Rock Hardness Research Articles

A Survey on the Shortcomings of the Current Rate of Penetration Predictive Models in Petroleum Engineering

In drilling engineering, the rate of penetration (ROP) is a prevalent indicator to evaluate the energy efficiency of drilling operations. Nowadays, ROP prediction has become more critical since the production from deeper hydrocarbon resources is unprecedentedly escalating. So far, a wealth of theoretical and practical investigations has been conducted to develop ROP models; however, the existing models have not been adequately updated with the new technological advancements or geological restrictions. This research strives to integrate the latest advancements, restrictions, and future requirements in ROP prediction. To do this, the existing empirical and data-driven ROP models are elaborated and compared. From the conducted research, it was deduced that four uncontrollable factors, including the rock permeability, wellbore inclination, temperature, and rock hardness, have not been adequately considered in ROP models. Moreover, although data-driven ROP models deliver more accurate results than the empirical models, the determination of the number and type of the input parameters is still challenging. To tackle this issue, it is recommended to develop a formation-based classification system of input parameters for future investigations. This inclusive review can be adopted by the companies and engineers involved in drilling operations to update and reform their drilling strategies.

Prediction of the stability of gob‐side entry formation by roof cutting by machine learning‐based models

AbstractGob‐side entry formation by roof cutting is a new technology for no pillar coal mining, which can maximize coal resources and reduce roadway drivage ratio. However, the mechanical behavior of the formed entry is complex while it is crucial to ensure the stability of the entry for mining safety. This paper proposed a machine learning‐based method for predicting the stability of the formed entry, which combined artificial neural network (ANN) with particle swarm optimization (PSO) algorithm or genetic optimization (GO) algorithm. The data set from 75 coal mining faces from 2009 to 2022 was employed to train and test the models. A descriptive variable of dynamic unstable distance was introduced to evaluate the stability state of the formed entry and six other parameters were chosen as influence parameters. The two intelligent models were compared with each other to have a comprehensive assessment. Model assessment indices such as R2, mean absolute error, mean absolute percentage error, and root mean square error were used to evaluate the accuracy of the models. The results of both developed models are promising, and the predictive accuracy of the PSO‐ANN model is higher than that of the GO‐ANN model. Through sensitivity analyses, it has been found that the coal seam thickness and roof rock hardness are the most important parameters for influencing entry stability. The developed method provides a practical tool for the prediction of entry stability and the optimization of entry design.

Hardness, strength and abrasivity of rocks: Correlations and predictions

AbstractSince ancient times, rocks and their geomechanical and mineralogical properties have played a fundamental role for realising construction and infrastructure projects. Workability and excavability of the material itself are still decisive factors controlling tool wear and advancement rates. In engineering geology, standardised tests and analyses related to the strength, hardness, abrasivity and mineralogical composition are commonly conducted in this context. The uniaxial compressive strength (UCS), CERCHAR Abrasivity Index (CAI) and equivalent quartz content are widely used parameters for such an assessment, in order to estimate and predict drillability and associated wear of drill bits, cutting discs or chisels. In this article, the correlations between strength, abrasivity and mineral content of various rock types are investigated. The concept of hardness in geotechnics and engineering geology is elaborated in greater detail, shedding light on hardness definitions, testing methods and how hardness parameters are interrelated. Under the aspect that the CAI shows a good correlation with the Mohs hardness commonly used in mineralogy, a novel approach for estimating the CAI is presented. It is suggested that the CAI of a rock can be estimated within 50 % of the actual value, if its UCS exceeds »60 MPa. On the data basis of various rock types analysed from national and international construction projects, the potential and limitations of this method are discussed.

A new method to improve rock breaking efficiency by using automatic unloading of bottom hole central stress

