bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Simulation on Damage Characteristics of Carbonate Rock Under Axial-Torsional Coupled Impact by PDC Cutters
Authors: Fu Xinkang , Huang Zhongwei , Sun Jinming , Shi Huaizhong , Zhang Jiawei , Ji Guodong , Song Hengyu
Published: September 2023

Carbonate rock is an important oil and gas reservoir rock. Deep carbonate rocks are of high strength, hardness, abrasiveness and poor drillability, which will cause various problems as follows: PDC cutter collapse, serious wear and low drilling efficiency. Axial-torsional coupled percussive drilling can improve the drilling speed of deep hard rocks including carbonate rock. Therefore, a numerical model of axial-torsional coupled impact rock breakage was established for carbonate rocks. The damage characteristics of carbonate rock caused by spherical cutter and axe-shaped cutter under different impact parameters were analyzed. The simulation results reveal that the impact force of two kinds of cutters increases with the increase of impact speed. The torque of PDC cutters increases with the increase of torque amplitude. Under the same conditions, the tensile damage of carbonate rock caused by axe-shaped cutter is more obvious under the action of axial-torsional coupled impact and the tensile damage area under axe-shaped cutter tends to be connected. Additionally, the damage caused by PDC cutter to carbonate rocks is mainly tensile damage, supplemented by compressive damage. This work is instructive in revealing the effect of PDC cutter on breaking carbonate rocks under different impact parameters.

INTRODUCTION

As an important oil and gas reservoir rock, 50% of the world's proven oil and gas resources are stored in carbonate rocks (Al-Shargabi et al., 2022). Deep carbonate rocks are of high strength, hardness, abrasiveness and poor drillability. Conventional rotary bit drilling method has many problems in rock breaking, such as serious wear and low drilling efficiency (Jamali et al., 2019). Axial-torsional coupled percussive drilling has higher penetration rate in breaking hard rock, such as carbonate rock (Saai et al., 2020). And some oil companies designed some special-shaped PDC cutters (Zhu et al., 2022), such as axe-shaped cutter and spherical cutter. However, the characteristics of breaking carbonate rock by PDC cutters under the action of axial-torsional coupled impact is not clear. Therefore, it is of great significance to investigate the effect of different impact parameters on breaking carbonate rock by PDC cutters under the action of axial-torsional coupled impact.
Formation Sampling Optimization Using Borehole Images and Acoustic Data in Tight Gas Sand Reservoir
Authors: Hamadas Manel , Sokhal Abdallah , Bazine Badreddine
Published: September 2023

In this paper, we present an integrated workflow designed to characterize the natural fracture systems and to optimize fluid sampling in a very complex and heterogeneous fractured tight sandstone reservoir. Conventional techniques are not suited for tight formations, and to overcome this challenge it is imperative to implement and enhance new technology. This advanced workflow aims to decrease uncertainties at different stages of the reservoir lifecycle through combined advanced tools. Electrical and Acoustic borehole images, Full Acoustic Waveform, and MDT Dual packer were incorporated in this study to optimize fluid sampling and increase the success rate of economic recovery.

INTRODUCTION

Almost all hydrocarbon reservoirs are affected in some way by natural fractures, but the effects of fractures are often poorly understood and largely underestimated. In tight reservoirs, natural fractures created through complex tectonic activity help create secondary porosity and promote communication between reservoir compartments (Irofti et al., 2022). However, these highly permeable channels sometimes short-circuit fluid flow in the reservoir, causing premature production of water or gas and rendering secondary recovery initiatives ineffective (Bratton et al., 2006).Since the introduction of the wireline formation testers in 1955, these tools have provided the oil and gas industry with measurements for the determination of formation pressure, fluids in-situ as well as giving formation fluid samples for identification and analysis. This conventional technique is well suited for thick and permeable clastic formations. Under low porosity and low permeability conditions, the wireline formations testers become unsuitable as they may not be able to isolate the zone, may miss a thin zone or in a formation with very low permeability, the contact area with the packer may be too small to let the fluid flow. When testing is to be conducted out employing pricey rig operations, evaluation of heterogeneous reservoirs using open hole logs and cores carries some risk (Aoun, Soto, et al., 2022; Ifrene et al., 2023) The modest success rate of sampling operations, fluid sampling presents a challenge in tight reservoirs, therefore it is imperative to adopt innovative techniques. To overcome this challenge, we proposed in this article a workflow to increase and optimize the success rate. This includes the combination of borehole image logs and acoustic waveforms, two technologies aiming to enhance formation sampling and recovery rate.
Sand Production Evaluation: Experimental Testing, Analytical Solution, Numerical Simulation and Field Implications
Authors: Asadi Sadegh , Khaksar Abbas
Published: September 2023

