Rock Quality Research Articles

Effects of precipitation and dissolution of carbonate cements on the quality of deeply buried high-temperature and overpressured clastic reservoirs: XD 10 block, Yinggehai Basin, South China Sea

Clarifying the precipitation and dissolution processes of carbonate cement is of great significance for reconstructing the history of reservoir diagenesis, quantitatively evaluating reservoir quality, and enhancing acidification-related oil recovery. In this study, comprehensive experiments were performed, including thin section observation, cathodoluminescence, scanning electron microscopy, electron probe, QEMSCAN, carbon and oxygen isotopes, and fluid inclusions, to investigate the carbonate cements from the lower Huangliu Formation in the XD10 block of the Yinggehai Basin in the South China Sea. The types, distributions, paragenetic framework, and formation mechanisms of carbonate cements were studied systematically to further reveal their impacts on clastic reservoir quality. The results were as follows: (1) The average absolute content of carbonate cement was 7.5%, which accounted for 84.3% of the total authigenic minerals and showed a negative correlation with reservoir properties. The early-stage calcite and dolomite being controlled by sedimentation with the δ18OPDB values ranging from −12.28‰ to −5.28‰, filled in the intergranular pores with poikilotopic crystals, which resulted in almost 90% of the primary porosity to be lost; the late-stage ferrocalcite and ankerite that were characterized by δ18OPDB values varying from −17.0‰ to −9.02‰ filled up approximately 30% of the secondary pores caused by minerals dissolution. (2) The overpressure of deeply buried clastic reservoirs resulted from CO2-rich fluid charging, water saturation, and their distributions, which directly affected the stability of the reservoir carbonate cements. For a gas layer containing bound water, spot-like selective dissolution developed because the film-type bound water contained only a small amount of CO2. For the water layer, the carbonate cements suffered from strong dissolution owing to the long-term sufficient CO2 supply. (3) In situ dissolution experiments of carbonate cement, periodically monitoring the relationship between injection pressure and dissolution time, indicated that the intrusion pressure represented a periodic leaping change. This is attributed to the periodic leap between the “throat blocking” resulted from the precipitation of dissolved carbonate grains and the “throat opening” caused by the further dissolution of carbonate grains. (4) Analysis of 3D petrophysical properties and 2D images showed that the early-stage carbonate cements dissolution enhanced the porosity of the reservoir from 3% to 28%, which increased the permeability by 2000–4000 times. The dissolution of late-stage carbonate cement improved the porosity from 8% to 16% and resulted in a permeability increase of 5–100 times. The mean radius of 2D pores were extended from 68.4 to 266.3 μm, the mean radius of 2D throat were increased from 14.9 to 35.1 μm. Systematic analysis of the influence of carbonate cements in deep tight reservoirs with high temperature and overpressure (HTOP) on the rock quality provided some insights into the analysis of quality control factors and the later development plan of carbonate cement-rich deep tight reservoirs.

Study on Well Logging Technology for the Comprehensive Evaluation of the “Seven Properties” of Shale Oil Reservoirs—An Example of Shale Oil in the Lucaogou Formation in the Jimsar Sag, Junggar Basin

The shale oil reservoir in the Lucaogou Formation in the Jimsar Sag, Junggar Basin has a complex lithology, complex pore structure, and strong heterogeneity. Conventional logging reservoir evaluation methods have difficulty accurately evaluating the lithology, physical properties, electrical properties, oil-bearing properties, source rock characteristics, brittleness, and horizontal in-situ stress. Starting from the logging response characteristics of complex lithologies, a multilevel, multi-logging parameter combination shale oil logging evaluation is proposed, and an evaluation model of the “seven properties” of shale oil reservoirs is established. This model is used to interpret the data in the study area and achieves good results, effectively solving the “three types of quality” evaluation problems of shale oil source rock quality, reservoir quality and engineering quality. Based on the comprehensive evaluation of the seven properties of logging, it is proposed that the lithology of the Lucaogou Formation shale oil determines the physical properties, brittleness, source rock properties, rock mechanical properties, contact relationships with the reservoir and physical property controls on the oil-bearing property; the “sweet spot” of shale oil in the Lucaogou Formation is clarified, providing effective support for the development of shale oil in the Jimsar Depression.

