bluuebunny/crossref_metadata_2025 huggingface dataset:
Research on the Prediction Method of Formation Pore Pressure Based on Ensemble Methods
Authors: An J. T. , Huang H. L. , Li J. , Yang H. W. , Gao R. Y. , Zhang G. , Liu P. L. , Luo M. , Li W. T.
Published: September 2022

Accurately predicting overpressure can effectively guide oil and gas exploration and development. However, the parameterization method is limited by the model form, and the accuracy is low. In this study, a new method to predict pore pressure using ensemble method was proposed. For the first time, acoustic velocity, porosity, mud content, and density were used as input variables and effective stress as output variable, and then accurate prediction of pore pressure was achieved based on the effective stress theorem. Compared with the predictions of traditional models, the performance of the developed models was verified. The results show that compared with the traditional models, the developed method has the advantages of high accuracy, and its absolute average percentage deviation (AAPD), root mean square error (RMSE) are 0.76 % and 0.030 g/cm3, and the coefficient of determination (D2) is 0.992. In addition, the Random Forest (RF) is superior to the Gradient Boosting Machine (GBM) in goodness of fit, generalization and prediction accuracy, which AAPD and RMSE are only 0.13 % and 0.013 g/cm3, and D2 is 0.993. The model developed in this study is not affected by the mechanical depositional environment and is applicable to sandy mudstone formations, such that it can be a useful and highly accurate alternative to the traditional prediction methods with fixed parameter forms.

1. INTRODUCTION

Formation pore pressure, refers to the pressure possessed by the fluid (oil, gas, or water) in the empty space of the formation. During the drilling process, if the pore pressure is accurately predicted, complex downhole accidents such as blowouts and drilling fluid leaks will be effectively reduced and drilling costs will be lowered (Zoback, 2007; Tingay et al., 2009). When the pore pressure is less than the hydrostatic pressure, it is called anomalous low pressure; and when the pore pressure is greater than the hydrostatic pressure, it is called anomalous high pressure (Fig. 1). More than 47.7% oil or gas reservoirs are classified as anomalous high-pressure reservoirs around the world. The mechanisms of anomalous high-pressure genesis include, but are not limited to, the effects of disequilibrium compaction, hydrothermal pressurization, clay mineral transformation, permeation, hydrocarbon generation, hydrocarbon cracking, and pressure transfer. Among these mechanisms, disequilibrium compaction is one of the most dominant loading mechanisms for overpressure formation, and fluid expansion (hydrothermal pressurization, hydrocarbon generation, and hydrocarbon cracking are all part of fluid expansion) is one of the most dominant unloading mechanisms (Vernik. 2016).
Study on the Wellbore Shrinkage During Drilling Operation in Frozen Soil
Authors: Li Y. , Cheng Y. F. , Yan C. L. , Yang H. L. , Gao Q. , Han S. C.
Published: September 2022

During drilling operations in cold regions, the frozen soil obtains strong creep characteristics, which will result in wellbore shrinkage. An unreasonable drilling fluid density may cause engineering accidents, such as blocking and stuck drills. To solve this problem, a drilling fluid density calculation model based on the power law model of frozen soil creep was established in this paper for controlling the wellbore shrinkage in frozen soil. The factors affecting the wellbore shrinkage rate in frozen soil were analyzed. A map of the wellbore shrinkage and drilling fluid density was also established. The results show that the wellbore shrinkage rate is affected not only by the physical properties of the frozen soil but also factors such as the temperature, formation depth, drilling fluid density, wellbore size, and well opening time. The density of the drilling fluid required for safe drilling in frozen soil increases as the formation depth increases, and the magnitude gradually decreases. As the temperature decreases, the drilling fluid density at the same depth gradually decreases, and the magnitude also gradually decreases. The results provide an important basis for determining the drilling fluid density and the selection of drilling methods in cold regions.

1. INTRODUCTION

As the oil and gas energy situation becomes increasingly severe, unconventional oil and gas is gradually becoming the focus in the petroleum industry. In 2008, the US Bureau of Geodetic Survey conducted exploration in the Arctic and confirmed that the Arctic region is rich in oil and gas resources (Bird et al., 2008). The petroleum industry began to pay attention to the polar regions (Li et al., 2019; Vernikovsky et al., 2013; Sekretov, 2001). Compared with the conventional stratum, there are ice particles in the pores of the frozen soil. The change of temperature and pressure will cause the ice to melt, seepage and secondary solidification, which greatly changes the structure and mechanical properties of the frozen soil and makes the frozen soil own strong creep property. The unique mechanical properties of frozen soil, especially its creep characteristic, would bring a significant impact on drilling operations (Yu et al., 2013; Yang et al., 2010), such as wellbore shrinkage, stuck drilling, and casing could be crushed after cementing. These seriously hinder the exploration and development of oil and gas resources. Therefore, research on the wellbore shrinkage of frozen soil is of great significance for the development of petroleum in cold regions.
Analysis of Cement Plug Bonding Surface Integrity in CO2 Geological Storage
Authors: Dian Wang , Jun Li , Xianbo Liu , Tuo Li , Penglin Liu , Wei Lian
Published: September 2022

