bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Study on True Triaxial Pressure-Bearing Plugging in Micro-Fractured Formation
Authors: Jiang Kaining , Hou Bing , Qiu Xiaojiang , Cao Wenqi , Wei Yimeng , Cui Zhuang , Elsworth Derek
Published: September 2022

The pressure-bearing capacity of the micro-fractured formation is low, and the leakage phenomenon is serious under the action of high-density drilling fluid. The pressure-bearing plugging can improve the pressure-bearing capacity of the formation that is vulnerable to leakage. In this paper, the sandstone outcropping was made into 295mm×297mm×300mm samples to conduct true triaxial pressure-bearing plugging physical experiments. The result shows:(1) Large granular lost circulation materials were accessible to accumulate around the open hole to form a dense mud cake, which greatly improved the pressure-bearing capacity of the formation. (2) Under the action of high pressure, the small granular lost circulation materials were rapidly pumped into the micro-fractured formation, forming a plugging layer in the fracture. (3) It was found that when increasing the plugging slurry injection rate, the plugging zone was more prone to instability, and the final pressure-bearing value of the formation was reduced. This study explored the plugging effect of lost circulation materials with different particle sizes and ratios on the fractures and optimized the corresponding construction parameters by carrying out true triaxial pressure-bearing plugging experiments in micro-fractured sandstone formations under different working conditions, in order to provide guidance for pressure-bearing plugging site construction of micro-fractured sandstone formations.

1. INTRODUCTION

Loss circulation refers to a phenomenon in which the drilling fluid in the wellbore is lost to the formation under the action of the pressure difference during the drilling operation[1]. Economic losses to the global oil industry due to lost circulation run into millions of dollars each year. Pressure-bearing plugging technology[2,3,4] is a method for plugging lost formations and fractures when lost circulation occurs during the drilling process, which is significant to drilling engineering. The pressure-bearing plugging technology is to use different shapes and sizes of lost circulation materials to mix with drilling fluids in different concentrations by bridging, stacking and filling to form a plugged zone. The plugged zone can block fluid pressure transmission and fluid medium passing through. It reduce leakage, meet the needs of control and cementing by improving the pressure bearing capacity and bottom-hole pressure of low-pressure and weak formations[5,6]. Because of the low pressure-bearing ability of the micro-fracture formation and the serious leakage phenomenon under the action of high-density drilling fluid[7]. This paper carried out pressure-bearing plugging experiments based on the actual micro-fracture formation stress field, researched the effect of plugging slurry injection speed on the pressure-bearing capacity of the sandstone formation and different particle size ratio on the plugging effect of induced cracks by analyzing the pumping pressure curve.
A Rock Mechanical Experimental Study on the Effect of Hydrostatic Pressure and Directional Stress on Rock Sample Permeability
Authors: Almakrami A. H. , Alqam M. H. , Algarni S. H.
Published: September 2022

The objective of this study is to compare the laboratory sensitivity of the permeability of cylindrical plugs to variations in hydrostatic and directional stress to determine how different stress evolution trajectories may affect fluid flow for different rock types. To achieve this, series of Stress-Dependent Permeability (SDk) tests were performed on suites of rock mechanical plugs, covering a wide range of lithology and permeability, employing different types of transients simulating varying geo-mechanical conditions during loading and/or unloading processes. Specific laboratory testing procedures were adapted depending on the permeability of the rock sample. Steady-state method was deployed for medium to high permeability samples, while complex transient (subjecting a sample to a pressure pulse and monitor the resulted pressure decay) methods were deployed to estimate the permeability of tighter rock samples. Test results suggest that plug samples with low to ultra-low permeability are more sensitive to changes in hydrostatic pressure, and less affected by directional stress variations. For more permeable samples, permeability responses to a dynamically evolving state of stress may exhibit a degree of sensitivity to directional stress variations difficult to predict without conducting a proper laboratory analysis.

1. INTRODUCTION

In some cases, during the production cycle of a reservoir, the net effective reservoir stress is increasing due to reduction of the pore pressure; this continuous process induces an alteration of the reservoir flow capacity of the reservoir. Published studies (Fatt, 1952) showed that while the change in porosity with pressure is small, the effect of overburden stress is significant on permeability. Another study (Jones, 1979) showed that tight sand samples are profoundly affected by evolving net reservoir stress more than the permeable samples. Moreover, the effect of changing stress environment has been investigated on different formations, i.e. sandstone and carbonates (Dautriat et al, 2007) to accentuate a relationship between effective stress and permeability. In this lab study, we are attempting to investigate the stress-dependent permeability in tight and medium-to-high permeability rock samples, in response to the effect of both hydrostatic pressure and directional stress, aiming to program the relationships between hydrostatic pressure and directional stress in order to optimize permeability and maximize fluid flow.
A Machine Learning-Based Microseismic Event Location and Wave Velocity Prediction
Authors: Maksut Z. , Meiramov R. , Yazici A. , Suorineni F.
Published: September 2022

