Presence Of Microfractures Research Articles

Experimental study on hydraulic fracture propagation behavior in heterogeneous shale formations

The study of fracture propagation in heterogeneous shale is a crucial prerequisite for the investigation of heterogeneous cluster and perforation parameters optimization. In this paper, we conduct a physical simulation fracturing experiment on heterogeneous shale to investigate the effects of various influencing factors, such as shale bedding, near-wellbore fractures, lithological changes, and the presence of fractures surrounding the perforation hole, on fracture propagation law and morphology. Our research demonstrates that during shale fracturing, shear dislocation typically occurs between layers, resulting in the separation of different layer planes. The main fracture primarily propagates through layers in a stepped manner. The presence of sandstone in heterogeneous shale significantly impedes fracturing fractures, causing significant distortion and deviation. As the scale of natural fractures increases, it tends to cause the fracturing fracture to twist and change direction. The natural fractures network can also lead to the distortion of fracturing fractures, albeit to a lesser extent than large-scale natural fractures. The presence of micro fractures parallel to the perforation axis surrounding the perforation hole enhances the ability of the main fracturing fractures to pass through natural fractures.

Study on Electrical Characteristics and Formation Mechanism of Micro-Ion Capacitor in Shale Pore and Fracture Structure

Due to the micro/nanoscale and intricate pore structure, poor connectivity, complex pathways, the presence of microfractures, and the coexistence of organic and inorganic pores, shale and other tight reservoirs exhibit increasingly complex conductive characteristics. This paper mainly studied the electrical properties of shale based on experimental test data, including scanning electron microscope (SEM) thin-section and microtomography CT (μ-CT) images, and analyzed the shale pore type, pore structure characteristics, and development of fracture. Then, the distribution of pyrite, content, and graphitization of organics and their influence on the electrical properties are discussed. Furthermore, the double electrical layer and zeta potential in the shale zone were discussed in depth. The results revealed that, within the content of pyrite, organics, and its graphitization, the vitrinite maturity are inversely proportional to shale resistivity. It was also found that in the presence of an external electromagnetic field, the fluid in shale pores is subjected to the combined strength of pore pressure and external field potential difference. Thus, its response equation should be an improved Navier-Stokes equation, which considers pore pressure, zeta potential, and Coulomb force. When shale is subjected to an external electromagnetic field, due to the complex pores structure and organic and inorganic minerals, it will represent more of a dielectric-like property than electricity. So, it will form special microscopic ionic capacitors, which are different from common plate capacitors. There are three special kinds of microscopic ionic capacitors, they are (I) the intergranular pore microscopic ionic capacitor model, (II) the particle with isolated pore microscopic ionic capacitor model, and (III) the pyrite or graphite or other organics microscopic ionic capacitor model. Finally, the characteristics of microscopic ion capacitors are summarized: irregular polar area and varying distance between poles, varying charges with time, and salinity of the formation water.

Pointed bone tools from the Hamangia culture on the Lower Danube: Manufacture and function

This study focuses on the pointed bone tools from the Hamangia culture (Chalcolithic period, late 6th millennium BC - early 5th millennium BC), obtained from the long bones of caprines by quadri-partition with double grooving. The pointed bone tool assemblage from the Hamangia settlement at Cheia-Vatra Satului (Romania) was selected as a case study for analysing technological manufacturing sequences of these specific typological tools. Our study was two-fold: 1. identifying the sequence of the procedures in the chaîne opératoire from the original bone to the finished pointed tool; 2. determining the functionality of these tools (as multipurpose items or designed for a single activity only). For the purposes of this research, we focused on the diagnostic pieces such as preforms, blanks, and manufacturing waste, rather than the finished pieces, in an attempt to reconstruct the entire sequence of the manufacturing procedures, to visualize their transformation from the bone to the finished object. This experimental approach resulted in detailed records of the raw material used, the time necessary for each type of procedure, the tools used, and the results obtained during each operation. Next, the experimental tools were tested in various scenarios, such as hide perforation, bone breaking, shell perforation, wood processing, vegetable fiber production, etc. The active ends of these items were then observed, with variables such as frequency and distribution of polish, micro-topography, pattern of functional striations, presence of microfractures, and use-wear areas. The observations made on the archaeological specimens were compared to those on the experimental replicas in order to test the hypotheses regarding the ways the pointed tools were manufactured and used by the prehistoric communities.

