bluuebunny/crossref_metadata_2025 huggingface dataset:
Microscale Observations of Hydraulic Fractures in Gonghe Granite Subjected to Low- and High-Cycle Fatigue Experiments
Authors: Zhuang L. , Zang A.
Published: September 2022

Previous studies have revealed that hydraulic fracturing behavior in crystalline rock (i.e., granite) is highly dependent on mineral content, grain size, and heterogeneity and is therefore quite different from the behavior observed in sedimentary rock (i.e., sandstone). We investigated hydraulic fracture paths generated via continuous and cyclic injections in Gonghe granite through uniaxial testing. Hydraulic fracturing breakdown pressure was reduced by 20% in cyclic injection. In particularly, we analyzed the role of biotite grains and pre-existing defects - such as natural fractures - in controlling hydraulic fracture propagation. We found that hydraulic fracture crossed natural fracture either without or with an offset in continuous injection, while hydraulic fracture was arrested by natural fracture in cyclic injection, and NF is more likely to be activated during high-cycle injection. This can be beneficial if hydro-shearing of pre-existing flaws is anticipated.

1. INTRODUCTION

The grain-scale heterogeneity of rock has been found to greatly affect crack growth behavior in mechanical fracturing tests. Previous studies have revealed that hydraulic fracturing in crystalline rock for geothermal purposes (i.e., granite) is highly dependent on mineral present, grain size and heterogeneity (Ishida et al., 2000; Zhuang and Zang, 2021). A growing number of work has employed grain-based model coupled with fluid flow in pore network to investigate hydraulic fracturing behavior in crystalline rock, and has reported unique results in terms of asymmetric fracture growth, zipper-mode crack initiation, and the partial re-closure of previous cracks (Kong et al., 2021; 2022), and influence of spatial distribution of minerals (Li et al., 2021). Cracks can be induced in mineral grains (intragranular or transgranular cracking) or along grain interfaces (grain-boundary or intergranular cracks). An in-depth analysis of the hydraulic fracture path can shed some light on the fluid-driven fracture nucleation and fracture propagation process (Chen et al., 2015; Ishida et al., 2016; Zhuang et al., 2020).
Investigating Influence of Freeze-Thaw Cycles on the Degradation of Sandstone Through Discrete Method Modeling
Authors: Huang C. , Zhu C. , Aluthgun Hewage S. , Ma Y. , Shi X.
Published: September 2022

In this study, the discrete element modeling (DEM) method is adopted to evaluate the influence of freeze-thaw cycles (FTC) action on the mechanical properties of sandstone. A water-rich rock model is built, with rock and water compositions represented by particles of different sizes. To overcome the intrinsic porosity limitation of DEM model and better represent the saturated rock sample, we develop an algorithm to include particles of smaller and varying sizes into rock pores. The ice-water phase change and the resulting accumulation of residual strain are considered by developing a modified elastoplastic model. The rationality of this model is first validated by comparing numerical results with published experimental data. Numerical results highlight the FTC deterioration of the uniaxial compressive strength and Young’s modulus of rock specimens. We establish exponential correlations between the freeze-thaw cycle number and the rock’s mechanical properties such as the uniaxial compressive strength, Young’s modulus and strain energy, showing consistent trends with experimental observations. Inter-particle contact damage also reflects the formation and distribution of micro-fractures in rock specimens under freeze-thaw cyclic treatments. This study provides new insights into the rock degradation process under changing climate and could contribute to the future design and assessment of climate-resilient infrastructure.

1. INTRODUCTION

Rock is a typical porous material that allows water to enter pores and micropores by capillary action in a water-rich environment (Amirkiyaei et al., 2021). When the temperature is below 0 °C, the water in pores is frozen as ice whose volume increases by about 9% (Hou et al., 2021). During the phase change of water, the frost heaving pressure increases due to the constrain of surrounding rock (Wang and Zhou, 2018). The frost heaving pressure produces additional microcracks or enlarges the pre-existing discontinuities of rock, allowing more water to fill into rock after ice thawing and producing more damage under the subsequent freezing. As a result, the physical and mechanical properties of rock are deteriorated after increasing freeze-thaw cycles. Therefore, understanding and predicting rock behaviors after freeze-thaw cycles are critical to prevent the damage of rock-based infrastructures in cold regions and regions that undergo seasonal weather changes.
Mesomechanics-Based Crack Evolution Modeling in Shale Reservoirs Stimulated by Cryogenic Liquid Nitrogen
Authors: Han Songcai , Gao Qi , Yang Junchao , Li Yang , Cheng Yuanfang
Published: September 2022

