bluuebunny/crossref_metadata_2025 huggingface dataset:
Detecting and Characterizing Fluid Leakage Through Wellbore Flaws Using Fiber-Optic Distributed Acoustic Sensing
Authors: Anwar Ishtiaque , Carey Bill , Johnson Paul , Donahue Carly
Published: September 2022

A primary concern associated with the utilization of subsurface systems is that there may be pathways for fluid leakage from the resource storage, or disposal reservoir. Leakage of any fluid can contaminate groundwater, cause geo-environmental pollution, generate hazardous surface conditions, and potentially compromise the functionality of the subsurface system. A low-cost, innovative technique to detect and characterize fracture leakage that is functional over many years is needed for applications such as geothermal reservoirs, CO2 sequestration wells, deep borehole storage of nuclear waste, and strategic petroleum reserve caverns. In this experimental study, we investigate the use of fiber-optic distributed acoustic sensing (DAS) to measure dynamic strain changes caused by acoustic signals induced by fluid flow with an eventual goal of developing instrumentation and analytical techniques to detect and characterize the movement of fluids through leaky wellbores. In the first phase of the experiments reported here, we conducted fluid flow tests in a porous medium as an analog to a fracture filled with comminuted material. The measured effective permeability is then compared with the signals generated by the fiber-optic cable. The study indicated that acoustic signals generated from fluid flow through porous media could be effectively captured by the fiber-optic cable DAS technology.

1. INTRODUCTION

Wellbores are used for gaining access to various subsurface systems such as underground fluid reserve (Miyazaki, 2009), CO2 sequestration (Watson and Bachu, 2008; Zhang and Bachu, 2011), geothermal energy development (Shadravan, Ghasemi, & Alfi, 2015), waste disposal, oil and gas exploration (Davies et al., 2014), etc. A primary concern associated with the utilization of subsurface systems is that there may be pathways for fluid leakage from the resource storage, or disposal reservoir through the wellbore flaws. Leakage of any fluid from a leaky wellbore can contaminate groundwater, cause geo-environmental pollution (Davies et al., 2014; Ingraffea, Wells, Santoro, & Shonkoff, 2014; Jackson, 2014), generate hazardous surface conditions, and potentially compromise the functionality of the subsurface system (Gasda, Celia, Wang, & Duguid, 2013). Researchers has identified different potential leakage pathways, including fractures in the cement or micro annuli from de-bonding at the cement-casing or cement-formation interface (Celia, Bachu, Nordbotten, Gasda, & Dahle, 2005; Theresa L Watson & Bachu, 2009) or the casing corrosion product (Anwar, Chojnicki, Bettin, Taha, & Stormont, 2019; Beltrán-Jiménez et al., 2021).
Microbially Induced Calcite Precipitation for Sealing Anhydrite Fractures with Gouges
Authors: Sang Guijie , Lunn Rebecca J. , Minto James M. , El Mountassir Grainne
Published: September 2022

Caprock formation forms a natural barrier for geological storage of CO2, nuclear wastes, and hydrocarbon resources. Fault and natural/artificial fractures that crosscut the storage systems represent potential leakage pathways. Sealing of caprock fractures/faults is of great importance to ensure its long-term sealing integrity. In this study, we conduct microbial-induced-calcite-precipitation (MICP) experiments for sealing anhydrite fractures (artificially cut) with gouges. MICP involves a bio-chemical reaction for calcite precipitation using ureolytic microorganism – Sporosarcina pasteurii. The precipitated calcite, which occurs initially from finer pores to larger pores, induces a 10.7% decrease of porosity inside the fracture after the 1st 12 cycles of MICP treatment. After 18-21 cycles of MICP treatment, the fracture permeability of the two fractured core samples effectively decreases by 2-3 orders of magnitude. Our study also indicates that the MICP sealing efficiency could be improved by lowering the injection rate, optimizing fluid chemistry for a better bacteria retention inside the fracture. The study provides a baseline for using MICP technique to seal anhydrite fractures.

1. INTRODUCTION

Caprocks with very low permeability and ultra-fine pores, consisting of massive consolidated clay-rich sediments (shale) and evaporites such as anhydrite and halite, create natural barriers for hydrocarbon resources, CO2 sequestration, disposal of radioactive wastes, hydrogen storage, etc. However, natural fractures/faults or injection-induced fractures that crosscut subsurface storage systems (Figure 1) could provide potential leakage pathways 1,2. Seeking an efficient technique to seal caprock fractures is crucial.