Using depleted oil and gas reservoirs to sequester CO2 is an effective means of mitigating the greenhouse effect. The long-term integrity of the cemented surface of the cement plug is directly related to the risk of CO2 leakage. In this paper, based on the cohesive element method, a new finite element model of cement plug-reservoir-cover is established, considering the sealing location, sealing length, and nature of cementing surface of cement plug. The influence of each factor on the long-term integrity of cementing surface is quantitatively analyzed. The simulation results show that: the cement plug sealing location should be mainly in the cover area, and the minimum length should be not less than 50m; the normal strength and failure displacement of the cemented surface determine the tendency, speed, and height of fracture development, and the fracture development slows down after penetrating the layer. This study is necessary in reducing the leakage risk of CO2 geological storage.

1. INTRODUCTION

Using depleted oil and gas reservoirs to sequester CO2 is a potential method to mitigate the greenhouse effect (Leung et al., 2014). Ensuring the long-term sealing of depleted oil and gas reservoirs is the focus of attention and the key to the success of CO2 geological storage (Shukla et al., 2010; Newell et al., 2013). The types of oil and gas reservoir seal failure can be divided into two categories: wellbore containment barrier seal failure and oil and gas reservoir top cover seal failure (Zhang et al., 2011).

As shown in Fig. 1, the wellbore barrier failure mode can be divided into (1) Formation/cement sheath interface failure, (2) Cement sheath/casing interface failure, (3) Casing/cement plug interface failure, (4) Cement plug body failure, (5) Cement sheath body failure, and (6) Casing body failure according to the CO2 leakage path; For the open hole section, there are two failure ways (1) and (4). (Gasda et al, 2004; Bai et al, 2015; Nagelhout et al, 2010)
Experimental Investigation on the Conductivity of Micro-Frac Dynamic Proppant Transport Process
Authors: Fen Peng , Hongtao Liu , Shiyong Qin , Longcang Huang , Longqiao Hu , Zhikun Pan , Yixuan Liu , Fujian Zhou , Ben Li , Junling Wang , Zhaokun Yuan , Haijun Gao
Published: September 2022

Fractures are primary flow channels of unconventional oil and gas reservoirs; thus, hydraulic fracturing is widely used to promote the productivity of unconventional formations. Hydraulic fracturing can induce numerous tiny fractures and stimulate some weekly cemented natural fractures. Since the width of the induced fractures is less than 200μm, large size proppants (less than 140 mesh) could not be transported and propped the tiny fractures efficiently. This paper proposed a dynamic proppant transportation and conductivity test method to investigate the flow resistance of the propped tiny fractures. Silica powder, tiny glass beads, and TBA powder were considered potential proppants that could be used to prop the tiny fractures. Proppant diameter and crushing ratio were evaluated. The conductivity of the tiny fractures was evaluated at various slurry concentrations and closure stresses. Results showed that the optimal slurry concentration should not be higher than 9%. Silica powder can be used in higher fracture closure stress formations while TBA and tiny glass beads are more suitable for propping tiny fractures under lower fracture closure stress conditions.

1. INTRODUCTION

As more wells are drilled and completed in the tight formations, operators rely more on small-sized proppants to help ensure the created fractures are propped and to maintain the conductive flow paths for the well production. Micro-proppants may refer to proppants smaller than 100 mesh, mainly including silica flour and ceramic micro-proppant, fly ash are also available.

In 1998, Standard oil applied 100 mesh grains of sand as propping agents in the Upper Morrow formation. Recent studies show that proppants that are smaller than 100 mesh proppants could further improve the permeability of unconventional reservoirs. It was also proposed that microproppant enjoyed a proppant transport benefit into complex fracture systems, in areas where 100 mesh and 40/70 could not travel.

Dahl et al., (2015) undertook permeability tests on split core samples to investigate the effect of micro-sized proppants and tackifying agents in improving secondary fracture conductivity,
3D Geomechanical Modeling for Accurate In-Situ Stress Characterization of a Reservoir in Mahu Oilfield, China
Authors: Xianglong Meng , Yongcun Feng , Wenjun Cai , Shiqi Wang , Wei Yan , Qingping Jiang
Published: September 2022

Mahu oilfield in Junggar basin is rich in oil and gas resources, because the reservoir has the characteristics of low porosity and permeability, horizontal well and stimulated reservoir volume were applied here. The conventional 1D MEM (mechanical earth model) obtained from single well logging data can not reflect the changes of reservoir geomechanical parameters in long horizontal section, 3D geomechanical modeling is carried out to achieve the fine description of reservoir geomechanical characteristics. During the development of the 3D geomechanical model, a variety of field data and core experimental data were used to ensure its accuracy. The 3D structural model of the reservoir was interpreted from seismic data and a fine description of wave velocity distribution in the 3D model was obtained by a combined interpretation of the acoustic logging data and seismic data. Rock mechanical test data were used to calibrate the predicted geomechanical parameters in the model, ensuring its accuracy. Furthermore, the in-situ stresses determined from diagnostic fracture injection tests and 1D MEM were used to verify the stress calculation results of the 3D geomechanical model. The modeling results show that the direction of the maximum horizontal principal stress in the Mahu oilfield is east to west. The minimum and maximum horizontal principal stresses are in the regions of 42-55 MPa and 63-70 MPa, respectively. There are obvious stress differences between layers, in the range of 10-25 MPa. A comparison of the modeling results against the micro-seismic signals and instantaneous shut-in pressure data recorded during hydraulic fracturing operations further confirmed the reliability of the model. The 3D geomechanical model can provide important guidelines for new drilling and hydraulic fracturing designs in this oilfield.