A statistical meso-damage thermo-hydro-mechanical (T-H-M-D) coupling model is developed to investigate the trans-scale progressive cracking progress of the shale reservoir stimulated by liquid nitrogen (LN2). The theoretical framework encompasses: (1) local thermal equilibrium theory to formulate the heat exchange between the rock matrix and the cold fluid; (2) compressible fluid seepage theory to simulate the mass transport in pore media; (3) classical thermo-poroelatic theory to analyze the matrix deformation characteristics; (4) elastic-brittle-damage theory with double damage functions to identify the shale failure process at low temperatures; (5) two independent statistical distribution functions to describe the shale meso-scale heterogeneity in both thermos-mechanical and hydraulic properties; (6) Helmholtz free energy functions to calculate temperature- and pressure-dependent fluid thermodynamic properties; (7) experimental and empirical formulas to characterize temperature- and damage-related rock properties. The numerical model is implemented into the finite element method to analyze the underlying response mechanisms of the shale reservoir interacting with LN2. Simulation results show that LN2-shale interaction can induce multiple initiation positions, resulting in multiple asymmetric macro-cracks. The failure mechanism of the shale during cryogenic fracturing is dominated by elastic-brittle tensile-induced damage. Cryogenic shock to enhance shale elastic modulus and induce thermal tensile stress is beneficial to forming complex fracture networks. It is also found both fracture initiation pressure and induced fracture morphology are more sensitive to fluid injection temperature relative to reservoir temperature.

1. INTRODUCTION

At present, large-scale hydraulic fracturing is widely used in shale gas reservoirs to generate complex fracture networks. However, microseismic monitoring results have shown that some gas-rich shale layers with high temperature, high stress, low brittleness, and undeveloped natural fractures, cannot create a sufficient stimulated reservoir volume (SRV) when adopting conventional hydraulic fracturing (Zou et al. 2015; Lei et al. 2021). Additionally, the use of water-based fracturing fluid will bring about many serious problems such as formation secondary damage, fluid backflow difficulty, and water resource waste etc. (Middleton et al. 2015; Kalam et al, 2021).
Mechanism Study of a Novel Expandable Deep-Water Drilling Conductor on Improving the Bearing Capacity of Subsea Wellhead
Authors: Zhang Minghe , Yang Jin , Wang Yuanming , Li Lei , Zhou Yisu , Yang Yuxiang , Xu Dongsheng , Zhu Guojing
Published: September 2022

A novel expandable drilling conductor is designed to enhance the bearing capacity and stability of subsea oil and gas wellheads in deep-water. The basic structure is the annular expandable materials installed on the surface of the conductor, which changes the interaction between cylinder foundation and surrounding soil, and the annular expandable materials are connected with the conductor to form a similar variable section rack structure, which increases the occlusive effect between the conductor and the stratum. This study proposes the method of combining theoretical models with laboratory experiment and field test for analysis. A new model of expandable conductor and soil strata is presented. Results show that the novel expandable conductor can significantly increase the vertical bearing capacity of the foundation, the end of the annular material area (the thickness of the material), the length of the annular material, and the distance of ring material are the main factors affecting the vertical flexural bearing capacity, and the distance and the thickness of the material and the shear strength of soil are closely related. This kind of variable section pipe can be used as a new type of pile foundation structure to effectively increase the stability of the foundation.

1. INTRODUCTION

Structural casing jetting operations have become commonplace in deep-water environments. Having originated in the Gulf of Mexico (GOM), the structural casing is now jetted in most deep-water basins in the world (Akers, 2008). Very little literature has been published regarding jetting practices. The lack of published literature, combined with the complex mechanics and hydraulics of the process and the lack of detailed soil data at most locations, results in an operation that is heavily dependent on the experience and expertise of the rig-site team.

The deep-water drilling conductor is an important bearing device, and the bearing capacity generated between conductor and soil is used to carry the wet weight of wellhead, mud mat, cam-actuated drill-ahead (CADA), etc. There are usually three methods of conductor installation: jetting, drilling, and hammering (Yang et al., 2010; Pereira, M. F. et al., 2013; Roy et al., 2018). In recent years, the conductor anchor node (CAN) has been developed and successfully applied as an emerging technology (Sivertsen et al., 2011; Kan et al., 2018). In contrast, the jetting method is most mature and has been widely used in the Gulf of Mexico (GOM), West Africa, Southeast Asia, and South China Sea (Jeanjean, 2002; Mackenzie et al., 2012; Prasertamporn, 2016; Wei et al.,2019). In this paper, we focus on the jetting method.
Integrated Assessment of Interaction Between Supercritical CO2 and Shale During Thermal Maturation
Authors: Onwumelu C. , Kolawole O. , Bouchakour I. , Ozotta O.
Published: September 2022

Recent studies have shown that shale reservoirs can be an attractive candidate as a CO2 storage reservoir or a caprock for geologic CO2 sequestration. Shales are source rocks, and thus, there is a continuous diagenetic process that can alter its properties during thermal maturity before it reaches maturity. However, there is a significant knowledge gap in the geochemical and other alterations in shales due to the interaction with supercritical CO2 (ScCO2) stored in the underlying reservoir unit during the diagenetic process. This study investigates the changes in mineralogical properties and Total Organic Carbon (TOC) of shales during thermal maturation when it is exposed to ScCO2 at in-situ conditions and assesses its implications for CO2 utilization and sequestration. Here, we used Bakken shale and Green River shale samples, and exposed the samples to ScCO2 for a specific period. This was followed by inducing the samples to thermal maturity for the pyrolysis process. Subsequently, we evaluated the mineralogical properties and TOC of the pre- and post-CO2 treatment of mature shale samples. Our results from this study provide novel insights for CO2 utilization and sequestration in the mature Bakken and Green River shale formations.