Sand production prediction is essential from the early stages of field development planning for well completion design and later for production management. Unconsolidated and weakly consolidated sandstones are prone to fail at low flowing bottom hole pressures during hydrocarbon production. To predict the sand-free drawdown, a robust sand production prediction model that integrates near-wellbore and in-situ stresses, rock mechanical properties, well trajectory, reservoir pressure, production and depletion trends is required. Sanding prediction models should be calibrated with field data such as production and well tests observation. In the absence of field data, numerical techniques can provide a reliable estimate on potential onset and severity of sanding at various reservoir pressures. In this study, analytical and finite-element numerical models are independently used to predict the onset and extent of sanding in a high-rate gas well with poorly consolidated sandstone reservoir. The analytical method uses a poro-elastic model and core-calibrated log-derived rock strength profiles with an empirical effective rock strength factor (ESF). In the study, the ESF was calibrated against documented field sanding observation from a well test extended flow periods at the initial reservoir pressure at low drawdown pressure. This sanding model ESF was then verified against the sanding observation from another well test with higher drawdown pressure as well as with the sand-free production conditions from two producers that have been on line for several years.

The numerical method uses a poro-elasto-plastic model defined from triaxial core tests. The rock failure criterion in the numerical method is based on a critical strain limit (CSL) corresponding to the failure of the inner wall of thick-walled cylinder core tests that can also satisfy the existing wells sanding observations in the production well tests. To validate the onset and severity of sanding predicted by the analytical model, numerical simulations for an identical sandstone interval were developed to investigate the corresponding CSL. Results showed the numerical model is also able to predict the onset of sanding observed during well tests with plastic strains exceeding the CSL defined from TWC test simulations. This combined analytical and numerical modelling calibrated with field data provided high confidence in the sanding evaluation and their application for future well completion and sand management decisions. The analytical model was finally used for sanding assessment over field life pressure condition because of its processing simplicity, speed and flexibility in assessing various pressure and rock strength scenarios with sensitivity analysis over the whole production interval in compared with the numerical method which is more suitable for single-depth and pressure condition modelling.
Prediction of Heavy Oil Production Based on Geomechanical Analysis in Coupled Thermo-Hydro-Mechanical Process
Authors: Li Dengke , Gao Yanfang , Chen Zupeng , Li Yanchao , Shen Shijie
Published: September 2023

The geomechanical response concerning porosity, permeability, and water saturation, will directly affect the prediction of oil production as well as the economic benefits of oil fields. In this paper, different models are established for the three stages of the SAGD (Steam Assisted Gravity Drainage) production period by using the coupled thermo-hydro-mechanical method, aiming to understand the influence of rock volumetric dilation or compression on oil yield. The relationship between the evolved flow parameters and the deformation parameter characterized by volumetric strain is proposed, then a mathematical model with the dilation volume and yield is derived. The ratio of the new model considering reservoir geomechanics and the conventional model is exhibited as a function with the bulk strain, and it is found that different volumetric strains will affect the yield ratio of the new model and the conventional model. The stages of the steam chamber ascending and expanding laterally are selected for typical case studies, and the predicted yield of the new model is about 1.24 and 1.17 times, respectively, that of the conventional model when the volume strain is 2%. This method can help oilfield engineers obtain more accurate heavy oil production, and quantitatively evaluate the significance of reservoir geomechanics in the process of SAGD.

INTRODUCTION

In recent years, with the massive exploitation of conventional oil and gas and the discovery of unconventional reservoirs, how to exploit unconventional reservoirs efficiently has become increasingly important. For example, hybrid thermal recovery processes have become a primary technique used to improve the recovery performance of post steamed heavy oil reservoirs and been used to exploit marginal heavy oil reservoirs effectively. SAGD (steam assisted gravity drainage) is an efficient technique for recovering heavy oil, as proposed by Butler et al. (1981). SAGD technology develops heavy oil through the use of steam drive and gravity flooding. By transforming the reservoir through the injection well, water and heated heavy oil flow downward under the action of gravity, so as to improve the efficiency and production of heavy oil exploitation (Butler et al., 1981).
Numerical Simulation of Hydraulic Fracturing Process of Shale in Jiyang Depression Layer
Authors: Chen Zupeng , Gao Yanfang , Li Yanchao , Shen Shijie , Li Dengke
Published: September 2023