1. INTRODUCTION

Mahu Oilfield is a large conglomerate tight oil reservoir under development. It is difficult to apply conventional reservoir development methods because of the deep burial of the reservoir, the large natural in-situ stress difference, the low porosity and permeability, and the strong heterogeneity which is caused by the poor particle size sorting of the gravel in the rock mass. In order to realize the efficient development of the Baikouquan Formation conglomerate reservoir in Mahu Oilfield, Mahu Oilfield engineers applied horizontal wells and volumetric hydraulic fracturing based on the idea of multi-layer 3D development.
Exploring Advanced Neural Network Architectures for Synthetic Well Log Generation
Authors: Wong Yian , Wong Sau-Wai
Published: September 2022

A majority of well logs contain basic petrophysical and lithological values which usually include density, porosity, resistivity, gamma-ray, and customarily, the caliper measurement of borehole diameter to provide an indication of borehole and log quality. Acoustic log acquisition may not be routine, especially in older wells and in cost-constrained environments. Attempts can be made to create synthetic acoustic logs relying on petrophysical and empirical models. However, their reliability and applicability do depend to a large extent on the complexity, variability, and uncertainty of subsurface rocks and fluids. Often, the challenge is compounded by rock heterogeneity such as the highly variable and laminated unconventional shales. In this paper, we tackle the problem of predicting acoustic log response given common well log values such as gamma-ray, resistivity, porosity, and density. Well logs from two very different/contrasting rock formations are used in this exercise. Accurate prediction of acoustic response is shown to be considerably more challenging when rock formation is highly heterogeneous. We consider the generation of synthetic acoustic well log values as a regression task between lithological/petrophysical parameters and acoustic well log values. We survey common machine learning methods in this problem and introduce the use of more advanced techniques. Our methodology includes the adoption of Monte Carlo dropout as a probabilistic inference alternative to regular inference, which allows the quantification of uncertainty in model prediction without having to significantly increase the model’s computational complexity. We survey the performance of all models using two well log datasets, one set of well logs is taken from a relatively homogenous rock formation and the other from a heterogenous rock formation. Out of all models, the proposed Convolutional Neural Network coupled with Monte Carlo dropout provides the most robust results with adequate quantification of prediction uncertainty.
Prediction of Reaming Performance of Vertical Raises Using Rock Properties and Operational Parameters
Authors: Shaterpour-Mamaghani A. , Copur H. , Tumac D. , Balci C. , Erdogan T. , Dogan E. , Altintas E. , Kocbay A. , Sirin O.
Published: September 2022

Raise Boring Machines (RBMs) as the fast and safe operation of drilling excavation of vertical and/or inclined shafts are used in the mining and construction projects for different purposes. Proper selection and accurate performance prediction of these machines are very important issues affecting the cost estimation and planning of the projects. However, reliable models for predicting performance of these machines have been quite limited. This study aims to develop empirical performance prediction models for vertical reaming operations (unit penetration rate and specific energy parameters) by using physical-mechanical properties of rock samples, operational parameters of pilot hole drilling and reaming. Field performance data of the pilot hole drilling and reaming operations, as well as rock samples, are collected in different raise excavation projects in Turkey. The evaluation indicates that weight on bit of pilot hole drilling with the rotational speed of reaming and static elasticity modulus of the rocks could be used to predict the field specific energy. Consumed reamerhead torque and indirect tensile strength and dynamic elasticity modulus of rocks could be used to predict the unit penetration rate.

1. INTRODUCTION

Raise Boring Machine (RBM) has now become the most preferred machine for drilling / excavation of inclined and vertical shafts in mining and tunnelling projects. RBMs excavate a circular hole between two levels of underground or between an underground level and surface without using explosives and provides improved personnel safety over the drill and blast method as well as a significant reduction in labor requirement. However, the capital investment is higher and higher level of operator training is required (Wake 2000).