The main reasons for slow drilling speed in deep formation are poor rock drill-ability, low cutting efficiency and short service life of rock breaking tools caused by high hardness and intensity of bottom hole rock and stress concentration of surrounding rock. In view of this, the concept of releasing rock stress in the bottom hole center to improve rock breaking efficiency and prolong the service life of rock breaking tools is put forward, the bit for automatic unloading of bottom hole central stress (AUOBHCS) is designed, and the testing experiment of rock breaking effect is carried out. The experimental results show that, firstly, the rock breaking efficiency can be improved by using automatic unloading of bottom hole central stress. When the rock breaking energy of the special bit is sufficient, the rock breaking efficiency in limestone and red sandstone can be increased by 1.59 and 4.36 times respectively, and the rock breaking effect still has a large room for improvement. Secondly, WOB and rotating speed are the key factors affecting the rock breaking effect of the bit used for automatically unloading bottom hole central stress, whose speed-up effect increases with the increase of rotating speed and WOB. Thirdly, the bit for automatic unloading of bottom hole central stress has the function of prolonging the service life of the cutter, and it should have some restraining effect on the “holing” of the bit. The above proposed idea of improving rock breaking efficiency and the laboratory test of rock breaking bit provide a new idea for the research and development of downhole rock breaking speed-up technology and tool, and open up a new direction for the drilling speed-up research in deep formation difficult to drill.

Study of the Jet Output Characteristics under Multi−Source Coupling

The challenges posed by elevated rock hardness, deficient drillability, excessive friction torque, and significant underpressure in extended−reach horizontal wells are the primary factors that contribute to low ROP (Rate of Penetration) and limited horizontal reach during the drilling operation. Reducing drag and friction is one of the primary methods of addressing the aforementioned challenges. To augment the pulse output characteristics of the oscillating jet and bolster the energy of the hydrodynamic impact load, we developed and designed a multi−source impact oscillation speed−increasing tool coupled with blade rotation disturbance and multi−order oscillation cavity self−excitation. We utilized fluid dynamics software to model and conduct numerical analysis on the multi−source pulsed jet generator. Furthermore, we constructed a prototype and subjected it to testing. This paper examines the impact of dimensionless structural parameters on the pressure output characteristics of the multi−source pulse−jet generator. Specifically, we used three dimensionless quantities (cavity length ratios, cavity diameter ratios, and inner wall collision angle ratios) to study this effect. The findings indicate that the multi−source impact oscillation speed−increasing tool is capable of augmenting the pulse oscillation amplitude, and the frequency of pulse oscillation can be adjusted within the range of 5 Hz to 15 Hz. During the study, we determined that the optimal oscillation output characteristics can be achieved when the cavity diameter ratio is 0.8, the cavity length ratio is 1.0, and the inner wall collision angle ratio is 1.5. These findings present a novel approach for the development of downhole hydraulic impact oscillation speed−increasing tools.

Experimental investigation on rock microfabrical effects on dynamic hardness behaviour

Rocks' physicomechanical properties are influenced by their textural qualities. Rock mass stability is largely governed by these variables. Many geoengineering facilities depend on the capability to assess both short and long-term rock behaviors based on the interaction between different textural characteristics and physicomechanical properties. Rock hardness is frequently used in excavation engineering and is measured by several methods with different mechanisms. The Leeb dynamic hardness method has been recently applied in rock engineering applications as a new, inexpensive, non-destructive, and portable technique. This paper aims to show the interaction between the quantitative microfabrical features and the Leeb hardness of the rocks. For this purpose, 33 various rock types were chosen by considering their physicomechanical properties and microfabrical characteristics. Thin sections were prepared and their microphotographs were analyzed by ImageJ software. Four basic characteristics, including grain perimeter and area and length of minor and major axes, were measured. Then, seven textural quantities, including equivalent diameter (Dequi), shape factor (SF), aspect ratio (AR), grain compactness (C), texture coefficient (TC), interlocking index (g), and grain size homogeneity (t), were calculated based on those basic parameters and statistically analyzed. The findings indicate that the Leeb hardness increases with increasing the grain area and perimeter, equivalent diameter, grain compactness, homogeneity of grain size, grain interlocking, and TC. In contrast, Leeb's hardness decreases with increasing the AR and SF. The texture coefficient reveals the most meaningful relationship (R2 > 0.8) with the Leeb dynamic hardness test among the studied microfabrical characteristics in sedimentary and igneous rock samples.