As an important unconventional energy source, shale oil is widely distributed and has huge reserves, which is one of the alternative energy sources in the future. In this paper, the shale reservoir of the grain-bearing layer of Well A in the Jiyang Depression in the Bohai Bay Basin was used as the research object to carry out the numerical simulation of hydraulic fracture. Firstly, the 3D geological model of shale oil reservoir is simplified, the spatial distribution of reservoir fracturing fractures and textured shale is described, and the 3D geometric model of the reservoir is established by ABAQUS. Secondly, based on the stress balance equation and fluid continuity equation, traction separation criterion and flow equation of cohesion unit, a coupled numerical model of reservoir stress-seepage flow is established. Then, using on-site production and original in-situ stress as the loading conditions, the required parameters are input into the model, and the finite element method is used to solve the model. It is found that the simulated fracture morphology is similar to the actual single hydraulic fracture morphology, the pressure accumulation is mainly concentrated in the reservoir, and the Mises stress at the water injection point reaches a maximum at the beginning of injection, and the stress gradually decreases as the fracturing progresses. The pore pressure in the reservoir is mainly distributed near textured shale reservoirs and fractures, and the pore pressure near the fractures can reach more than 10MPa. The width reaches a stable value after cracking, indicating that hydraulic fracturing can effectively control fracture spread and produce a relatively stable fracture structure.

INTRODUCTION

The Jiyang depression in the Bohai Bay Basin is rich in shale oil resources, with a resource volume of 4.1 billion tons. The shale in the Jiyang Basin is characterized by low evolution, deep burial, large shale thickness, and abnormally high temperature and high pressure. The shale structure is so dense that there are almost no cracks in the cores taken from deep underground, but there are obvious lamellar structures. Lamellar shale bedding developed a lot, the height of the development of bedding joints, bedding joints in the state of unfilled or semi-filled, with high seepage capacity. The lamellar facies has the characteristics of high porosity, large pore diameter, and good connectivity. According to the oil testing results, the shale facies with the most industrial oil flow are shale laminar argillaceous limestone and limestone argillaceous mudstone intercalated with thin sandstone or carbonate rock.
Constitutive Relation of Shale Poroplasticity Based on Prandtl-Reuss Model
Authors: Qi Jiayue , Zhang Guangqing , Wu Anan , Ma Fanxin , Zhang Yiwei
Published: September 2023

The effective stress coefficient is important to describe the stress and deformation of rock. However, when shale is in the plastic stage, the mathematical expression of the effective stress coefficient is still unclear. Based on Prandtl-Reuss model, the incremental constitutive relation of shale in elastic-perfectly plastic condition is established. The plastic effective stress coefficient of shale in this condition is obtained. The mathematical expression of the effective stress coefficient tensor elastoplasticity is proposed by defining the correction function. The mechanical properties of shale are analyzed when the correction function is a power function by combining the specific parameters of shale. The results show that when 0 ≤ n ≤ 0.932, the effect of pore pressure on the plastic effective stress coefficient is related to the confining pressure. The plastic effective and Biot effective stress coefficients are gradually equal with the increase of pore pressure. Combining the four basic operations and composite operations of specific functions allows a correction function suitable for the plastic effective stress coefficient of shale.

INTRODUCTION

Shale is a typical rock used to develop oil and gas resources. With the decreasing reserves of conventional oil and gas resources and the increasing proven reserves of unconventional oil and gas resources such as shale oil, the study of shale is getting more and more attention. There are many natural microcracks and micropores in shale (Zhang et al., 2022), which cannot be ignored for their impact on rock mechanical properties. The free-moving fluid in micro cracks and micropores changes the mechanical behavior of porous media (Cheng, 1993; Xu 2001) and generates pore pressure in micro cracks and micropores of porous media. The difference between the effective stress and the total stress acting on the rock solid skeleton makes the mechanical properties of saturated rock with pore pressure significantly different from those of dry rock (Berryman, 1992; James G, 1992; Diansen Yang, 2010; Dong Chen, 2016; Liu, 2018; A M Gorshkov, 2020). Schrefler (1998) extended the saturated single-phase theory to the multiphase fluid. Zimmerman (2000) applied the pore elasticity theory to the fractured media. However, none of the above scholars have considered the issue of deep shale deformation entering the stage of pore plastic deformation. In the process of deep oil and gas extraction, the brittle shale will transform into elastic-plastic shale due to the high-temperature field, high-stress field, and high pore pressure field (Li et al., 2020; Crisci Eleonora, 2021).
Study on Filtration Control Strategies of Guar Fracturing Fluid
Authors: Wen Xiaoyong , Bai Hao , Li Zhiwen , Yin Taiheng , Liu Qian , Wang Wenzhong , Wu Junlin , Yao Erdong , Zhou Fujian
Published: September 2023

