Shale Beds Research Articles

Reservoir Structure and Remaining Oil Distribution of Meandering River Channels: A Case Study of the Y92-1 Layer in the Jingbian Oilfield, Ordos Basin

The majority of the Jurassic Yan’an Formation reservoirs in the Jingbian Oilfield of the Ordos Basin have entered the middle and late stages of development, and fine characterization of reservoir heterogeneity and recognition of the remaining oil distribution are the keys to formulating comprehensive adjustment for production. The Y92-1 layer of the Yan’an Formation Y9 reservoir in the LZ area of Jingbian Oilfield is taken as the research object in this paper. Based on logging data, core, and other basic data, the reservoir morphology in the study area is finely characterized. Single-channel units are subdivided, and the reservoir configurations of different hierarchical levels are analyzed qualitatively and quantitatively. Finally, combined with the plane profile consistency analysis of the morphological characteristics of the sand body in layer Y92-1 and the injection production mode, it is concluded that there may be four distribution patterns of the remaining oil in the study area, and the composite river channel sand body dominated by the lateral superimposed mode under the inverse lateral accretion shale bed tendency injection condition is a favorable enrichment area for the remaining oil. This mode is mainly distributed in the boundary part of the different periods of single sand body superposition. These understandings not only deepen the significance of tapping the potential of the remaining oil in frequently rerouted meandering rivers but also could provide effective guidance for the further remaining oil tapping.

Geochemical and organic petrographic analyses on the Late Eocene bentonitic shale bed in Kom Aushim vicinity, Fayum, Egypt; Paleoenvironmental implications

Bentonitic shale bed of the Qasr El Sagha Formation (Late Eocene), located in the Kom Aushim area, Fayum, composes of 5 m thickness of dark massive stratum intercalated with veins of iron bands. In this study, twelve fresh shale samples from this area were investigated concerning their geochemical and organic petrography analyses. A total organic carbon (TOC) of the shale samples is very low (0.07–0.39 wt%). Petrographic analysis of samples shows a dominance of the vitrinite maceral followed by either inertinite and/or liptinite macerals. Theoretically derived vitrinite reflectance (Rocal) reveals that the bentonitic shale bed was subjected to an abnormal increase in maturity toward the invasion of Fe, Zn, Cu rich hydrothermal sulfates and oxides veins. The composition and origin of these veins were investigated by XRD and XRF analyses. The results suggested that the Fe brought from these veins affected the main composition of the shale stratum to exhibit Fe-shale facies. Moreover, rich iron in the depositional medium leads to form siderite minerals (≈10%) and the replacement of lime associated clay to Ferron dolomite (≈25%). The dominant paleoclimate synchronized shale deposition was semi-arid and arid climate, as confirmed by the domination of evaporite (≈44% gypsum). The source area of shale components was found to be sedimentary and mafic igneous (basaltic trend). CIA-ICV and A–CN–K ternary diagrams showed that the shale components were subjected to a variety of intensive to moderate weathering, and they were probably coming from gabbro and tonalite igneous rock. An active sedimentary margin and island arc are the main provinces, as the shale was deposited at the marine anoxic condition of a clay-lime platform of a pro-delta sedimentary setting. The organic matters (OM) are of type III and IIIC that refer to terrestrial OM of marginal aquatic environments. Organic and inorganic studies confirm a prevailing depositional environment of a shallow shelf, shoreline mudflats and evaporitic lagoon that reflects the general trend of sea-level regression in the Fayum area during the Late Eocene.

Fracture Propagation Behavior of Bedding Shale in the Process of Multistage Cluster Fracturing considering the Intercluster Stress Interference

Abstract Staged multicluster fracturing in horizontal wells is the key technology for forming complex fractures in shale reservoirs. The existence of shale bedding plays a conspicuous role for the propagation path of hydraulic fractures, affecting the propagation of the fracture height direction prominently. A 3D finite element model containing three clusters signed as side clusters and middle cluster was established based on the cohesive zone model and the dynamic distribution mechanism of interfracture flow. And the correctness of the model was verified by literature comparison. Some factors including cluster spacing, horizontal stress difference, shale bedding strength, perforation density, injection rate, and viscosity of fracturing fluid which influenced fracture propagation behavior of bedding shale were simulated. The results indicate that the stress interference of the middle cluster by the clusters on both sides will be prominently obvious when the cluster spacing is less than 10 m. Multiclusters will penetrate across the shale bedding when the horizontal stress difference is more than 4 MP, which will conspicuously reduce the activated probability of discontinuities and the complexity of fracture geometry. In correspondence with increase of horizontal stress difference, the interference between clusters also increases prominently, which will conspicuously decrease the propagation of the middle cluster. In order to comprehensively equalize the length of multiclusters, the inhibition of intercluster stress interference on the middle cluster propagation can be counteracted by improving pressure drop in perforation. The high injection rate and viscosity of fracturing fluid will contribute to the shale bedding shear slip increasingly, which is conducive to the formation of complex fractures in areas with well-developed bedding. The study has a certain guiding significance for the operation parameter design of multicluster fracturing in bedded shale.

