Development Characteristics Of Fractures Research Articles

Study on the Influence of Coal Face Advancing Distance on Fault-Activated Water Inrush

Water inrush in deep coal mines is closely related to geological structures such as faults. In order to study the influence rule of different coal face advancing distance (CFAD) on floor fault activation and floor rock mass failure characteristics, a mechanical model of fault activation was established with the 1315 working face of Anju coal mine as the research background, and the distribution of shear stress on fault plane and the influence rules of fault-activation under different CFAD are obtained. Then, FLAC3D numerical simulation software was used to further simulate and study the activation rules of floor faults and the characteristics of fracture development in floor rock mass. The results show the following: (1) When the coal face advances to a position of 80 m from the fault, the shear stress on the fault plane is sufficient to cause the fault-activation; When the coal face advances to a position of 20 m from the fault, the shear stress reaches a maximum and the fault-activation will reach a peak; (2) In the fault zone, fractures are derived from the bottom up under mining operation, but the development degree of fractures in the deep part is weaker than that in the shallow part. (3) Due to the existence of faults, a “oblate” damage and failure zone extending to the front and bottom of the coal wall gradually forms in the working face floor. The extension of this damage zone is first connected with the activated fault crack, and has formed advanced water inrush.

Experimental Investigation of the Mechanical Failure Behavior of Coal Specimens With Water Intrusion

Mining fissures formed by coal mining can easily open bedrock and loose aquifers, and can thus cause water–rock interaction. The strength of coal and rock and their pore and fracture development characteristics are closely related to moisture content. Therefore, this paper studied the strength, porosity, fracture propagation, and failure characteristics of coal with different moisture contents. Six representative coal samples with different moisture contents of 0, 3.79, 6.10, 9.17, 11.01, and 11.68% were prepared by non-destructive water immersion experiment, and the samples were analyzed using. Nuclear Magnetic Resonance (NMR) and uniaxial compression acoustic emission experiments. Then, the temporal and spatial variation of the moisture contents of the coal samples and the characteristics of crack propagation and failure were studied before and after water immersion. Non-destructive water immersion and NMR analysis showed that the moisture content increases exponentially with increasing water immersion time and immersion frequency. Additionally, it was shown that with increasing water immersion time and frequency, the moisture distribution within the coal samples changes from uneven to uniform, and micro-pores and meso-pores develop into larger pores with increasing water content. A coupled uniaxial compression–acoustic emission experiment showed that acoustic emissions are closely related to the macroscopic failure mode of the coal sample. With increasing sample moisture content, the degree of sample destruction and the concentration of acoustic emissions both reduced. The increase of moisture content promotes the change of the macroscopic failure mode of the coal samples from tensile failure to tensile–shear composite failure. The results of this study have important reference value for analyzing the stability of coal pillars and surrounding rock under the action of water, and especially for the design of coal pillar dam bodies in coal mines with groundwater reservoirs.

Research on Characteristics and Laws of Fracturing Dust Production in Deep Coking Coal with Similar Material Structure

The coking coal under the influence of the earth stress is easily broken in the mining process, the dust production is large, the particle size is small, and the dust is serious. Considering the characteristics of primary coal dust, the ratio of similar materials is determined by experiment of single axial compressive strength. By means of acoustic emission detection device, uniaxial compression test is carried out for the coking coal similar materials in different structural forms, and the characteristics of fracture development and dust production of the primary dust are studied. The experimental results show that: pulverized coal: gypsum: cement is 0.81:1:1 and 0.2:1:1 the compressive strength of similar materials is 3.5 mpa and 1.5mpa, which meets the test requirements. The regularity of structural pressure of similar material is shown, similar materials with smaller strength and heavy loading deformation, however, the fracture and fracture development, the primary dust production is large. When the axial pressure is low, the low-intensity similar coal test block is vulnerable to crushing deformation at the upper part, the lower high strength coal needs to be damaged when the force is larger.