When using guar fracturing fluid for Chang 8 tight oil reservoir, it has been found that the filtration problem is severe and can cause significant damage to the reservoir.In this paper, the formula for guar fracturing fluid has been optimized, and the strategies to control filtration and damage have been recommended. Firstly, laboratory studies are conducted to investigate the filtration performance and damage mechanisms of reservoir materials. Secondly, three filtration control strategies are selected and optimized to reduce filtration. Finally, the optimized fracturing fluid system is selected for field test to evaluate the stimulation effect. The results show that the filtration and damage of the guar system are far higher. The clay migration after the filtration is the main reason for the high damage. Three filtration control strategies are optimized, namely: increasing the guar concentration to 6.0 kg/m3, adding 1.0 wt% nano plugging particles (100 mesh), and 5 wt% KCl soluble salt. Their static filtration coefficients are 0.47, 0.91, and 1.03 × 10−3 m/min1/2. The dynamic filtration damage test found that the damage rates (R) are 15.9 %, 39.8 %, and 40 %, and filtration loss (V) are 13.0, 28.2, and 28.6 mL, respectively. Among them, improving the guar concentration is the best method for filtration control and its cost is the lowest. In addition, there is a certain linear relationship between the filtration loss and damage rate (R=1.4067*V). In the field application, it is found that under the same displacement, when fracturing with optimized formula, the pressure response is faster and higher than the original system, up to 44 MPa under the same displacement.

INTRODUCTION

Ordos Basin in China is the second-largest sedimentary basin Chang 8 reservoir of the Yanchang Formation in Changqing oilfield is one of the most important supplementary layer series for future oil and gas exploration and development in Ordos Basin (Liu et al., 2022). Its formation distribution is stable, the thickness is generally 80-90 m, and the bury depth is 2260-2750 m (Luan et al., 2022). Chang 8 reservoir has good physical properties and is characterized by low porosity and permeability (Liu et al., 2018). The shale content of the Chang 8 reservoir is high, consisting mainly of intergranular and feldspar-dissolved pores that exhibit considerable heterogeneity (Yang et al., 2020).To achieve the goal of effective fracturing to form fissures and oil-well stimulation, and to avoid the problems such as fluid filtration and incompatibility, Chang 8 reservoir is currently using guar fracturing fluid for hydraulic fracturing production.
Predicting Weathered Rock Properties Using Integrated Geophysics and Laboratory Testing Approach
Authors: Shi Z. , Flottmann T. , Millen M. , Pidgeon B. , Huang Y. , Chen Z.
Published: September 2023

Reliable estimation of the mechanical properties (e.g., strength) of weathered rocks is critical for taking proactive measures, such as adjusting mud weight density and/or drilling parameters, to achieve safe and efficient infill drilling. Currently, the common approach to estimating rock properties is to use its regression with geophysical logs, particularly the sonic velocity. Such an approach was challenged when applied to shallow and heavily weathered bedrocks accompanied by high porosity and water saturation and low density. The preserved drill cores from the previous drilling of adjacent wells offer an opportunity to correlate laboratory results with borehole logging data to estimate rock properties and identify high-risk drilling horizons. In this work, 196 cylindrical drill cores with an average diameter of 89.25 mm from the depth between 35 m and 396 m were prepared for uniaxial compressive strength (UCS) and Brazilian tensile strength tests. Sample characterization on each sample was also conducted, including density, porosity, P- and S-wave velocities, and static and dynamic elastic moduli. Compared with the logging P-wave velocities, the laboratory P-wave values are consistently lower by 32.7% on average, especially for shallow rock cores, which likely reflects the impact of water moisture, further weathering and in-situ compaction stress. The characterisation results were then used as inputs to correlate with rock UCS and tensile strengths, which were found to be well correlated with the depth, density, P- and S-wave velocities, and elastic modulus. This work provides useful experimental data sets of shallow weathered bedrocks and highlights the importance of including rock depth in the regression model for rock strengths. The developed correlations could be directly employed to forecast the mechanical properties of shallow rocks as well as being a guideline to identify high-risk weak rock facies, where are severely weathered and expect significant drilling challenges.

INTRODUCTION

The mechanical properties of in-situ reservoir rocks are key input parameters for geomechanical assessment of geoengineering applications such as well drilling and completion and wellbore stability. Accurate site investigation, including assessing rock strength, is essential prior to drilling to plan the most efficient and cost-effective activities (Kolapo, 2021). However, weathering constantly affects the strength of rock masses, particularly near the ground surface, posting a significant challenge for conducting engineering activities, such as drilling (Gupta and Seshagiri Rao, 2000; An et al., 2020). It is therefore crucial to reliably predict the mechanical strength of each rock layer, particularly the weathered zones, to achieve a safe and successful drilling operation.
Advance Prediction of Rockmass Conditions During TBM Tunnelling Based on Cost-Sensitive Learning Under Imbalance Dataset
Authors: Hou Shaokang , Cheng Li , Liu Yaoru
Published: September 2023

The uncertainty of rockmass conditions will be an important factor causing TBM construction risks and inefficient tunnelling. Currently, prediction of rockmass classification based on TBM operation data and machine learning models has been proved feasible by many researchers. However, due to the sample imbalance problem in the previous studies, the prediction accuracy of minority rockmass class is relatively poor. To overcome the above problem, this study proposed a rockmass classification advance prediction method based on cost-sensitive learning (CSL), which using 10 key TBM operation parameters as model inputs. First, through the data preprocessing, the dataset consists of 7538 TBM tunnelling cycles and corresponding rockmass classification information is obtained. And the sample set was divided into training set and test set by stratified sampling. Then, eight classifiers, i.e., support vector machine (SVM), decision tree (DT), random forest (RF), k-nearest neighbors (KNN) and their cost-sensitive version are established to construct the mapping relationship between key TBM operation parameters and rockmass classes. The case study results show that AdaBoost classifier presents the best prediction performance, with the accuracy and F1-score more than 0.9. And CSL can improve the prediction accuracy of minority class samples to some extent to achieve more reasonable prediction results.