GEOPHYSICS AND BUILDING DIAGNOSTICS, THEIR POSSIBILITIES AND LIMITS

Abstract. Geophysical methods and building diagnostics are non-destructive measurements which can inform about quality of rocks, hydrology conditions or artificial objects (tunnels, foundation of buildings, constructions etc.). Such methods are in the principle non-destructive without impact on environment or population. Hence these methods are mostly cheaper and more acceptable than drilling, travelling with heavy machines etc. Geophysical methods study physical fields and are divided as follows: Geoelectrical methods, Seismics, Gravimetry, Magnetometry, Thermometry, Radiometry. The geophysical measurements can be carried out on a surface of terrain, in boreholes (well logging) or in the air (airborne geophysics). The non-destructive building diagnostics use mostly measurements relating to the seismic, acoustic or nuclear properties. The time-lapse monitoring of administered objects by non-destructive methods can bring beneficial and fundamental source of information about actual conditions of managed objects. Presented non-destructive measurements use sophisticated technologies and respected geophysical companies follow international and domestic standards and legislation. Geophysicists are organized mainly in international associations such as EAGE (European Association of Geoscientists and Engineers) or SEG (Society of Exploration Geophysics). This paper presents typical examples of application of non-destructive methods to our practice.

Geoengineering characteristics modeling of Eocene limestone beds of the upper plateau of Mokattam area, Egypt using GIS techniques

This research paper is concerned with studying and evaluating the geoengineering characteristics and potential risks of the area under study with a view to safe urban expansion using GIS techniques. Different geotechnical laboratory tests such Atterberg limits, free swelling test, swell potential and pressure tests, chemical analysis, unconfined compressive strength (UCS) and rock quality designation (RQD) were performed on 28 boreholes (95 samples). Furthermore, X-ray diffraction was used to examine clay minerals. The results indicate that the UCS values range from 47 to 72.2 kg/cm2 and RQD from 21 to 45%. The free swell ratio ranges from 110 to 120%, and the plasticity index from 34 to 37%. In addition, GIS techniques have been utilized to produce the distribution layers of different geotechnical characteristics, then integrated to produce a geospatial urban model for delineating and defining safe and dangerous urban zones. This model is classified into four classes: low, moderate, high and very high hazard. Low and moderate hazards were detected in the major portion of the survey area. These portions are suitable for foundations at shallow depths. However, high to very high hazards were detected in the southeast portion of the study area. The reason for these areas is caused by the existence of heavily fractured limestone and highly swelling clay layers. This model would aid in the creation and discovery of the best areas of shallow base in the region under consideration.

Studi Geologi Bawah Permukaan Menggunakan Core Logging Dan Water Pressure Test: Studi Kasus Bendungan Lolak Bolaang Mongondow

North Sulawesi has one large dam located in Pindol Village, Lolak District. The research objective was to determine the lithology of the bedrock of dam foundation, rock class, and subsurface permeability of the dam. The method used in this research is core logging analysis and water pressure test. The bedrock foundations of the dam consist of andesite units, shale units, sandstone units which are included in the Sedimentary Facies Tinombo Formation and Volcanic Tinombo Formation. The formations are included in the Eocene to Early Oligocene Eocene and deposited in the deep marine environment. The foundation rock class is predominantly CM class (rather soft and fairly weathered rock), the rock quality of design is poor-medium characterized by the presence of broken cores and easily crushed in some parts. Lolak Dam has 6 permeability zones, namely very high, high, medium, moderate, low, and very low permeability with an average lugeon value of 4.43 liters/minute. The standard of lugeon value for the dam foundation is Lu3, so it is necessary to repair the drill points on the pilot hole which has a lugeon value of 3 (low, moderate, medium, high, and very high permeability zones) to increase the capacity foundation rock.