Developmental characteristics, influencing factors and prediction of fractures for a tight gas sandstone in a gentle structural area of the Ordos Basin, China

The study of fracture development characteristics of tight gas sandstone reservoirs in the gentle structural areas of the Ordos Basin has been a research hot spot in recent years. However, there remains a lack of systematic research in this field. In this paper, we conducted a systematic study of the developmental characteristics, influencing factors and prediction of fractures for the tight gas sandstone in the Lower Permian He 8 Member in the Sulige gas field. The results show that the development degree of the fractures in the He 8 Member is mainly concentrated between 1 and 2 m−1. Meanwhile, the degree of fracture development is positively correlated with the cumulative sand body thickness, porosity and permeability and that it has no obvious correlation with gas saturation. Moreover, the degree of fracture development is positively correlated with the energy storage coefficient (Esc), indicating that the enrichment of natural gas in the tight gas sandstone reservoir of the He 8 Member is obviously affected by the fractures. The fracture index U is constructed by the finite element method and has a positive power exponential correlation with the fracture linear density. Finally, the fracture index was divided into three grades according to its coupling relationship with the gas well productivity.

Fracture development characteristics and controlling factors for reservoirs in the Lower Silurian Longmaxi Formation marine shale of the Sangzhi block, Hunan Province, China

The Lower Paleozoic Silurian Longmaxi Formation shale in the Sichuan Basin of southern China has achieved commercial exploitation in recent years, and has become a rapid growth point in China's oil and gas reserves. However, the Longmaxi Formation on the peripheral margin of the Sichuan Basin has not made substantial progress. Fractures, that as fluid reservoirs and migration channels, play an important role in shale gas exploration and development. Based on data from field outcrop observations, drilling cores, well logging, geochemical tests, microscopic thin section observations, and focused-ion-beam scanning electron microscope (FIB-SEM) combined with energy dispersive spectrometer (EDS) analysis and cathodoluminescence microscope (CLM) technology, the development characteristics of fractures in the Longmaxi Formation SY-5 well in the Sangzhi block of Sangzhi-Shimen synclinorium, NW Hunan Province, are described systematically, and their important roles, primary dominant factors and effectiveness in the shale gas reservoir are discussed. The results showed that tectonic factors are the dominant factors of shale fracture development, followed by nonstructural factors. The fractures were mainly ductile shear fractures, with short extension distances and small apertures, and they had at least two openings in the burial history. The statistical fracture density of drilling cores was no obvious correlated with the contents of total organic carbon (TOC) (0.18%–2.49%) and clay minerals (13.0%–34.0%), and the same with brittle quartz (32%–72%), which indicates that they have controlled the development of fractures together. These results could be used for estimating effective permeability, fracture distribution, numerical simulation, well deployment, hydraulic fracturing and reserves calculation in the processes of exploration and development.

Characterization of fracture formation in organic-rich shales - An experimental and real time study of the Permian Lucaogou Formation, Junggar Basin, northwestern China

Abstract Volume fracturing in horizontal wells represents a principal technology in the effective economic development of unconventional shale oil and gas reservoirs. However, the number of intervals that were effectively fractured by this technology only account for 20%–50% of the total designed sections. Fracture development characteristics and the factors controlling artificial fracture growth in organic-rich shale require detailed evaluation; this cannot be accomplished using microseismic monitoring technology or well-log interpretation. In this paper, formation processes of micro- and nano-scale fractures in the organic-rich shales of the Lucaogou Formation, Jimusaer Sag, Junggar Basin, northwestern China, were presented. The study focused on the application of in situ, nano-scale CT imaging technology, in combination with rock mechanics analysis. Results allowed the visualization of fracture growth in the shale reservoirs, and revealed the influence of organic matter, mineral composition, and pore structure on the formation and distribution of artificial fractures. A three-dimensional dynamic growth model of micro- and nano-scale fractures in organic-rich shale was established. Data revealed that the Lucaogou Formation shale contained dolomite and illite intragranular pores, with a low abundance of organic pores; pore size ranged between 200 nm and 2 μm. Micro- and nano-size fractures were observed. The development of artificial fractures was positively correlated with the loading stress. The initiation stress of new generated fractures was 475 mN, which could break up dolomite, albite, K-feldspar, and the original pore system as well. The original pore system was generally conducive to fracture extension and expansion. As loading stress increased from 50 mN to 515 mN, the sample extension increased from 2 μm to 14 μm, and artificial fracture width increased from 0.3 μm to 10 μm. The total porosity increased from 5.45% to 8.35%, and volume growth rate reached 53.2%. These findings provide valuable insights into the study of fracture growth in organic-rich shales, and have implications for the design of hydro-fracturing in organic-rich shales.