Microbially induced calcite precipitation (MICP) has been considered as an environmentally sustainable grouting technology for soil improvement 3,4, fracture sealing 5-8, removal of contaminants in ground water 9,10, among others. It has several advantages over conventional cement grouting methods, including (1) the low viscosity of the bacterial solution and cementing solution (urea and calcium chloride), which allows to penetrate with relatively low pumping power; (2) micron-size microbes for sealing fine pores and fractures; and (3) potential low carbon footprint. MICP is a naturally occurring metabolic phenomenon that can be found in soil and marine environment 11. This bio-inspired grouting technique also have advantages over conventional chemical grouting method (such as acrylates, acrylamides, and polyurethanes, etc.) due to its lower cost of the raw materials and less toxicity to the environment 12.
Comparison of Measured and Modeled Casing Deformations of a Test Well in a Longwall Abutment Pillar
Authors: Zhang Peter , Su Daniel , Kim Bo Hyun , Midler Evan
Published: September 2022

Shale gas wells in longwall abutment pillars are subjected to longwall-induced ground movements, and their stability has been a safety concern for both the mining and gas industries. To understand the impact of longwall mining on the stability of gas wells in longwall abutment pillars, a test well was drilled in a longwall pillar between two adjacent longwall panels to measure longwall-induced deformations in the casing as the two longwall panels on either side were retreated. The longwall panels extracted the Pittsburgh coal seam under a depth of 177 m and were developed with a 3-entry system with 52-m-wide chain pillars and 366-m-wide panels. Casing deformations induced by longwall mining were measured by 60-arm caliper logging and observed with a downhole camera. A FLAC3D model was set up to predict longwall-induced stresses and deformations in the test well casing. The model was validated with measured surface subsidence and longwall-induced pillar pressure. Both measurements and numerical modeling showed that the casing deformed because of relative displacements along distinct weak rock interfaces. Casing deformations at the weak claystone interfaces were found to be confined within the claystone layers. In this case, casing deformations occurred at the same locations with both first and second panel mining. Second panel mining caused additional deformations as the relative horizontal displacements along the interfaces shifted in the opposite direction. The FLAC3D model proved to be capable of predicting the locations and magnitudes of casing deformations with reasonable accuracy.

1. INTRODUCTION

When gas wells penetrate through coal seams in the vicinity of longwall gob, they are subjected to longwall-induced subsurface movements, and casing deformations could occur at certain locations. Longwall-induced deformations in the casing could diminish casing integrity and allow high pressure gas to leak into the mine, potentially causing fires or explosions. Over the past decade, over 1,500 shale gas wells have been drilled in the coal reserves of the Pittsburgh coal seam. The influence of longwall mining on these shale gas wells will have to be evaluated as future longwall panels are mined. Therefore, it is important to understand how longwall-induced subsurface movements and stresses affect gas well casings and to develop engineering guidelines to safely mine-by these shale gas wells.
Application of Machine Learning to the Estimation of Intact Rock Strength from Core Logging Data: A Case Study at the Newcrest Cadia East Mine
Authors: Thielsen C. , Furtney J. K. , Valencia M. E. , Pierce M. , Orrego C. , Stonestreet P. , Tennant D.
Published: September 2022

Understanding the spatial variation in intact and defected rock strength is critical to geomechanical mine design. At the Newcrest Cadia East mine, systematic point load testing (PLT) was used to measure the strength of intact rock and individual defects (e.g., veins) at regular closely spaced intervals along several boreholes. The systematic PLT data collection covers only 1.3% of the 590 km of hole logged at Cadia East. A procedure was developed to homogenize the available geotechnical and geological logging data, infill missing values, and encode raw data into engineered features for use in a machine learning model. A random forest classifier was applied to predict point load index (Is50) from core logging data where tests were not performed. The random forest model predicts the rolling average Is50 value within 1 MPa 48% of the time. The model gives insights into which core logging quantities have the strongest controls on rock strength and provides the basis for developing more detailed geospatial models of intact and defected rock strength.

1. INTRODUCTION

Understanding the spatial variation in intact and defected rock strength is critical to many aspects of geotechnical mine design, including predicting caveability, and excavation and ground support design. At the Newcrest Cadia East mine, there are 590 kilometers of logged core. Having measurements or estimates of point load index at every meter along each core enables the production of geospatial models of rock strength for use in predictive models. While point load testing is relatively quick and inexpensive, performing point load tests at a tight spacing along 590 km of core would be costly and impractical (see ASTM, 2016 and Franklin, 1985).