Multiscale petrophysical characterization and flow unit classification of the Minnelusa eolian sandstones

Integration of petrophysical and geological information is critical to simulation of subsurface carbon storage (GCS). In this sense, two depositional facies were identified from the core description and well-log interpretation, namely massive (MS) and cross-bedded (CB) facies groups. Additionally, pore-scale characteristics were studied by a combination of techniques, e.g. Nuclear Magnetic Resonance (NMR) and mercury intrusion capillary pressure (MICP). Scanning electron microscope (SEM) and petrographic analyses show that the pore structure is dominantly controlled by the depositional environment and dolomite cementation. NMR-T2 distributions of MS and CB facies show triple and quadruple modes, respectively. In addition, MICP of high- and low-permeability MS facies samples, and their CB facies group mixtures were collected. The MS sample pore-throat size distribution is uni-modal, while the triple-modal characteristic of the mixtures indicates heterogeneous pore structures at the sub-core scale for CB facies. The reliably estimates of porosity and permeability for both facies groups via NMR techniques and the MLR (Multiple Linear Regression) approach demonstrate the applicability of these techniques to eolian sandstone. Moreover, irreducible water saturation via the T2-cutoff method correlates strongly with T2LM instead of porosity. Finally, the rock quality index and flow zone indicator were calculated based on Combinable Magnetic Resonance (CMR) log interpretations. This provides direct connection to properties measured in the well. Four flow units were classified for both facies groups. Results show that better reservoir quality with significant heterogeneities is observed in the CB facies. This study highlights the importance integrating multiscale petrophysical properties including facies, pore architecture and diagenesis analysis with core- to log-scale property characterization. The results herein validate our reservoir characterization and flow unit classification in eolian reservoirs.

Rock mass classification prediction model using heuristic algorithms and support vector machines: a case study of Chambishi copper mine

The rock mass is one of the key parameters in engineering design. Accurate rock mass classification is also essential to ensure operational safety. Over the past decades, various models have been proposed to evaluate and predict rock mass. Among these models, artificial intelligence (AI) based models are becoming more popular due to their outstanding prediction results and generalization ability for multiinfluential factors. In order to develop an easy-to-use rock mass classification model, support vector machine (SVM) techniques are adopted as the basic prediction tools, and three types of optimization algorithms, i.e., particle swarm optimization (PSO), genetic algorithm (GA) and grey wolf optimization (GWO), are implemented to improve the prediction classification and optimize the hyper-parameters. A database was assembled, consisting of 80 sets of real engineering data, involving four influencing factors. The three combined models are compared in accuracy, precision, recall, F1 value and computational time. The results reveal that among three models, the GWO-SVC-based model shows the best classification performance by training. The accuracy of training and testing sets of GWO-SVC are 90.6250% (58/64) and 93.7500% (15/16), respectively. For Grades I, II, III, IV and V, the precision value is 1, 0.93, 0.90, 0.92, 0.83, the recall value is 1, 1, 0.93, 0.73, 0.83, and the F1 value is 1, 0.96, 0.92, 0.81, 0.83, respectively. Sensitivity analysis is performed to understand the influence of input parameters on rock mass classification. It shows that the sensitive factor in rock mass quality is the RQD. Finally, the GWO-SVC is employed to assess the quality of rocks from the southeastern ore body of the Chambishi copper mine. Overall, the current study demonstrates the potential of using artificial intelligence methods in rock mass assessment, rendering far better results than the previous reports.

Petrophysical core-based zonation of OW oilfield in the Bredasdorp Basin South Africa