INTRODUCTION

With the continuous improvement of construction mechanization level, tunnel boring machine (TBM) plays an increasingly important role in tunnel construction. Especially for long tunnels, the efficiency and economic advantages of TBM will be more obvious (Liu et al., 2020a; Hou et al., 2022b). TBM is sensitive to the change of rockmass condition, and depends on the empirical adjustment of TBM driver to achieve adaptive tunnelling (Hamidi et al., 2010). However, the geological exploration boreholes before the construction of the long tunnel are relatively sparse, which cannot effectively describe the rockmass conditions of the whole tunnel. Therefore, it is urgent to propose an advanced prediction method of rockmass classification during TBM construction.
An Analytical Probabilistic Approach for Fault Stability Analysis During Fluid Injection: The SAFE Tool Methodology
Authors: Petroselli S. , Rotella D. , Gemelli F. , Frabotta G. M. , Brignoli M. , Grilli D. , Corradi A. , Mantica S. , Bottazzi F.
Published: September 2023

The increasing interest in CO2 storage and geothermal projects leads Oil & Gas companies to develop tools and methodologies to support the operation design phase. SAFE (Stability Analysis Fault Eni) is an in-house developed code based on an analytical probabilistic approach to provide a fast preliminary assessment of fault stability for fluid injection strategies in reservoirs or aquifers. The method allows to identify: i) the impact of the uncertain parameters affecting the fault stability; ii) possible critical cases for which a refinement of the analyses is required, e.g. 3D Finite Element Model focused on critical faults. Beyond a summary of the theoretical framework of the fault reactivation process, the paper will highlight the assumptions and the computation workflow of the developed tool, as well as the output data resulting from the analysis.

INTRODUCTION

In the present days, the attention for the climate change and the environment protection are the engines that guide civil and industrial development, through regulations aiming to reduce global warming, wastes and pollution while promoting health and safety. To this aim, CO2 storage in deep underground layers will be of paramount importance to reach the world net-zero emissions by 2050. Amongst all the known phenomena related to fluid injection, fault destabilization is one of the main effects to be evaluated with a geomechanical study for the reservoir exploitation. In fact, fault reactivation with induced seismicity has been observed for wastewater disposal in the US (Ellsworth, 2013; McGarr et al., 2015; Rubinstein and Mahani, 2015) where the earthquake rate increase since 2001 is partially due to the injection of the water resulting from hydraulic fracturing operations in nearby disposal wells. Likewise, CO2 disposal (Espinoza and Santamarina, 2011) or geothermal projects (Buijze et al., 2019) may generate seismic events. Moreover, the thermal effect due to the injection of a fluid with lower temperature than the formation temperature leads to additional stress variation that may reduce the fault resistance (Rutqvist, 2012; Kim and Hosseini, 2014). Geochemical effects might also be an issue, due to possible rock properties degradation in presence of reactive fluids such as CO2 saturated water. This aspect is still not fully understood, and results are sometime contradictory, i.e. some papers evidence effects of CO2 on the rock mechanical properties, other show little or almost no effects (e.g. Bateman et al., 2013; Rathnaweera et al., 2015, 2017, Menezes, 2019).
Reuse a Depleted Reservoir as a Storage Site: How May Reactivation of a Sealing Fault Affect the Storage Site's Integrity?
Authors: Xie Xiyang , Cerasi Pierre , Stroisz Anna
Published: September 2023

The focus of this study is to investigate how the change of pore pressure inside the reservoir may affect the yield of a bounding fault. The following factors are considered: fault width, active/inactive fault, fault inclination, internal structure of the fault and loading-unloading history. The fault has a higher tendency to yield in the following situations: 1. Active fault; 2. Wider fault; 3. Injection beyond a pressure threshold; 4. Inclined fault; and 5. More cycles of loading-unloading. The study also reveals the distribution of the yield pattern in different conditions above and how the yield area in the fault grows. Although the research was based on a synthetic case, the findings can serve as fundamental principles to assess whether a particular reservoir and its sealing faults are suitable for storage.