Human-induced seismicity from underground mining can have a huge effect on the safety, productivity, and operating costs of mines. Hence, the mining industry demands an accurate predictor for microseismic source locations in order to mitigate unexpected catastrophes such as rockbursts. The aim of the research is to use machine learning models to predict real-time seismic wave velocities in the constantly degrading underground mine environment. In underground mining, the rockmasses and voids are constantly changing in quality, geometry, and content with mining. Data for this study is generated by a research team at the laboratory of the School of Mining and Geosciences at Nazarbayev University. The research on the application of machine learning is also conducted at the School of Engineering and Digital Sciences in the same university. The main findings of the data analysis were that due to the heterogeneity and constant change of the mining environment, the seismic wave velocity is not constant, hence the traditional methods of calculating the event source location using a constant velocity model in the seismic monitoring systems will often lead to inaccurate results. The accuracy of determining seismic event source locations depends on the seismic wave velocity assumed in the algorithm for the event source location. Thus, the ability to predict real-time seismic wave velocity allows the better location of seismic events. Several machine learning approaches have been explored for the prediction of seismic wave velocities under various conditions. Linear Regression models were trained from the data obtained in the laboratory to predict the wave velocity. The deep learning approach was also used to achieve higher precision, in particular, Deep Artificial Neural Networks (ANN) have been applied. Compared with Linear Regression which could only learn linear relationships between the features and the target, Deep ANN could learn complex relations due to the activation function in each layer.
Numerical Investigation of Geopolymer Displacement in an Enlarged Wellbore: Implications for Wellbore Integrity
Authors: Wu Yuxing , Salehi Saeed , Khalifeh Mahmoud
Published: September 2022

Cement-mud displacement plays a crucial role in the sealability of cement sheaths. An unstable interface between two fluids always leads to mud channeling, interfluid mixing, and cement contamination and degrades the cement quality. In this study, a 3D computational fluid dynamics (CFD) model is developed to simulate the time-dependent mud removal in a horizontal casing-formation annulus. The annulus geometry is created based on the caliper log data from an unconventional shale well in the Tuscaloosa Marine Shale (TMS). A water-based mud (WBM) is displaced by i) neat class G and ii) rock-based geopolymer, respectively. The results present that geopolymer has a lower residual mud resulting in a lower risk of cement failure. Cement slurries injected with a high flow rate cause better displacement performance. Reducing density ratio causes few residual drilling fluids in the annulus due to buoyancy effects. Compared with the density ratio, the flow rate has a lower sensitivity on the cement volume fraction. The model can be extended to any other mud-slurry displacement procedure as the annulus geometry and the properties of the drilling fluid and cement slurry are known.

1. INTRODUCTION

Cement sheath is an important barrier to maintaining wellbore integrity, preventing formation fluids from flowing into an unintended area. After setting a casing into an openhole, the cement slurry is injected into a target depth to displace the annulus’s original drilling fluids (mud). The cement is drying from a slurry to a solid, developing a set cement. A recent study estimated that 13% of loss well control accidents in the past resulted from cement failure during drilling and 30% occurred during production (Patel et al., 2019a). The failure could lead to costly and complicated remedial operations and trigger environmental contaminations (i.e., groundwater contaminations). An excellent cement design should satisfy the requirements of long-term and short-term well integrity. The long-term design considers structural failures of the cement sheath in the form of circumferential cracking, axial cracking, debonding, shear failure, and cement crushing (Patel et al., 2019b; Wu et al., 2020a, 2021; Kimanzi et al., 2020). Short-term integrity factors such as cement slurry quality, rheology, and pumpability during cementation are the major considerations (Bittleston et al., 2002; Maleki and Frigaard, 2017; Kimanzi et al., 2019; Etrati and Frigaard, 2019; Foroushan et al., 2021).
Linking Microscale Processes to Macroscale Salt Creep with a New THMC Model
Authors: Hu M. , Steefel C. I. , Rutqvist J.
Published: September 2022

Microscale grain deformation and reorganization, fracturing, and pressure solution at interfaces have been identified as the mechanisms for the creep of salt at larger scales. However, it remains uncertain how each mechanism quantitatively impacts creep in granular salt that are naturally under complex geometric and multi-physical conditions. In this study, we have developed a new microscale thermal-hydro-mechanical-chemical (THMC) model that accounts for coupled deformation, dynamic contacts, chemical reaction, and fluid transport in heated environment in granular systems where the microscale geometry can be realistically represented. The THMC model was achieved by linking a microscale mechanical code based on the numerical manifold method (NMM) to the reactive transport model Crunch. Using this first-of-its-kind microscale model, we show the results of compaction within a halite aggregate. We found that sharp corners of salt grains can dominate the contact dynamics, microfracturing, and pressure solution, thus governing the porosity loss of the system. We show that pressure solution and structural changes can be further accelerated by a thermal gradient due to changes of thermodynamics and reaction rates. We show that grain reorganization and pressure solution can occur repeatedly and continuously, thus contributing to longer-term creep of salt at larger scales.