Study of nonlinear flow mechanisms and microfracture networks in low-permeability reservoirs

As efficient technologies boost oil yields and economic benefits, horizontal wells and hydraulic fracturing are widely used in low- permeability reservoirs. To better evaluate the reserve and improve recovery, it is essential to determine fluid flow patterns and transport mechanisms. Laboratory experiments, field operations, and analytical studies have identified nonlinear flow and microfracture networks during the fluid flow in a reservoir with fractured horizontal wells. However, the interactions between nonlinear flow and microfracture networks are still not fully understood. In this study, nonlinear flow experiments and triaxial compression tests were carried out to analyze nonlinear flow characteristics in the vicinity of microfracture networks. By analyzing the effects of microfracture networks on nonlinear flow, two-phase flow, rock stress sensitivity, and artificial fractures, we found that fluid capacity in capillaries with smaller dimensions decreased along with a drop in the pressure gradient, generating a nonlinear flow pattern. The area of nonlinear flow was diminished by the presence of microfractures, which improved flow efficiency and reservoir quality. Considering the size of fracture apertures, microfractures behave more like matrix pores than natural fractures. Also, microfractures significantly increase rock stress sensitivity and reduce the threshold permeability, which enhances fluid flow capacity. This study contributes to our understanding of flow behavior, predicting production and improving recovery in low-permeability reservoirs.

Reservoir quality and facies modeling of the early Eocene carbonate stratigraphic unit of the Middle Indus Basin, Pakistan

The early Eocene carbonate reservoir, Sui Main Limestone (SML), is the largest gas reservoir in Pakistan. In the Qadirpur area, more than 30 wells have been drilled, some of which have been declared dry or abandoned due to poor reservoir characteristics or facies variation. The aim of this work is to re-evaluate the reservoir characteristics and facies identification of Sui Main Limestone by using petrophysical analysis and rock physics modeling in the Qadirpur field in the Middle Indus Basin of Pakistan. The reservoir characterization of carbonate rocks is difficult because of their complex pore networking. The well data on five exploratory wells drilled in this gas field are used. The log-derived porosity varies in a very large range of 2%–36%, with an average of 14%–34%. The average porosity of the clean carbonate intervals is 10%–14% and thus has the capacity to accumulate hydrocarbon. The high porosity value indicates the presence of micro-fractures in Sui Main Limestone. These micro-fractures and secondary pores are well interconnected and allow the pore fluids to communicate. The secondary porosity is mainly due to the presence of vuggs and fractures in Sui Main Limestone. The volume of shale varies from 11%–38% in the wells. The net pay zones have hydrocarbon saturation in the range of 40%–70%, which is mainly gas. Using wireline log response, the formation is divided into three facies: limestone, shale, and shaly limestone. This comprehensive work may help improve the prediction of the reservoir quality in heterogeneous carbonate reservoirs and optimize field development.

Laboratory Testing and Evaluating of Shale Interaction with Mud for Tanuma Shale formation in Southern Iraq

Rock failure during drilling is an important problem to be solved in petroleum technology. one of the most causes of rock failure is shale chemical interaction with drilling fluids. This interaction is changing the shale strength as well as its pore pressure relatively near the wellbore wall. In several oilfields in southern Iraq, drilling through the Tanuma formation is known as the most challenging operation due to its unstable behavior. Understanding the chemical reactions between shale and drilling fluid is determined by examining the features of shale and its behavior with drilling mud. Chemical interactions must be mitigated by the selection of suitable drilling mud with effective chemical additives. This study is describing the laboratory methods that concern testing and evaluating the shale instability encountered while drilling operations. The cutting samples are collected from the targeted formation and used to categorize shale reactivity levels and the required additives to inhibit the clay instability. These tests include the descriptive method with the various analytical technique of standard laboratory equipment. The shale testing techniques are the Scanning Electron Microscope (SEM), X-ray Diffraction, X-ray Fluorescence, Cation-Exchange, Capacity (CEC), and Capillary Suction Timer test (CST). Also, Linear swelling meter test (LSM) was performed to enhance the development plan. Tanuma formation contains moderately active clay with the presence of microfractures and micropores in its morphology. And it is controllable by using polymer muds with 8 % of inorganic inhibitor (e.g., KCL), filtration controls additives, and poly amino acid hydration suppressant which showed minimum swelling percentage.

Dental microfracture detection using wavelet features and machine learning.