This study aims to generate rock units based on core permeability and porosity of OW oilfield in the Bredasdorp Basin offshore South Africa. In this study, we identified and classified lithofacies based on sedimentology reports in conjunction with well logs. Lucia's petrophysical classification method is used to classify rocks into three classes. Results revealed three lithofacies as A (sandstone, coarse to medium-grained), B (fine to medium-grained sandstone), and C (carbonaceous claystone, finely laminated with siltstone). Lithofacies A is the best reservoir quality and corresponds to class 1, while lithofacies B and C correspond to class 2 and 3, which are good and poor reservoir quality rock, respectively. An integrated reservoir zonation for the rocks is based on four different zonation methods (Flow Zone indicator (FZI), Winland r35, Hydraulic conductivity (HC), and Stratigraphy modified Lorenz plot (SMLP)). Four flow zones Reservoir rock types (RRTs) were identified as RRT1, RRT3, RRT4, and RRT5, respectively. The RRT5 is the best reservoir quality composed of a megaporous rock unit, with an average FZI value between 5 and 10 µm, and HC from 40 to 120 mD/v3, ranked as very good. The most prolific flow units (RRT5 and RRT4 zones) form more than 75% of each well's flow capacities are supplied by two flow units (FU1 and FU3). The RRT1 is the most reduced rock quality composed of impervious and nanoporous rock. Quartz is the dominant framework grain, and siderite is the dominant cement that affects flow zones. This study has demonstrated a robust approach to delineate flow units in the OW oilfield. We have developed a useful regional petrophysical reservoir rock flow zonation model for clastic reservoir sediments. This study has produced, for the first time, insights into the petrophysical properties of the OW oilfield from the Bredasdorp Basin South Africa, based on integration of core and mineralogy data. A novel sandstone reservoir zonation classification criteria developed from this study can be applied to other datasets of sandstone reservoirs with confidence.

Investigation of the Enrichment and Accumulation of Normal Pressure Shale Gas in Anchang Syncline Outside of Sichuan Basin

Gas shales from the Wufeng-Longmaxi Formation in Anchang syncline in northern Guizhou area of Sichuan Basin are stable in distribution and can be classified as type I shale gas reservoir with great resource potential. The exploration and development of shale gas in Anchang syncline have achieved great progress. However, the enrichment rule and accumulation pattern of shale gas in Anchang syncline are still not clear at the present. Gas pressure in the Wufeng-Longmaxi Formations shales in Anchang syncline is normal. The maximum tested gas production in the field ranges from 18,000 to 58,000 m3/d. However, gas production of shale wells varies greatly even in the same platform. In order to understand the shale gas enrichment and accumulation and improve the effective development of shale gas in this area, comparative analyses on the geological characteristics and preservation conditions of gas reservoirs in several typical wells were carried out from the perspective of geology, petrophysics, geophysics, and well logging. Results show that shale gas in Anchang syncline has the characteristics of accumulating in both deep central position and gently wings. Tectonic preservation condition is the key factor to high gas production in Anchang area. The hydrological conditions, syncline structure, fault distribution, and cap rock quality comprehensively control the gas pressure and gas bearing capacity of shale reservoir. According to these factors, an accumulation model of shales with normal gas pressure is established: syncline controls the distribution of gas reservoir; fault controls the boundary of the reservoir; preservation conditions controls gas capacity. The results are benefit for the rolling exploration and development of shale gas in Anchang area, and are important for the development of normal pressure shale gas in northern Guizhou area.

Paleoenvironment of the Lower Ordovician Meitan Formation in the Sichuan Basin and Adjacent Areas, China

The quality of hydrocarbon source rocks is affected by the sedimentary paleoenvironment. A paleoenvironment with anoxia and a high paleoproductivity is beneficial to source rocks. The paleoenvironment of the Lower Ordovician Meitan Formation of the Sichuan Basin and its adjacent areas is lacking, restricting the oil and gas exploration of the Ordovician in the Sichuan Basin and its adjacent areas. In this paper, the content of major and trace elements of 50 samples was tested to clarify the paleoenvironment of the Meitan Formation. The paleoclimate, paleosalinity, paleoredox, and paleoproductivity during the deposition of the Meitan Formation were analyzed. The control effect of the paleoenvironment on the development of source rocks was clarified, and the favorable paleoenvironment for source rock development was pointed out. The results show that the paleoenvironment of the Meitan Formation has the following characteristics: humidity, brackish water, oxygen depletion, anoxia environment, and high paleoproductivity. These characteristics are conducive to the development of poor and moderate source rocks. The source rocks of the Meitan Formation were developed in the north, west, and south of the Sichuan Basin and its adjacent areas. The organic matter of the source rocks is mainly typed II1 kerogen, and the quality is evaluated as poor-medium source rocks having the potential of generating oil and gas. This study can provide fundamental parameters for the further exploration of Ordovician petroleum.