1. INTRODUCTION

Understanding the creep of salt is essential for effectively predicting the long-term evolution of nuclear waste repositories and strategic storage caverns for oil and gas in salt rocks (Urai et al., 1986). Different underlying mechanisms of creep in different stress/temperature ranges have been theoretically proposed (Urai et al., 1986). Based on laboratory tests micrograph, Urai and Spiers (2007) suggested that grain dislocation, microfracturing and pressure solution (Spiers et al., 1990; Zhang et al., 2007) are mechanisms acting during long-term creep. Identifying these mechanisms has advanced the understanding of salt creep. However, an important scientific question remains:

How does each mechanism quantitatively impact creep in granular systems that involve such complex geometric and multi-physical conditions?
The Influence of Failure Criterion on the Estimation of In-Situ Stress Using the Wellbore Breakout of Inclined Well
Authors: Peng Hao , Tang Zhiqiang , He Mingmin , Chen Yousheng , Gao Jiajia , Chen Yifan
Published: September 2022

The accurate information of in-situ stress at depth is critical for oil and gas drilling and hydraulic fracturing operations. The technique of estimating the maximum horizontal stress using the wellbore failure detected from imaging logging has been widely recognized, but the influence of different rock failure criteria on the calculation results has always been neglected. Therefore, according to the linear poroelastic mechanics theory and different failure criteria of rock, the maximum horizontal stress in southwest Sichuan of China is calculated using the wellbore breakout of an inclined well from literature. The calculation results show that magnitudes of the maximum horizontal stress calculated with different failure criteria vary greatly. The frequent earthquakes in southwest Sichuan indicate that the preexisting faults in this area are close to the friction limit state under the in-situ stress, which agrees well with the results calculated by using Mogi-Coulomb criterion.

1. INTRODUCTION

In-situ stress is very important for oil and gas drilling and hydraulic fracturing stimulation operations. Accurate knowledge of stress magnitude and orientation can help us to design reasonable drilling fluid density and borehole trajectory for preventing wellbore failure and associated underground complexity. Wellbore shape reflects the stress state that the wellbore wall is being or has been subjected. Thus, according to the wellbore shape and drilling history data, we can determine the stress distribution around the wellbore wall and calculate the magnitude of the in-situ stress. Although the final shape of wellbore may be different from the shape after the initial wellbore breakout, the breakout width remains unchanged (Zoback et al., 1985; Barton et al., 1988). Therefore, the breakout width can reflect well the initial stress state of wellbore when breakout occurs.

Many authors preferred using wellbore failures to calculate the in-situ stress, even established several models to calculate the maximum horizontal stress combining Mohr-Coulomb criterion (Barton et al., 1988; Vernik & Zoback, 1992; Zoback et al., 2003; Frydman & Ramirez, 2006), but these models are only suitable for the failures of vertical wellbore. Peska and Zoback (1995) also adopted Mohr-Coulomb criterion to determine the maximum horizontal stress magnitude according the wellbore failure of inclined well. However, the effect of intermediate principal stress on rock strength is ignored in Mohr-Coulomb criterion, which may lead to unrealistic calculation results of in-situ stress. Although Tang et al. (2021) chose Mogi-Coulomb criterion in the analysis of the influence of thermo-poroelastic coupling operation on the calculation results of in-situ stress, the author did not analyze the differences caused by other criteria, which may have significant influence on in-situ stress estimation.
Parent-Child Well Interaction in Multi-Stage Hydraulic Fracturing: A Bakken Case Study
Authors: Merzoug A. , Chellal H. A. K , Brinkerhoff R. , Rasouli V. , Olaoye O.
Published: September 2022

Multi-stage hydraulic fracturing technology for enhanced production from unconventional reservoirs has improved significantly during the past decade. However, multi-stage fracturing for multiple closely spaced long horizontal wellbores introduces several technical challenges. One example is Fracture Driven Interaction (FDI). In this study, we document a Fracture Driven Interaction case study where the primary well was drilled into the Three Forks Formation, and an offset well was drilled in the Middle Bakken. The idea of this study was to investigate the possibility of frac-hit as the cause of low production rate in the offset well. This study estimates the stress in the petroleum system, and combine that with petrophysical analysis to construct a fully coupled hydraulic fracturing, geomechanics and reservoir numerical model. The model matched the production data and the results show the occurrence of a fracture driven interaction was a result of stress decrease due to depletion. Fracture driven interaction effect was severe on the offset well reducing the stimulated reservoir volume, thus jeopardizing the production.