Characteristics of Fossil Wood from Loa Sugarcane Kutai Kertanegara, East Kalimantan

Fossil Wood is wood that has petrified as a result of wood or plant stems being buried by sediment, does not experience decay and the cells in the wood are replaced by minerals over a long period of time but the outer structure of the wood is still maintained. One way to find out the characteristics of fossil wood is by observing or characterizing it both macroscopically and microscopically. This study aims to determine the macroscopic characteristics, hardness values and density of wood fossils found in Loa Tebu Village, Kutai Ketanegara. Observations of macroscopic characteristics included the color of wood fossils which were observed by naked eye and with a loupe magnification of 10x, and observations of other macroscopic characteristics were carried out on three areas of wood namely transverse, tangential and radial planes using a Nikon SMZ 645 Stereo Microscope with a magnification of 30x. The hardness value uses a rock hardness test tool, namely "Diamond Selector II", while the density uses the general calculation standard for specific gravity, namely the ratio of mass to volume. The results showed that the characteristics of the fossil wood from Loa Tebu were light brown, dark brown, white and gray in color. The color found in the wood fossils is probably caused by cells that have been mixed with inorganic elements and mineral constituents around the location of the wood fossils. Another macroscopic structural observation was the discovery of wood cells such as those of broad leaf wood in general, namely the presence of vessel cells (pores), radius cells and clusters of fiber cells. Axial intercellular channels (SIA) were also found in the samples and arranged tangentially. The AIA characteristics found here are common characteristics that are often found in several species of wood from the Dipterocarpaceae family. The results of the measurement of the sample hardness value were 4 on the Mohs scale and the density was 3.26

Conductivity Enhancement of Fractured Carbonates through High-Temperature Diammonium Hydrogen Phosphate Consolidation: A Preliminary Study

SummaryIn well stimulation operations, the ability to sustain long-term conductivity of hydraulic/acid fractures defines an efficient and effective hydrocarbon production operation. However, it is challenging to keep the fracture conductive in the soft and weak carbonate formations due to many challenges. For example, the plastic deformation of rocks causes proppant embedment or asperities failure, resulting in fracture conductivity reduction. Consolidating chemicals, particularly diammonium hydrogen phosphate (DAP), have shown to be effective in rock consolidation and could reduce the decline in fracture conductivity if applied to carbonate formations. The previous research tested DAP at ambient conditions, whereas this work involves studying the hardening properties of DAP at reservoir conditions. The solutions with two initial concentrations (1 and 0.8 M) were tested at 77°F (ambient), 122°F, and 176°F. Furthermore, a post-treatment analysis was conducted to compare the performance of the chemical under different conditions. The analysis included understanding the changes in carbonate rocks’ (limestone and chalk) hardness (impulse hammer test and indentation test), porosity (helium porosimeter), permeability (steady-state and unsteady state nitrogen injection), and mineralogy [X-ray diffraction (XRD) and scanning electron microscopy (SEM)]. Results demonstrated that both rock lithologies reacted efficiently with the DAP solution, presented in terms of the noticeable improvements in their hardness. The elevated temperatures positively affected rock hardness, leading to a more than 100% increase in hardness for most samples. After obtaining successful results from experiments at various temperatures, the pilot American Petroleum Institute (API) conductivity experiments were conducted, testing the conductivity sustenance through the rock hardening concept. Preliminary API conductivity experiments have demonstrated that treated rock samples with DAP provided higher conductivity values than the untreated samples at high stresses. The results shown in this study provide a good foundation for further studies on the implementation of DAP in actual acid/hydraulic fracturing field operations.

Seismic microzonation based on HVSR inversion results for shear wave (Vs30) mapping and soil vulnerability in West Sulawesi and South Sulawesi Regions

The Horizontal to Vertical Spectral Ratio (HVSR) is a seismic analysis method used to obtain information about subsurface geological characteristics. This method is based on the spectral ratio analysis between the horizontal and vertical components of earthquakes or other vibration sources. HVSR has proven to be an effective tool in evaluating rock layers, layer thickness, rock hardness, and geological hazard potential. In the HVSR method, seismic data are measured using three-component seismometers, the N-S (North-South), E-W (East-West), and vertical components. In this study, the HVSR method is used for Vs30 inversion to estimate shear wave velocity at a depth of 30m. The southern and western regions of Sulawesi have various geological conditions and high seismicity levels. The results of this inversion can be used for mapping soil vulnerability indices. The obtained soil vulnerability index values indicate high vulnerability near the Palu-Koro fault, with A0 9.27 at F0 3.2 Hz. This method can also estimate shear wave velocity values at a depth of 30 m. Vs30 values range from 200 m/s to 500 m/s at various measurement points, indicating variability. According to the SNI 1726:2012 and NEHRP classifications, the research area has soil conditions ranging from medium to soft.