1. INTRODUCTION

Carbon dioxide is injected into the subsurface in a supercritical phase. Supercritical CO2 (ScCO2) state is found above the critical point of temperature and pressure of 31.8 C and 7.38 MPa, respectively where CO2 behaves like a gas but with a density like that of a liquid (Span and Wagner, 1996). There are three recognized geological formations for CO2 storage: saline aquifers, depleted oil and gas fields, and coalbeds. Basically, injection of CO2 has two economic benefits (Baines and Worden, 2004): first, a financial incentive with the carbon tax credits by solely capturing and storing CO2 (e.g., saline aquifers) to reduce CO2 emissions in the atmosphere. Secondly, utilization of CO2 injection in enhanced oil recovery (EOR) or coalbed methane production. Once the CO2 percolates the reservoir, it migrates upward due to buoyancy reaching the top of the formation where it is contained by a cap-rock layer. Trapping mechanisms play a key role in preventing leakage risk (Fatah et al., 2020) and are not only structural and stratigraphic, but also related to the trio CO2-fluid-rock interactions. In residual or capillary trapping, CO2 droplets are immobile and trapped in pore space (Kang, 2011). Solubility trapping happens when CO2 dissolves in brine water or is trapped by mineral precipitation.
A New Approach for Mitigating Subsidence Influences on High Tower Structures Induced by Longwall Mining Based on Foundation Grouting Reinforcement Mechanism
Authors: Zhao Gaobo , Guo Wenbing
Published: September 2022

This paper presents a new approach for mitigating high tower structures slope based on the foundation grouting reinforcement (FGR) mechanism. Three effects of the FGR approach were analyzed: the reinforcement provided by grouting to improve the strength of the foundation, filling gaps around foundation provided by grouting materials to compensate for the subsidence, and adjusting foundation elevation provided by controlling grouting pressure to reduce the slope of the high tower structure body. The FGR mechanism was further developed by the theoretical analysis of foundation vertical deformation under the effect of grouting, and the theoretical equation for calculating the grouting pressure based on the subsidence induced by longwall mining was derived. Finally, the systematic procedure of the FGR approach was presented, and applied to mitigate the slope of a radio signal tower located above the active panel of a Chinese coal mine. The results of this application show that: after implementing the FGR approach, the tower foundation slope decreases as face advances, and is less than its limited deformation.

1. INTRODUCTION

High tower structures refer to structures with large height and relatively small cross section. Taking horizontal load (especially wind load) as the main basis for structural design (Shu et al. 2018), they can be divided into self-supporting tower structure, guyed mast structure, and lattice steel tower structure (Lee and McClure 2007), mainly including high-voltage transmission tower, radio signal tower, and communication tower, as shown in Fig.1. Particularly, in China, there are abundant recoverable coal reserves buried under high-voltage transmission lines in coal mines(Guo and Yong 2011). For examples, in Yanzhou Coal Mine, 21.89 million tons of recoverable coal reserves is under high-voltage transmission lines; the first 1000 kV high-voltage transmission line passes through Sihe Coal Mine, affecting a total coal reserves of 30.78 million tons. It is a technical and engineering problem to mining these coal reserves without affecting the high-voltage transmission lines safety.
Analysis and Modeling of Explosive Fracturing Process in a Transparent Surrogate of Jointed Rock
Authors: Vorobiev O. Yu. , Robey R. E. , Pope J. , Torres S. M. , Hargather M. J.
Published: September 2022

The paper describes analysis and modeling of dynamic experiments with 1-foot-size blocks of a transparent rock substitute, PMMA. During these experiments, 0.3-0.7 grams of explosive was detonated in the middle of a multilayered cubic block in single and double tamped wells. Numerical modeling in combination with the analysis of experimental records helps to understand the mechanism of dynamic fracture and to predict the fracture process for a nontransparent granite block with and without preexisting fractures which will be used at the next stage of this study.

1 INTRODUCTION

In traditional hydraulic fracturing stimulation, the effective conductivity of low permeability rock is increased by generating or activating fractures through injection of pressurized fluid. Methods that use dynamic loading from explosives and propellants potentially extend stimulation to previously unrealized geological resources. In contrast to traditional fracturing methods (e.g. hydraulic fracturing) the stresses in the source region may be significantly larger than the in-situ stress, which helps to create fractures not oriented with the maximum in-situ stress. Fractures initially are generated by the diverging stress wave propagating from the energy release zone. It has been shown in the past that the crack area and final extend depend on the ability of explosive products to flow into the cracks after the wave propagation, e.g., Kutter and Fairhurst (1971); Nilson (1981). The gas flow fracture mechanism has been confirmed in the current work utilizing high - speed schlieren imaging, and Photon Doppler Velocimety (PDV) during recent explosive fracturing experiments with 0.3-0.7 g of high explosive source detonated in prestressed 1-foot PMMA cubic blocks with and without artificial joints (Hamm et al. (2021)). Modeling of these experiments presents a significant computational challenge, as various physical scales need to be resolved to address this problem. Thus two major temporal scales to be resolved are the shock wave propagation scale related to the time required for the shock wave to propagate across the sample and the time required for the explosive products to escape into the cracks generated around the cavity by the outgoing shock wave.
Special Triaxial Experiments on the Fracture Behavior of Hollow Rock Salt Specimens
Authors: Baumgärtel L. , Körner F. , Leuger B.
Published: September 2022

The knowledge of possible fracture formation and propagation in rock salt caverns is an important aspect for their safety-oriented design. During gas withdrawal the internal pressure is reduced and the associated cooling of the gas and thus also of the cavern wall cause a reduction of the principal stresses and can possibly lead to finite fractures at the cavern wall, if the minimal stress component falls below the present gas pressure by a certain stress difference.