1. INTRODUCTION

During the last decade, hydraulic fracturing designs included larger proppant volume injection in order to maximize stimulated reservoir volume (SRV) and increase recovery (Gupta et al., 2021; Seth et al., 2020; Xu et al., 2018). Along with implementing multi-pads, a connection between fractures from adjacent wells was noted. This connection was defined as Fracture Driven Interaction (FDI) so called frac-hit or frac-bashing (Daneshy, 2020). Miller et al. (2016) reported the impact of FDI on production for several plays of the United States. This effect can cause either an increase or a decrease in production. FDI was first reported in the work of Ajani and Kelkar (2012). Gupta et al. (2021) reported the factors that can cause an FDI to occur. In this work, we focus on two factors: stress profile and depletion.

When depleting the primary well (parent well), pore pressure decreases, leading to a decrease in the total stress around the primary well. Consequently, the new distribution of stress will cause asymmetric growth in the fractures of the offset well (child well). The asymmetric growth reduces the stimulated reservoir volume because the fractures grow in the direction of the depleted zones, i.e., the zones with the least resistance to propagation (Ajani & Kelkar, 2012; Daneshy & King, 2019; Gupta et al., 2021; Kumar et al., 2020; Manchanda et al., 2017; Rezaei et al., 2019; Seth et al., 2020; Zheng et al., 2019)
Simulation of Wellbore Annular Closure in Salt Formations Due to Creep: Potential for Natural Sealing Barrier
Authors: Mazon A. L. , Fontoura S. A. B.
Published: September 2022

Wellbore construction through salt formation remains a great challenge in drilling engineering in many oil provinces around the world due to its unique mechanical properties, in particular, its high creep mobility under downhole conditions. However, once the casing is defined, the behavior of salt is ideal and can provide a necessary and economically beneficial natural barrier. The aim of this paper is to model wellbore closure in salt throughout uncemented casing zones considering both open annular. For this, a computational methodology was developed and evaluated through the reproduction of studies about the formation of natural barriers in salt, reported in the literature, is presented. The comparison between the results obtained demonstrates a time difference for annular closure ranging from a few months to a few years, depending of depth and type of salt. Discussions on specific aspects of creep simulation in salt rocks are also discussed at this stage. From that, we present an analysis to estimate the wellbore closure time under field conditions representative of the Brazilian basins, coupling the adjustments made in the previous stage. As a result, it is observed the potential for the formation of natural barriers in saline layers for permanent plug and abandonment operations.

1. INTRODUCTION NATURAL BARRIERS FOR PERMANENT ABANDONMENT OF WELLS

The operations carried out at the end of the useful life of any kind of well (exploration, production or injection) are called permanent plug and abandonment of the well (PP&A) whose objective is to prevent the flow of fluids from the formation and resist the pressures acting above and below.

According to Khalifeh & Saasen (2020), the PP&A operations of a well can reach up to 25% of offshore well drilling costs. Due to the high costs of these operations and the high number of wells to be permanently plugged and abandoned in Brazil in the coming years, the oil and gas industry has increasingly invested in the development of alternative PP&A techniques.
Modelling Failure Propagation Using the Phase-Field Method with the Assistance of Ultrasonic Wave Velocity Measurement
Authors: Li Xu , Si Guangyao
Published: September 2022

The phase-field method in damage mechanics is attracting increasing popularity in rock engineering, which bridges the gap between microcracking fracture mechanics and quantified damage mechanics. The quasi-static microcrack propagation can be well captured in FEM via the phase-field damage method. However, the determination and calibration of the length scale parameter (ℓ) in the phase-field method are still unclear since the influence width of microcracks cannot be directly observed. In this research, the phase-field damage method is applied to simulate rock behavior under triaxial tests. The length scale parameter is calibrated via ultrasonic wave velocity measurements. The results show that the phase-field damage can be calibrated via ultrasonic wave velocity measurement, which empowers the application of phase-field damage in engineering applications.

1. INTRODUCTION

The microcracks, joints and faults are always challenging geotechnical issues for underground engineering (X. Li et al., 2022; Si et al., 2020). A novel approach to capture microcracking activities is to use the phase-field damage mechanics (Francfort & Marigo, 1998; You et al., 2021). The phase-field damage mechanics enables to simulate crack propagation and coalescence without introducing complicated remeshing in FEM, plausible problem-specific functions (e.g. in XFEM) or any other thermodynamically unfriendly auxiliary equations or mesh manipulations.