New Facies Model and Carbon Isotope Stratigraphy for an Ediacaran Carbonate Platform From South America (Tamengo Formation—Corumbá Group, SW Brazil)

The Ediacaran is a period characterized by the diversification of early animals and extensive neritic carbonate deposits. These deposits are still not well understood in terms of facies and carbon isotope composition (δ13C). In this study we focus on the Tamengo Formation, in southwestern Brazil, which constitutes one of the most continuous and well-preserved sedimentary record of the late Ediacaran in South America. We present new detailed lithofacies and stable isotopes data from two representative sections (Corcal and Laginha) and revise the paleoenvironmental and stratigraphic interpretation of the Tamengo Formation. The Corcal section consists of neritic deposits including shallow-water limestone beds, alternated with shale and subordinate marl beds. These facies yield specimens of the Ediacaran fossils Cloudina lucianoi and Corumbella werneri. On the other hand, the Laginha section shows more heterogeneous facies, such as impure carbonates, breccias, marls, and subordinate mudstone beds, as well as no evidence of Corumbella werneri. The stable carbon isotope record is also different between the two sections, despite belonging to the same unit. The Corcal section displays higher and more homogeneous δ13C values, consistent with those of Ediacaran successions worldwide. The Laginha section, instead, displays more variable δ13C values, which suggest the influence of local and post depositional processes. The difference between the two sections was attributed to the different distance from the shore. We propose that the difference is due to topographic variations of the continental platform, which, at the Laginha site, was steeper and controlled by extensional faults. Therefore, the Corcal section is a better reference for the Tamengo Formation, whereas the Laginha is more particular and influenced by local factors. Besides, the lithofacies associations of the Tamengo Formation are like those of the Doushantuo and Dengying formatios, in South China, with no significant biogenic carbonate buildups, and different from those of other important Ediacaran units, such as the Nama Group in Nmibia and the Buah Formation in Oman. Our work highlights the complexity and heterogeneity of Ediacaran carbonate platforms and of their carbon isotopic composition. In addition, we characterize the Corcal section as a possible reference for the Ediacaran in South America.

Experimental Study on Hydraulic Fracturing of Bedding Shale Considering Anisotropy Effects.

The bedding plane formed by sedimentation makes shale anisotropic. To clarify the influence of bedding on the hydraulic fracturing of shale, the fracture characteristics of bedding shale were first clarified by conducting a hydraulic fracturing experiment on large-scale shale samples with different bedding angles. Subsequently, combined with the experimental results, based on the theory of elasticity, a new fracture initiation criterion for shale hydraulic fracturing considering its anisotropic characteristics was established. The influence of the bedding angle on the hydraulic fracture initiation pressure and initiation angle was analyzed. The results showed that the pump pressure curve during hydraulic fracturing can be divided into four stages: continuous pressurization, internal pressure drop, internal pressure attenuation, and internal pressure equilibrium stage. Corresponding to the four stages of the pump pressure curve, the evolution of hydraulic fracture has four processes: microfracture development, fracture initiation, fracture propagation, and fracture network equilibrium process. When the direction of the maximum principal stress is perpendicular to the bedding, a complex fracture network is easily formed. Depending on whether the bedding plane is open or not, the modes of interaction between the hydraulic fractures and bedding plane could be divided into eight types. Hydraulic fractures initiate in two ways: from the matrix and from natural fractures. During fracturing, with the increase in the bedding angle, the initiation pressure decreases gradually and the initiation angle decreases first and then increases. The knowledge gained in this study can provide data and theoretical support for drilling direction design and fracture pressure evaluation in the field of hydraulic fracturing.