A probability prediction method for the classification of surrounding rock quality of tunnels with incomplete data using Bayesian networks

The classification of surrounding rock quality is critical for the dynamic construction and design of tunnels. However, obtaining complete parameters for predicting the surrounding rock grades is always challenging in complex tunnel geological environment. In this study, a new method based on Bayesian networks is proposed to predict the probability for the classification of surrounding rock quality of tunnel with incomplete data. A database is collected with 286 cases in 10 tunnels, involving nine parameters: rock hardness, weathering degree, rock mass integrity, rock mass structure, structural plane integrity, in-situ stress, groundwater, rock basic quality, and surrounding rock level. Moreover, the Bayesian network structure is built using the collected database and quantitatively verified by strength analysis. Then, the accuracy, precision, recall, F-measure and receiver operating characteristic (ROC) curves are utilized for model evaluation. The average values of accuracy, precision, recall, F-measure, and area under the curve (AUC) are approximately 89.2%, 91%, 92%, 91%, and 0.98, respectively. These results indicate that the established classification model has high accuracy, even with small sample size and imbalanced samples. Ten additional sets of tunnel cases (incomplete data) are also used for verification. The results reveal that compared with the traditional Q-system (Q) and rock mass rating (RMR) classification methods, the proposed classification model has the lowest error rate and is capable of using incomplete data to predict sample results. Finally, sensitivity analysis suggests that the rock hardness and rock mass integrity have the strongest impact on the quality of tunnel surrounding rock. Overall, the findings of this study can serve as a useful reference for future rock mass quality evaluation in tunnels, underground powerhouses, slopes, etc.

Antakya Mevsimler Mozaiğinin Arkeometrik Yönden İncelenmesi

Hatay Arkeoloji Müzesinde sergilenen Mevsimler Mozaiği dokuz panelden oluşur ve dört köşesinde mevsimler tasvir edilir. Mevsimler Mozaiğinin üzerinde bulunan cam ve taş tesseraların renkleri Munsell Renk Kataloğu esas alınarak, dijital Odak marka Capsure Portatif Renk Eşleştirme cihazı ile belirlenmiştir. Taş tesseraların petrografik analizleri ince kesit optik mikroskop analizi ile cam tesseraların kimyasal analizi de Polarize Enerji Dağıtımlı X-Işını Floresan Spektrometresi kullanılarak, arkeometrik yönden karakterize edilmiştir. Elde edilen sonuçlar ışığında taş tesseraların kayaç türü, dokusu, sertlik derecesi, agregayı oluşturan kayaç ve mineraller tanımlanmıştır. Cam tessera analizlerinin sonuçları çerçevesinde bulunan elementlerin kimyasal kompozisyonu belirlenmiş ve renk özellikleri hakkında öngörülerde bulunulmuştur. Taş tesseraların petrografik analizi ile kayaç türleri biyosparitik, biyomikritik ve mikritik kireçtaşı olarak belirlenmiştir. Cam tesseraların PED-XRF analizi sonucunda soda kireç camı olduğu, bitki külünün kullanılmadığı, bünyesinde bozulmanın ve direncinin düşük olduğu belirlenmiştir. Cam tesseraya yeşil rengi veren elementin bakır olduğu anlaşılmıştır. Stronsiyum (Sr) ve zirkonyum (Zr) içeriklerine bakılarak cam tesseraların yapısında karasal hammadde kullanıldığı sonucuna varılmıştır.

The effect of rock hardness and integrity on the failure mechanism of mortar bolt composite structure in a jointed rock mass

The appearance of joints in the rock tunnels makes the rock mass show obvious anisotropy and discontinuity in physical and mechanical properties, which has a significant influence on the rock mass stability. As one of the normally used support structures in tunnel construction, Mortar bolts will inevitably pass through the joints during the process. Both rock hardness and rock mass integrity will directly affect the anchorage performance of mortar bolts in joint rock. Considering this, this paper designs a mortar bolt pull-out experiment to investigate the joint rock failure modes and bolt-grout composite structure failure modes under different conditions of rock hardness and rock mass integrity. By taking the two influencing factors of rock hardness and integrity into account, the calculation formulas of ultimate pull-out load and ultimate displacement of mortar bolts in joint rock are established and proposed.