The usage of the phase-field damage model involves two additional parameters, namely the critical fracture energy per unit of area (Gc) and length scale parameter (ℓ). Gc describes the energy consumed to form the unit length of fractures, which is related to fracture toughness (Kc). The fracture toughness can be easily captured by ISRM suggested SBC test. However, the length scale parameter represents the influencing width of microcracks, which is hard to be directly measured. Some previous studies casually selected the minimum size of the mesh as the length scale parameter, which may influence the accuracy of the entire phase-field damage model (Duda et al., 2015). In this paper, we proposed an approach to calibrate parameters in the phase-field damage model via ultrasonic wave velocity measurement. The bridge between traditional damage mechanics and phase-field damage mechanics has been established. The proposed model shows great consistency with laboratory results.
Numerical Simulation of Metal Jet Shape and Rock Damage Law near Tunnel Under Confining Pressure
Authors: Liu Xianbo , Li Jun , Liu Gonghui , He Long , Liu Wei
Published: September 2022

Different scholars had carried out much research on rock damage during perforation by numerical simulation or experiment, but they had not considered the influence of metal jet morphological feature on rock damage during perforation. In this paper, a numerical calculation model of perforation is established. The paper reveals the change characteristics of metal jet shape during perforation and carries out the influence law of metal jet shape on the rock near pore damage. The results show that: In the process of perforation, the shape of the metal jet changes dynamically with time, and the penetration process of the metal jet is divided into three stages. It is clear that the near channel damage of rock mainly occurs in the third stage of shape change. The paper also clarifies that the rock failure form in the perforation process is compression shear failure and reveals the influence law of confining pressure on the rock damaged area in the perforation process. The paper defines the influence law of metal jet shape and confining pressure on rock damage during perforation, which is of great guiding significance for perforation operation design.

1. INTRODUCTION

Unconventional oil and gas resources such as shale gas and shale oil often exist in low porosity and low permeability rocks. Due to the limitation of the storage environment, oil and gas exploitation is difficult (Wu et al.,2016; Lian et al.,2020; Gao et al.,2019). Aiming at the problems in the exploration and development of such oil and gas resources, the combination of perforation completion and reservoir stimulation is used to improve the oil and gas seepage conditions (Zhang et al.,2021; Albert et al.,2018; Wang et al.,2019; Behrmann et al.,1991;). However, while the perforation forms a channel for oil and gas migration, the rocks around the perforation channel will also be damaged to varying degrees. The transient nature of perforation and the complex and diverse rock damage around the perforation channel have severely restricted the stable and efficient development of unconventional and complex oil and gas reservoirs.
Determination of Continuous Kinematic Criticality in Structurally Homogeneous Zones – A New Approach
Authors: Rahman T. , Niyogi A. , Sarkar K.
Published: September 2022

The sectional kinematic feasibility analysis of a slope does not demonstrate the continuous kinematic criticality of a curve-faced rock slope prone to discontinuities. The geotechnical engineers and geologists need to understand the zones carefully where roads, tunnels, bridges or dams are to be constructed, which might be affected by slope failures along discontinuities. In the present study, a continuous model has been proposed considering structurally homogenous zones along a road-cut in tectonically deformed terrains. These structural zones are defined in such a way that homogeneity of discontinuity orientations are preserved, which may or may not be defined by geological boundaries such as faults, river tributaries, lithology etc. The proposed model accounts for the deviation of strikes along slopes, the dip amount in a particular section which varies from bottom to top of the slope and the criticality of slope failure. The criticality (%) depends on the number and orientation of discontinuities that are present in a particular structural zone. The variation in kinematic criticality due to deviation in slope direction and dip amount are obtained and presented in the model. This continuous criticality model for different slope failures postulates a precise and quick zonal kinematic criticality. The sections of different locations from a particular zone are plotted as an array of lines in the continuous criticality model, which are joined together (top-top and bottom-bottom) to precisely represent the potentiality of kinematic failures of the entire zone.

1. INTRODUCTION

The slope stability condition of any rock slope is controlled by the orientation of the discontinuities present in the rock mass. Therefore, to evaluate the rock slope stability, the kinematic analysis is used where the geometry of the discontinuities and the rock slope is considered, and it does not include any external force acting on the slope (Park and West, 2001).

Kinematic analysis is the most preliminary analysis for any rock slope to evaluate the potentiality of structurally controlled failure of rock slopes in various modes (such as, planar, wedge or toppling failure). The kinematic analysis used Markland’s test which was first introduced in 1972, and has been widely used in various studies (Gokceoglu et al., 2000; Smith, 2015; Niyogi et al., 2020, etc.). A number of assumptions of kinematic analysis were suggested by Markland (1972), but the assumption of a uniform slope angle and slope aspect have been addressed in the present paper. In nature, the slope angles are not found as planar or uniform rather the slopes are rugged and uneven. There is another problem related to sloping direction of the slope. Mountainous slopes are large and are sinuous in terms of sloping direction.
Upscaling of Mechanical Properties in Heterogeneous Reservoirs for Compaction Analysis
Authors: Hajiabadi M. R. , Nick H. M.
Published: September 2022

Geo-mechanical grid of reservoir simulations often contains coarse cells (×1 m thick) compared with the sub-metre resolutions of well log data. The geomechanical properties of the coarse cells are usually assumed to be representative of well log data although a rigorous upscaling process is lacking. A few existing studies on the upscaling methods have mainly focused on elastic properties. Guidelines for the upscaling of other mechanical properties related to plasticity are still required.

