Microstructures And Nanomechanical Properties Of The Bakken Shale

The focus of this thesis is to study the pore structures along with mechanical properties of the shale rocks from the Bakken Formation. The pore structures that are obtained from the SEM image analysis method showed that total surface porosity of the studied samples is less than 12% and that organic porosity is not the main contributor to total porosity for the samples analyzed. Clay minerals and feldspar have a positive influence on porosity while quartz, pyrite, and TOC has a negative impact. The results from the multifractal theory and lacunarity methods based on the segmented SEM images indicated that pores distribution and size in Bakken shale are heterogeneous. Regarding gas adsorption analysis, the results showed that all range of pore sizes: micro (\u3c2 nm), meso (2-50 nm) and macro-pores (\u3e50 nm) exist in the Bakken shale samples. Meso-pores and macro-pores are the main contributors to the porosity for these samples. In comparison with the Middle Bakken, samples from the Upper and Lower Bakken exhibited more micro pore volumes. The deconvolution of the pore distribution function from the combination of N2 and CO2 adsorption results proved that five typical pore size families exist in the Bakken shale samples: one micro-pore, one macro-pore and three meso-pore size clusters. In order to analyze the heterogeneity of the pore structures from gas adsorption, multifractal method was applied to analyze adsorption isotherms (CO2 and N2). The results explained that the generalized dimensions derived from CO2 and the N2 adsorption isotherms decrease as q increases, demonstrating a multifractal behavior. Samples from the Middle Bakken demonstrated the smallest average H value and largest average α10-- α10+ for micropores while samples from the Upper Bakken depicted the highest average α10-- α10+ for the meso-macropores. This indicated that the Middle Bakken and the Upper Bakken have the largest micropore and meso-macropore heterogeneity, respectively. The impact of rock composition on pore structures showed that organic matter could increase the micropore connectivity and reduce micropore heterogeneity. This study was followed by mechanical analysis of shale samples from the Bakken. Statistical grid nanoindentation method was applied to analyze mechanical properties of the Bakken. Then the Mori-Tanaka scheme was carried out to homogenize the elastic properties of the samples and upscale the nanoindentation data to the macroscale. The discrepancy between the macro-mechanical modulus from the homogenization and unconfined compression test was less than 15% which was found acceptable. The creep analysis of the samples describes that minerals with various mechanical properties exhibit different creep behavior. Under the same constant load and time conditions, the creep displacement of hard minerals would be smaller than the soft ones. On the contrary, the changes in mechanical properties (storage modulus, loss modulus, complex modulus, and hardness) of hard minerals are larger than soft minerals. The results from curve fitting led us to conclude that the changes in creep displacement, storage modulus, complex modulus and hardness with respect to the creep time would follow a logarithmic function

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

Quantitative prediction of fractures in shale using the lithology combination index

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. Fractures, which are related to tectonic activity and lithology, have a significant impact on the storage and production of oil and gas in shales. To analyze the impact of lithological factors on fracture development in shales, we selected the shale formation from the Da’anzhai member of the lower Jurassic shales in a weak tectonic deformation zone in the Sichuan Basin. We defined a lithology combination index (LCI), that is, an attribute quantity value of some length artificially defined by exploring the lithology combination. LCI contains information on shale content at a certain depth, the number of layers in a fixed length (lithology window), and the shale content in the lithology window. Fracture porosity is the percentage of pore volume to the apparent volume of the rock. In the experiment, fracture porosity was obtained using 50 samples from six wells, by observing rock slices under a microscope. The relationship between LCI and fracture porosity was analyzed based on machine learning, regression analysis, and weighting methods. The results show that LCI is able to represent the impact of multiple lithological factors (i.e., shale content at a certain depth, the number of layers in the lithology window, and the shale content in the lithology window). The LCI within a thickness of 2 m for the lithology window demonstrates a good linear relationship with fracture porosity. We therefore suggest LCI be used for fracture predictions of shale formations from weak tectonic deformation zones. Our proposed LCI and fracture prediction methods also provide implications for sandstone, mudstone, or carbonate formations under similar processes

Investigation of Properties Alternation during Super-Critical CO2 Injection in Shale

The low recovery of oil from tight liquid-rich formations is still a major challenge for a tight reservoir. Thus, supercritical CO2 flooding was proposed as an immense potential recovery method for production improvement. While up to date, there have been few studies to account for the formation properties\u27 variation during the CO2 Enhanced Oil Recovery (EOR) process, especially investigation at the micro-scale. This work conducted a series of measurements to evaluate the rock mechanical change, mineral alteration and the pore structure properties\u27 variation through the supercritical CO2 (Sc-CO2) injection process. Corresponding to the time variation (0 days, 10 days, 20 days, 30 days and 40 days), the rock mechanical properties were analyzed properly through the nano-indentation test, and the mineralogical alterations were quantified through X-ray diffraction (XRD). In addition, pore structures of the samples were measured through the low-temperature N2 adsorption tests. The results showed that, after Sc-CO2 injection, Young\u27s modulus of the samples decreases. The nitrogen adsorption results demonstrated that, after the CO2 injection, the mesopore volume of the sample would change as well as the specific Brunauer-Emmett-Teller (BET) surface area which could be aroused from the chemical reactions between the CO2 and some authigenic minerals. XRD analysis results also indicated that mesopore were altered due to the chemical reaction between the injected Sc-CO2 and the minerals