Damage and Failure Evolution Mechanism for Coal Pillar Dams Affected by Water Immersion in Underground Reservoirs

In coal mines, underground reservoir systems can increase the availability of water and are an effective technical approach for the protection and utilization of water resources. The stability of coal pillar dams is the key factor in the safety and stability of these underground water storage systems. However, coal pillar dams must operate in complex environments that combine dynamic-static superimposed stress fields and water immersion; moreover, coal pillar dams subjected to both stress and seepage are more susceptible to damage and even collapse. In this study, a seepage-stress coupling model of a coal pillar dam was constructed using the Universal Distinct Element Code (UDEC) simulation software. This model provides a platform for analyzing the characteristics of fracture development in surrounding rock in active mines and the coupled development of crack fields and seepage fields in coal pillar dams. Methods were developed for (1) calculating the water content for the coal pillar dam numerical simulation model and (2) reducing water immersion weakening. The maximum seepage width of a coal pillar dam subjected to water immersion was obtained, and a damage and failure evolution mechanism for coal pillar dams experiencing flooding was developed. The results provide a scientific basis for enhancing the stability control of coal pillar dams and are of great significance for realizing water conservation in coal mines.

PL油田花岗岩潜山储层裂缝发育特征厘定

PL油田花岗岩潜山储层裂缝发育特征厘定

3D paleotectonic stress field simulations and fracture prediction for marine-continental transitional facies forming a tight-sandstone reservoir in a highly deformed area

Marine-continental transitional facies coal measures that form tight sandstone reservoirs in heavily deformed areas are a new field for natural gas exploration. The degree of fracture development is the core factor in the enrichment of the potential natural gas resources. In this paper, using the Shanxi Formation in southern Qinshui Basin of eastern China as an example, the characteristics of fracture development in tight sandstone reservoirs were studied, and the paleotectonic stress field was reconstructed. On this basis, the fracture development of the area was predicted. The results show that fractures of different scales were developed in the tight sandstone of the Shanxi Formation and that over 90% of these fractures are tectonic. The types of fracture filling in the tight sandstone of the Shanxi Formation are mainly unfilled and half-filled. Fine geological modeling and paleostress simulations were conducted on the II sand group of the Shanxi Formation using the 3D finite element method. The geostress field of the target layer in the Himalayan period was restored. Considering that the tensile and shear fractures of the target layer were both well developed, we constructed a comprehensive rupture rate (IF) to quantitatively predict the level of development of the fractures by combining the Griffith tensile failure criterion and Mohr–Coulomb shear failure criterion. The prediction results are in agreement with the actual geological conditions, which indicates that this method is suitable for characterizing the fractures in marine-continental transitional facies coal measures that form tight sandstone reservoirs in heavily deformed areas.