Microfractures (cracks) are the third most common cause of tooth loss in industrialized countries. If they are not detected early, they continue to progress until the tooth is lost. Cone beam computed tomography (CBCT) has been used to detect microfractures, but has had very limited success. We propose an algorithm to detect cracked teeth that pairs high resolution (hr) CBCT scans with advanced image analysis and machine learning. First, microfractures were simulated in extracted human teeth (n=22). hr-CBCT and microCT scans of the fractured and control teeth (n=14) were obtained. Wavelet pyramid construction was used to generate a phase image of the Fourier transformed scan which were fed to a U-Net deep learning architecture that localizes the orientation and extent of the crack which yields slice-wise probability maps that indicate the presence of microfractures. We then examine the ratio of high-probability voxels to total tooth volume to determine the likelihood of cracks per tooth. In microCT and hr-CBCT scans, fractured teeth have higher numbers of such voxels compared to control teeth. The proposed analytical framework provides a novel way to quantify the structural breakdown of teeth, that was not possible before. Future work will expand our machine learning framework to 3D volumes, improve our feature extraction in hr-CBCT and clinically validate this model. Early detection of microfractures will lead to more appropriate treatment and longer tooth retention.

Influencing factors and fracability of lacustrine shale oil reservoirs

Abstract Shale oil reservoirs need fracturing in order to achieve commercial productivity. A key factor influencing the fracturing process is shale fracability, which is strictly dependent on rock fabric. In this study, we focused on the lacustrine shale oil reservoirs located in Dongying Sag and in the Northern Jiangsu Basin. Through X-ray mineral diffraction, rock mechanics, and P- and S-wave velocity tests, we analyzed the mineral components and mechanics of different shale lithofacies, as well as the influence of these factors on shale fracability. Based on the results of these analyses, we proposed a new method for the evaluation of lake shale oil reservoir fracability. The experimental results showed that the macrostructure, mineral content, lithology, and the presence of microfractures in the shale are the main factors affecting its brittleness. Shales with lamellar and sheet-like structures have relatively high Young's modulous or lower Poisson's ratios; under these conditions, netted fractures are easily formed during fracturing. The fracture toughness of carbonate minerals in shales is strong. An appropriate amount of carbonate minerals seemed to increase the shale brittleness; however, an excessive amount increased its cohesiveness, hindering the initiation and propagation of fractures. Felsic minerals are characterized by high brittleness, a characteristic that favors shale fracability; on the other hand, clay minerals and organic matter are highly plastic, a characteristic that hinders fracturing transformations. Due to their macrostructure and composition, lamellar clayey limestones (dolomite), lamellar clayey siltstones, layered calcareous (dolomitic) siltstones, sheet-like clay-containing siltstones, and flaggy carbonate-containing siltstones are expected to be easily fractured. In addition, the presence of microfractures seemed to reduce the overall fracture toughness of the rock: they penetrated artificial fractures, enhancing rock fracability. Taking into account the fracture toughness of carbonate minerals and the plastic effect of organic matter, we proposed a new formula for the calculation of the brittleness index, based on the mineral component method. The brittleness indices obtained through the proposed formula were linearly correlated with those calculated through the rock mechanics parameters method. Overall, our results indicate that the silty lithological successions occurring in the study area should be the easiest to fracture, followed by the calcareous (dolomitic) and clayey or organic-rich lithological successions. This study is of guiding significance for evaluating the fracability of different lithofacies in lacustrine shale oil reservoirs. Especially, the new proposed brittleness index containing the plastic effect of organic matter can correctly evaluate the actual brittleness of organic-rich shale.

Experimental study on fluid flow in micro-fractures contained within matrix

Two types of artificially fractured cores were prepared in the lab to investigate the effects of micro-fractures on fluid flow in matrix. Using naturally fractured cores and cores without fractures as control group, we analyzed fluid flow behavior in micro-fractures and their effects on fluid flow in matrix itself from the perspectives of threshold pressure gradient and oil displacement efficiency. The experimental results show that, for cores with similar physical properties, the presence of micro-fractures can significantly reduce the real threshold pressure gradient but has negligible effect on pseudo threshold pressure gradient. The matrix system should be classified as parallel or series system based on its relative location to macro-fractures. Pressure gradient in parallel matrix system is usually small and its real threshold pressure gradient is the key factor deciding whether the oil inside can be recovered. On the other hand, pressure gradient in series matrix system is generally large and its pseudo threshold pressure gradient is the key factor deciding whether effective injection-production communication can be established between two wells. Oil displacement efficiency in micro-fractures increases with the increase of micro-fracture aperture and their relationship can be approximated by a quadratic function within a certain range. However, the relationship between oil displacement efficiency in matrix and micro-fracture aperture is not monotone. Pore volumes of micro-fractures, oil displacement efficiency in micro-fractures, and water channeling within micro-fractures are the three main reasons accounting for this phenomenon.