Application of Cascade Forward Backpropagation Neural Networks for Selecting Mining Methods

Decision-making is very important in many fields, such as mining engineering. In addition, there has been a growth of computer applications in all fields, especially mining operations. One of these application fields is mine design and the selection of suitable mining methods, and computer applications can help mine engineers to decide upon and choose more satisfactory methods. The selection of mining methods depends on the rock-layer specification. All rock characteristics should be classified in terms of technical and economic concerns related to mining rock specifications, such as mechanical and physical properties, and evaluated according to their weights and ratings. Methodologically, in this study, the criteria considered in the University of British Columbia (UBC) method were used as references to establish general criteria. These criteria consist of general shape, ore thickness, ore plunge, and grade distribution, in addition to the rock quality designation (ore zone, hanging wall, and foot wall) and rock substance strength (ore zone, hanging wall, and foot wall). The technique for order of preference by similarity to ideal solution (TOPSIS) was adopted, and an improved TOPSIS method was developed based on experimental testing and checked by means of the application of cascade forward backpropagation neural networks in mining method selection. The results provide indicators that decision makers can use to choose between different mining methods based on the total points given to all ore properties. The best mining method is cut and fill stopping, with a rank of 0.70, and the second is top slicing, with a rank of 0.67.

Mapping the maturity of organic-rich shale with combined geochemical and geophysical data, Draupne Formation, Norwegian Continental Shelf

The evaluation of the interplay between kerogen maturation and total organic carbon (TOC) content within an organic-rich shale is critical to characterizing source rock shale's elastic behavior. We analysed a comprehensive database of twenty-nine well logs, two 3D seismic surveys, and geochemical data of eighteen wells within the Draupne Formation in the Norwegian North Sea. The Upper Jurassic (Kimmeridgian) Draupne Formation Shale is found throughout the Norwegian Continental Shelf and is the primary source, carrier, and seal rock in several oil and gas fields. The dataset explores a broad range of depths (2–5 km), total organic contents (0–20 wt%), and various maturation states ranging from immature to overmature. We study the relationship between rock brittleness and organic content as shale matures. We use calculated values of Young's modulus and Poisson's ratio. Academic and industry-standard cutoffs are applied to both the TOC content and maturation level. We introduce here the Organic Maturation Product (OMP) rock physics template, which incorporates a relationship between organic content and any subsequent maturation. The degree of brittleness or ductility is impacted directly by the kerogen maturation process, with lower TOC content having a dampening effect on the transition towards increasing brittleness. This template allows one to classify rock maturation in nine categories, from low to high maturation, and link these categories to rock geomechanical properties. Seismic inversion of the 3D surveys and mapping utilizing the OMP classification reveal how large-scale depositional environment and subsequent diagenetic events influence both placement and quality of source rock broadly within the basin context of the North Sea.

Investigation and Evaluation of Reservoir Quality of the Upper Cretaceous Rocks (Belqa Group) in North Jordan (NW Irbid)

Upper Cretaceous rocks are widespread in Jordan, covering uncomfortably the rocks of the Lower Cretaceous (Kurnub Sandstone). These rocks consist mainly of limestone, marls, chalk, biogenic chert and phosphate of primarily marine origin. Age determination of these rocks was based on the abundance of macro and microfossil content. Man researchers subdivided the Upper Cretaceous succession. The classification of Powell (1989) is considered in this study, particularly for the Belqa Group, which is of Coniacian to Maastrichtian in age. This investigation will focus on the geology of Wadi Es Sir, Umm Ghudran; Amman silicified limestone, and Al Hisa phosphorite formations, that belong to Ajlun and Belqa Groups respectively. Moreover, sedimentary structures, lithological characteristics Microfacies types and fossil content were evaluated. Using the lithostratigraphy and the fossil content of those formations, 6 Microfacies types were identified. Evaluation of the cross-section, the tectonic processes and the prepared rose diagrams indicate that the study area was affected by compressional as well as by tensional forces. Two major tends of fracturing can be identified in the rose diagrams. The measured physical properties show that the porosity is variable and ranges between 30% and 48%. The bulk density has values between 1.8 and 2.2 g/cm3. The different obtained results point out that the studied rocks represent good reservoir.