Fracture development characteristics in the Upper Triassic Xujiahe Formation, western Sichuan depression (China)

Abstract Field joint surveys and electro-imaging logging were performed to study the characteristics of fracture development in the Upper Triassic Xujiahe Formation, which is located in the western Sichuan depression. Systematic joints with extensional characteristics have been identified; these likely formed as natural hydraulic fractures. There are four sets of fractures: nearly E–W trending fractures, nearly N–S trending fractures, NW–SE trending fractures, and NE–SW trending fractures; the nearly E–W trending fractures are the most widely developed. We analyze the timing of tectonic events of the Longmenshan orogenic belt and the western Sichuan foreland basin, terminal relationships, and filling characteristics of the joints. Based on this analysis, we proposed that some nearly E–W striking joints (after restored to the horizontal) formed during the Early Cretaceous (~100 Ma), although the Cenozoic (~60 Ma and onward) has been another important period for the formation of joints. This implies that horizontal, N–S wells could be drilled to reopen the sealed E–W striking joints and to gain access to the more permeable E–W striking joints.

Mineralogy and fracture development characteristics of marine shale-gas reservoirs: A case study of Lower Silurian strata in southeastern margin of Sichuan Basin, China

Mineral contents and fractures of shale from well Yuye-1 and outcrops were examined mainly based on systematic description of the cores and outcrops, and data from experimental analyses. The data enabled us to thoroughly explore the mineralogy and developmental features of shale of the Lower Silurian Longmaxi Formation in the study area. The results show that, the Lower Silurian Longmaxi Shale (SLS) in the southeastern margin of Sichuan Basin, China, is primarily characterized by a high content of brittle minerals and a relatively low content of clay minerals. The total content of brittle minerals is approximately 57%, including 27% quartz, 12.2% feldspar, 11.2% carbonate and 2.4% pyrite. The total content of clay minerals reaches 41.6%, composed of illite (23.8%), mixed-layer of illite and smectite (I/S) (10.8%) and chlorite (7.0%). The SLS accommodates the widespread development of various types of fractures, including tectonic fractures, diagenetic fractures, inter-layer fractures and slip fractures. The developmental level of the fracture in the SLS is mainly influenced by faults, lithology, mineral contents and total organic carbon content (TOC) in study area.

Distribution Characteristics of Fractures in the Ultra-Low Permeability Reservoirs of Anpeng Oilfield, China

Using the data of cores, outcrops, thin sections and image logs, we analyzed the fracture distribution characteristics in the ultra-low permeability reservoirs of Anpeng oilfield. Then, we analyzed the main factors controlling the development of fractures. There are tectonic fractures and diagenetic fractures in the ultra-low permeability reservoirs. Tectonic fractures with high dip-angle are the main fracture type. Under the stresses resulting from horizontal tectonic compressions in the Early and Late Himalayan movements, three sets of tectonic fractures formed in the study area, i.e. E-W, NE-SW and NW-SE orientations. The E-W oriented fractures are developed better, then the NE-SW and NW-SE oriented fractures. The formation and distribution of these tectonic fractures are controlled by the tectonic stress field, lithology, porosity, permeability, layer thickness and structures. Under the same controlling factors, the origin types, occurrences and development characteristics of fractures in shallow-to mid-depth reservoirs are similar to that in the deep reservoirs. But fractures developed better in the deep reservoirs than that in the shallow-to mid-depth reservoirs.

Study on Underlying Coal and Strata Pressure-Relief Principle and the Gas Extraction Technique under Upper Protective Layer Mining

By study on underlying coal and strata pressure-relief principle and the gas extraction technique under upper protective layer mining, we obtain the stress change and distribution law of underlying coal-rock mass. We analyze the deformation law and fracture development characteristics of underlying coal-rock mass movement. With mining proceeding ahead, the total floor coal and rock experiences compression deformation first, then expansion deformation and re-compaction of the continuous periodic destruction. Based on different development characteristics and status of underlying coal-rock mass, the underlying coal-rock mass under an effect of upper protective layer mining was divided into the floor heave fracture zone and the floor heave deformation zone in this paper. The permeability coefficient of change law of underlying the coal seam as follows: the original value-small decreasing-increasing greatly-reducing-stability at last. The field test for upper protective layer mining of Zhang-ji coal mine of Huainan shows that the effect of pressure relief of protected seam is very good. So it eliminates the risk of gas outburst, ensuring safety mining of the protected seam. The research has an important significance for safety and efficient production under similar exploitation conditions of low-permeability with high gas and outburst risk coal seam.

