Drillability Of Rocks Research Articles

Optimised design of downhole turbodrills with bending-torsional tilting blade

Deep dry hot rock mining drilling operations confronts several challenges including the high-temperature conditions in the downhole environment, the unreliable performance of surface drives, and the difficult drillability of geothermal reservoir rocks. As a result, the use of downhole motors in drilling operations becomes an imperative selection. The all-metal turbodrill presently stands as the sole downhole motor capable of effectively drilling in deep wells with temperatures exceeding 180°. Although it can withstand high temperature, the output torque is small. In this study, we focus on the optimised design of the turbodrill blades with the objective of enhancing the output torque, addressing the prevailing issue of limited output torque. First, the structural parameters and parametric design are determined based on turbine blade design theory. Utilizing the quintic polynomial method, a two-dimensional blade profile is generated. Subsequently, an output torque model applicable to unsteady flow conditions is derived using the moment of momentum theorem in its integral form; Utilizing the designed blade profile, the Unigraphics NX (UG) software is used to illustrate the combination of spatial twisting, bending and tilting of the blade for the combination of the modelling method, and a three-dimensional bending-torsional tilting blade was achieved. Subsequently, numerical analysis, employing Computational Fluid Dynamics (CFD) simulation software, was conducted to examine the impact of intricate spatial modelling on the blade's flow field parameters. The optimal parameters for blade spatial modelling were ascertained to be a twist lofting radius of 63 mm, a bending angle of 30°, and an inclination angle of 3°. The blade in the hydraulic efficiency is reduced by only 2.7% of the case, the output torque increased by 18.87%, improving the performance of the turbodrill, so that the turbodrill in the geothermal development drilling has a good application prospect.

Evaluation of Rock Brittleness Based on Complete Stress–Strain Curve

As a basic mechanical property of rocks, brittleness is closely related to the drillability, wellbore stability, and rockburst characteristics of reservoir rocks. Accurate evaluation of rock brittleness is of great significance for guiding oil and gas production and reservoir reconstruction. This paper systematically introduced the commonly used brittleness evaluation methods based on the stress–strain curve and analyzed their theoretical background and mathematical models. Combined with practical engineering application, the characterization effect of commonly used brittleness indexes in various rock samples is verified and optimized, and it is obtained that the brittleness index (B17 in this paper), based on the stress–strain curve and considering energy conversion, has the best characterization result for rock brittleness, which has good differentiation for different rock samples. At the same time, considering that the stress–strain curve under high confining pressure may result in a significant yield plateau phenomenon before and after the peak strength, the endpoint of the plastic yield plateau is used to replace the peak point as the starting point for the drop of bearing capacity. The revised brittleness index is consistent with the changing trend of the original curve, which verifies the reliability of the model. Finally, the method for characterizing the brittleness of Class II curves is supplemented, and the combined brittleness index of rock is established, which verifies the rationality and correctness of the index and provide a more general evaluation method for rock brittleness in engineering.

Zoning of quarry and mine fields according to categories of drillability and explosiveness, in order to reduce the cost of drilling and blasting by 20% or more

Mathematical models (theory) have been developed, surfaces of the lowest values of the criteria for drillability and explosibility of ores and rocks of curvilinear-anisotropic rock massifs of ore deposits, and a method for zoning quarry and mine fields by categories of drillability and explosibility. The purpose of the research is to reduce the cost of drilling and blasting by 20% or more, based on the development of projects for the production of drilling and blasting operations, in accordance with their actual properties of geological bodies that make up mineral deposits. The application of the proposed technical solutions will make it possible to achieve high-quality crushing with the lowest possible costs for blasting, as well as to ensure resource saving, energy saving during blasting, to ensure the stability of the slopes of worked-out spaces, etc. The proposed method has been tested in the quarries of the Republic of Kazakhstan. There are no analogues of the proposed method.