A New Conulariid (Cnidaria, Scyphozoa) From the Terminal Ediacaran of Brazil

Paraconularia ediacaran. sp., the oldest documented conulariid cnidarian, is described based on a compressed thin specimen from the terminal Ediacaran Tamengo Formation near Corumbá, Mato Grosso do Sul State, Brazil. The conulariid was collected from a laminated silty shale bed also containingCorumbella werneriand vendotaenid algae. The specimen consists of four partial faces, two of which are mostly covered, and one exposed corner sulcus. The two exposed faces exhibit 32 bell-curve-shaped, nodose transverse ribs, with some nodes preserving a short, adaperturally directed interspace ridge (spine). The transverse ribs bend adapertureward on the shoulders of the corner sulcus, within which the ribs terminate, with the end portions of the ribs from one face alternating with and slightly overlapping those from the adjoining face. This is the first Ediacaran body fossil showing compelling evidence of homology with a particular conulariid genus. However, unlike the periderm of Phanerozoic conulariids, the periderm ofP.ediacaralacks calcium phosphate, a difference which may be original or an artifact of diagenesis or weathering. The discovery ofP.ediacarain the Tamengo Formation corroborates the hypothesis, based in part on molecular clock studies, that cnidarians originated during mid-late Proterozoic times, and serves as a new internal calibration point, dating the split between scyphozoan and cubozoan cnidarians at no later than 542 Ma. Furthermore,P. ediacarareinforces the argument that the final phase of Ediacaran biotic evolution featured the advent of large-bodied eumetazoans, including, possibly, predators.

Time-Sensitive Characteristics of Bedding Shale Deterioration under the Action of Drilling Fluid

Abstract Fractures continuously extend and expand along bedding shale formations under the action of drilling fluid and eventually form a complex fracture network, which greatly reduces the strength of the rock. To determine the effect of the drilling fluid action time on the physical and mechanical properties of shale, nuclear magnetic resonance tests are carried out on shale soaked in oil-based drilling fluid for different soaking times. The fluid absorption rate of shale takes the form of a power function. The equations relating the shale mass, porosity, and permeability to soaking time are established. Then, in a sonic time-difference test, the change in the dynamic elastic parameters with the immersion time are observed. According to a triaxial strength test, the failure form and the deterioration degrees of the layered shale in different loading directions with immersion time are analyzed. Numerical simulation of the deterioration degree of shale under different water content conditions is carried out. When the water content increases, the collapse density of the surrounding rock of the well wall increases significantly. Finally, considering the anisotropy of bedding shale, the inclination angle, azimuth angle, and drilling fluid immersion time are substituted into the rock mechanics parameter deterioration model, the three pressure profiles of the formation are corrected, and the safe drilling fluid density window of the target interval is given.

Dynamic Brazilian splitting experiment of bedding shale based on continuum-discrete coupled method

Exploring the dynamic mechanical properties of shale helps to understand the damage of shale reservoirs under dynamic loading. In this paper, static Brazilian splitting experiments were conducted to obtain the static Brazilian strength of shales with loading angles ranging from 0° – 90° (at 15° intervals), and corresponding bedding shale models were developed based on the discrete element method. Then, the split Hopkinson pressure bar system was simulated according to the coupling of the continuous-discrete method, and dynamic Brazilian splitting simulations with loading rates of 100 – 170 GPa/s were carried out. The experimental results show that the Brazilian strengths of the shale at a loading rate of 0.05 mm/s are 4.73 MPa – 7.63 MPa, and the shale matrix and bedding can be simulated using the Flatjoint and Linearpbond models, respectively. The model parameters calibrated by static Brazilian splitting experiments were not applicable under dynamic loading; the microscopic tensile strength of the matrix and bedding increased by 2.7 and 3.1, and the microscopic cohesion increased by 1.8 and 1.7, respectively, for dynamic simulation tests. At a loading rate of 115 GPa/s, the dynamic tensile strength of the shale was 14.5 MPa – 18.2 MPa and increased with the increase of loading angle. There is a linear correlation between the dynamic tensile strength and the loading rate. The loading rate affects the damage pattern of the shale, with cracks tending to expand along the bedding. As the loading rate increases, the primary energy dissipated changes gradually from fracturing the rock to an increase in kinetic energy of the broken fragments.