RBM uses a small diameter drill rod, around 230-350 mm, to drill a pilot hole down to the required depth. Once the pilot hole has been drilled, the pilot drill bit is removed and replaced with a large diameter reamerhead. The reamerhead is then pulled back up to the upper level by enlarging the hole diameter. In the pilot drilling, the cuttings (muck) are removed from the hole with the aid of flushing. However, in the reaming stage, the muck falls by gravity to the bottom of the hole in where it is continuously removed.
Intelligence-Driven Prediction of Shear Wave Velocity Based on Gated Recurrent Unit Network
Authors: Li Xuechen , Ma Xinfang , Xiao Fengchao , Xiao Cong , Wang Fei , Zhang Shicheng
Published: September 2022

Accurate shear wave velocity (Vs) is essential for determining reservoir rock mechanical parameters and pore structure of tight oil, whereas it is usually not measured in well logging due to high costs. The simple conversion of compressional wave velocity (Vp) to Vs introduces significant uncertainty to calculation results. To this end, we propose to construct an efficient and robust Vs prediction model using Gated Recurrent Unit (GRU) network, which could deeply mine and connect the information in Vs sequence and other logging sequences (e.g., gamma ray, resistivity, neutron porosity and Vp). Based on the collected logging data set, the GRU prediction model for predicting Vs is trained, validated and tested, which can predict Vs in the missing sections according to synthetic logging data and depth correlation. The optimal hyperparameters of the proposed model, including the number of layers, the number of neurons, activation function and learning rate are explored via the Bayesian optimization algorithm. The field experiment shows that the proposed Vs prediction model outperforms the traditional conversion method and can be used as a promising tool for accurate Vs prediction.

1. INTRODUCTION

The key to the successful development of shale gas and tight oil lies in horizontal drilling and hydraulic fracturing techniques that can break up tight reservoirs for effective connectivity (Junfeng et al., 2015; Longde et al., 2019; Ning et al., 2018; Wendong et al., 2013; Xiaoxia et al., 2014). Notably, rock elasticity, such as Young’s modulus and Poisson’s ratio, plays an important role in the fracturing and production effectiveness of fractured horizontal wells. Most of the parameters characterizing the elastic properties can be calculated from compressional wave velocity (Vp), shear wave velocity (Vs) and density. Currently, Vp and density can usually be easily obtained by conventional logging, but Vs are often missing due to the high cost of the dipole acoustic wave measurements (Gholami et al., 2014; X.-Y. Li & Zhang, 2011; Jun Wang et al., 2020). On the other hand, not only rock elastic properties but Vs is also important for prestack inversion (Pei-Jie & Xing-Yao, 2008), reservoir description (Wegener et al., 2006) and the identification of fluid distribution (Hamada, 2004). Therefore, the urgent need for accurate prediction of Vs has raised the wide interest of foreign and domestic researchers.
Experiment Study of Stress and Pore Pressure in Setting Cement Paste
Authors: Hu Zhao , Yonglin Shan , Jiadi Ding , Ligang Shi , Dayang Wen , Wenke Cao , Wei Liu
Published: September 2022

Cement sheath integrity plays an important role in ensuring the wellbore safety. Shear failure, tensile crack or debonding may happen in the cement sheath during the cement sheath setting, later pressure test or production period. The failure condition is up to the effective stress state and strength criterion. However, how to determine effective stress in the cement sheath is difficult. In this paper, a device is designed to test the lateral and pore pressures evolution of a cement column when a constant pressure is applied on the top of the column. Meanwhile, the cement paste volume change could be monitored. It is found that the lateral and pore pressures show a three-stage feature of stable-decrease-stable when the cement paste is hardening. The lateral and pore pressures are nearly equal at the early time and then separate without water charging, both the ultimate pressures are pretty low. The lateral pressure and pore pressure are nearly same too when suppling water to cement. However, the ultimate pressures are slightly higher than the former case. The experiment study could provide an inspect to research cement behavior, which contributes to developing and verifying the cement sheath stress evolution model.

1. INTRODUCTION

Cement sheath integrity is important to the wellbore safety. In order to study the cement integrity condition, the stress state is needed to know first. However, it is not easy to determine the cement sheath stress state. Since cement sheath is originally slurry, the cement solidification process changes its material physical and mechanical properties. The stress state of cement sheath may be different from its initial hydraulic pressure.

The casing-cement-formation is normally regarded as an entire system, the cement sheath stress state could be obtained based on the thick wall cylinder theory. The methods include theoretical solution (Li et al., 2015; Liu et al., 2017; Thiercelin et al., 1998; Yuan et al., 2013) and numerical simulation (Nabipour et al., 2010; Wong and Yeung, 2005). Both of them could provide an exact cement stress distribution. However, these models usually assume the system is a one-part structure, it means that the stress or displacement boundaries are applied on the model at one time. Actually, such treatment disagrees the fact, the well construction, the hole drilling, casing running and cement pumping are done separately and step by step (Gray et al., 2009). Therefore, the cement stress state should consider the procedure and the cement setting process (Agofack et al., 2019; Amziane and Andriamanantsilavo, 2004; Zuo and Bennett, 2019).
Improvement of Lost Circulation Control While Drilling by Using Managed Pressure Drilling System
Authors: Zhang Pei , Ma Tianshou , Liu Yang , Li Zhilin , Zhong Chengxu
Published: September 2022