Research of water control technology for horizontal wells in water-driven reservoirs

Horizontal wells are widely applied in the Bohai offshore oil fields due to their large oil drainage areas and high yields. However, water coning is a significant problem existing in water-driven reservoirs. To control water coning, this paper introduces a stinger completion method which can be applied in the horizontal wells. Based on the principle of mirror reflection and mass conservation law, a mathematical model coupling fluid flow both in the reservoir and in the horizontal wellbore has been developed. Using the new proposed model, the well production profile and bottom hole flowing pressure distribution along the horizontal well, considering the influence of flowing in the wellbore, are calculated successfully. Moreover, the influence of the stinger completion on the inflow profile is investigated. According to the results of the sensitivity analysis, a 160 m’ 2-7/8 tubing is designed to be built in the horizontal section. The field-test results show that the stinger completion could be used to improve the wellbore inflow profile and decrease the possibility of water-cut thus increasing the effective enhanced oil recovery (EOR).Cited as: Shang, B., Han, X., Li, S., Liu, K. Research of water control technology for horizontal wells in water-driven reservoirs. Advances in Geo-Energy Research, 2018, 2(2): 210-217, doi: 10.26804/ager.2018.02.0

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

Quantifying the Pore Heterogeneity of Alkaline Lake Shale during Hydrous Pyrolysis by Using the Multifractal Method

Distinguishing itself from marine shale formations, alkaline lake shale, as a significant hydrocarbon source rock and petroleum reservoir, exhibits distinct multifractal characteristics and evolutionary patterns. This study employs a combination of hydrous pyrolysis experimentation, nitrogen adsorption analysis, and multifractal theory to investigate the factors influencing pore heterogeneity and multifractal dimension during the maturation process of shale with abundant rich alkaline minerals. Utilizing partial least squares (PLS) analysis, a comparative examination is conducted, elucidating the disparate influence of mineralogical composition on their respective multifractal dimensions. The findings reveal a dynamic evolution of pore characteristics throughout the maturation process of alkaline lake shale, delineated into three distinct stages. Initially, in Stage 1 (200 °C to 300 °C), both ΔD and H demonstrate an incremental trend, rising from 1.2699 to 1.3 and from 0.8615 to 0.8636, respectively. Subsequently, in Stages 2 and 3, fluctuations are observed in the values of ΔD and D, while the H value undergoes a pronounced decline to 0.85. Additionally, the parameter D1 exhibits a diminishing trajectory across all stages, decreasing from 0.859 to 0.829, indicative of evolving pore structure characteristics throughout the maturation process. The distinct alkaline environment and mineral composition of alkaline lake shale engender disparate diagenetic effects during its maturation process compared with other shale varieties. Consequently, this disparity results in contrasting evolutionary trajectories in pore heterogeneity and multifractal characteristics. Specifically, multifractal characteristics of alkaline lake shale are primarily influenced by quartz, potassium feldspar, clay minerals, and alkaline minerals

Indentation size and loading rate sensitivities on mechanical properties and creep behavior of solid bitumen

Creep behavior of rocks could impair fracture conductivity and wellbore stability during gas production from highly matured organic-rich shales in South China, of which the organic matter is mainly in the form as solid bitumen and is thought to be a major contributor for the creep deformation. To get a better insight into this phenomenon, this paper for the first time characterizes the mechanical properties and creep behavior of a millimeter-sized solid bitumen sample by using quasi-static state creep tests and Dynamic Mechanical Analysis in nanoindentation, and reports their dependences on indentation size and loading rate, respectively. Mechanical properties (including hardness and Young's modulus) are found to be negatively related with both indentation size and loading rate. The extremely small creep strain rate sensitivity (m) of solid bitumen indicates a localized shear flow inside. And m exhibits slightly positive dependences on indentation size and loading rate. The potential mechanisms controlling the deformation of solid bitumen under indentation are also discussed

Pore structure and adsorption hysteresis of the middle Jurassic Xishanyao shale formation in the Southern Junggar Basin, northwest China

In order to understand the pore structures of the Middle Jurassic Xishanyao Formation in the Junggar Basin, 11 shale samples from a single well were picked and were subjected to several analyses including mineralogy, (programmed) Rock-Eval pyrolysis for geochemical and N 2 adsorption for pore structure analysis. The results showed that the mean value of total organic carbon (TOC) content of these samples is around 1.54% while T max varies between 429 to 443°C, indicating they are in the oil window. Mineral assemblages of the samples is mainly quartz and clay (illite, chlorite and kaolinite). Moreover, negative correlations between the K-feldspar/plagioclase and micro-mesopore volume was found, depicting that few of such pore sizes exist in these two abundant minerals. In contrast, micro, meso and macro pores all were detected in clay minerals. Particularly, the pores with radii of around 5.35 nm were abundant in clay minerals and there was not a robust relationship between the organic matter, surface area and pore volume. Finally, fractal analysis was performed to better delineate heterogenous characteristics of pore structures which showed that D 2 (representing the larger pores) is greater than D 1 (smaller pores). In addition, the differences between the fractal dimensions of the adsorption and desorption (D 2d –D 2a ) branches to better interpret the hysteresis, was defined. The positive correlation between the (D 2d –D 2a ) and the meso-macro pore volume, pointed out that the meso-macro condensation is the main reason for hysteresis that was observed in N 2 adsorption experiments in the Xishanyao Shale samples