Study on vertical multifracture propagations in deep shale reservoir with natural fracture network

AbstractBuilding a complex fracture network in a deep shale gas reservoir during multicluster hydraulic fracturing is challenging. On the one hand, hydraulic fractures tend to propagate along shale bedding planes, thus leading to a limited hydraulic fracture height; on the other hand, under the effect of high in situ stress and weak plane of fractures, multihydraulic fractures can hardly initiate and propagate uniformly. All of these will jeopardize the performance of hydraulic fracturing. This paper considers the influence of natural fracture network, shale beddings, fracturing‐fluid leakage, and high in situ stresses, and builds a numerical model to simulate multifracture propagations along vertical direction during hydraulic fracturing in deep shale reservoirs. The model is developed based on the theory of solid–fluid coupling and finite element method with pore pressure cohesive element technique are used to simulate multicluster fracture initiation and propagation. Moreover, the model is verified by published experiment data, and a good agreement between model and experiment results is observed. The influences of pump rate and fracturing‐fluid viscosity on fracture geometry are discussed, and the flow dynamics around each perforation cluster under different perforation parameters are also studied. Finally, suggestions to improve overall outcome of multicluster hydraulic fracturing are also provided.

Revisions to the Wanakah Formation in and around Ouray County, Colorado

A well-defined type section and consistent nomenclature are fundamental requirements for successful correlation of stratigraphic units and subsequent accurate paleogeographic reconstructions. The exact location of the original type section of the Middle Jurassic Wanakah Formation of the eastern Colorado Plateau was found in Ouray County, Colorado. Two nearby reference sections are described that supplement the lithologic descriptions in the original type section. These reference sections and the original type section constitute a “principal reference section” for the Wanakah Formation. Informal names “upper shale beds”, “marl member”, and “beds at Sawpit” have been used to describe the uppermost member of the Wanakah. These terms are replaced with the formal name, “Crooked Tree Member of the Wanakah Formation”, in the reference sections to avoid confusion, foster consistent usage, and facilitate litho-stratigraphic correlation.

Petrography and mineralogy of rinded ferrous‐carbonate concretions in the Middle Eocene carbonate rocks: A case study from the Avanah Formation in north‐east Erbil City, northern Iraq

The spherical rinded concretions in the carbonate rocks of the Avanah Formation (Middle Eocene) in the Gomaspan section, north‐east Erbil City, northern Iraq have been studied for the first time. They occur within the grey to yellow thick‐bedded marly dolomitic limestone interbedded with thin beds of grey shale in the lower part of the formation. Their diameter varies between 1 and 3 cm, are mostly spherical in shape, and characterized by brownish to dark grey isopachous rinds or cortex and brownish‐red centre. The microscopic study shows a similarity between the concretions and their host rocks in their microfacies and fossils contained. The main microfacies is dolomitized lime mudstone and contains Valvulina sp. and pelecypods. The concretions are affected by selective diagenesis that resulted in the colour changes between the centre and outer rims of the concretions. X‐ray diffraction (XRD) analysis of concretions and host rocks revealed the presence of dolomite, calcite, quartz, illite and haematite, the latter exists only in the concretions. Meanwhile, the scanning electron microscopy (SEM) and EDX analyses supported the results of (XRD) and show that the iron oxide (haematite) mainly was concentrated in the outer orange to red cortex. While the dark black inner cortex is characterized by containing less amount of Fe as compared with Ca, O, C and Mg in addition to illite, which also exists in the carbonate host rocks. Field observations, petrographic and mineralogical analyses revealed that the studied concretions had undergone four stages of formation, which consequently include dissolution of semi‐rounded fossils such as pelecypods and forming the moulds during the deposition of the carbonate rocks of the Avanah Formation, precipitation of calcite minerals in the moulds and formation of calcite concretions syn‐depositionally, partial dolomitization of calcite in meteoric‐marine mixed zone and formation of rinded iron oxide concretion by selective diagenesis in meteoric water conditions by the action of acidic water to form haematite.