The described induced thermal stress reduction is subject of investigations in laboratory tests on hollow cylinders of natural rock salt. For this purpose, the specimen is loaded with a quasi-hydrostatic pressure (axial, circumferential and internal gas pressure) and unloaded in a pressure-controlled manner by reducing axial pressure (corresponding vertical rock pressure) at constant gas pressure in order to mechanically reproduce the expected result of thermal cooling. The unloading of the hollow cylinder by one stress component shows a so-called infiltration fracture at mechanically absolute compressive stresses in all three main stress directions due to the existing constant inner pressure and the mechanical loading situation.

Four specific laboratory tests and their impact on the rock mechanical design of a gas storage cavern are discussed and compared in this paper. The measured stress differences are investigated with respect to the influences of test and specimen parameters under stress conditions during gas withdrawal in rock salt caverns. Ultrasonic measurements provide a first basis for estimating possible stress differences for these four tests. The initial pressure as well as the selected pressure reduction rates do not correlate with the level of the stress difference.

1 INTRODUCTION

The consumption of natural gas in Germany in 2020 was 86.50 billions m3, which represented 25.74 % of primary energy consumption (BP, 2021). Rock salt caverns are suitable for storing energy sources such as natural gas or mineral oil. At the end of 2020, the working natural gas volume, which could be stored in underground caverns in Germany, totaled 15.19 billions m3 (Vn, Conditions: 273.15 K, 1.01325 bar) (EID, 2021) respectively 17.50 % of the natural gas consumption without requiring large areas above ground. Energy storage caverns thus have the potential to store and hold large amounts of primary energy efficiently.
Using 3D Printing to Improve Fracture Conductivity Experiments
Authors: Obi C. E.
Published: September 2022

Using 3D printing technology, fracture conductivity test samples are replicated to overcome the challenge of sample inconsistency. The fracture surface topography data is acquired with three different techniques. A laser profilometer, Computed Tomography (CT) scans, and photogrammetry. The developed and printed 3D models are used to make Fracture conductivity test sample replicates of cement/shale and cement/sand mixtures. We validate the replicability of these samples based on visualized scans of the fracture surface of the original rock samples, 3D molds, and concrete replicates. We also used the joint roughness coefficient (JRC) to quantify and validate the replicability of these samples. The Meremac and Austin chalk formations are the source of the original Fracture conductivity samples used. The replicability ranged from 78% to 99% for the Meremac core using CT scans and Digital Light Processing (DLP) printer. The replicability range was from 75% to 93% for the Austin chalk core using the photogrammetry technique and Fused Deposition Modeling (FDM) printer. The final check for replicability and validation of methodology used is with the fracture conductivity results. The cement/sand mixture replicates C5 and C7 that cured for relatively similar conditions produced similar fracture conductivity results. The cement/sand mixture replicates C5 and C7 which were cured for relatively similar conditions produced very similar fracture conductivity results. We also observed that the mechanical properties and surface topography of rock samples dominate fracture conductivity behavior significantly at closure stress within the peak strength of the rock sample. This study provides a methodology for achieving consistency of fracture conductivity test samples.

1. INTRODUCTION

3D printing has gained traction in recent years as a very applicable technology to many fields such as biological medicine, education, aviation, engineering, and most recently rock mechanics. The true investigation of the mechanical and related behavior of rock samples by conventional methods over the years has been limited by the replicability of these tests which is mainly due to significant heterogeneity (Jiang et al. 2016).
Sand Production Prediction of Depleted-Reservoir Underground Gas Storage – An Experimental Study
Authors: Ma Lijun , Dong Jingnan , Wang Jinzhong , Zhang Yuda , Li Guotao , Li Jingcui
Published: September 2022

Depleted-reservoirs underground gas storage are playing an increasingly important role in seasonal demanding of natural gas, ensuring stable gas supply and strategic reserves. However, due to the physical properties of the reservoir, the problem of sand production in the wellbore cannot be ignored. This research is based on the high-pressure sand production experiment simulation device system. Using the in-situ core, through experiments to simulate the sand production of the high-pressure gas storage reservoir, the sand production is systematically and quantitatively studied. The experimental results show that there are two key pressures for sand production caused by the destruction of the inner wall of the wellbore, namely, the equivalent initial sand production pressure (critical confining pressure) and the strength of the thick-walled barrel (collapse pressure). After the confining pressure exceeds the equivalent initial sand production pressure, the core begins to produce sand, and the sand has a relatively smaller particle size. The sand production is found to increases linearly with the equivalent confining pressure. After the confining pressure exceeds the strength of the thick-walled barrel, the core body is damaged and a large amount of sand is produced. The sand body has a relatively large particle size. The particle size distribution indicates the increase of sand particle size during the sand production of the gas storage well. This experiment provides guidance for the operation and completion of gas storage in depleted oil reservoirs, and also provides a quantitatively experimental method for the study of wellbore sand production.