3D Slicing of Weighted Index for Reservoir Quality of Lower Miocene Rocks, Belayim Marine Oil Field, Gulf of Suez, Egypt

This study aims to have a single coefficient resulting from the integration of all the reservoir parameters through which a decision can be taken to determine the best quality places in the reservoir. The conventional well logging data in nine wells were used to determine the reservoir parameters in the study area. Seven different parameters were calculated, five of them were directly proportional to the quality of the reservoir, while the remaining two parameters which represent shale volume and water saturation were inversely proportional to the reservoir quality. The index of each parameter was calculated. A new value was created from the integration of the seven different parameter indexes called the weighted index for the reservoir quality. The reservoir quality values were sliced in the three dimensions depending on the effect of all reservoir parameters and not on any single parameter. It is clear from the results of this study that horizontal and vertical slicing, as well as cut-off values, illustrates that the middle and upper parts are the best places for the reservoir to explore hydrocarbons, where the values of the weighted index of the reservoir quality range from 0.65 to 0.9. Meanwhile, the quality of the reservoir decreases in its lower parts.

Mechanical model and numerical analysis of a method for local rock reinforcing to control the floor heave of mining-affected roadway in a coal mine

A floor heave controlling method has been substantiated, based on the creation of locally reinforced zones of a special shape in roadway floor, which allows saving stability even with a low rocks friction ratio. The method is based on a mechanical model of the formation of an increasing strength spacer system in roadway floor. The numerical analysis of rock massif stress-strain state carried out by the finite element method, has made it possible to determine the mechanism of floor heave during rock reinforcing. The performed analysis of the simulation results has made it possible to determine the influence of the quality of rock reinforcing and the rock friction ratio value on the amount of heave in the mining-affected roadway. The effectiveness of the proposed method has been proven as a result of a comparative analysis.

Evaluation of Petroleum Source Rock Using Well Logs in the Niger Delta Region of Nigeria

Petroleum source rock potential evaluation was carried out on eight wells of the Niger Delta’s Agbada Formation by assessing some source parameters from well logs and through well logs analysis based on organic matter effect and relationship on and with well logs. Source rock determinant parameters such as volume of shale (Vsh), total organic content (TOC), and amount of generated hydrocarbon (S2) was derived from well logs following established organic matter effects on well logs using Schlumberger's Techlog 2015 software, while hydrogen index (HI) was calculated using known values of TOC and S2 respectively. Well log data used for the analysis includes GR log, resistivity, sonic and Density logs. Apart from deriving source rock parameters, the well logs were also used to identify, quantify and somewhat determine mature zones of source rock intervals by correlation with TOC, S2, and HI source parameters respectively, Therefore, a source interval criterion composing of high GR log, high resistivity, high sonic and low-Density was developed alongside source rock parameters. Results of the analysis in the area show poor to fair source rock quality rating in terms of TOC, and insufficient source rock thickness fit for sustaining a world-class petroleum province like the ND petroleum province and further depicting that the Agbada FM may not be the principal source rock of the area. It is, therefore, recommended that more elaborate studies covering greater depths of the Agbada FM be carried out in other to properly represent the true source quality of the area.

Evaluation of Source Rock Quality in Chang-7 Member of Major Oil Fields in Ordos Basin

Petroleum is an important natural resource for human. The demand for petroleum in the next several decades will continue to be high, which requires the exploitation of more petroleum sources. Chang-7 member of the Ordos basin is proven to be the richest among all members, and this research took a focus on making a comparison between the source rock quality of Chang-7 member in four major oil fields in the basin, to provide a simple view of priority oil fields for exploitation in the future. The research analyzed the source rock quality of four oil fields from the TOC, A%, rock type, R0, sedimentary environment, and kerogen type, and found a superiority of source rock quality of Maling and Zhidan oil field in multiple dimensions, mainly triggered by their sedimentary environment. The research concluded that the oil fields that was once in the central-southern region of the basin contains a slightly better source rock quality among the chosen fields, which is related to a more stable sedimentary environment in the early Triassic.