Feed hardness and acoustic emissions of autogenous/semi-autogenous (AG/SAG) mills

In this study, the relationship between acoustic emissions and hardness of different rock types (model quartz, model calcite, and real iron ore) coupled with binary mix ratios of model quartz and iron ore (1:3, 1:1 and 3:1) was investigated in a laboratory-based AG/SAG mill. The acoustic emission response and sensitivity of the mill were compared, along with its product particle size distribution. Features, such as discrete wavelet transform (DWT), power spectral density estimate (PSDE), and statistical root mean square (RMS) were extracted from the mill acoustic emissions. From the results, it was evident that the mill acoustic emission response can be used to classify different rock hardness and their binary mixtures. Model quartz emitted the highest acoustic response, followed by iron ore and model calcite at the initial stages of grinding (5 min). The results further indicated that as the rock feed sizes reduced, the average mill noise also increased as a function of grinding time. Accordingly, the acoustic emission presented a contrasting effect with model calcite and model quartz emitting the highest and lowest noise emission, respectively. The proportions of different mineral types (model quartz and iron ore) were reflected well in the acoustic emissions. The study has demonstrated that the integration of acoustic sensing techniques in AG/SAG mills can serve as a proxy for online measurement of different rock/ore hardness, for example, as a fast track method to determine the hardness of ores in comparison to the traditional ore grindability procedures, such as Bond work index (BWI) and SAG power index(SPI) tests which can be laborious and time-consuming.

Comprehensive Identification of Surface Subsidence Evaluation Grades of Mines in Southwest China

Due to their complex geological structure, it is difficult to systematically analyze the surface subsidence of coal mines in southwest China, and the factors that cause surface subsidence are also different from other coal mines. Focusing on the problem of surface subsidence caused by mining in southwest China’s mines, a grade evaluation system for surface subsidence of southwest mines is constructed based on the analytic hierarchy process, and ten evaluation indicators are established from the perspectives of mining disturbance and geological structure. A matter–element model of surface subsidence based on matter–element extension theory and a cloud model of surface subsidence based on cloud theory are then constructed. A coal mine in Anshun, Guizhou, is taken as an example to calculate the evaluation level of surface subsidence and thus verify the scientificity of extension theory and cloud theory. The results show that the main factors that affect the surface subsidence of southwest mines are the number of coal seam layers, mining height and comprehensive Platt hardness of rock, similar to that of northern plain coal mines. Surface slope and subsidence area are also very important. The comprehensive correlation degree of each grade of the coal mine is −0.29836, 0.192232, −0.1093 and −0.46531, and the coal mine is concluded to be in grade 2. The calculated similarity of the overall index evaluation cloud map of the coal mine and each grade is 0, 0.3453, 0.7872 and 0, respectively. The coal mine is in grade 2, which is a relatively safe state. Consistent with the calculation results of the extension model and in line with the field situation, the extension matter–element model and cloud model built in this paper can verify each other and have a certain scientificity.

Evolution of fracture permeability and its compressibility in proppant-supported shale

Hydraulic fracturing is one of the effective means to increase shale gas production, during which, proppant was widely used to support fractures to form stable seepage channels. Following which changes in permeability are restricted by the compaction and embedding of proppants, so studies on the impact of proppant on the fracture conductivity are highly desirable. In this paper, we discussed first the change mechanism relating to fracture compressibility under the combined effects of proppant compaction and embedding, and, further, proposed a new calculation model denoting fracture compressibility. Moreover, the effect of the number of proppant paving layers was also incorporated in that model. Secondly, based on the exponential relationship between stress and permeability, and by taking fracture compressibility as a bridge, a shale fracture permeability model, that considered the combined effects of proppant compaction and embedment, was constructed. The proposed permeability model was, then, verified through evidence from publicly released test data, and the permeability change law relating to shale fractures. In addition, we focused on the influence of factors: such as, stress, proppant types, rock types, and the number of proppant layers on fracture compressibility. The results revealed that fracture compressibility in its original state was a higher than that of the supporting fracture. The increased stress leads to reduced fracture compressibility It was found that the higher the rock hardness, the smaller the corresponding fracture compressibility, the smaller the elastic modulus of the proppant, and an increase in fracture compressibility. The analysis of these results would help to improve the basic understanding of the key processes affecting proppant performance and permeability alterations.

