Shale Gas Rock Research Articles

Use of Cluster Analysis to Group Organic Shale Gas Rocks by Hydrocarbon Generation Zones

In the last decade, exploration for unconventional hydrocarbon (shale gas) reservoirs has been carried out in Poland. The drilling of wells in prospective shale gas areas supplies numerous physicochemical measurements from rock and reservoir fluid samples. The objective of this paper is to present the method that has been developed for finding similarities between individual geological structures in terms of their hydrocarbon generation properties and hydrocarbon resources. The measurements and geochemical investigations of six wells located in the Ordovician, Silurian, and Cambrian formations of the Polish part of the East European Platform are used. Cluster analysis is used to compare and classify objects described by multiple attributes. The focus is on the issue of generating clusters that group samples within the gas, condensate, and oil windows. The vitrinite reflectance value (Ro) is adopted as the criterion for classifying individual samples into the respective windows. An additional issue was determining other characteristic geochemical properties of the samples classified into the selected clusters. Two variants of cluster analysis are applied—the furthest neighbor method and Ward’s method—which resulted in 10 and 11 clusters, respectively. Particular attention was paid to the mean Ro values (within each cluster), allowing the classification of samples from a given cluster into one of the windows (gas, condensate, or oil). Using these methods, the samples were effectively classified into individual windows, and their percentage share within the Silurian, Ordovician, and Cambrian units is determined.

Key factors of marine shale conductivity in southern China—Part I: The influence factors other than porosity

The complicated response mechanism of shale gas requires that a systematic pilot study is conducted on the influences of resistivity log responses to help with subsequent experimental design, physical derivations and resistivity simulations. The purpose of this research is to clarify the main controlling factors of the resistivity log responses and their semi-quantitative relationship in certain marine shale gas formations. Comprehensive analyses on large core test datasets, including mineralogical, petrological, geochemical and other data, from the Longmaxi-Wufeng Formation and the corresponding resistivity logging curves are carried out in 4 shale gas areas. This research indicates that in low-porosity systems, the mineral composition directly impacts the resistivity. Carbonates with large intergranular pores and even intragranular pores increase the resistivity. The hydrophilic nature of clay decreases the resistivity. When organic matter is not carbonized, the organic matter and biosilica will further increase the resistivity. Pyrite layers will cause a large drop in resistivity (to less than 5 Ω m). The resistivity is lower where organic matter carbonization has occurred in the studied reservoir. Mineral components, especially clay minerals and organic matter, are the main controlling factors of the conductivity of low-porosity systems. In addition, we propose that elemental logging can be used to characterize the comprehensive effect of low-porosity s systems on the resistivity of the reservoir and can accurately reflect the resistivity. These results will help to develop future studies on the relationship between the electrical conductivity and saturation of shale gas rocks, which is of great significance. • Combining the data of 14 wells in four shale gas blocks, the resistivity characteristics of marine shale are systematically and quantitatively analysed. • Shale mineral composition is the non-porous factor that mainly affects resistivity, among which carbonate minerals and combinations of biosilica and (uncarbonized) organic matter will increase the resistivity. • The layered pyrite greatly reduces the resistivity, but the dispersed pyrite has little effect on the resistivity of marine shale. • A semi-quantitative characterization technology for non-porous systems is proposed (ER curve), and the effect of mineral composition on resistivity is semi-quantitatively characterized.

Petrophysical Investigations of Shale Gas Formations in Poland

Results of laboratory experiments and well logging processing and interpretation are described to show the investigation trial realized on the Polish Silurian and Ordovician shale gas deposits in the Baltic Basin, north Poland. Standard and sophisticated laboratory methods of density, porosity, permeability, mineral composition and geochemical parameters measurements on rock samples were performed. Statistical methods were applied to standard, dipole sonic and geochemical well logs to limit the processed data without loss of useful petrophysical information, avoiding redundancy and doing the primary classification. The goal of this specific trial was to build petrophysical model of shale gas rocks taking into account low porosity, very low permeability, TOC equal at least 2 wt.% and organic matter as a kerogen and bitumen, free gas gathered in the rock matrix effective porosity and in secondary porosity. The unique model for the Polish shale gas formations is explained by age, depth of occurrence and mineral composition of the Silurian and Ordovician shales, different from those described in the literature.