DRILLING MECHANISM BASED ON THE APPLICATION OF DIAMOND DRILLING BITS

In order to express how much the drill selection depends on the rock properties and its effect on the percentage of core and drilling speed, diamond drill selection and drilling mechanisms are taken. In diamond drills, enough pressure should be applied to ensure that each active diamond grain sinks into the rock for the best drilling speed. This stress should be above the surface strength of the rock to be drilled and below the diamond grain strength. The main goal to be achieved in engineering and research and development studies is to ensure that both the designed system and the product to be developed have maximum performance. In order to reveal the conditions in which the best results will be obtained, first the feature that determines the performance is determined and the factors affecting this feature are examined. Then, experiments are carried out to determine the effects of these factors on the feature that determines the performance and to find the most suitable combination (by taking into account the uncontrollable factors). Optimum conditions are determined by evaluating the performance indicator obtained as a result of the experiments. The experiments carried out within the framework of this approach can be perceived as the question asked to the system, and the results of the experiments as the answer given by the system. The critical point is that it is necessary to ask the right question in order to get the right answer. Drilling is a complex event influenced by many factors. Drill type and diameter, rotational speed, pressing force, torque and circulating fluid are controllable parameters. On the other hand, physic mechanical properties of the rock and geological conditions (uncontrollable factors such as discontinuities, bedding conditions, grain size, matrix structure, porosity, cementation degree and abrasive mineral ratio) affect rock drill ability. The drill ability of rocks can be defined by various concepts such as feed rate, drill wear amount, drill life and tooth penetration. Drilling method classification can be made according to several criteria. The most logical classification can be made according to the type of energy required for rock fragmentation. Drilling is done in rocks for many purposes and many methods developed for this purpose have successfully drilled into rocks. Many new methods in rock drilling have been acquired through experience. The most successful practical application is the mechanical shredding process. Although it is mostly applied for cutting soft rocks, this method is done by spraying high pressure water. Keywords: drilling, drilling process, drilling operations, tools, drilling mechanism, drilling diamond.

PDC bit optimization scheme for tight oil reservoirs in the Jidong Oilfield based on rock anti-drilling characteristics

The tight oil reservoir rocks in the Jidong Oilfield are generally characterized by high hardness, strong abrasiveness, and extremely unevenness, which seriously reduce the drilling efficiency. In view of this problem, the mineral composition, hardness, plasticity coefficient, and drillability of rocks were first determined. It was found that the rock integrity, compactness and high hardness are the main factors leading to the low rock breaking efficiency of PDC bits. Then, combined with logging data, the drilling resistance profile of the whole well section was established. According to the crushing mechanism of rocks under the action of a PDC bit, the mechanical model of a single composite plate under rock crushing conditions is established, and the influence of the back angle on the cutting force is analyzed. The flow field distribution of the drill bit surface under different nozzle combinations was calculated by finite element numerical simulation. Based on the experimental and simulation results, the optimal backward angle was 15°, and the optimal nozzle combination was two 16-mm central nozzles and five 14-mm peripheral nozzles. Finally, a personalized PDC bit design scheme for tight oil reservoirs in the Jidong Oilfield was proposed. The scheme includes the following: (1) a 5-blade straight-arc crown is profiled; (2) anti-impact cylindrical components are set; (3) the nozzle hydraulic structure adopts a bending type; and (4) the diameter-retaining teeth and diversion grooves are optimized. The optimized bit reduces stick–slip, improves stability, and has an obvious acceleration effect.

Drillability prediction in some metamorphic rocks using composite penetration rate index (CPRI) – An approach

Assessment of drillability of rocks is vital in the selection, operation, and performance evaluation of cutting tools used in various excavation machinery deployed in mining and tunneling. The commonly used rock drillability prediction methods, namely, drilling rate index (DRI) and Cerchar hardness index (CHI) have limitations in predicting the penetration rate due to differential wear of the cutting tool in rocks with varied hardness and abrasivity. Since cutting tools get blunt differently in different rocks, the stress beneath the tip of the bit decreases until it reaches a threshold value beyond which the penetration rate becomes constant. In this research, a new composite penetration rate index (CPRI) is suggested based on the investigations on four metamorphic rocks viz. quartzite, gneiss, schist and phyllite with varied hardness-abrasivity values. The penetration-time behavior was classified into active, moderate, passive, and dormant phases based on the reduction in penetration rate at different stages of drilling. A comparison of predicted penetration rate values using DRI and CPRI with actual penetration rate values clearly establishes the supremacy of CPRI. Micro-structure and hardness-based index was also developed and correlated with CPRI. The new indices can help predict cutting tool penetration and its consumption more accurately.