Application of machine learning techniques for identifying productive zones in unconventional reservoir

Unconventional reservoirs are the productive zones in other words the rock quality and the mechanical properties of the rocks this process is devastating if humans or people try to search for the best reservoirs. So we can use machine learning (ML) algorithms to help us find and search easily and fast for the best reservoirs with less human interaction as possible. The objectives of this paper is to use machine learning (ML) techniques to predict and classify the reservoirs based on the properties of each reservoirs and choose the best reservoir. In this paper we have made a comparison between the different types of machine learning algorithm and described how we get the best and worst result for each one, the comparison we made gave us that the AdaBoost algorithm gave the worst performance measured in the accuracy while the random forest (RF) algorithm gave the best performance, this paper aim to make improvement of the process of searching for productive zones using ML algorithms.

Study of Instability Mechanism and Roof Caving Mode of Cementing Filling Stope: The Case Study of a Nonferrous Metal Mine in China

The downward layered cemented filling method, which is generally used in the mining of high‐value metal mines with poor surrounding rock quality, is widely believed to not cause large‐scale instability of the roof strata in the mining area. However, a nonferrous metal mine in northern China, which has been using the downward cemented filling method, suddenly suffered a violent collapse accident of the stope roof, and the surface is accompanied by significant subsidence on a large scale. The accident revealed that the roof collapse mechanism still needed further research. In this paper, field investigation and numerical simulation were combined to study the mechanism of roof collapse. Based on the input data including in‐situ stress state, geological occurrence pattern, and mining steps, the particle flow code (PFC) was used to simulate the stress and displacement changes of the rock mass under mining disturbance. These results indicate that the failure process of the overlying rock mass can be divided into four stages due to the special geological conditions of the mine: pillar stability stage, pillar chain failure stage, roof filling caving stage, and gneiss plug settlement stage. In the early stage of mining, the pillars between the mined‐out drifts could effectively support the overlying rock mass due to the small exposed roof. As more drifts were mined, the vertical pressure on the pillars was added. When the number of mining drifts reached five, one of the pillars was firstly destroyed due to overloading, and then the pressure of the overlying strata was transferred to the surrounding pillars, leading to the subsequent failure of other pillars. When pillars were damaged, arch caving appeared inside the roof filling material. Finally, the vertical shear resistance capacity of the gneiss mass above is insufficient, owing to the steeply dipping joints. Finally, the gneiss above was subject to sudden plug settlement along the vertical joints. It should be noted that the stope mining management of the mine has a significant impact on production safety. In order to ensure the stability of the stope formed by cemented filling method, the dense distribution of simultaneous mining drifts should be avoided and the mine‐out areas should be backfilled in time.

Numerical modeling in determining deformation characteristics of fractured rock massifs

The article proposes a method for determining the deformation modulus of fractured-block rock masses, weakened by mutually-orthogonal systems of cracks within the linear section (after the closure of cracks) of the deformation curve σ= f (ε). The values of normal - kn and tangential - ks stiffness are used as elastic characteristics of joints. It is shown that in such rocky structures it is impossible to apply the laws of mechanics of a solid elastic body to calculate their deformation characteristics, since the opening of interblock joints is observed when performing numerical experiments. The discrepancy between the results of numerical and analytical calculations is up to 40%. For these purposes, it is proposed to use the regression equation obtained on the basis of regression analysis based on the results of numerical modeling and the method of experiment planning. The presented equation allows, depending on three independent factors: the modulus of elasticity of the rock block - Eo, the rock quality designation index - RQD and the dip angles of systems of mutually orthogonal ioints – α, to determine the modulus of deformation ofthe rock mass.