Coupled Broad Ion Beam–Scanning Electron Microscopy (BIB–SEM) for polishing and three dimensional (3D) serial section tomography (SST)

Here we describe the first automated fully integrated in-microscope broad ion beam (BIB) system. Ar+-BIB has several advantages over Ga+ focused ion beam (FIB) and Xe+ plasma-FIB (PFIB) methods inducing less beam damage, especially for ion beam sensitive materials. It can mill areas several orders of magnitude larger (up to millimetre scale), and is not confined to the edge of the sample with associated curtaining issues. BIB is shown to have sputter rates up to five times higher than comparable FIB techniques. This new coupled BIB–SEM system (commercial name ‘iPrep™II’) enables in-microscope surface polishing to remove contaminants or damage for two dimensional (2D) imaging, as well as automated serial section tomography (SST) by milling and imaging hundreds of slices, cost and time efficiently. The milled slice thickness can be controlled from a few nanometers up to a micrometre. A novel sample transfer, handling and interlock system allows automated and sequential BIB polishing, scanning electron microscopy (SEM) and analysis by secondary electron (SE) imaging, electron back scatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) for 3D microstructure analysis. Furthermore, insulating surfaces can be sputter coated after milling each slice to reduce charging during SEM analysis. The performance of the instrument is demonstrated through a series of case studies across the materials, earth and life sciences exploiting the imaging, crystallographic and chemical mapping capabilities. These include the study of butterfly defects in bearing steels, meta-stable intermetallic phases in bronze bearings, shale gas rock, aluminium plasma electrolytic oxide (PEO) coatings as well as liver and mouse brain tissues.

Analyzing Tight Sand Characteristics and Its Influence on Aqueous Phase Removal by Gas Displacement

Tight sand gas is an important unconventional natural gas. Liquid filtration plays a main role in the formation damage. Flowback rate is closely related to the formation permeability recovery. Usually, the more flowback rate is, the better the permeability recovery is. In order to research the factors that influence the liquid flowback, the tight sand characteristics and gas displacement have been investigated. Some experiments were carried out, including porosity, permeability, XRD, casting thin section, SEM, stress sensitivity, rock expansion, and gas displacement. The results are shown as follows. Taiyuan formation has average high clay content than H8 formation. Illite is the main clay in Taiyuan formation; chlorite is the main clay in H8 formation. From the casting thin section, the rock has strong compression. Taiyuan formation has large pore. However, H8 formation mainly has small cracks. Through the SEM, there are many micro- to nanocracks as well as some pores in these two formations. In some cases, cracks penetrate the clays. From the permeability stress sensitivity, these tight sands have strong stress sensitivity. Permeability decreases quickly with increasing confining pressure. In the rock expansion experiment, the tight sand has less expansion rate compared with shale gas rock and volcanic gas rock. Rock expansion rate has positive relationship with clay content. In the gas displacement experiment, usually the higher the clay content, the lesser the liquid flowback. Some samples have high clay content. However, it has high flowback rate, which may account for good pore connection. Through the above study, the flowback rate has negative relationship with clay content and positive relationship with pore size, pore connection, and displacement pressure. In order to increase tight gas production in the study area, the formation should have high flowback rate and less liquid retention. Therefore, clay expansion preventing additives should be added into the fracturing liquid to reduce the liquid retention. The quick flowback should be implemented after hydraulic fracturing for reducing the interaction between liquid and rock. At the same time, the drawdown pressure should keep in a certain value to reduce stress sensitivity.

Investigation of static/dynamic moduli and plastic response of shale specimens

High clay and organic content, irregular voids, presence of micro-cracks, and obvious bedding planes are among features of shale gas rocks that affect their mechanical response. These sedimentary rocks exhibit substantial degree of anisotropy and inhomogeneity, with non-linear response when subjected to axial loading. The laboratory characterization of these rocks is necessary to design an optimized hydraulic fracturing programme, and for reliable constitutive and numerical modeling of these formations. In this study, a series of triaxial multi-stage elastic and cyclic tests were performed on Marcellus Shale specimens, retrieved from a deep well (~2270 m deep) located in West Virginia, to characterize their elastic-plastic and hysteresis response subjected to loading/unloading cycles. The experimental results indicated a nonlinear response, particularly at low differential stress levels. In addition, the specimens with higher clay content exhibited a softer response with lower estimated static and dynamic moduli at different stress levels compared to those of specimens with higher calcite/quartz content. The ultrasonic P- and S-wave velocity measurements were found to be sensitive to the changes in the micro-structure of the rock caused by variation in stress condition. In general, the estimated Young's modulus during unloading was found to be higher than loading. The plastic deformations were pronounced in the first cycle of loading, followed by a decreasing trend in the subsequent cycles.