A Probabilistic Approach for Prediction of Drilling Rate Index using Ensemble Learning Technique

Drillability is one of the significant issues in rock engineering. The drilling rate index (DRI) is an important tool in analyzing the drillability of rocks. Several efforts have been made by the researchers to correlate and evaluate DRI of rocks. The ensemble learning methods including the decision tree (DT), adaptive boosting (AdaBoost), and random forest (RF) are employed in this research work in order to predict DRI of rocks. A drillability database with four parameters is compiled in this work. A relationship between the input parameters and DRI is established using the simple regression analysis. In order to train the model, different mechanical properties of rocks incorporating the uniaxial compressive strength (UCS), Brazilian tensile strength (BTS), brittleness test (S20), and sievers’ J-miniature drill value (Sj) are taken as the input variables. The original DRI database is randomly divided into the training and test sets with an 80/20 sampling method. Various algorithms are developed, and consequently, several approaches are followed in order to predict DRI of the rock samples. The model performance has revealed that RF predicts DRI with a high accuracy rate. Besides, the Monte Carlo simulations exhibit that this approach is more reliable in predicting the probability distribution of DRI. Therefore, the proposed model can be practiced for the stability risk management and the investigative design of DRI.

Development models for the drill bit lifetime prediction and bit wear types

This study presents the relationships between drill bit lifetime and mechanics, drillability and abrasive properties of rocks. Additively, worn-out of bit types were investigated. For that purpose, rock blocks were collected from tunnels. Regression analyses were employed to derive models for estimating the bit lifetime based upon drillability, and mechanical and abrasive properties of the rock. Results indicated that the bit lifetime values decrease with the increases of the uniaxial compressive strength, point load index, Brazilian tensile strength, Schmidt hardness (L-type and N-type) and Cerchar abrasivity index. However, the drill bit lifetimes increased with the increasing of Micro Deval abrasion, Brittleness values, Sievers' J-miniature drill, and excavation volume values. The statistical results showed that the most important parameters on the drill bit lifetimes were uniaxial compressive strength, Schmidt hammer, Sievers’ J-miniature drill, and excavation volume. Furthermore, developed models would be used as guidance for practical applications.The paper also was investigated the bit wear types and their classification. Two new different bit wear types were investigated including, crack of the bit body and failure of bit rod. The crack of the bit body is a typical for the failure of tool materials. The failure of bit rod is typical for special wear types.

Measurement, Prediction, and Modeling of Bit Wear During Drilling Operations

Modelling wear of drill bits can increase the efficiency in the drilling operations. Related to the subject, it is aimed to investigate the wear mechanism of drill bits. Wear in drill bits is influenced by many factors related to the drilling and rock properties. The type and intensity of wear are dependent on several complicated factors that are required to be considered in anticipating the rate of wear in the field and laboratory conditions. The laboratory tests have been performed in order to specify the relationships between the bit wear rate and the physico-mechanical properties, drillability, abrasive properties, and brittleness of rocks. Statistical analysis has been used to obtain equations for estimating the bit wear rate based on the rock properties. In this work, an ensemble technique is used to estimate the confidence interval and the prediction intervals for the regression models. This paper summaries the rock properties and bit wear mechanism, and argues the options to determine the bit wear rate. The test models indicate that the rock properties can give an idea of bit wear. They also show a good correlation between the bit wear rates. Also some models are developed to represent the wear quantification, and an approach is suggested in order to estimate the bit wear rate. The results obtained from studying the developed models provide a good agreement with the performance evaluation of an efficient drilling, which provide an indirect evaluation of drill bit wear rate during a drilling process, which can help to reduce the specific energy consumption and lower costs for the exchange of drill bits.