Systematic point load testing (PLT) was used to measure rock strength based on guidelines provided by Pierce Engineering (Pierce, 2019; Pierce, 2020). It offers a means to quantify the variability in both intact and structural tensile strength over the largest possible volume with a minimum of bias. It involves performing diametral point load tests at strict 1-2m intervals along core from multiple boreholes and taking care to recognize both structure (which requires careful alignment of the test platens in order to test the strength of the structure itself) and pre-disturbed sections of core (which should be registered as having zero tensile strength, unless marked as an induced break). Axial tests are also conducted periodically to check for anisotropy. The testing is carried out in accordance with ISRM standards (ISRM, 1989), with careful attention paid to the maintenance of the radius of curvature on testing points to ensure a high proportion of valid tests.
Transient Inverse Analyses of Overcoring Data for Improved Stress Estimation
Authors: Valley B.
Published: September 2022

Overcoring is a common technique for measuring stresses in mining projects. Knowledge of the in-situ stress state is essential to ensure the stability of underground infrastructures as well as to assess the induced microseismic risk associated with deep mining operations. There are different types of overcoring probes. Some are bonded in the pilot hole with an epoxy resin and allow for 3D stress measurement (e.g. CSIRO-HI), and others are based solely on a mechanical coupling of the probe, but are limited to biaxial stress measurement (e.g. USBM). The need to glue the probe to obtain a 3D measurement limits the applicability of this technique to short boreholes because it is technically difficult to glue probes in deep boreholes. In any case, traditionally the data analysis is done only based on the final deformation obtained after overcoring. In this paper we propose to use the transient deformation response during overcoring to: (1) allow to evaluate the 3D stress field from a single biaxial overcoring measurement, and (2) add a quality control component by reproducing the entire overcoring response. The general principle of our approach is to simulate the transient response of overcoring by numerical elastic simulation. The principle of superposition is used to derive the responses for any set of parameters from a limited number of basic models and thus allows to limit the total number of model runs. Such approach allows for a systematic inversion procedure to be applied for determining the optimal parameter sets that best capture the transient response of the overcoring. This allows the estimation of a 3D stress tensor from biaxial measurements. It also allows to have a quality control on the measurements evaluating the quality of the fit between the model and the data. In this paper, we evaluate the robustness of the proposed approach by performing a systematic sensitivity analysis on the stress estimation from the inversion of transient overcoring data. We demonstrate the advantages of the approach but also its limitations. The preliminary results obtained in this study suggest that the transient overcoring response contains sufficient information for constraining efficiently the 3D stress tensor. The inversion must be performed using multiple starting point, and the mode of the obtained calibrated parameters is in close adequacy with expected values, while some outliers can be present in the calibrated set. Further work is required for confirming these encouraging results by testing of broader range of stress configurations and applying the method to actual field measurements.
Results from Benchmark Testing of Zonal Isolation Borehole Completions for Multi-Stage EGS Stimulation in the Bedretto Underground Rock Laboratory in Switzerland
Authors: Meier P. M. , Serbeto F. , Christe F. , Alcolea A. , Castilla R. , Bethmann F. , Dyer B.
Published: September 2022

Multi-stage stimulation concepts are considered the most promising approach for a breakthrough in EGS technology. Multi-stage stimulation enables a reduction of seismic risks and an increase of circulation flow rates through the reservoir and thus an increase of energy production. However, there is a debate about the best technology for zonal isolation. Some experts favor cementing a liner in the borehole and fracturing through perforations or sliding sleeves in order to obtain a hydraulic connection to the reservoir rock whilst others prefer open-hole packer solutions in order to avoid the possibility that cement could block access to fractures that are favorably oriented for shear stimulation. We have performed a benchmark in-situ test in the Bedretto underground rock laboratory in the Swiss Alps in granite at a depth below surface of 1100 m. The results show that multi-stage stimulation works well with both zonal isolation methods. Both methods were able to hydraulically connect the reservoir well to the borehole. This is good news for reservoir and drilling engineers as the choice between the two methods can now focus on drilling and completion aspects such as wellbore stability, completion deployment, reservoir temperature and completion system costs.

1. INTRODUCTION

Multi-stage stimulation concepts are considered the most promising approach for a breakthrough in EGS technology. Multi-stage stimulation enables a reduction of seismic risks and an increase of circulation flow rates through the reservoir and thus an increase of energy production (Meier et al., 2015). Therefore, a number of innovative projects throughout the world aim to demonstrate the multi-stage stimulation concept (e.g. FORGE, Utah; Haute-Sorne in Switzerland).

However, there is a debate about which is the best technology for zonal isolation (e.g. Walters et al. 2012; Guinot and Meier, 2019; Glauser et al., 2013; Gradl, 2018). Some experts favor cementing a liner in the borehole and fracturing through perforations or sliding sleeves in order to obtain a hydraulic connection to the reservoir rock. In contrast others prefer open-hole packer solutions in order to avoid the possibility that cement could block access to fractures that are favorably oriented for shear stimulation.
Characterization of Natural Discontinuities in Tight Sandstone and Gas Shale Unconventional Reservoir Rocks of Thrace Region in Turkey with DFN Method
Authors: Celen F. , Develi K.
Published: September 2022