Assessment of Chemical Effects on Wellbore Stability in Transition Shale Formation

Substantial research has been conducted on the mechanisms responsible for the wellbore collapse in shale formations. For chemically active soft shale formations, pore pressure and stress change due to osmotic pressure and hydration swelling are usually recognized as the primary mechanisms, while for chemically inactive hard shale formation, wellbore instability is generally attributed to weak structural planes like bedding planes and microcracks. In this paper, experimental testing and analytical and numerical analyses are performed to reveal the dominant mechanism of the frequently encountered severe wellbore instability events in the middle-deep shale formation in the Bohai oil field of China. It is evidenced from the physical and chemical experimental testing that the middle-deep shale features both medium chemical activity and abundant bedding planes and microcracks, indicating that the middle-deep shale is in the transition process from chemically actively soft shale to the chemically inactive but laminated and fractured hard shale. Mechanical testing also shows considerable strength degradation of the middle-deep transition shale due to drilling fluid-shale interaction. Analysis through a hydro-chemo-mechanical coupling theory shows that the extent of the damage zone around the wellbore is limited if only pore pressure change and hydration swelling caused by the chemical difference between the drilling fluid and the formation fluid are considered, which cannot explain the severe wellbore collapse in the drilling process. In contrast, when pore pressure increase and strength degradation of the shale due to drilling fluid penetration along the bedding planes and microcracks are taken into account, a damage zone of 3~4 times of the wellbore diameter can be generated, implying that the dominant mechanism of the wellbore instability in the middle-deep transition shale formation should be the pore pressure change and strength degradation resulted from drilling fluid penetration along the bedding planes and microcracks.

Field Description and Lithostratigraphic Study of Sargelu Formation in Rania and Sargelu sections, Northern Iraq

The Sargule Formation was deposited during the Middle Jurassic, representing one of the most important source rocks in Iraq and surrounding areas.
 Jurassic successions are commonly exposed as an isolated patch at some eroded anticline’s cores and limbs in the High Folded Zone, Imbricate Zone, and Thrust Zones of northern Iraq. The Sargelu section is located in Sargelu village at Surdash anticline, while the Rania section is located in Rania city near Hanjira village at Shaweri anticline. Fieldwork in Rania and Sargelu Sections allow to divide Sargelu Formation lithostratigraphically into many subdivisions as follow:
 The Sargelu Formation in the Rania section is about 40 m and can be divided into two lithologic units; the upper thin to medium bituminous limestone beds alternated with thin chert beds, and the lower thin to medium bituminous limestone beds alternated with fissile gray shale beds.
 the The Sargelu section comprises 180m and is divided into three units; the upper unit consists of dark gray limestone beds and thin brownish-black of laminated shale. The middle unit includes dark chert alternated with dark gray bituminous limestone and brownish-black laminated shale. At the same time, the lower unit consists of dark gray bituminous limestone beds alternated with medium brownish-black laminated shale beds.

Geochemical and hydrological evolution of mine impacted waters at the Argyle Diamond Mine, Western Australia

The lamproite ore body at the recently closed Argyle diamond mine is geochemically benign, but the surrounding sedimentary sequence contains two thin pyritic black shale beds that are prone to acidification. Although the waste rock is on average net neutralizing, some localized acidification and sulfate release was first observed in 1999 after 15 years of open pit mining. Contact and receiving water quality have been actively managed and monitored for 20 years to further limit the already low environmental risks posed by nitrate, nickel, magnesium and sulfate. All four solutes exhibited increasing concentrations across the monitoring network during or shortly after the period of peak waste rock production from 1999 to 2004. Since open pit mining and associated rock blasting ended in 2013, concentrations of nitrate have declined by 50%–90% at most locations. Nickel concentrations have also declined by 30% at the two acidic waste rock seeps which account for 80% of the dissolved nickel load. Sulfate and magnesium concentrations generally peaked between 2010 and 2013 and have not exhibited any significant increasing or decreasing trends since then. Annual offsite sulfate release since 2014 has averaged about 0.4% of the total sulfur inventory stored on site. Future sulfate concentrations and loads are predicted to gradually decline as the remaining sulfur inventory is depleted. Even during the 2005 to 2008 period, when all solute release rates were near their maximum, the receiving aquatic ecosystems remained intact with only subtle changes to some invertebrate communities. It is clear that peak release rates have already passed. The geochemical evolution at Argyle is much more rapid than at most other large mines. This is likely due to several factors including: 1. the low mean sulfur concentrations in the black shales; 2. the short geochemical lag time in the reactive units; 3. the high annual rainfall; and 4. a black shale segregation program that was initiated in roughly 1996.