1. INTRODUCTION

In recent years, with the increasing demand for natural gas, the stable supply of natural gas has attracted more and more attention. Therefore, gas storage as a means of peak regulation has developed rapidly in a short period of time in China. As a safe, efficient and convenient gas storage place, the stable operation of gas storage needs a higher technical level as a support (Wei et al., 2015;Dong et al., 2020). Compared with regular gas production wells, there are two main reasons why gas storage wells are more prone to sand production:

• The service life of gas storage wellbore is longer than that of production wells, generally about 30 to 50 years. The long-term stress alternation will weaken the cementation and connection between rock particles, resulting in instability of wellbore wall and the damage of the formation.

• When the supply and demand are unbalanced due to seasonal changes, the gas storage needs to meet the operating conditions of high throughput and large discharge. Under the action of the highspeed fluid dragging force, the wellbore has a higher probability of sand production. (Wei et al., 2019; Li et al., 2005; Liu et al., 2021)
Experimental Study on Fracture Propagation Morphology of Deviated Well in Tight Reservoir
Authors: Cui Zhuang , Hou Bing , Zeng Yue , Jiang Kaining , Zhuang Li , Elsworth Derek
Published: September 2022

The highly efficient production method of tight reservoir in complex fault block reservoir is large slope cluster well volume fracturing. The characteristics of fracture initiation and propagation in deviated Wells of tight reservoirs under different stress states and well trajectory parameters have not been clarified yet. Based on the laboratory physical simulation test of true triaxial hydraulic fracturing, this paper studies the law of fracture propagation caused by various factors such as horizontal stress difference, well inclination Angle and azimuth Angle. The experimental results show that under low horizontal stress difference coefficient, spiral perforation is beneficial to the initiation of multi-hole fracture, but the fracture near the wellbore is seriously distorted. Directional perforation under high horizontal stress difference coefficient is favorable for hole initiation, forming complex fractures dominated by single smooth fractures and supplemented by secondary fractures. 20° or 80° well oblique fracture is smooth fracture, fracture surface distortion degree is small, perforation orifice initiation point is less; When the inclination Angle is 40° and 60°, the perforating hole has more fracture initiation points, high fracture extension pressure and high fracture plane distortion. When the azimuth Angle is less than 80°, it is beneficial to add sand on site.

1. INTRODUCTION

China’s unconventional oil and gas reserves are rank among the top in the world, unconventional oil and gas are mainly tight oil and gas (Zhen et al., 2019; Sun et al., 2019). Due to its low porosity and low permeability characteristics, tight reservoirs are mainly stimulated by horizontal well volume fracturing (Zhu et al., 2019; Hou et al., 2021). However, the tight sand-mudstone reservoirs in complex fault-block reservoirs have many vertical layers, and the scale of the plane sand bodies varies greatly. The most effective mining method for vertical multi-layer tight sand-mudstone reservoirs is volume fracturing with high-angle cluster wells. The mechanism of hydraulic fracture initiation and propagation of perforated fracturing in highly deviated wells is unknown, and it is difficult to add sand during on-site construction, resulting in high construction pressure and small reservoir stimulation volume. Therefore, it is of great significance to study the fracture propagation pattern and construction influencing factors of perforated hydraulic fracturing in highly deviated wells for reservoir stimulation.
Investigation on Diversion Acidizing for Fractured Carbonate Reservoir: An Case Study from KOA Oilfield in Kazakhstan
Authors: He Jiayuan , Li Xiaolong , Li Feng , Mou Jianye , Wang Haibo , Zhang Lufeng
Published: September 2022

Fractured carbonate reservoir with thick formation and heterogeneous intervals, widely distributed in Kazakhstan, are the significant gas exploration and exploitation area and have a profitable development prospect. For such reservoirs, diversion acidizing is an indispensable technology to unlock the production potential and enable commercial productivity. Hence, this paper takes well X located in KOA oilfield as an example to perform diversion acidizing design by using an independently-developed acidizing software, aiming to provide an insight for efficient development of oilfield in Kazakhstan. Moreover, the investigation on geological reservoir and well logging mainly achieved the following findings: 1. The effective formation thickness and main production layer are 140.1m and 34.8m, respectively. 2. The average oil saturation, permeability, and porosity of main production layer are 81.1%, 0.3md, and 10.6%. 3. Local natural fractures are developed in the formation. According to the formation properties, acidizing numerical simulation shows that diversion acidizing technology can achieve the uniform distribution of acid in the entire production interval. The post-stimulation production of well X reaches 150 ton per day, marking a breakthrough in exploration and development for fractured carbonate reservoir of KOA oilfield.