Optimization and Establishment of Drillability in Gravel Formation

Rock drillability is a comprehensive index used to evaluate the difficulty of drilling and is a basic factor to determine drilling efficiency. Proper evaluation of rock drillability is vital to achieving high-quality and efficient drilling. Currently, the method for determining the drillability of rocks is primarily based on laboratory micro-bit experiments to measure the core’s resistance to drilling. This method requires a large number of cores and is expensive, takes a long time to start the test, and often lags behind drilling. Moreover, some people analyzed the relationship between the actual drilling parameters and the mechanics’ specific energy of drill bit and established a method to evaluate the rock drillability of rock. Field application proves that the drillability model is poorly correlated with the actual drilling conditions. The value of the actual drilling energy is usually 100–1000 times that of the theoretically calculated mechanics specific energy. Through laboratory experiments, we study the strength and failure characteristics of different gravel sizes and gravel contents with different impact speeds. The matching relationship between rock mass and impact speeds is analyzed. Based on the experimental results, a phase similar to a drill bit and a drilling tool is proposed to support drillability assessment. Under the condition of the same gravel size, with the increase of gravel content, the average cutting speed gradually decreases under the condition of certain cutting force, which is approximately exponential; with the increase of gravel particle size, under the condition of certain cutting force, the average cutting speed rapidly decreases, and the gravel particle size has a decisive impact on the average cutting speed. The above research results can be used to establish a new drillability evaluation model and to optimize the drill bits, drilling tools, and drilling parameters.

Finding velocity of roller-bit and rotary-percussive drilling using the energy conservation law

Velocity of roller-bit and rotary-percussive drilling depends on many factors distributed in 4 groups in technical literature: rock properties, bit parameters, bit-rock interaction conditions and drilling modes. Literature sources present some very complex formulas which need finding empirical coefficients before determining drilling velocity, i.e. the formulas are difficult to use. Moreover, the formulas neglect jointing of rock masses. At the same time, mathematical relations connecting drilling velocity, drilling mode and drillability of jointed rocks will make it possible to rate drilling processes and adjust blasting parameters. These studies aim to determine velocity of roller-bit and rotary-percussive drilling using the energy conservation law. The used method of mathematical modeling allowed obtaining formulas for rock drilling velocity with regard to drilling modes, bit parameters, factor of rock hardness (strength) and rock mass jointing. The validity of the relations of the roller-bit and rotary-percussive drilling velocity is proved. The reliability of the drilling velocity formulas can be determined by means of investigations performed in open pit mines, with recording of all parameters and with mathematical processing of the data. The mathematical relations connecting drilling velocity, drilling modes, drill bit parameters and drillability of jointed rocks will enable rating of drilling and adjustment of blasting patterns.

Modern Methods and Tools for Determining Drillability and Blastability of Rocks

Often in practice, the coefficients of Protodyakonov scale of hardness f are not established based on objective measurements that are very laborious, but by visual degree and rock family type, which naturally significantly reduces the accuracy of calculations.In this regard, it is proposed to obtain information on the physical-mechanical properties of rocks directly from rigs during drilling, based on the energy characteristics of this process; its main characteristic is the specific drilling energy, i.e., the energy required to destroy a unit of rock during drilling.For a quantitative assessment of the rock strength as resistance to mining - technological destruction - prof. M.M. Protodyakonov [2] proposed the coefficient of hardness f, in the first approximation, it is proportional to the rock break-down point under compressive load. The scale of hardness was developed, according to which all rocks are divided into ten categories. Many post-Soviet engineers use this coefficient. Since the coefficient of hardness mainly reflects only fracturing under compressive loads, and in real conditions tensile and shearing forces are often used, it becomes necessary to introduce other indicators to assess the properties of rocks.

The Effect of Mineralogical Composition on Strength and Drillability of Granitic Rocks in Hulu Langat, Selangor Malaysia

The strength characteristics of a rock is a criterion for its ability to be drilled and fractured. The uniaxial compressive strength (UCS) is the most commonly used strength index in construction industry and in rock engineering projects. Therefore, a comprehensive knowledge on the factors that could influence the value of the compressive strength is essential in any rock engineering project. This paper explores the effects of mineralogical composition on the UCS value of granitic rocks to evaluate its drillability. Samples of granitic rocks were collected from the Interstate Raw Water Transfer Tunnel project. The rock samples were characterized with quantitative Rietveld XRD method to quantify the relative mineral phase compositions in the rock samples. The relationships between mineralogical properties and UCS were investigated by simple regression analysis. The strength of the rock samples increases with higher composition of quartz and decreases with increase in the composition of feldspar and kaolinite. The positive effects of quartz on the strength of the granitic rock is more significant than the negative influence of feldspar and clay content. The drilling rate varies inversely with the UCS.