Engineering Properties and Slope Inventory of Clayey Soil from the Trusmadi Formation in Bundu Tuhan, Sabah

The study area in Bundu Tuhan mainly consists of the metasedimentary rock of the Trusmadi Formation. Trusmadi Formation aged Palaeocene to Middle Eocene made up of interbedded dark grey shale and thin sandstone that shows the characteristics of deep marine sediment. Shale bed also known as phyllite unit is the major rock unit in Trusmadi Formation that is usually weathered and produced clayey soil. Clayey soil is often acknowledged as problematic soil due to its properties that tend to hold water highly compared to sandy soil, which will cause further engineering problems due to the soil creeping. The study area is prone to landslide activity, especially in the Trusmadi Formation’s slopes. Hence, this paper aims to determine the engineering properties of clayey soil from the Trusmadi Formation. Two slopes were also selected for laboratory analysis and slope inventory. The results of the analysis show that both samples are poorly sorted materials which are classified as sandy and silty clay soil and silty clay soil. The moisture content is 25.98% to 37.10%, specific gravity ranged from 2.52 to 2.57, and plasticity index ranged from 21.68% to 23.91%. The soil has inactive to a normal type of clay (0.59 to 0.97) with a very high swelling capacity (9.29% to 11.43%). The maximum dry density (MDD) ranged from 1.44 to 1.53 and optimum moisture content (OMC) of 25.47% to 25.67%. Also, both samples are classified as impermeable soil with a K value ranging from 1.74x10−8 m/s to 2.45x10−8 m/s. While, based on the slope inventory inspection, both slopes were comprised of metagreywacke, where the first slope was an embankment slope, and the second slope was a cut slope. The first slope has poor vegetation cover which leads to moderate erosion and instability that further caused failure which is rotational debris slide. As for the second slope with average vegetation cover, there is only minor erosion and instability present on the slope, and this slope also shows slow soil movement that indicates soil creep.

Petrography and Geochemistry of Sandstones of Eocene Kopili Formation, Shillong Plateau: Implications on Paleo-weathering, Provenance and Tectonic Setting

ABSTRACT Geochemical and mineralogical studies were carried out on the sandstones of the Eocene Kopili Formation, Shillong Plateau to evaluate paleo-weathering, provenance and tectonic setting. The study area is located on the bank of the Lubha River along the Badarpur-Jowai road section. The Kopili Formation is represented by a sequence of alternation of shales and sandstones. The sandstone beds are usually thin, fine grained, ferruginous and mostly parallel laminated. The thickness of the sandstone beds increases towards the top with decreasing numbers of shale beds. Petrographically the sandstones are dominated by monocrystalline and polycrystalline quartz followed by feldspar, mica and rock fragments. The matrix content does not exceed 15% and hence the sandstones can be classified as arenite. The Kopili sandstones are classified as quartz arenite to sublitharenite, and arkose to sublitharenite based on their petrographic and geochemical parameters respectively. Arenitic composition and high SiO2/Al2O3 ratio indicate that the Kopili sandstones are highly mature. The weathering indices mostly indicate low to moderate (except 49D sample) degree of chemical weathering under arid to semi humid climatic conditions. The major and trace element-based diagrams and their ratios indicate that the Kopili sandstone received sediments from felsic dominated source with minor contribution from basic source rocks and are deposited in tectonically passive margin setting.

A novel wettability index and mineral content curve based shut-in time optimization approach for multi-fractured horizontal wells in shale gas reservoirs

An appropriate shut-in after fracturing can dramatically enhance performance of wells in shale gas reservoirs. In this study, a novel wettability index and mineral content curve based shut-in time optimization approach was proposed. First, a new experimental device for spontaneous imbibition of fracturing fluid under the conditions of formation temperature and confining pressure was designed. Then, we conduct a set of experiments on shale samples from the Longmaxi Formation (LF) in the Sichuan Basin to measure the water-wet index and determine the critical value of clay content. Finally, the low-field nuclear magnetic resonance (NMR) equipment was used to monitor the water flow in shale micro-pores, based on this, the shut-in time for different shale reservoirs was optimized. The experimental results show that LF shale hydration can induce new micro-fractures, and the fractures shape is mainly shale bedding fractures, with obvious directionality. The clay content is the key factor affecting water wettability of shale and the critical clay content value of type I and type II shale reservoirs was about 39.5%. The optimal shut-in time of type I and type II shale reservoirs are about 20 days and 15 days respectively. Field application of three wells showed that the average daily gas production was increased by 2–3 times. The research results can guide the development of shale gas reservoirs effectively.

Composition of Formation Water and Natural Gas as an Indicator of Shale Gas Preservation Conditions: A Case Study of the Marine Shale of the Niutitang Formation in the Cengong Block in Northern Guizhou, China.