1. INTRODUCTION

It’s well known that more than 60% of the world’s oil and 40% of the world’s gas reserves are held in carbonate reservoirs, which are the significant domains in oil and gas exploration and development. Matrix acidizing in carbonate reservoirs is one of the most successful methods to bypass the formation damage from the near-wellbore region and decrease the skin. Furthermore, acid- etched wormhole is a typical phenomenon in carbonate matrix acidizing, which determines the acidizing effects. Hence, numerous scholars have performed investigation by numerical simulation and core-flooding experiments [1-5]

For treatments on vertical well with thick formations and strong heterogeneity, acidizing effects mainly depends on the diversion efficiency of acid over the total production interval. Hence, diverting acid systems were widely used in the field application to enhance the stimulation effects. In general, it can be classified into two types: polymer-based and viscoelastic surfactant-based (VES). However, the former usually cause damage after flowback due to the residue of polymer, while the latter will break when it encounters the hydrocarbon, and flowback without any damage to formation. Moreover, the superiority of VES also has been proved by field application [6-11].
Omori Decay of Hydraulic Fracture Induced Seismicity
Authors: Maxwell S. C.
Published: September 2022

An important consideration for managing induced seismicity is the characterization of seismicity following operational changes made in an attempt to mitigate seismicity. When operational changes are made to control seismicity, ongoing activity associated with earlier activated structure can continue to generate seismicity regardless of the current operational status. It is therefore important to distinguish between ongoing ‘relaxation’ activity associated with a return to geomechanical equilibrium versus continued seismicity associated with ongoing activation and additional perturbation of the seismically active fault. In this paper, three case studies are described demonstrating that seismicity follows an Omori decay, consistent with earthquake aftershock sequences. The Omori decay characteristics thereby provides an important framework to distinguish between fault ‘relaxation’ seismicity and additional activation.

1. INTRODUCTION

Induced seismicity is an important consideration for many industrial activities, including mining, reservoir impoundment, geothermal extraction, petroleum production and subsurface injections. Unconventional reservoir development has recently resulted in an increase in reported cases of induced seismicity (see for example, Schultz et al., 2020), particularly in regions of North America that were previously seismically quiescent. Specifically, hydraulic fracture stimulations and subsurface disposal of produced connate saltwater have both resulted in anomalous seismicity in several regions. Injection induced seismicity results from increased subsurface pore pressure reducing the effective normal clamping stress on preferentially oriented, pre-existing faults (Ellsworth, 2013), triggering slip and associated seismicity along these reactivated faults. In response, regulatory agencies and industry have developed operational guidelines to ensure safe operations, and industry best practices have emerged to mitigate induced seismicity.

Seismicity patterns associated with hydraulic fracturing and saltwater disposal (SWD) typically have distinct patterns (Rubenstein and Mahani, 2015). SWD injections are generally located in the general vicinity of unconventional wells so that water coproduced with hydrocarbons can be safely stored in deep, permeable intervals. SWD wells normally operate for an extended time period resulting in pressure plumes within the target intervals which in some case encounter critically stressed faults that are activated resulting in associated seismicity. Seismicity can occur several kilometers from the causal injection well with variable onset times relative to the start of injection. Cumulative injection effects from interaction of different injection wells can further extend the spatial reach and timing of fault activation and associated seismicity. In general, SWD induced seismicity tends to be relative long lasting, since elevated pressure can persist after injections stop and may appear as a diffuse regional effect around groups of injection wells.
Prediction of Formation Properties Based on Drilling Data of Wells at Utah FORGE Site Using Machine Learning
Authors: Phelan Zachariah , Xing Pengju , Panja Palash , Moore Joseph , McLennan John
Published: September 2022

This study shows how predictions of formation properties can be obtained from bit-rock interactions without additional operational costs using machine learning models. The models are based on drilling data sets acquired from a highly deviated well at the Utah FORGE (Frontier Observatory for Research in Geothermal Energy) site in south-central Utah, U.S.A. The drilling data is preprocessed by removing outliers that are out of the meaningful range. The data shows that pump pressure and bit RPM are highly correlated with rate of penetration (ROP). Random forest model yields the best performance for ROP prediction with top drive torque as the most important feature. Pump pressure and downhole torque are clustering in different groups for different sections of the inclined well. Successful prediction of confined compressive strength (CCS) is also achieved through random forest model. The model was trained with CCS obtained from sonic logs and calibrated by core test data from a nearby offset well. The well-trained machine learning model can be used to predict CCS of other nearby geothermal wells with drilling data only.

1. INTRODUCTION

Understanding formation properties is important in drilling, completion, and production for geothermal reservoir development. Wireline logs, such as sonic logs, can be used to assess formation properties in oil/gas and geothermal reservoirs. However, logging after drilling can be expensive and is rarely conducted in geothermal wells. Alternatively, predictions of formation properties can be acquired from bit-rock interactions without additional operational costs using machine learning models (Jamshidi et al., 2013; Mahmoud et al., 2021). Drilling and logging contribute a significant portion of cost for Enhanced Geothermal System (EGS). Reliable estimation of formation properties without after-drilling wireline logging would reduce the cost EGS and enhance understanding of geothermal reservoirs.

