties of 3D printed rocks. Nanoindentation data were first captured from 3D printed gypsum powder and binder rock fragments followed by, triaxial compression tests on similar cylindrical core plugs to acquire modulus values in micro and macroscale respectively. Mori-Tanaka (MT) scheme, Self-Consistent Scheme (SCS) method and Differential Effective Medium (DEM) theory were used to estimate Young’s modulus in macroscale based on the results of nanoindentation experiments. The comparison demonstrated that M-T and SCS methods would provide us with more comparable results than DEM method. In addition, the potential applications of 3D printed rocks were also discussed regarding rock physics and the geomechanics area in petroleum engineering and geoscience

Applications of nano-indentation methods to estimate nanoscale mechanical properties of shale reservoir rocks

In order to study the mechanical properties of shale samples from Bakken Formation, nanoindentation method, an imaging technique borrowed from other engineering disciplines, was used. Different types of nanoindentation curves were analyzed and the applicability of the nanoindentation theories to study mechanical properties of shale samples at nanoscale was demonstrated. Elastic modulus and Hardness of different samples were calculated, compared and related to their mineral compositions and microstructures which are detected by 2D XRD and FESEM methods, respectively. Results showed that samples with more clay minerals (mainly composed of illite) and larger pore structures have less Young\u27s modulus. In addition, based on the energy analysis method, the fracture toughness at nanoscale was estimated and its relationships with Young\u27s modulus was quantified. It was observed that fracture toughness increases linearly with Young\u27s modulus. This paper presents the results and main findings of this study

Estimation of the Permeability of Rock Samples Obtained from the Mercury Intrusion Method Using the New Fractal Method

Rock permeability, defined as the ability of fluid to flow through the rocks, is one of the most important properties of rock. Many researchers have developed models to predict the permeability of rock from the porosity and pore size based on the mercury intrusion. However, these existing models still have some limitations. In this study, based on data regarding the fractal nature of the mercury intrusion of the rocks, we built a new model to predict the permeability of the rocks. In order to verify the new model, we extracted data regarding different kinds of samples from the literature and estimated the permeability using the new model. The results showed that the model could predict various types of rocks, such as tight sandstone, carbonates, and shale. The comparison of the calculated permeability using the new model is closer to the measured value than the value estimated from the existing models, indicating that the new model is better in predicting the permeability of rock samples

Stable Isotope Geochemistry of the Organic Elements within Shales and Crude Oils: A Comprehensive Review

Over time, stable isotopes have proven to be a useful tool in petroleum geochemistry. However, there is currently insufficient literature on stable isotope geochemistry of the organic elements within shales and crude oils in many petroleum systems around the world. As a result, this paper critically reviews the early and recent trends in stable isotope geochemistry of organic elements in shales and crude oils. The bulk and compound-specific stable isotopes of H, C, and S, as well as their uses as source facies, depositional environments, thermal maturity, geological age, and oil–oil and oil–source rock correlation studies, are all taken into account. The applications of the stable isotopes of H and C in gas exploration are also discussed. Then, the experimental and instrumental approaches to the stable isotopes of H, C, and S, are discussed

Evaluation of 3D printed microfluidic networks to study fluid flow in rocks

Visualizing fluid flow in porous media can provide a better understanding of transport phenomena at the pore scale. In this regard, transparent micromodels are suitable tools to investigate fluid flow in porous media. However, using glass as the primary material makes them inappropriate for predicting the natural behavior of rocks. Moreover, constructing these micromodels is time-consuming via conventional methods. Thus, an alternative approach can be to employ 3D printing technology to fabricate representative porous media. This study investigates fluid flow processes through a transparent microfluidic device based on a complex porous geometry (natural rock) using digital-light processing printing technology. Unlike previous studies, this one has focused on manufacturing repeatability. This micromodel, like a custom-built transparent cell, is capable of modeling single and multiphase transport phenomena. First, the tomographic data of a carbonate rock sample is segmented and 3D printed by a digital-light processing printer. Two miscible and immiscible tracer injection experiments are performed on the printed microfluidic media, while the experiments are verified with the same boundary conditions using a CFD simulator. The comparison of the results is based on Structural Similarity Index Measure (SSIM), where in both miscible and immiscible experiments, more than 80% SSIM is achieved. This confirms the reliability of printing methodology for manufacturing reusable microfluidic models as a promising and reliable tool for visual investigation of fluid flow in porous media. Ultimately, this study presents a novel comprehensive framework for manufacturing 2.5D realistic microfluidic devices (micromodels) from pore-scale rock images that are validated through CFD simulations

Molecular Weight Distribution of Kerogen with MALDI-TOF-MS

Kerogen is an amorphous organic matter (AOM) in fine grain sediments, which produces petroleum and other byproducts when subjected to adequate pressure and temperature (deep burial conditions). Chemical characteristics of kerogen by considering its biogenic origin, depositional environment, and thermal maturity has been studied extensively with different analytical methods, though its molecular structure is still not fully known. In this study, conventional geochemical methods were used to screen bulk rock aliquots from the Bakken Shale with varying thermal maturities. Organic matter was isolated from the mineral matrix and then a mass spectrometry method was utilized to quantify molecular weight distribution (MWD) of four different kerogens at various thermal maturity levels (immature to late mature). Furthermore, to complement mass spectrometry, Fourier transform infrared (FTIR) spectroscopy was employed as a qualitative chemical and structural investigation technique. The MWD of four samples was obtained by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, and the results are correlated with the absorption indices (CH3/CH2 ratio and aromaticity) calculated from the FTIR attenuated total reflectance (ATR) method. The results showed when the degree of maturity increases, the aliphatic length shortens, and the branching develops, as well as the aromatic structure becomes more abundant. Moreover, based on the MWD results, higher maturity kerogen samples would consist of larger size molecular structures, which are recognized as more developed aromatic, and aliphatic branching stretches. The combination of infrared spectroscopy (AFT-FTIR) and mass spectrometry (MALDI-TOF) provided MWD variations in kerogen samples as a function of maturity based on varying absorption indices and revealed the rate of change in molecular mass populations as a function of thermal maturity.</p