How microfracture roughness can be used to distinguish between exhumed cracks and in-situ flow paths in shales

Flow through fractures in shales is of importance to many geoengineering purposes. Shales are not only caprocks to hydrocarbon reservoirs and nuclear waste or CO2 storage sites, but also potential source and reservoir rocks for hydrocarbons. The presence of microfractures in shales controls their permeability and transport properties. Using X-ray micro-tomography and white light interferometry we scanned borehole samples obtained from 4 km depth in the Pomeranian shales in Poland. These samples contain open exhumation/drying cracks as well as intact vein-rock interfaces plus one striated slip surface. At micron resolution and above tensile drying cracks exhibit a power-law roughness with a scaling exponent, called the Hurst exponent H, of 0.3. At sub-micron resolution we capture the properties of the clay interface only, with H = 0.6. In contrast, the in-situ formed veins and slip surface exhibit H = 0.4–0.5, which is deemed representative for in-situ fractures. These results are discussed in relation to the shale microstructure and linear elastic fracture mechanics theory. The data imply that the Hurst roughness exponent can be used as a microstructural criterion to distinguish between exhumation and in-situ fractures, providing a step forward towards the characterization of potential flow paths at depth in shales.

Scaling Laboratory-Data Surfactant-Imbibition Rates to the Field in Fractured-Shale Formations

Summary By use of existing methods, typical oil-recovery factors from the Bakken and other shale formations are low, typically less than 5% of original oil in place (OOIP). We are investigating the use of surfactant imbibition to enhance oil recovery from oil shale or other tight rocks. Much of our previous work has measured surfactant-imbibition rates and oil-recovery values in laboratory cores from the Bakken shale, Niobrara chalk/shale, and Eagle Ford formations. With optimized surfactant formulations at reservoir conditions, we observed oil-recovery values up to 20% of OOIP incremental over brine imbibition. However, whether surfactant imbibition will be a viable recovery process depends on achieving sufficiently high oil-production rates in a field setting. This, in turn, depends on three factors: the area of formation contact (through fractures and microfractures) when/where the surfactant formulation is introduced; the rates of surfactant imbibition; and the distances of surfactant imbibition into the rock and ultimate oil-displacement effectiveness. In this paper, we use analytical models to scale laboratory surfactant-imbibition rates to a field scale in fractured-shale formations. In laboratory cores, we observed imbibition rates that varied inversely with time. Dimensionless scaling groups were applied that compensate for the effects of sample size and shape, boundary conditions, permeability, porosity, and viscosity. Calculations were made of available fracture area, assuming typical horizontal-well lengths and transverse-induced-fracture spacing in typical Bakken wells. These fracture areas were coupled with our imbibition-scaling groups to estimate oil-recovery rates in a field setting. Considering realistic timing, surfactant imbibition will generally not proceed more than a few meters into the low-permeability shale/chalk formations. These calculations indicate insufficient fracture area to provide a viable imbibition process if only the induced-fracture area is considered. However, recent results from geological, microseismic, and pressure-transient studies indicate considerably greater area associated with natural microfractures in our target formations. When the increased area suggested by the presence of microfractures is included in our analyses, the surfactant-imbibition process appears quite promising.

Acute Kidney Injury Secondary to NSAID Diagnosed on 99mTc MDP Bone Scan

A bone scan with 99mTc MDP was obtained to rule out the presence of micro fractures in a patient with the diagnosis of idiopathic osteoporosis. There was not any sign of micro fractures, but interestingly, both kidneys were diffusely very active. A differential diagnosis of acute kidney injury secondary to the use of nonsteroidal anti-inflammatory drug was made and reported after elimination of other clinical situations. The renal functions obtained following bone scan were impaired. The anti-inflammatory drug was discontinued. The renal functions were recovered starting with the following day.Conflict of interest:None declared.

Stress-induced anisotropy in brine saturated shale

SUMMARY This paper reports the intrinsic and crack-induced anisotropic properties of a set of preserved, brine-saturated shale samples and their response to external stresses. We used undrained multistage triaxial tests to evaluate how the ultrasonic wave velocities and their anisotropy changed with increasing isotropic and differential stress conditions. In addition, the impact of stress orientation with respect to fabric orientation was evaluated. An array of ultrasonic transducers allowed to measure five independent wave velocities which were used to calculate the elastic properties of the shale. Results indicate that in this shale P- and S-wave velocities vary with stress in a different manner dependent on the maximum principal stress orientation with respect to the fabric. Where the maximum stress is normal to bedding, Vpv and Vs1 increase monotonically with increasing effective stress. However Vph and Vsh decrease during individual loading stages but increase from stage to stage as confining pressure increases. The reverse occurs when the microfabric is parallel to the maximum principal stress. Where the maximum stress is bedding normal, velocity anisotropy decreases as differential stress increases; when maximum stress is fabric parallel, anisotropy increases. Elastic anisotropy is related to the initial composition and the spatial distribution of the different minerals (fabric) in the sediment and the presence of microfractures, while changes in elastic anisotropy result from the applied stresses, their orientation with respect to the rock fabric and the degree of stress anisotropy.