The drilling data sets investigated were acquired from the Utah FORGE (Frontier Observatory for Research in Geothermal Energy) site in south-central Utah, U.S.A. Utah FORGE is a geothermal field laboratory intended to develop and test tools and technologies required for creating, sustaining, and monitoring EGS reservoirs. Since 2017, five vertical monitoring wells, the deepest to 9500 ft, and a highly deviated injection well, 16A(78)-32, have been drilled. In this study, we focus on the drilling data from the injection well, 16A(78)-32, which is highly deviated at a tangent of 65° to the vertical.
A New Rate of Penetration Prediction Model Based on the XGBoost and Explanation Method
Authors: Chen Gang , Yang Henglin , Yang Dianru , Guo Kaijie , Feng Ming , Wang Yuan , Fu Li , Jiang Hailong
Published: September 2022

Machine Learning (ML) studies are carried on in the hydrocarbon exploration and production. The Rate of penetration (ROP) is one of the investigations related to ML. Previous studies could not fit the ultra-deep well and most of machine learning ROP prediction are black box models lacking of enough explanation. ML ROP prediction need more accurate ROP models.

In this paper a new and reliable calculation method of ROP prediction for well drilling is proposed by Extreme Gradient Boosting (XGBoost) algorithm. It is compared with Random Forest Regression algorithm on ROP prediction models. According to the importance ranking, rotating torque is the most impacted factor in this dataset. The conclusion is that ROP prediction model based on XGBoost has smaller prediction mean square error and shows higher efficiency than Random Forest, showing the superiority of the XGBoost. Considering the SHAP value ranking, torque and RPM are the most important features and they are positively impacted ROP in well drilling

ROP model established by XGBoost machine learning method can make reasonable use of drilling parameters and provide reference for well drilling optimization. ROP model optimization derived from XGBoost can extremely reduce the expenses by reducing the drilling time.

1. INTRODUCTION

Hydrocarbons, including oil and gas, are often stored in sedimentary rocks in deep formations (Yang et al. 2017). wells are drilled both onshore and offshore with more drilling cost (Ma, Ping, and Jian 2016). One of the most important factors is the Rate of Penetration (ROP) which affect the drilling efficiency and cost (Eskandarian, Bahrami, and Kazemi 2017).

It is difficult to enhance ROP for ultra-deep well drilling as the well depth is increased. Increasing ROP is an approach of drilling optimization and cutting costs by reducing the drilling time. Existing ML models considering less parameters and data points cannot meet current ultra-deep well developing demands. In order to cut costs and increase the amount of drilling operations, finding out the relationship between the ROP and parameters is very important. Make it clear that variables and their relationships with ROP are the main factors which is helpful to promote ROP.

Traditionally instantaneous ROP refers to the drilling footage in time unit, like the data used in this work. And some researchers want to estabilish some models of ROP using drilling parameters. These models can be used to make real time drilling optimization by equipment design guidance and drilling parameters selection.
Geophysical Response of Saturated Rock Joints During Shear
Authors: Han K. , Pyrak-Nolte L. J. , Bobet A.
Published: September 2022

Monitoring the frictional behavior of rock discontinuities is essential for the identification of potential natural hazards caused by mechanical instability. Active seismic monitoring of changes in transmitted and/or reflected compressional (P) and shear (S) waves has been used as a non-destructive method to assess the degree of damage inside rock and to monitor slip along a discontinuity. The objective of this study is to explore the geophysical response of a saturated rock joint undergoing shear. Laboratory shear tests are conducted on prismatic Indiana limestone specimens. Induced tension fractures resulted in specimens composed of two blocks (152.4 mm × 127.0 mm × 50.8 mm) with rough contact surfaces. Direct shear experiments were performed inside a metal confinement chamber under an effective normal stress of 2 MPa on water-saturated specimens. During the experiments, the chamber pressure, the total normal load, the shear load and the slip displacement were monitored. During the tests, continuous pulses of P-and S-waves were transmitted through the specimen and the amplitudes of the transmitted and reflected waves were recorded. The paper provides results of the mechanical and geophysical response of saturated joints and compares them with those obtained from similar, but dry, joints. For dry joints, both transmitted and reflected P- and S-waves show a distinct peak wave amplitude prior to shear failure. However, for saturated joints, a distinct peak in amplitude is only observed in both transmitted and reflected S-waves. Transmitted and reflected P-waves, propagated through saturated rock, displayed a continuous decrease and increase in amplitude, respectively, but had a sudden change in the rate of amplitude change that can be taken as a seismic precursor to joint shear failure.