This study aims to assess the upscaled material parameter including elastic and visco-plastic material in heterogeneous chalk reservoirs for compaction analysis which are optimized from history matching the surface subsidence. To this purpose, a 2D simulation case with over-burden, reservoir and under-burden is considered as a conceptual model. A random distribution of porosity field defines different mechanical properties within the reservoir based on porosity dependent functions which are collected for chalk. Our results demonstrate the shape of yield surface is the most important parameter for upscaling while other upscaled material parameters are relatively close to the values obtained from the material with mean porosity. These imply that the heterogeneity has the most effect on the shape of the yield surface.

1. INTRODUCTION

Reservoirs are geologically heterogeneous with vertical and lateral changes in petrophysical and lithological properties that are not investigated in laboratory. Standard triaxial tests such as uniaxial / hydrostatic compaction tests are commonly carried out on 1~3cm long core plugs and are designed to characterize one single lithology or reservoir rock type in the laboratory. Therefore, upscaling technics from the geo-statistical description to reservoir grid are necessary to use laboratory data acquired at the cm-scale for reservoir modeling carried out at the km-scale. Studies on the upscaling methods for geo-mechanical properties are limited to the elastic properties for multi-layer reservoirs (Fokker, 2012 and Settari et al., 2013, Zhang et al., 2020). However, the effect of heterogeneity on the upscaled material parameter is missing in literatures.
Influence of Stress Field Evolution on Refracturing of Horizontal Wells
Authors: Fen Peng , Hongtao Liu , Shiyong Qin , Longcang Huang , Zhikun Pan , Yakai Tian , Longqiao Hu , Yixuan Liu , Fujian Zhou , Ben Li , Junling Wang , Zhaokun Yuan , Haijun Gao , Yao Jiang
Published: September 2022

After the initial fracturing, the stress field around the wellbore will become more complex, and even stress reversal will occur. The depletion of oil and gas will lead to the reduction of reservoir pore pressure, and then increase the effective stress of the reservoir, leading to the decreases of fracture conductivity and production, thus refracturing is often needed. Most of the studies focus on the initial fracturing of the reservoir, but there are few investigations on refracturing. In this paper, based on the global Cohesive zone model, the finite element method is used to simulate the initial fracturing and oil and gas production and extract the stress field distribution. Then the stress field is introduced into the new model to simulate the initiation and extension of the fracture during refracturing. The results show that under the influence of the initial fracturing, the stress concentration problem exists in the refracturing area, and the uniform repeat reconstruction can be achieved by adjusting the perforation position or combining it with the temporary plugging process.

1. BACKGROUND

Tight oil exploration has broad prospects and has become the key object of unconventional oil and gas development, with great exploitation potential(Bing et al. 2016). The large section and multi-cluster hydraulic fracturing technology of horizontal wells is an effective means to develop low permeability oil and gas fields. With the progress of production, the production capacity is gradually reduced, and the refracturing operation needs to be carried out (Zhou et al. 2017). The implementation of refracturing technology measures can effectively expand the reservoir reconstruction volume and supplement formation energy, improve the single well production and extend the stable production period. But different from the initial fracturing, the stress field around the wellbore is disturbed by the initial fracture, and the long-term production has an impact on the formation pressure, which leads to a more complex formation environment for refracturing(Xu et al. 2020).
Experimental Study and Analysis of Proppant Conductivity Under Gas-Liquid Alternation
Authors: Zhikun Pan , Longqiao Hu , Yixuan Liu , Bin Wang , Ben Li , Dunhao Chen , Fujian Zhou , Fen Peng , Hongtao Liu , Shiyong Qin , Longcang Huang , Junling Wang , Zhaokun Yuan , Haijun Gao , Yao Jiang
Published: September 2022

The conductivity of the proppant filling layer is directly related to the quality of fracturing construction and stimulation effect. The existing diversion experiments are mainly pure liquid measurement diversion, but there is oil-gas mixed flow in actual oil well production, so it is necessary to carry out systematic experiments on the actual conductivity of the formation. To make the experimental results closer to the actual formation production, this paper carried out the gas measurement conductivity test, liquid measurement conductivity test, and gas-liquid alternating conductivity test of different proppants. The experimental results show that the conductivity of gas measurement is much greater than that of liquid measurement, and the former is about 12.6 times that of the latter. Related to the air permeability and water-resistance of the experimental film coating material, the conductivity of gas-liquid alternating measurement is lower than that of pure liquid measurement.