Shale gas (SG) wells in the Wufeng–Longmaxi Formation in northern Guizhou differ considerably in their production capacities. Preservation conditions are a crucial factor affecting the formation of SG reservoirs. In this study, the formation water (FW) in four wells in northern Guizhou was analyzed to determine the type; the Cl/Mg, Ca/Mg, and Na/Cl coefficients (CCl/Mg, CCa/Mg, and CNa/Cl, respectively), and the coefficient of desulfurization (Cd); the SG in these wells was tested to identify its composition and the sources of its components. The results show the following: wells TX1 and CY1 contain two types (CaCl2 and NaHCO3) of FW, each with low CNa/Cl, low Cd, high CCa/Mg, and high CCl/Mg, suggesting a high level of FW retention. The FW in wells TM1 and CD1 is of NaHCO3 type and is characterized by high CNa/Cl, high Cd, low CCa/Mg, and low CCl/Mg, indicating a high level of connectivity between the FW and surface water. Nonhydrocarbon gases account for a high proportion of SG in composition; with a 15N value between −8.7 and −4.2‰, N2 accounts for approximately 20% of SG in wells TX1 and CY1, respectively. This result indicates the ammonification of organic matter (OM) during pyrolysis and hydrocarbon generation as sources of N2. In contrast, with a 15N value between −3.6 and 0‰, N2 accounts for over 70% of SG in wells TM1 and CD1, respectively, indicating the atmosphere, the ammonification of OM, and the presence of poor preservation conditions due to a high level of connectivity between the shale bed and the atmosphere acted as the sources of N2. The metamorphism coefficients of FW in conjunction with the SG composition and the sources of nonhydrocarbon gases can serve as auxiliary indicators of the quality of SG preservation conditions.

Fracturing of shales with slots under uniaxial compression parallel to bedding layers

Shale samples containing single pre-machined slots were tested under uniaxial compression directed along with bedding layers. Observed fractures were initiated by the slots, however, their further development was controlled by the shale bedding layers. Slots normal to the loading direction also cause fracturing and reduce failure load. The fracturing is initiated by tensile stress generated at the slot surface due to convergence of the initial open slot. Therefore, the presence of initial aperture in pre-existing cracks or fractures is capable of changing the mechanism of fracture generation. Three kinds of macroscopic fractures are observed: tensile fractures along bedding layers, shear fractures along the inclined direction, and buckling fractures at the final failure moment. Three types of microcracks are identified: shear microcracks produced from initial weak micro-planes unrelated to the bedding layers, tensile microcracks produced from the shear microcracks in the form of wing cracks, and shear microcracks induced by high uniaxial compression stress. The growth of wing cracks can be assisted by the bedding layers. The research results will contribute to the understanding of failure mechanisms of rocks containing prominent bedding layers.

Mechanical Properties of Lamellar Shale Considering the Effect of Rock Structure and Hydration from Macroscopic and Microscopic Points of View

It is well known that the effect of shale hydration causes wellbore instability due to water phase invasion of drilling fluid to lamellar shale rich formations. This is because of that the mechanical properties (compressive strength, elastic modulus, etc.) of lamellar shale surrounding the borehole, which is rich in clay minerals, will decrease significantly after hydration. In this study, using the lamellar shale in the continental stratum of the southern Ordos Basin, the mechanical properties of lamellar shale were studied by compression tests considering the effect of lamellar structure and hydration from a macroscopic point of view. In addition, the mechanical mechanism was discussed combined with the CT scanning tests results from a microscopic point of view. The results demonstrate the following points. Lamellar shale has stronger anisotropy than bedding shale, the compressive strength (deviatoric stress) and elastic modulus of lamellar shale are both lower than those of bedding shale, and it is more prone to tension fracture. With the increase in the angle (β) between the lamina and the axial direction from 0° to 90°, the compressive strength of lamellar shale decreases when β < 30° and then increases, the elastic modulus of lamellar shale decreases greatly when β < 30° and then tends to flatten. With the increase in hydration time, the compressive strength and elastic modulus of lamellar shale both gradually decrease, and the rates of their decrements reduce. The mechanical properties of lamellar shale are more affected by hydration than those of bedding shale. The hydration of lamellar shale leads to the formation of new fractures and the expansion of existing fractures in the junction area between the laminae and rock matrix, resulting in easy tension fracture along the laminae of shale.