1. INTRODUCTION

The stability of a rock mass depends on the presence of pre-existing discontinuities, such as fractures, joints, and faults, and on their response to changes in stress, pressure, fluids and geochemical conditions. The ability to evaluate and assess the mechanical stability and frictional behavior of rock joints has been considered a longstanding goal for the design of geotechnical structures and prevent potential natural hazards. Active seismic monitoring is a non-destructive method to probe inside rock to detect fractures and to monitor their evolution with changes in stress, as well as the degree of damage over time. Changes in transmitted and reflected compressional (P) and shear (S) waves have been used as seismic precursors to shear failure of rock joints (Hedayat et al., 2014a, 2014b, 2018; El Fil et al., 2019, 2021). Recent laboratory experiments have shown that converted waves (S to P & P to S) may be used to track the mechanical behavior of rock joints and detect precursors to shear failure (Nakagawa et al., 2000; El Fil et al., 2021, Gheibi et al, 2021).
Effect of Mineralogy, Pore Geometry, and Fluid Type on the Elastic Properties of the Bakken Formation
Authors: Malki M. L. , Rasouli V. , Saberi M. R. , Mellal I. , Ozotta O. , Sennaoui B. , Chellal H.
Published: September 2022

The velocity-porosity relationship depends on many factors such as grain shape, sorting, chemical composition, and diagenesis processes. Fluids always occupy the pore space and pore shape plays a significant role in rock stiffness. The Bakken Petroleum System (BPS) in the Williston Basin, North Dakota, consists of the Bakken formation and three folks reservoirs. Bakken formation is divided into three members: Upper Bakken (UB), Lower Bakken (LB), and Middle Bakken (MB), while Three Forks (TF) formation divided into five different units. Although clastic formation’s pores spaces are homogeneous, carbonates present pores heterogeneity which makes their elastic properties estimation complex. This heterogeneity is represented by different pore shapes captured from thin sections, where the aspect ratio (α) defines multiple pore types such as cracks, intergranular, and moldic pores. Furthermore, the pore filling material is a mixture of gas, oil, water, and kerogen in organic-rich shale. This study aims to analyze the effect of mineral composition, pore shape, and fluid type on rock properties of the BPS using various rock physics models. Our results showed that both compressional and shear velocities decreased for all fluid types. We also observed that filling pores with different fluids affect the elastic properties differently, based on their pore geometry, porosity, and lithology.

1. INTRODUCTION

The Bakken Petroleum System (BPS) of the Williston Basin is at the forefront of this new oil boom and contains mature source rocks (the Bakken Shales) and proven reservoirs (Middle Bakken and Three Forks). The BPS is composed of three different formations in ascending order: Three Forks, Bakken, and Lodgepole (LeFever & LeFever, 2005; Nordeng et al., 2010). This would include the upper and parts of the middle Three Forks (Bazzell, 2014). The Bakken formation is divided into nine lithofacies where the upper (UB) and lower Bakken (LB) members are formed from the same lithofacies which is a thinly laminated black organic rich mudstone, while the other eight were identified within the middle Bakken (MB) member (Smith & Bustin, 2000). The UB and LB are mainly composed of illite, silt and clay-size quartz, and orthoclase feldspar. The MB is mainly composed of limestone and sandstone (Smith & Bustin, 2000). The middle Bakken member is characterized by a bigger grain size (sandstone and oolite) and more significant calcite percentages. The Three Forks (TF) consists of mudstones, silty to sandy dolostones, and anhydrites (Gutierrez & Sonnenberg, 2013). The TF Formation is formally divided into five units in which Units 4 and 5 are the primary producing units (Baah, 2015).
Effect of CO2 on Mineralogy, Fluid, and Elastic Properties in Middle Bakken Formation Using Rock Physics Modeling
Authors: Malki M. L. , Rasouli V. , Saberi M. R. , Sennaoui B. , Ozotta O. , Chellal H.
Published: September 2022

The Bakken Petroleum System in the Williston Basin consists of three main members: Upper Bakken Shale (UB), Middle Bakken (MB), and Lower Bakken Shale (LB). The Middle Bakken is a calcareous siltstone and fine-grained sandstone which is a proven reservoir within the Williston Basin. Injection of Supercritical CO2 (ScCO2) may cause changes in the elastic behavior of the rock and therefore may reactivate the fault in the Bakken Formation. This research aims at studying the changes in the MB mechanical properties when flooded with ScCO2 for 30 days, and to quantify the physical and chemical effects of ScCO2 saturation on the mineralogy composition and fluid properties of the formation. This will be achieved using different rock physics models before and after ScCO2 injection. Two different samples were taken from MB clastic and carbonate portions of well 24123 in McKenzie County. X-Ray Diffraction (XRD) analysis was applied on two samples pre-and post-CO2 saturation. The results of post-CO2 modeling resulted in a decrease of densities and elastic properties of both lithologies at different rates. Only change in mineralogy and fluid properties were considered for the post-CO2 injection modeling.

1. INTRODUCTION

The Bakken formation consists of three main members: Upper Bakken Shale (UB), Middle Bakken (MB), and Lower Bakken Shale (LB). The UB and LB shales are proven source rocks with a similar lithology and are composed of an organic-rich, siliceous, pyritic black shale. The UB covers a larger areal extent than the LB within the Williston Basin. The MB member is a calcareous siltstone and fine-grained sandstone which is aproven reservoir within the Williston Basin (Lefever & Helms, 2006; Nordeng et al., 2010; Pitman et al., 2012). Despite the enormous hydrocarbon reserves within the Bakken formation, the Bakken is classified as unconventional oil-bearing reservoir due to its ultra-low porosity and permeability characteristic (Sorensen et al., 2008). The variation of MB lithology across the Williston Basin from mainly clastic to carbonate led to divide it intoseven distinct lithofacies (Lefever & Helms, 2006; LeFever & LeFever, 2005; Ozotta et al., 2021c).