Surface Drilling Parameters and Drilling Optimization Techniques: Are They Useful Tools in Gas Hydrate Detection?

This paper examines the application of surface drilling parameters and drilling optimization techniques, such as mechanical specific energy (MSE) and equivalent strength (EST), in detecting hydrate gas-bearing sediments during drilling operations. Gas hydrates are usually detected from 3D seismic imaging and later confirmed with Measurement-While-Drilling (MWD)/Logging-While-Drilling (LWD) data and collected core samples. Here, we describe an analysis of the time-based surface drilling parameters recorded from two wells drilled during the International Ocean Discovery Program (IODP) Expedition 372A offshore of New Zealand and the Indian National Gas Hydrate Program Expedition 02 (NGHP-02) offshore of India. The investigation revealed that drilling parameters, as well as MSE/EST methods, can and should be used to monitor and optimize the drilling process and to detect lithological/tectonic (fractures, fault zones, rock hardness, etc.) changes in the drilled substrata and signs of the dynamic changes in the downhole environment (tool vibration, washouts, pack-offs, etc.). However, surface drilling parameters with MSE models cannot explicitly determine the hydrate gas-bearing sediments. This qualitative analysis of whether the gas-bearing sediments consist of hydrates can only be accomplished with the use of the MWD/LWD suite, preferably located as close as possible to the bit.

A review on rock hardness testing methods and their applications in rock engineering

A review on rock hardness testing methods and their applications in rock engineering

Spatial and temporal trends in California coastal cliff retreat

Airborne LiDAR datasets were used to measure erosion and retreat along 866 km of California coastal cliffs between 2009–2011 and 2016. Erosion exceeding the level of detection was observed at 55% of cliffs. State-averaged cliff face and top retreat rates were both 0.06 m yr−1, and varied alongshore, with more erosion in northern California compared to southern and central California. Retreat rates were higher for unarmored cliffs and cliffs fronted by sandy beaches. Mean cliff top retreat rates were lower than in previous studies conducted between the 1920s–1930s and 1998/2002, and between 1998 and 2009–2011. However, the average cliff face retreat rates and statistical distributions were similar (r2 = 0.99) for the 1998 to 2009–2011 and 2009–2011 to 2016 time periods. No significant correlation was found between cliff erosion rates and environmental factors such as rock hardness, rainfall, groundwater, waves, and relative sea level change. However, the highest cliff face retreat rates (>1 m yr−1) occurred at locations with weak rocks.

Behaviour of electric drive of roller-bit drilling rig swivel head with fuzzy control

The efficient consumption of electric power in mining is an important task in power consumption optimization. The use of high-performance drilling rigs requires special attention to the development of energy-saving electric drive for open-cut mining operations. The increase of the efficiency factor and energy performance of a drilling rig is achieved through controlling the electric drive which allows the specific resonance frequency and limiting the current and velocity amplitudes to be regulated. The main idea of the study lies in the application of fuzzy controllers in the systems of automatic control of processes and equipment modes in mining production. The use of fuzzy controllers is aimed at improving the characteristics of PI and PID controllers. The calculations and simulation of transients based on simulation models in the MatLab 7.11 Simulink software package allowed reliable analysis of modes of a swivel head electric drive operation to be carried out. In the course of simulating the transients of swivel head velocity varying with the use of a fuzzy controller, fuzzy variables including mismatch of rotation velocity, mismatch change speed, velocity setting voltage were justified. The analysis allowed for the term-sets of fuzzy variables and the membership functions for each term-set of fuzzy variable to be defined. The simulation results showed that the control time (response) of transients of the swivel head motor torque and current change when using the swivel head velocity control by a fuzzy controller with increasing load depending on the rock hardness decreased by a factor of 2. Implementation of a system of automatic control of swivel head velocity with the application of a fuzzy controller allows drilling rig vibration to be reduced and provided effective protection of the swivel head electric (motor) drive from overload, thus increasing reliability of the equipment, and increasing drilling productivity.

A new rock hardness classification system based on portable dynamic testing

A new rock hardness classification system based on portable dynamic testing