Due to the complex lithology and geological conditions of deep and ultra-deep formations, fractured formations with narrow safety mud weight window are often encountered during drilling, which is easy to cause lost circulation and gas kick. In order to enhance the pressure bearing capacity of fractured formations, lost circulation material (LCM) is usually employed as an effective measure. However, the wellbore pressure fluctuation in conventional drilling can affect the fracture width, which makes LCM difficult to enter into the fracture, consequently, lost circulation occurs frequently. In this paper, we aim to explore the improvement of lost circulation control while drilling by adjusting the fracture width using managed pressure drilling (MPD) system. Firstly, the range of wellbore pressure fluctuations in the process of drilling operation was investigated. Then, the changes of fracture width with dynamic wellbore pressure were calculated and analyzed. Finally, the relationship between fracture width and choke pressure of the MPD system was analyzed, and the proper choke pressure was optimized to enhance the effect of lost circulation control while drilling. The results indicated that with the increase of wellbore pressure, the fracture width gradually increases, and the stronger the nonlinearity of fracture width distribution. The fracture width is highly dependent on the wellbore pressure, and the dynamic wellbore pressure will cause dynamic fracture width. The improper particle size distribution of LCM is difficult to match the dynamic fracture width in the process of lost circulation control while drilling. An appropriate stable wellbore pressure is a useful way to adjust the fracture width, which can make the LCM enter into the fracture to bridge, consequently, the lost circulation control while drilling can be improved by adjusting the fracture width using the MPD system.

1. INTRODUCTION

With the development of oil and gas resources of deep and ultra-deep formations, more and more deep and ultra-deep formations with complex lithology and geological conditions will be encountered during drilling (Ma et al. 2016). Due to the deep and ultra-deep formation is generally featured by well-developed fractures, so as to the pressure bearing capacity are significantly reduced by the existing of weak fractures (Liu et al. 2020a). Consequently, the deep and ultra-deep formations usually have a narrow safety mud weight window, and even the caving pressure or pore pressure is much higher than the fracturing pressure (Chen and Ma 2014; Ma et al. 2018, 2019; Liu et al. 2019, 2020b; Zhang et al. 2021). For the drilling operation in such a fractured formation, the conventional drilling operations are prone to a series of downhole conditions and drilling problems, such as lost circulation and gas kick.
Temporal Variation in CT Values of Pore Water in Granite Fracture Subjected to Long-Term Confining Pressure and Temperature
Authors: Nakashima S. , Isumi Y. , Song C. , Kishida K.
Published: September 2022

The groundwater flow in rock fractures and the chemical dissolution of rock minerals, which are greatly affected by the confining pressure and temperature, are important issues in the geological disposal of radioactive waste and geothermal studies. In order to investigate the long-term deformation and permeability of rock fractures, the authors conducted long-term permeability tests using a granite specimen with a single split fracture, with parameters of confining pressure and temperature. The saturated specimen was subjected to confining pressure of 3 MPa at 20°C for more than six months, and then the temperature was raised to 60°C. Microfocus X-ray computed tomography (CT) images were taken at predetermined times. The results showed that the CT values inside the fracture rapidly increased with time for 60 days at 20°C and 3 MPa, and then became stable. However, after the temperature was raised to 60°C, the CT values started to increase again. This evolution in the CT values implies pressure solution at the fracture interface due to the long-term confining pressure and temperature.

1. INTRODUCTION

The groundwater flow in rock fractures and the chemical dissolution of rock minerals, which are greatly affected by the confining pressure and temperature, are important issues in studies on the geological disposal of radioactive waste (Tsang et al. 2012; Watson et al. 2018), enhanced geothermal systems (EGS) (Pruess 2006; Gerber and Marechal 2012), and petroleum extraction (Chen and Spiro 1994; Gan et al., 2009).

In order to investigate the long-term deformation and permeability of rock fractures, the authors conducted long-term permeability tests using a granite specimen with a single split fracture, with parameters of confining pressure and temperature (Song et al. 2021). The saturated specimen was subjected to confining pressure of 3 MPa at 20°C for more than six months, and then the temperature was raised to 60°C. The results revealed that the permeability measured in the short-term tests showed reversible behavior and a dependence on the applied confining pressure and temperature. In contrast, the permeability measured in the long-term tests displayed irreversible behavior and an irregular change under constant confining pressure. In the experiment, microfocus X-ray computed tomography (CT) images were taken at predetermined times to observe the changes in the internal fracture structure.
Numerical Investigation of Shock Wave Stimulation and Comparison to Experimental Work
Authors: Khalaf Mina S. , Gordon Peter , Rezaei Ali , Soliman Mohamed Y. , House Waylon
Published: September 2022

Efficient production from tight and unconventional reservoirs requires drilling horizontal wells with an extensive network of high-quality (i.e., long, and conductive) fractures. However, the hydraulic fracturing process may be costly and may not be economically the best option when the oil and gas prices are low. The shock wave (pulsed loading) stimulation is a potential complement to traditional stimulation (e.g., hydraulic fracturing), that can add additional permeability enhancement to rock (via dynamic fracture processes) and bypass the skin region. The shock wave technique generates multiple fractures propagating in different directions around the wellbore, and is waterless, and less impacted by stress shadowing.