INTRODUCTION

With the goal to achieve a low-carbon emission society, research in CO2 capture, utilization and storage (CCUS) and hydrogen storage and utilization keeps evolving. Repurposing a depleted reservoir as a storage site can explore its long-term value. Still, there are concerns regarding the integrity of the reservoir and the caprock, particularly in the presence of faults. For example, pressure variation inside the reservoir affects the stress state in the caprock. The stress change may cause the fault to yield and create high-permeable channels. These channels can result in hydraulic connectivity between the depleted reservoir and the surroundings. Consequently, the yielded fault is converted into a potential fluid leakage path (Rongved and Cerasi 2019).

The simulation of fault reactivation induced by CO2 injection has been studied for a while. Two representative works are from Cappa and Rutqvist (2011) and Rinaldi et al. (2014). Both use the coupled fluid flow and geomechanical simulator TOUGH-FLAC to study the synthetic cases. Other researchers focus on field development. Injecting CO2 into a depleted reservoir or an aquifer with normal pressure is becoming matured by field case studies in Australia (Underschultz et al. 2011), the North Sea (Hansen et al. 2013), and North America (Duong et al. 2019). For example, a fault, as the sealing boundary of a producing reservoir, may yield due to the pressure depletion in the reservoir (Candela et al. 2019).
Experimental Study of Tight Reservoir Rock Failure Process Based on Unsupervised Machine Learning
Authors: Wu Shan , Zhao Qi , Yang Hui , Ge Hongkui
Published: September 2023

Understanding the failure process of tight reservoir rocks is essential for reservoir stimulation with hydraulic fracturing. The acoustic emission (AE) technique has proven to be an effective tool to monitor fracture propagation and thus characterize the rock failure process. In the present study, AE is employed to investigate different rock failure processes under uniaxial compression. We use four typical tight reservoir rocks from oil and gas production fields in China. We employed an unsupervised machine learning method to classify the recorded AE waveforms and evaluated the results via two score systems: elbow and silhouette scores. The cluster number is approximately constant for all samples, indicating that this method provides a more precise and reliable interpretation of the rock failure process. Machine learning results demonstrate that AE events could be distinguished into three clusters, which could relate to the mechanisms of the microscopic ruptures, i.e., tensile, shear, and mixed cracking types. Our results reveal that cracks formed under low-stress conditions are predominantly in tensile failure mode, and the failure mode transit into shear ruptures before the compression peak strength. In addition, different strength of weak planes could diversify the process of tensile-to-shear rupture transition by affecting the local stress concentration. This research may help understand the failure mechanism in tight reservoir rocks and shed light on further hydraulic fracturing technology in reservoir development.

INTRODUCTION

Tight reservoirs contain rich oil and gas resources, collectively known as tight oil and gas (Abdel-Aal et al., 2003; Jia et al., 2022; Ma and Holditch, 2015; Pang et al., 2015; Zendehboudi and Bahadori, 2016). The vast reserves of tight oil and gas are an important supplement to conventional oil and gas, and their effective commercial development relies on new underground reservoir engineering technologies such as horizontal drilling and hydraulic fracturing (Rezaei et al., 2020; Zhang et al., 2016; Zou et al., 2018). The development of these technologies requires the support of rock mechanics, with the most important part being the problem of fracture propagation in tight reservoirs. Different from conventional oil and gas reservoirs, tight reservoirs have abundant layers of weak planes formed by geological sedimentation, leading to the special rock mechanical behaviour (Heng et al., 2020; Kumar et al., 2019; Suo et al., 2020). Based on traditional rock mechanics models, it is not possible to predict the underground fracture expansion, which brings various disasters such as fracturing failure, reservoir pollution, and induced earthquakes (Ham and Kwon, 2020; Wu et al., 2022; Xie et al., 2020). Therefore, it is necessary to conduct in-depth research on rock fracture in tight reservoirs.
Two-Phase Unsaturated Microscopic Seepage Simulation Study Based on Digital Core Technology
Authors: Wu Xudi , Zou Xuefei , Wang Yunhan , Liu Wei , Xiao Jianfeng
Published: September 2023

In recent years, digital core technology as an emerging numerical simulation method has been widely used in many fields. In this paper, the three-dimensional pore throat structure of sandstone was extracted by digital core computed tomography (CT) technique. And establish a core's realistic 3D pore network model by 3D visualization image processing technique for gas-liquid two-phase unsaturated microscopic seepage simulation. To realize two-phase unsaturated seepage simulation, using two-phase volume averaged momentum equation and continuity equation to construct the two-phase unsaturated seepage coupling equation. The two-phase saturation and relative permeability variations were described by the saturation relation and the VG-Mualem model. The results show that the saturation variation is uneven during unsaturated seepage due to the finger-in phenomenon. There is a dominant channel in the two-phase fluid seepage process, the flow lines are more intensive in the large pore throat, and the flow velocity varies widely. The relative permeability of two-phase at different parts is similar, the relative permeability curve is concave downward. The final relative permeability of the non-wetting phase is closed to 1.