A sampling study on rock properties affecting drilling rate index (DRI)

Drilling rate index (DRI) developed in Norway is a very useful index in determining the drillability of rocks and even in performance prediction of hard rock TBMs and it requires special laboratory test equipment. Drillability is one of the most important subjects in rock excavation. However, determining drillability index from physical and mechanical properties of rocks is very important for practicing engineers such as underground excavation, drilling operations in open pit mining, underground mining and natural stone production. That is why many researchers have studied concerned with drillability to find the correlations between drilling rate index (DRI) and penetration rate, influence of geological properties on drillability prediction in tunneling, correlations between rock properties and drillability. In this study, the relationships between drilling rate index (DRI) and some physico-mechanical properties (Density, Shore hardness, uniaxial compressive strength (UCS, σc), Indirect tensile strength (ITS, σt)) of three different rock groups including magmatic, sedimentary and metamorphic were evaluated using both simple and multiple regression analysis. This study reveals the effects of rock properties on DRI according to different types of rocks. In simple regression, quite high correlations were found between DRI and uniaxial compressive strength (UCS) and also between DRI and indirect tensile strength (ITS) values. Multiple regression analyses revealed even higher correlations when compared to simple regression. Especially, UCS, ITS, Shore hardness (SH) and the interactions between them were found to be very effective on DRI values.

Effects of microfabric on drillability of rocks

Quantifying the rock microfabric is one of the most important research interests and challenges in the field of rock drilling and excavation. In this study, the relationship between quantitative parameters of the rock microfabric and the rate of top-hammer drilling was studied. For this purpose, a drilling operation was performed on five soft rock samples and three hard rock samples and drilling rates were recorded. Thin sections from the rock samples were prepared and micrographs of the sections were reviewed using AutoCAD software and digital boundaries of the grains were clarified and drawn. After preparation and filtering of the digitized thin sections, four basic properties of each grain, including maximum diameter, minimum diameter, area and perimeter, were calculated with the help of the software. These parameters were then analyzed statistically and used to obtain six indexes including equivalent diameter, grain compactness, shape factor, aspect ratio, interlocking index and grain size homogeneity index. Finally, the microfabric and drilling rate data were subjected to regression analysis and the significance of the relationships between the grain parameter values and drilling rate were explored. Drilling rate was found to increase with increasing equivalent diameter and shape factor. In contrast, drilling rate decreased with increasing grain compactness, aspect ratio, grain size homogeneity and grain interlocking index. Grain compactness showed the most significant relationship (R 2 = 0.8) with drilling rate among the studied parameters.

Relationships between the drilling rate index and physicomechanical rock properties

The drilling rate index (DRI) is an important parameter that influences the drillability of rocks. It can easily be used when estimating the economics of any excavation operation. Therefore, in the current study, an attempt was made to investigate the rock properties that govern the DRI. The relationships between the DRI and some physicomechanical rock properties were investigated based on data obtained from experimental work and in situ studies performed in different tunnels. Regression analysis was employed to develop models for estimating the DRI based on physicomechanical rock properties. The derived models were verified based on the behavior of the determination coefficient, the t test, and the F test. The study showed that the DRI decreases with increasing uniaxial compressive strength, point load strength, Brazilian tensile strength, and Schmidt rebound hardness. It was also concluded that the DRI increases with increasing apparent porosity and void ratio. Additionally, modeling results revealed that the proposed models can be successfully used as tools to forecast the DRI.