This work presents a numerical study on the effects of rock properties (e.g., rock density, Young’s modulus, and Poisson’s ratio) on the shock wave fractures quality (e.g., length, type, and number). In addition, repetitive application of shock waves in wellbore is investigated. Pulsed fracturing in the cased and perforated wellbore is compared with open-hole situation. This work examines the effect of confining stresses and stress anisotropy. The shock wave propagation in rock along with the developed fracture types (shear and tensile) are discussed. Furthermore, the numerical results are compared with previous experimental observations. The main objective of this study is to determine the dominant parameters that influence the rock behavior in pulsed fracturing.

Fracturing tight reservoirs by shock waves is an extremely high strain rate problem that we studied numerically using the combined finite discrete element method (FDEM). The results show that rock of large modulus of elasticity develops longer fractures. However, Poisson’s ratio has minor effects. Multiple application of high-rate loading can be used to control the wellbore collapse issue. The presence of wellbore casing reduces the shockwave strength and minimize the wellbore integrity issue. The confining stresses have a significant impact of the developed fractures length. However, stress anisotropy does not affect the created fracture orientations.
Thermo-Poroelastic Analysis of Cold Fluid Injection in Geothermal Reservoirs for Heat Extraction Sustainability
Authors: Khalaf Mina S. , Rezaei Ali , Soliman Mohamed Y. , Farouq Ali S. M.
Published: September 2022

Geothermal reservoirs are usually deep and have low permeability. Hydraulic fracturing stimulation of the production and injection wells can help increase the heat extraction from these reservoirs. However, injecting cold fluid in the hot formation can cause considerable volumetric changes in reservoir rock and pore pressure distribution in the reservoir. As a result, thermal loading occurs, and local stresses change in magnitudes and directions. Such redistribution of pore pressure, stresses, and temperature may result in an unwanted propagation of the created fractures or failure of pre-existing rock discontinuities and create an unwanted closed loop circle. In this study, we investigate the effect of heat exchange between the circulating fluid and the hot rock on redistribution of local stresses.

Our approach employs the thermo-poroelasticity theory to study cold fluid injection in geothermal reservoirs. The displacement discontinuity method (DDM) is used to solve the field equations of the thermo-poroelasticity theory. The DDM is a numerical technique that belongs to the boundary element methods (BEM). This numerical technique is suitable for studying hydraulic fracture (discontinuities in displacement) simulation problems. The thermo-poroelastic theory used here accounts for three fully coupled processes (mechanical, hydraulic, and thermal) in porous media saturated with fluid.

Our results show the effect of cold fluid injection on stress magnitude and orientation. Furthermore, the evolution of fracture aperture and cold fluid injection efficiency are discussed. The results presented in this work show that the difference in temperature between the cold treatment fluid and hot reservoir help create a wider fracture in less pumping time. In addition, it can cause a 90° re-orientation of the minimum principal stress direction, which controls the propagation of fractures. Also, cold fluid injection in hot formation creates thermal (tensile) stresses that can be much higher than iso-thermal fluid injection.
Inferring Compressional Wave (P-Wave) Travel Time from Conventional Well Logs Using Supervised Machine Learning: Case Studies from Texas Basins
Authors: Kholy S. M. , Lee Hunjoo P.
Published: September 2022

Compressional wave (P-wave) travel time is an important sonic log that can be utilized to indirectly estimate geomechanical properties such as Young’s modulus and Poisson’s ratio. Unfortunately, sonic travel-time logs are often absent in the conventional well log suites and need to be interpreted from other well logs. Many previous studies have developed data-driven models to synthesize the sonic logs, but those models have not been thoroughly evaluated for its prediction accuracy using field cases from several depositional basins. The objective of this study is to develop supervised machine learning models to accurately predict the P-wave travel time from the conventional well logs (i.e., Gamma Ray, bulk density, and neutron porosity) for a wide range of lithologies with the incorporation of confining stress effect by including the true vertical depth as an input. A linear regression model and an artificial neural network model have been trained using conventional well logs obtained from the Volve oil field in the Norwegian North Sea then the prediction accuracy of the two models have been tested using several cases from the Eagle Ford and the Midland basins. The testing results show that the inferred P-wave travel time yields a root mean square error range of 1.5 μs/ft-6.7 μs/ft indicating a good agreement with the actual sonic log measurements. Eventually, the findings of this study are essential in optimizing the design of drilling and well completion by providing accurately estimated geomechanical properties when the actual sonic logs are unavailable.

1. INTRODUCTION

Compressional wave, P-wave, transit time (DTC) is critical in estimating rock mechanical properties when the subsurface cores are unavailable due to financial and operational constraints on core retrieval. For many subsurface engineering applications, including drilling and well stimulation, geomechanical properties are important in understanding subsurface deformations to prevent hazards and enhance the recovery of resources. Wyllie et al. (1956) measured DTC with dry core samples, saturated core samples of several fluids, and hence developed an empirical equation to estimate the formation porosity. Nielsen et al. (1979) utilized the acoustic travel times to estimate rock mechanical properties including Poisson’s ratio and dynamic shear moduli.