INTRODUCTION

There are numerous seepage simulation methods, which can be mainly divided into macroscopic and microscopic from the seepage scale. Macroscale simulation methods are used to determine the fluid state in porous media by solving differential equations, including flow rate, saturation, etc. (Lei et al. 2018). The micro-scale simulation method is to simulate the seepage flow based on the pore network structure using finite element, lattice Boltzmann and other methods. And the boundary conditions are considered in the process (Liu et al. 2017). Due to the complexity of pore structure and fluid flow characteristics in porous media, conventional core images obtained by scanning methods are difficult to use for micro-scale seepage simulation. However, with the widespread application of CT scanning technology in the oil and gas industry, a three-dimensional high-precision digital core characterization technology based on CT scanning imaging has been formed, which is to extract rock structure data from digital rock, and process the data through a three-dimensional visualization image processing software to construct a pore network model. It has the advantages of high spatial continuity and high visualization, and the pore network model can better reflect the rock core pore structure characteristics and pore throat topological relationship than the random network defined only by porosity and permeability, which can greatly improve the accuracy of seepage simulation (Baldwin, 1996; F. Arzilli, 2016).
A PD-FEM Coupling Approach for Modeling Cracks Propagation in Brittle Rock Under Compressive Load
Authors: Hu Xiaokun , Yan Xiao , Yu Haitao
Published: September 2023

Considering the low computational efficiency and accuracy of peridynamics (PD), a concurrent multiscale method coupling PD and finite element method (FEM) is proposed for modeling crack propagation of brittle rock under compressive load. In the coupling method, the fracture behavior is solved by the ordinary state-based peridynamics (OSBPD), while the elastic deformation of the rock mass is simulated by FEM. The implementation of the approach is as the following steps: first, a hybrid region is introduced in the framework to realize the strain energy equivalence between PD and FEM and eliminate the boundary effect. Then, the short-range force is utilized in PD to model the contact of the crack surface and prevent the particles from penetrating each other. In addition, the tangential force of the short-range force is introduced to simulate the friction sliding effect of the crack surface. Finally, the dynamic relaxation method is used to solve the displacement in the PD-FEM coupling model. The crack propagation of rock samples with a single pre-existing closed fracture under uniaxial compression is simulated by the PD-FEM coupling approach, and the numerical calculation results are in good agreement with the rule of the experimental results. The proposed coupling approach can capture the failure process of rocks under uniaxial compression and reduce the computational cost, simultaneously.

INTRODUCTION

The failure of rock can lead to structural damage, landslides, and rock bursts, which can endanger human life and property. Therefore, studying the fracture behavior of rock is essential for ensuring the safety of engineering projects. The law of crack growth in the brittle rock remains a great challenge in rock mechanics. In order to predict crack growth, many numerical methods have been developed. The numerical methods often used in the field of rock fracture mechanics include continuous methods and discontinuous methods. Common continuous methods include the extended finite element method (Eftekhari et al., 2016), the cohesive force element method (Zhou & Molinari, 2004), and the phase field method (Zhou et al., 2018). However, traditional continuum mechanics theory-based modeling makes it difficult to deal with complex crack propagation problems. The discrete element method (Cundall, 1971) is a commonly used technique in discontinuous methods to simulate fracture problems in rocks. However, the microscopic parameters of the discrete element method lack physical meaning and need to be calibrated through experiments.
Study on Consolidation and Drainage Characteristics of Fine-Grained Tailings in Geotextile Bags Under Dynamic Load
Authors: Ma Lei , Zhang Chao , Guo Xiaogang , Ma Changkun
Published: September 2023

Tailings dams built using tailings have faced several technical challenges in recent years. However, the use of the geotextile bag method can provide a good solution to issues such as difficult dam construction, poor drainage, and stability of the dam. In order to address these concerns, the author conducted an experimental study using the geotextile bag built-in air bag pressurization test method. Nine sets of horizontal test conditions were designed, which included the air bag application position, air bag pressure variation range, and air bag pressure application mode. The research aimed to investigate the dynamic load on the geotextile bag of fine-grained tailings consolidation drainage characteristics. The results showed that the peak pressure of the loaded air bag has the greatest impact on the degree of tailings consolidation drainage rate. The loading method of variable peak pressure was found to be more conducive to increasing tailings retention and practical operation of the project.