Impact of fabric, microcracks and stress field on shale anisotropy

Few data are available on shales in terms of seismic to ultrasonic properties and anisotropy, although all are important with regards to imaging problems often encountered in such lithologies. Additionally, mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions. The results presented here are derived from a set of experiments run on Muderong Shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response. This shale was noted to have a linear velocity-mean effective stress response and extremely high anisotropy, both likely the result of the presence of fluid-filled cracks in a low- permeability porous medium. Changes in velocity and Vp/Vs ratios are dependent on both stress and smectite content. S-wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress (<20 MPa) is less sensitive to stress change than P-wave velocity. Vp/Vs ratios are noted to increase in this shale up to 20 MPa effective stress, then decrease slightly due to stress-induced loss of interlayer water in smectite. Intrinsic anisotropy comes from composition, a strong compaction fabric and the presence of microfractures; changes to ultrasonic anisotropy are the result of the magnitude of the stresses, their orientation with respect to the fractures and the degree of stress anisotropy.

Jackfork Group, Southeastern Oklahoma: New Gas Play in the Ouachita Overthrust

Recent discovery of natural gas in sandstones of the Jackfork Group of southeastern Oklahoma has led to revised interpretations of hydrocarbon potential in the Ouachita thrust province of eastern Oklahoma. Jackfork reservoirs consist of fine to very fine grained, submarine-fan lobe sandstones with low matrix permeability and porosity. Production is dependent upon natural fractures, enhanced in most cases by artificial stimulation. Individual wells have produced at rates of 1.3-5.7 Mcf (FOOTNOTE *) per day and possess estimated reserves in the range of 2.0-7.6 Gcf (FOOTNOTE *). Hyperbolic decline suggests gas contribution from the matrix, possibly due to the presence of microfractures. Drilling has concentrated on two subsidiary thrust blocks in the hanging wall of the Ti alley thrust, southern Latimer County. However, recent outcrop, petrographic, and sedimentological analyses have shown that traditional depositional models of the Jackfork are overly limited and that potential Jackfork reservoirs exist across a broad area to the south of the current play. Such analyses have revealed the existence of thick, medium-coarse-grained channel sequences at a number of localities in the Lynn Mountain syncline. Similar sequences may also exist well to the south, in the Boktukola syncline, where thick sand intervals have been identified. Petrographic study of samples from the Lynn Mountain syncline suggests that channel sequences may have significantly higher reservoir quality than is found in productive Jackfork sandstones to the north. Traditional assumptions pos ulating low regional hydrocarbon potential for the Jackfork therefore stand in need of revision.

Microstructure of Coal and Its Influence on Flow of Gas

Abstract This article discusses the results of an investigation carried out to study the microstructure of coal enhancing the understanding of the phenomena involved in release of methane and its movement in coalbeds. Scanning electron microscopy and transmission electron microscopy were used to obtain information about the structure of bituminous coal and its surface properties tike roughness, size and distribution of pores, and presence of microfractures and cleats. Micrographs obtained clearly showed the highly porous nature of coal. Most of the pores, however, appeared to have a very small volume and large surface area, which explains the large quantities of methane that are retained in adsorbed form in coalbeds. On the surface of coal, a network of microfractures and cracks was seen, indicating that it is these fractures that are chiefly responsible for flow of gas. The results illustrate the use of physical models for simulation of gas flow in coalbeds, involving dual flow behavior. Once the gas desorbs, there is diffusion flow in the coal matrix toward the fractures, followed by flow of gas in these fractures toward the gas producing wells. Furthermore, the microstructure of coal explains some of the unusual gas flow characteristics of coal.

Magnitude frequency study of rockfall in Co. Antrim, N. Ireland

AbstractDetailed observations of rockfall were kept on a weekly basis over a two year period. The results indicate that there is both a seasonal distribution of rockfall with maxima in February‐March and November–December, and continuous small scale rockfall throughout the year.There appears to be some connection between rockfall and frost processes, while the peculiar geotechnical properties of the lava flows in county Antrim, in particular the presence of microfractures, would seem to be the main underlying cause for the observed activity.