Integrated petrophysical log characterization for tight carbonate reservoir effectiveness: A case study from the Longgang area, Sichuan Basin, China

Ultra-low porosity and permeability, inhomogeneous fracture distribution, and complex storage space together make the effectiveness evaluation of tight carbonate reservoirs difficult. Aiming at the carbonate reservoirs of the Da’anzhai Formation in the Longgang area of the Sichuan Basin, based on petrophysical experiments and logging response characteristics, we investigated the storage properties of matrix pores and the characteristics of fracture development to establish a method for the characterization of effectiveness of tight reservoirs. Mercury injection and nuclear magnetic resonance (NMR) experiments show that the conventional relationship between porosity and permeability cannot fully reflect the fluid flow behavior in tight matrix pores. Under reservoir conditions, the tight reservoirs still possess certain storage space and permeability, which are controlled by the characteristic structures of the matrix porosity. The degree of fracture development is crucial to the productivity and quality of tight reservoirs. By combining the fracture development similarity of the same type of reservoirs and the fracture development heterogeneity in the same block, a three-level classification method of fracture development was established on the basis of fracture porosity distribution and its cumulative features. According to the actual production data, based on the effectiveness analysis of the matrix pores and fast inversion of fracture parameters from dual laterolog data, we divided the effective reservoirs into three classes: Class I with developed fractures and pores, and high-intermediate productivity; Class II with moderately developed fractures and pores or of fractured type, and intermediate-low productivity; Class III with poorly developed fractures and matrix pores, and extremely low productivity. Accordingly log classification standards were set up. Production data shows that the classification of effective reservoirs is highly consistent with the reservoir productivity level, providing a new approach for the effectiveness evaluation of tight reservoirs.

The Pressure Relief and Permeability Increase Mechanism of Crossing-Layers Directional Hydraulic Fracturing and Its Application

Abstract In order to reduce roof-floor blind area of hydrofracture in underground mines, expand influenced range of fracturing, improve the effect of hydrofracture, a pressure relief and permeability increase model of hydraulic fracturing was built on the basis of analysing the mechanism of crack initiation and the characteristics of fracture development. After discussing the mechanism of directional hydraulic fracturing and carrying out related numerical simulation, a directional hydraulic fracturing technique was proposed. The coal fracture development distribution rule in the process of directional hydraulic fracturing was analysed, and the directional hydraulic fracturing technique was applied in the F 15 -31010 mining workface of The Twelfth Coal of Pingdingshan Coal Mining Group. The results show that single-drill hole fracturing effective radius rise to 6m under the pressure of 27Mpa, which is 3-5 times more than before, and the average concentration of single-drill hole gas drainage promote to 87.5%, average flow up 55.6% than no-directional hydraulic fracturing. All these suggest that the technology obtains remarkable effect, and has a high application value.

Development characteristics and quantitative prediction of reservoir fractures in the Chaoyanggou oil field

The Chaoyanggou oil field is a fractured low-permeability reservoir, where the distribution of oil and gas is controlled by the distribution and development of fractures. Based on outcrop, drilling core, thin section and log data, the development characteristics of fractures in this area are described. On this basis, the degree of fracture development was predicted by quantitative analysis of fracture strength and numerical simulation. The result shows that four groups of structural fractures, i.e., in near NS, and EW directions and in due NW and NE directions, were developed in the reservoir, with the nearly NS and EW fractures dominant, which are the along bedding decollement fractures formed by compressive folding action, while low angle shear fractures are related to thrusts. These fractures are mainly formed in the reversed tectonic stage at the end of the Mingshui formation during the Cretaceous period. The degree of fracture development is controlled by such factors as lithology, stratum thickness, faults, folds and depth. The fractures are developed with a clear zonation and are best developed in the northern zone, moderately developed towards the south and poorly developed in the middle zone. These prediction results are in good agreement with interpretation results from logs.