1. BACKGROUND

Since the advent of hydraulic fracturing technology in the 1940s(Raghavan et al. 1972), due to the remarkable stimulation effect, the process technology has been widely valued, the research has been deepened and the application scope has been expanded(Mckay et al. 1993). Especially in recent years, with the large-scale development and utilization of unconventional oil and gas resources such as tight oil and gas and shale oil and gas, oil and gas wells combined with hydraulic fracturing has become one of the key technologies for the efficient development of unconventional reservoir stimulation and transformation(Zheng et al. 2013). The key of hydraulic fracturing technology is to form artificial fractures with a certain length and width and keep the fractures open by pumping proppant(Gao et al. 2006). Therefore, the performance of proppant is related to whether it can play an effective supporting role in fractures, and will also directly determine the fracturing construction effect and the productivity of oil and gas wells after stimulation(Yang et al. 2016).
Strength of Utah Coal Evaluated Using Laboratory Tests with an Unloading Path
Authors: Walton G. , Kim B. H. , Larson M. K.
Published: September 2022

Conventional triaxial tests evaluate rock strength using a load path where the minimum principal stress is held constant and the maximum principal stress is increased monotonically until the peak strength is achieved. Although brittle rock mechanical behavior is relatively well-understood under this loading path, in many practical rock mechanics applications, damage and failure develop as a result of loss of confinement (decreasing minimum principal stress). Since the inelastic behavior associated with damage that develops prior to rock failure is not path-independent, it is possible that rock damage and strength characteristics may differ depending on the loading path considered. To investigate this potential path-dependence, a series of tests were performed on Utah Coal where, following attainment of a target confining stress and deviatoric stress, the confining stress was decreased at a constant rate until failure while maintaining the deviatoric stress. These results were then compared to previously published data for Utah Coal obtained using conventional laboratory testing procedures. The previously published data for Utah Coal showed highly confinement-dependent strength behavior, where the peak strength increased by as much as up to 50% of the mean unconfined compressive strength (UCS) with confining stresses as low as 1% of the UCS applied. The peak strength and crack damage (CD) envelopes were found to be highly anisotropic, but the crack initiation (CI) envelope was relatively insensitive to the angle between the loading direction and the primary cleat orientation. In this study, although the CI stress, CD stress, and peak strength envelopes are all generally similar for both the loading and unloading tests, some of the test results for samples with cleating inclined at 30° to the loading direction suggest the CD stress envelope may be slightly lower under unloading conditions than under loading conditions.
Hydraulic ‘Fracturing’ Mechanisms of Unconsolidated Sands from Novel Laboratory Experiments and Modelling
Authors: De Borst K. , Tan C. P. , Mat Piah F. , Groenenboom J.
Published: September 2022

"Fracturing" mechanisms and associated injectivity impairment were investigated by means of large-scale laboratory experiments on sand packs built from synthetic sands and highly unconsolidated downhole core materials. Various fines were mixed into the sands around the perforations to mimic effects of plugging, formation damage and fines migration. Generation, closure and re-opening of "fractures" were monitored through in-situ CT-scanning. The water injection resulted in the opening of cavities rather than the formation of sharp fractures. The cavities opened at injection pressures exceeding the confining stress and enlarged against very low cohesive strength of the sands. This suggests material fluidization rather than shear failure as the immediate cavity initiation mechanism. A significant influence of the laminar and turbulent flow regimes on cavity shape and cavity closure characteristics was observed. It was found that injectivity was controlled by the interplay of fines transport away from the injection region, resulting in injectivity increase, and the permanent compaction of the sand around the cavity, resulting in injectivity decrease. The first-time insights highlight the role of sand fluidization and cavity formation in matrix vs "fractured" injection in unconsolidated sand reservoirs. The knowledge of the sand destabilization and mobilization processes enable design and operation optimization of water injectors with implications on sand control strategies and remediation measures.