TEST BACKGROUND

China is a nation that is rich in mineral resources, and as the mineral processing technology and recovery efficiency continue to improve, the size of tailings has become smaller and smaller. Traditional upstream and midline methods of constructing tailings dams can result in low permeability, high infiltration line, poor drainage, consolidation difficulties, and sand imbalance, which can all affect the stability of the dam. The geotextile bag method has recently gained attention from scientific researchers and engineers due to its good permeability and sand retention function, and has seen some success in the field of tailings dams. However, the role of the geotextile bag method in ensuring the safety and stability of tailings dams needs further exploration. Additionally, there are several new problems that need to be addressed. This paper aims to examine the use of built-in pressurized airbags for accelerating the solidification and drainage of fine-grained tailings in molded bags.
Benchmarking Analytical and Numerical Simulation of Induced Fault Slip
Authors: Novikov A. V. , Shokrollahzadeh Behbahani S. , Voskov D. , Hajibeygi H. , Jansen J. D.
Published: September 2023

Quantification of the poromechanical response of subsurface formations due to human-induced pore pressure fluctuations is critical for the performance and stability assessment of many geo-energy systems. In particular, natural faults in the subsurface introduce the hazard of induced seismicity. Numerical modeling of fault reactivation is challenging, while the specific details of induced stresses and fault slip in reservoirs with displaced (i.e. non-zero offset) faults may cause additional challenges depending on the type of numerical formulation employed. To facilitate the systematic development and testing of numerical tools for the simulation of induced seismicity in faulted reservoirs we developed a set of semi-analytical test problems of increasing complexity, based on inclusion theory and Cauchy singular integral equations. With these we investigate the accuracy of two recently developed Finite Volume (FV) schemes with collocated and staggered arrangements of unknowns. One of them employs a conformal discrete fault model (DFM) which can guarantee sufficient accuracy at the cost of adaptive mesh refinement but may suffer from modelling and computational challenges when addressing large-scale realistic geological configurations. The second one employs an embedded (or non-conformal) discrete fault model (EDFM) which avoids the need for excessive mesh refinement, but of which the accuracy and the range of applicability are still to be investigated. We found that both numerical schemes accurately represent the pre-slip Coulomb stresses, but show different degrees of accuracy in representing the resulting depletion-induced fault slip. The semi-analytical benchmark data are available via DOI 10.4121/22240309.

INTRODUCTION

The kernel of this paper is formed by a series of semi-analytical poro-mechanical test problems of increasing complexity with the aim to systematically compare the capacities of two poro-mechanical finite-volume-based simulation codes: one developed by Novikov et al. (2022b) which employs a discrete fault model (DFM). It forms part of a comprehensive porous media simulation package, the Delft Advanced Reservoir Terra Simulator (DARTS) and will be referred to with that acronym. The second code, developed by Shokrollahzadeh Behbahani et al. (2022), is based on a smoothed version of the embedded discrete fault model (sEFVM) and will be referred to with that last acronym. Both codes are being developed as part of the DeepNL Science4Steer project (NWO, 2017), and Appendices A and B give a brief overview of their characteristic features. The grids used in this study are presented in Appendix C.
Experimental Study of Flow Field in Solution Mining Period of Salt Cavern Gas Storage
Authors: Guan Yueyang , Ban Fansheng , He Qi , Che Yang , Xu Nan
Published: September 2023

Solution mining is the most critical process for salt cavern gas storage. So far Salt cavern gas storages in China are mainly single-well-cavity, and the time of dissolution between two and four years. Solution mining utilize intermediate hanging string and long hanging string for direct circulation and reverse circulation. In the process of solution mining, fluid velocity and different mode of circulation have a great influence on the longitudinal and transverse expansion of the cavity. Therefore, in order to research the changes of the flow field in different working conditions, this study uses Particle Image Velocimetry for flow field analysis. In the laboratory, using large-scale transparent models for laboratory simulation experiments. According to the experimental results, under direct circulation mode, water flows through the right space to upper part. Angle of reflux was in the range of 28° − 56°. The maximum influence radius of direct circulation flow field is 48.1m. In reverse circulation mode, if the distance from the intermediate hanging string to the insoluble matter is less than 2.44m, insoluble matter may affect circulation.

INTRODUCTION

Underground gas storage is important composition part of transporting pipeline system of natural gas (LI Tie et al., 2000). In 2021, China's natural gas consumption will reach 372.6 billion m3, and the reserves required for peak adjustment of natural gas will be 55 billion m3, but the actual reserves only reach about ⅓ of the demand (YANG Chunhe and WANG Tongtao, 2022). Overcoming the unstable supply of clean energy requires accelerating the construction of underground space for large-scale storage (WANYAN Qiqi et al., 2020). Salt cavern gas storage is the main technology for natural gas storage due to its stronger integrity, higher gas injection efficiency and less gas consumption of cushion (ZHU Xingshan et al., 2022). It can not only satisfy the peak regulation demand of the city, but also optimize the production system and gas pipeline network (ZHENG Yali and ZHAO Yanjie, 2010).

A mass of cavity completion data shows that the final cavity volume of salt cavern gas storage deviates from the initial design volume to some extent (WANG Yuangang et al., 2020), so it is particularly important to research the influence of different factors on solution mining.