Elastic, viscoelastic, and strength properties of Marcellus Shale specimens

Shale gas rocks are characterized as clastic sedimentary rocks, with features such as obvious bedding planes, presence of micro-cracks, and high clay and organic content. These rocks are anisotropic and inhomogeneous, exhibiting a nonlinear response under loading. In order to optimize the required energy for hydraulic fracturing operation and production in shale gas reservoirs, and for constitutive and numerical modeling, it is important to characterize these shale gas rocks. In this study, the hysteresis, elastic-plastic, viscoelastic, and strength properties of Marcellus Shale specimens, retrieved from a deep well located in West Virginia, were evaluated through performing a series of creep, cyclic, and triaxial multi-stage failure tests on these specimens. Both elastic moduli and plastic deformations show significant levels of pressure dependency. Moreover, higher creep compliance and lower Young's modulus values were observed for clay-rich specimens. Both Power-Law and Burgers' models were found to capture the creep response of these specimens reasonably well. The dynamic moduli estimated from the ultrasonic velocity measurements at different stress levels were found to be higher than static moduli estimations. In addition, the changes in the internal micro-structure of the specimens resulted from variations in the stress condition, were found to affect the ultrasonic velocity measurements. The strength properties of the calcite/quartz-rich specimen, using multi-stage triaxial failure test, were estimated through both Mohr-Coulomb and Hoek-Brown failure criteria.

Comprehensive Interpretation of the Laboratory Experiments Results to Construct Model of the Polish Shale Gas Rocks

More than 70 rock samples from so-called sweet spots, i.e. the Ordovician Sa Formation and Silurian Ja Member of Pa Formation from the Baltic Basin (North Poland) were examined in the laboratory to determine bulk and grain density, total and effective/dynamic porosity, absolute permeability, pore diameters size, total surface area, and natural radioactivity. Results of the pyrolysis, i.e., TOC (Total Organic Carbon) together with S1 and S2 – parameters used to determine the hydrocarbon generation potential of rocks, were also considered. Elemental composition from chemical analyses and mineral composition from XRD measurements were also included. SCAL analysis, NMR experiments, Pressure Decay Permeability measurements together with water immersion porosimetry and adsorption/ desorption of nitrogen vapors method were carried out along with the comprehensive interpretation of the outcomes. Simple and multiple linear statistical regressions were used to recognize mutual relationships between parameters. Observed correlations and in some cases big dispersion of data and discrepancies in the property values obtained from different methods were the basis for building shale gas rock model for well logging interpretation. The model was verified by the result of the Monte Carlo modelling of spectral neutron-gamma log response in comparison with GEM log results.

Nanopore structures, statistically representative elementary volumes, and transport properties of chalk

AbstractDual focused ion beam‐scanning electron microscopy (FIB‐SEM) is frequently being used to characterize nano‐scale pore structures observed in carbonate and shale gas rocks. However, applications are limited to qualitative analysis of nanopore structures. Herein, the concept of statistical representative elementary volumes (SREV) is applied to FIB‐SEM data of a Cretaceous chalk sample. Lattice‐Boltzmann (LB) simulations with multiple relaxation time and topological analysis show that the size of the SREV for this chalk sample can be established at ~ 10 microns based on anisotropic permeability, tortuosity, and specific surface area. This work confirms that the FIB‐SEM technique can be used for the quantitative analysis of nanopore structures and highlights nano‐scale basis for strong anisotropy in the presence of fractures. In addition, nanopores and pore throats are not resolved at voxel dimensions less than ~ 80 nm, resulting in significant underestimation of surface area and permeability.

Shale Gas Rock Properties Prediction using Artificial Neural Network Technique and Multi Regression Analysis, an example from a North American Shale Gas Reservoir

Estimation of reservoir parameters has always been a challenge for shale gas reservoirs. This study has concentrated on neural network technique and multiple regression analysis to predict reservoir properties including porosity, permeability, fluid saturation and total organic carbon content from conventional wireline log data for a large North American shale gas reservoir. More than 262 core analysis data from 3 wells were used as ``target' and ``response' for neural network and multiple regression analysis. Common log data available in three wells including GR, SP, RHOB, NPHI, DT and deep resistivity were used as ``input' and ``predictor'. This study shows that reservoir parameters could be better estimated using the neural network technique than through multiple regression. The neural network method had a correlation coefficient greater than 80% for most of the parameters. Although providing a set of algorithms, multiple regression analysis was less successful for predicting reservoir parameters.