1. INTRODUCTION

Rapid injectivity decline has frequently been observed during injection into unconsolidated sand formations. The rapid decline usually happens in "fractured mode", where the injection pressure surpasses a critical limit at which the flow localizes, and a dynamic propagating "fracture" is created. This mode is contrary to the matrix mode, during which water enters the formation through infiltration into the pore space without (significant) damage to the formation. The creation of void space i.e., "fracture" in the sand is a consequence of grain displacements, for which shear failure and fluidization have been considered as potential mechanisms. The injection rate can be increased without further increase in injection pressure, which indicates the creation of additional space, i.e., "fractures", to accommodate the injected water. However, it remains unclear under what condition the mechanisms prevail, and how the leak-off from the void space into the highly permeable sand affects the local stress state and, thus, propagation of the "fractures". The "fractures" adversely affect the performance of injector well and either the injectivity rapidly declined or the injector failed prematurely soon after initiating the ‘fractures’ (Dambani et al., 2014, Khodaverdian et al., 2010 and Sharma et al., 2000).
Integrated Experimental and Numerical Study on Debonding of Cement-Casing Interface
Authors: Wang Yangang , Feng Yongcun , Deng Jingen , Cai Wenjun , Wang Kongyang , Ma Chengyun , Wanyan Qiqi , Li Kang , Bai Song , Li Haitao
Published: September 2022

During the hydraulic fracturing process, the cement-casing interface may debond to be a severe leakage pathway for fluids. It is of significant interest to understand how stresses cause the debonding of cement-casing interface. A casing-cement interface normal bond parameters test device was fabricated to test normal bond parameters of the interface, including normal bond strength, normal stiffness and normal fracture energy. And a casing-cement interface shear bond parameters test device was fabricated to obtain the shear bond strength, shear stiffness and shear fracture energy of the interface. A 3D finite element numerical model of casing-cement-formation assembly was established to study the casing-cement interface debonding during hydraulic fracturing process, cohesive elements were employed to model the debonding of casing-cement interface, and the parameters obtained from our experiments were assigned to the cohesive elements. The simulation results demonstrate that the debonding of casing-cement interface is highly influenced by the normal bond parameters, shear bond parameters, situ stresses and internal casing pressure. The possibility and degree of casing-cement interface debonding and plastic deformation of cement increases with the increase of internal pressure.

1. INTRODUCTION

Well integrity plays an important role in conventional and unconventional oil and gas development. With the development of unconventional resource the application of hydraulic fracturing is becoming more and more widespread (Vengosh et al.,2014). During the hydraulic fracturing process, the casing will be subjected to significant internal pressure fluctuations, the casing-cement and cement-formation interfaces may debond to be a severe leakage pathway for fluids (Xi et al.,2020; Li et al.,2020).

Debonding between casing-cement and cement-formation interfaces can be decomposed into normal failure and shear failure. Debonding will occur once the normal and/or shear stress exceed the normal and/or shear bonding strength of the interface. It is of significant interest to understand how stresses cause the debonding of cement-casing and cement-formation interfaces. Since the difference of mechanical properties between cement and formation is higher than that between cement and casing, the casing-cement interface is more inclined to debond than the cement-formation interface. This paper focus only on the casing-cement interface debonding.
Analysis of Strain Responses in Vertical Monitoring Wells for Low-Frequency Distributed Acoustic Sensing
Authors: Srinivasan Aishwarya , Liu Yongzan , Wu Kan , Jin Ge , Moridis George J.
Published: September 2022

Fiber-Optic sensing has become invaluable for analyzing stimulation effectiveness and fracture geometries in unconventional reservoirs. Distributed Acoustic Sensing (DAS) is a Fiber Optic-based technique that provides dynamic strain variations along the length of the fiber during well stimulation. The strain measurements can provide critical information about the fracture characteristics. Most of the studies on Low-Frequency DAS (LF-DAS) focus on interpreting data measurements from horizontal monitoring wells. The LF-DAS measurements from vertical monitoring wells can provide valuable information about fracture propagation. However, strain responses from vertical monitoring wells are not fully understood. Therefore, it is necessary to model LF-DAS signals from vertical offset wells to understand the mechanisms and fracture characteristics. In this paper, we use a complex hydraulic fracture simulator to generate dynamic fracture geometries and obtain strain variations during fracture propagation at vertical monitoring wells located at different offsets. The mechanism of various strain-rate patterns with different fiber-to-fracture distances is illustrated using the spatial distribution of induced stresses during stimulation. A 1D feature, the sum of absolute strain rate along the fiber length is developed to determine the fracture passing time. This study provides valuable insights into the LF-DAS signals obtained from vertical monitoring wells and helps to better interpret the field LF-DAS measurements.

1. INTRODUCTION

Multi-stage hydraulic fracturing with horizontal drilling has made the development of unconventional reservoirs economically feasible. However, the questions about the exact hydraulic fracture geometry are still one of the most argued topics in the industry (Singh et al., 2020). A better understanding of the fracture geometry is vital for optimizing completion designs and improving well production. Various fracture diagnostic tools are available with their own limitations, such as, microseismic (Maxwell, 2014; Zhang et al., 2018), tiltmeters, tracers, pressure gauges, and time-lapse seismic (Byerley et al., 2018; Hull et al., 2019). Distributed fiber optic technique is recently being used for fracture diagnostics due to its capacity to provide continuous, real-time measurements along the length of the fiber optic cable (Sherman et al., 2019).