Furthermore, the interpreted geomechanical properties (e.g., Poisson’s ratio and Young’s modulus) of the subsurface formations from the DTC could help improve the prediction of the in-situ stress conditions. Eaton (1975) estimated pore pressure by comparing the compressional transit time well logs to its normal trend. Others have also analytically derived the in-situ stresses from the interpreted mechanical properties (Blanton et al., 1999).
Characterization of Fracture Process Zone in Short Beam Compression Tests on Barre Granite
Authors: Garg Prasoon , Hedayat Ahmadreza , Griffiths D. V.
Published: September 2022

Due to rock mass being commonly subjected to compressive or shear loading, the mode II fracture toughness is an important material parameter for rocks. Fracturing in rocks is governed by the behavior of a nonlinear region surrounding the crack tip called the fracture process zone (FPZ). However, the characteristics of mode II fracture are still determined based on the linear elastic fracture mechanics (LEFM), which assumes that a pure mode II loading results in a pure mode II fracture. In this study, the FPZ development in Barre granite specimens under mode II loading was investigated using the short beam compression (SBC) test. Additionally, the influence of lateral confinement on various characteristics of mode II fracture was studied. The experimental setup included the simultaneous monitoring of surface deformation using the two-dimensional digital image correlation technique (2D-DIC) to identify fracture mode and characterize the FPZ evolution in Barre granite specimens. The 2D-DIC analysis showed a dominant mixed-mode I/II fracture in the ligament between two notches, irrespective of confinement level on the SBC specimens. The influence of confinement on the SBC specimens was assessed by analyzing the evolution of crack displacement and changes in value of mode II fracture toughness. Larger levels of damage in confined specimens were observed prior to the failure than the unconfined specimens, indicating an increase in the fracture resistance and therefore mode II fracture toughness with the confining stress.

1. INTRODUCTION

The fracturing in laboratory-scale rock specimens is often characterized by the deformation of the inelastic region surrounding the crack tips, also known as the fracture process zone (FPZ) (Backers et al., 2005; Ghamgosar and Erarslan, 2016). While the influence of the FPZ on mode I fracture in rocks has been extensively investigated, there are limited studies on FPZ development in rocks under pure mode II loading (Ji et al., 2016; Lin et al., 2020; Garg et al., 2021; Li et al., 2021).
Meshfree Penetration of Projectiles into Rock
Authors: Foster C. D. , Atif M. , Chi S.-W.
Published: September 2022

The penetration of projectiles into soil and rock has many applications, both military and civilian. In this research we are concerned with depth of penetration of projectiles at formerly used munitions sites. We examine here a numerical framework for analyzing such penetration.

Because the penetration problem includes large deformation, fracture, fragmentation, and other phenomena, standard finite elements cannot adequately capture the interaction. In this research, we employ a semi-Lagrangian Reproducing Kernel Particle Method (RKPM) framework. This meshfree technique updates the influence functions for each particle with deformation, naturally extremely large deformation, particle rearrangement, separation, and contact. The method has recently been updated with improved nodal integration for enhanced stability and accuracy.

At this a stage a Drucker-Prager with damage model is used the capture the hardening and softening of the material. The damage function is modified to only include damage in tension.

1. INTRODUCTION

Penetration of projectiles into rock and soil is of interest in many applications. These include protection of sensitive underground structures, projectile design, nuclear waste disposal, investigate of properties of remote sites, and mining. The ultimate goal in this research is the prediction of penetration depths at formerly used munitions sites, in order to detect buried projectiles and remediate the sites.

While there are many empirical, analytical, and numerical approaches to this problem, it remains a challenging endeavor. Reasons for this include uncertainty in material properties, complicated physics of interaction between projectile and rock, and limitations of numerical methods. In the last case, challenges include large deformation inelastic behavior of the material, multiple discontinuities, and tracking of complex evolving contacts among the interacting projectile and rock mass.

Standard finite elements struggle with penetration problems, due to the basic assumption of continuous deformation. While many enhancements have been created to handle fracturing [1-6] and large deformation [7-8], the problem of multiple rapidly changing contacts of parts creates a computationally intense problem. Discrete elements [9-11] can handle separation and contact more easily but are computationally expensive.
Determining the Mechanical Property of Martian Rocks Using Accurate Grain-Based Model
Authors: Zhang Yiheng , Xu Jingjing , Tang Xuhai , Paluszny Adriana
Published: September 2022

In the future, the extraterrestrial human activities, such as resources exploitation and base construction on Mars needs the aid of geotechnical engineering technology. Currently, there are only two approaches to obtain the Martian rock samples: sample-return activities by spacecraft and the collection of meteorites. However, meteorites are rare, expensive, arbitrarily sized and shaped, so it is difficult to process them into standard rock samples required by the traditional macroscale rock mechanics experiments (macro-RME). In the present work, the mechanical property of small-sized Martian meteorites was obtained by Accurate Grain-Based Modelling (AGBM) based on the microscale rock mechanics experiments (micro-RME). Firstly, the mineral composition and microstructure of NWA12564 Martian meteorites are achieved by the TESCAN Integrated Mineral Analyzer (TIMA). Secondly, the micromechanical properties of rock-forming minerals in meteorites was measured using nanoindentation tests. Thirdly, with the combination of micro-RME results and AGBM, the macroscale mechanical property of Martian rocks was able to be achieved using small and any-shaped meteorites. The present methodology is potential useful to estimate the mechanical property of Martian rocks using only arbitrarily shaped samples.

