bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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.