Experimental investigation into rate of penetration in carbonated rocks

Rate of penetration (ROP) in drilling processes is dependent on various operational variables (bit weight, rotary speed etc.), in addition to rock drillability. Drillability of rock depends on physical, mechanical and micro-structural properties of rock. Evaluation of rock drillability is not a trivial task and different methods have been proposed to measure it. In this study we measure the drillability of several carbonate rocks and evaluate them with ROP experiments. Using a laboratory drilling setup, ROP was measured at different weights on bit (WOB) and rotary speeds for ten different carbonate rocks (other operational variables were constant). Experiments showed that unconfined compressive strength (UCS) is not applicable for characterising the drillability, while drilling rate index (DRI) leads to more reliable evaluations of drillability of carbonate rocks. The experiments demonstrated that there is a relationship between WOB exponent, rotary speed exponent in ROP equation and DRI. Using DRI as the indicator of drillability, we performed 500 tests to correlate ROP as a function of WOB, rotary speed and drillability for carbonate rocks. [Received: February 1, 2014; Accepted: September 17 2014]

Prediction of Drillability of Rocks with Strength Properties Using a Hybrid GA-ANN Technique

The purpose of this paper is to provide a proper, practical and convenient drilling rate index (DRI) prediction model based on rock material properties. In order to obtain this purpose, 47 DRI tests were used. In addition, the relevant strength properties i.e. uniaxial compressive strength and Brazilian tensile strength were also used and selected as input parameters to predict DRI. Examined simple regression analysis showed that the relationships between the DRI and predictors are statistically meaningful but not good enough for DRI estimation in practice. Moreover, multiple regression, artificial neural network (ANN) and hybrid genetic algorithm (GA)-ANN models were constructed to estimate DRI. Several performance indices i.e. coefficient of determination (R2), root mean square error and variance account for were used for evaluation of performance prediction the proposed methods. Based on these results and the use of simple ranking procedure, the best models were chosen. It was found that the hybrid GA-ANN technique can performed better in predicting DRI compared to other developed models. This is because of the fact that the proposed hybrid model can update the biases and weights of the network connection to train by ANN.

Performance Assessment of Hard Rock TBM and Rock Boreability Using Punch Penetration Test

Rock indentation tests are often called punch penetration tests and are known to be related to penetration rates of drilling equipment and hard rock tunnel boring machines (TBMs). Various indices determined from analysis of the force-penetration plot generated from indentation tests have been used to represent the drillability, boreability, and brittleness of rocks. However, no standard for the punch penetration test procedure or method for calculating the related indices has been suggested or adopted in the rock mechanics community. This paper introduces new indices based on the punch test to predict the performance of hard rock TBMs. A series of punch tests was performed on rock specimens representing six rock formations in Korea with different dimensions, i.e., the core specimens had different lengths and diameters. Of the indices obtained from the punch tests, the peak load index and mean load index showed good correlations with the cutting forces measured in full-scale linear cutting machine tests on the same rock types. The indices also showed good linear correlations with the ratio of uniaxial strength to Brazilian tensile strength, which indicates the brittleness of rock. The scale effect of using core specimens was investigated, and a preferred dimension for the punch test specimens is proposed. This paper also discusses the results of the punch test and full-scale rock cutting tests using LCM. The results of this study confirm that the proposed indices from the punch tests can be used to provide a reliable prediction of the cutting forces that act on a disc cutter. The estimated cutting forces can then be used for optimization of cutter-head design and performance prediction of hard rock TBMs.

Investigating the Mechanical Properties and Drill-ability of Rocks

Drilling is the primitive and common operation in rock excavation industry, starting from exploration to exploitation. Effect of mechanical properties of the formation on drilling operation is a big challenge to mining engineers. The primarily problem is rock drill-ability. The objective of this paper is to correlate the drill-ability of certain rocks with their mechanical properties including Compressive strength, Tensile strength, Schmidt hammer rebound number and Shore Hardness number, in order to establish rate of penetration. This paper contains investigation of both laboratory experiments and Field examination of drilling equipment. Laboratory work includes the determination of various mechanical properties of rock specimen, determined according to ASTM procedure using Universal Testing Machine. Schmidt hammer test conductedon the rock specimen by clamping into the UTM loading platens. Shore hardness test performed on disc specimen carefully prepared for the purpose. The test was conducted as per standard procedure.