1. INTRODUCTION

Mars shares many similarities with Earth: hard crust, dense cores and similar materials composition, which is also most likely to be the second home of individuals living in Earth (Horne, 2015). Study of rock samples from the Mars provides useful information for the future human activities on Mars, such as resources exploitation and base construction (Steele, 2007). Now, many countries and organizations are working on geological exploration of Mars (Changela et al., 2021). The US National Aeronautics and Space Administration (NASA)’s Perseverance rover is paving the way for the first Mars-sample return (Farley et al., 2020; Witze, 2021). The China National Space Administration’s (CNSA)’s Zhurong rover (Wan et al., 2020) also focus on investigating the geology of Mars. SpaceX has an ambitious program for human settlement on Mars (Bramson et al., 2021). The use of resources on the Mars may provide a means of reducing the cost and risk of Martian exploration (Sanders and Duke, 2005). Understanding and controlling the mechanical behavior of Martian raw material (e.g. rocks and soil) is critical for the construction and expand colonization on the Mars.
A New Analysis Method for the Influencing Factors of Surrounding Rock Stability and its Application
Authors: Li Peitao , Liu Quansheng
Published: September 2022

A new orthogonal matrix analysis method is proposed firstly in this paper, which aims at providing a new idea for the stability analysis of surrounding rock. Based on the orthogonal test method, a three-level analysis model is established. In detail, the geological conditions and rock masses grade, the variation of different factors, the deformation and fracture field of surrounding rock is regarded as the factor layer, the level layer, and the index layer, respectively. This method not only inherits the advantages of orthogonal test, but also avoids the influence of artificial weighting by establishing the index layer. Then, based on the combined finite-discrete element method, the influence of lateral pressure ratio, vertical stress, and rock mass grade on the surrounding rock stability is carried out by three analysis methods. The results show that the orthogonal matrix analysis method can well reflect the influence of factors, which is consistent with the result of the range analysis method and variance analysis method. In addition, this method can also realize the quantitative sensitivity analysis of factors under different levels, which is essential for the surrounding rock support design.

1. INTRODUCTION

As we all know, the stability of roadways is affected by many factors. So, the research on the stability of surrounding rock is still a challenge. It has an important reference value for roadway excavation and support. Currently, many experts and scholars have proposed different methods and models to evaluate the stability of surrounding rocks. These include experimental methods (Chen et al., 2021), numerical simulation (Jia et al., 2022), theoretical analysis (Zhang et al., 2022), etc.

The laboratory experiment is the basic method that evaluates the stability of surrounding rock based on the test result of rock samples. The numerical simulation is the most common method, which includes continuum mechanics methods (Tian et al., 2021; Wang et al., 2022), discontinuous mechanics methods (Lisjak and Grasselli, 2014; Zhao et al., 2022), continuum and discontinuous combination method (Deng et al., 2020; Mahabadi et al., 2012). The theoretical analysis method is based on the analysis of the stress field and deformation field, which contains the longitudinal deformation profile (Sugimoto et al., 2019), ground reaction curves (Fang et al., 2013), the pressure arch (Kong et al., 2018) and the stand-up time (Nguyen and Nguyen, 2015).
Optimization Design Method of Excitation Parameters and Installation Parameters of Downhole Vibration Drag-Reduction Tools
Authors: Shi Xiaolei , Huang Wenjun , Gao Deli , Ye Yuchen
Published: September 2022

In the sliding steerable drilling operation in long horizontal wells, vibration drag-reduction tools can greatly reduce tubular drag, effectively alleviate frequent pipe sticking problem and improve rate of penetration. The excitation parameters and installation parameters of vibration drag-reduction tools are closely related to the drag reduction efficiency. At present, the parameter design of vibration drag reduction is mainly based on field experience but lacks a set of effective theoretical methods. To solve this problem, the strong nonlinear effect of friction force is considered, the dynamic model of drill string with vibration drag-reduction tools is built and the drag reduction laws under different parameters are revealed. On the basis of vibration propagation distance model and inverse torque action distance model, an optimization design method of excitation parameters and installation parameters of vibration drag-reduction tools is established with drag reduction efficiency as the objective function and drill string fatigue failure and hydraulic energy loss as constraint conditions. Finally, the above theoretical model is applied to practice. The results show that the amplitude of the excitation force, the number and installation position of the drag-reduction tools are the main factors affecting drag reduction efficiency. With the increase of excitation force amplitude and the number of drag-reduction tools, the drag reduction efficiency increases continuously., the drag reduction efficiency when the vibration propagation ranges of multiple tools are independent is higher than that when not independent. There is a risk of tubular fatigue failure if the amplitude of the oscillator excitation force is too large, and a risk of too high hydraulic energy loss if too many drag-reduction tools are installed. In the case study, it is recommended to use three vibration drag-reduction tools with excitation force amplitude of 50kN, which are installed at 435m, 1038m and 1641m away from the drill bit respectively, and then a drag reduction efficiency of 54% is achieved.