The gas shales of the Mezardere formation and the tight sandstones of the Teslimköy unit in this formation in Thrace region have been considered for many years to be some of the remarkable sources of hydrocarbon production from unconventional reservoir rocks in Turkey. Design of a hydraulic fracturing (HF) operation is the only way for oil and gas recovery from such unconventional reservoirs. As hydraulic fractures induced by HF operations are likely to interact with natural ones, geometrical characterization of natural discontinuities gains importance to be able to build a more realistic reservoir model including structural geological components before an operation. This may reduce operational costs as well as increase production efficiency. Discrete fracture network (DFN) modeling of natural discontinuities (e.g., fractures, joints, faults, bedding, planes etc.) is quite a strong tool for this kind of purpose. In this study, preliminary results of an ongoing study on the development of DFN models in FracMan from outcrop data of natural discontinuities of tight sandstone and gas shale unconventional reservoir rocks of Thrace region are presented. These models can serve as unique database in reservoir modeling prior to potential HF operations probable to be performed in Thrace region in the future.

1. INTRODUCTION

Unconventional reservoirs are becoming more important in natural gas and petroleum industry in Turkey as well as in the world. Especially, Thrace Region, in NW Turkey, has a big potential to produce oil-gas from unconventional reservoirs. While Hamitabat Formation and Ceylan Formation have been defined as host rock for natural gas and limited oil (Soylu et al., 1992), Mezardere Formation is defined as good host rock for gas and oil (Bürkan, 1992 and Soylu et al., 1992). A DFN model is essential for the reservoirs to run processes. Traditional data in structural geology from natural discontinuities, which are location, orientation and position, are used to create a DFN model. The originality of this study is that it is the first DFN modelling attempt being carried out on aforementioned unconventional reservoir rocks of Thrace region in Turkey. The DFN models defined through this study can serve as valuable database in terms of reservoir modelling studies prior to potential HF operations probable to be performed in the study area in the future.
Development, Creation and Application of a Reinforcement Systems Database
Authors: Thompson A. G. , Villaescusa E.
Published: September 2022

Reinforcement systems are used widely for the stabilisation of excavation surfaces associated with surface and underground excavations in mines and civil engineering projects. To be effective the reinforcement must be of sufficient length to pass beyond the boundary between the unstable and stable rock regions and transfer the loads imposed by the unstable zone to the stable zone. The behaviour of the reinforcement in the unstable zone is quite different as it involves a direction of loading away from the excavation surface towards the stable rock. In order to attempt reinforcement design it is necessary to take into account the differences between the behaviours of both the toe and collar zones. A generic reinforcement system configuration specification has been developed which clearly defines the geometric differences between these zones. In order to create a database of reinforcement system force-displacement responses to loading it was found necessary to combine mechanical testing and model simulations. Details are provided as to how the database has been developed for many different reinforcement systems. In addition, it is shown how the database is applied to the stability assessment of a reinforced block for the purposes of design.

1. INTRODUCTION

Rock reinforcement systems are used widely to improve the stability of both open pit and underground excavations in both civil and mining projects. These reinforcement systems have a wide range of materials and configurations. Consequently, the performance of these reinforcement systems measured in terms of force and displacement responses to loading vary considerably. Whilst the reinforcement systems may be subjected to formal proof and suitability testing according to codes (e.g. BS8081 1989, AS5100.3:2017), many reinforcement systems are not subjected to formal evaluations, even for a limited number of configurations and small range of rock mass conditions.

When it comes to design of reinforcement systems, it soon becomes apparent that for many reinforcement systems the yield and ultimate force capacities of the "element" may be the only reliable parameters that are available from the manufacturer. Note that the term "element" was developed by Windsor and Thompson (1993) to describe the manufactured item of a rock reinforcement system. Consequently, it then becomes necessary for a potential client to perform testing for a single configuration in local ground conditions, often without the necessary understanding of what tests are needed and how the results may be analysed and interpreted for the purposes of design.
Development and Application of a Novel Dynamic Loading Simulation for Reinforced Blocks of Rock
Authors: Thompson A. G. , Villaescusa E.
Published: September 2022

Ground support consisting of integrated internal rock reinforcement systems and surface restraint systems are used widely for the stabilisation of excavations associated with underground excavations in mines. As mines have developed at ever increasing depths below the earth’s surface, so have the stability problems associated with stresses exceeding the rock strength with resulting violent failures and high energy demands imposed on the ground support. In order for excavations to remain open, ground support schemes have to dissipate the energy being imposed on them. A previous static analysis technique using a force equilibrium approach was modified to enable dynamic loading simulated by an initial vector of block velocity followed by small increments of displacement in the direction of the velocity vector. The dynamic loading technique is described in detail and demonstrated by an example using the SAFEX software package developed by the writers and their colleagues. An importance observation from the analysis results is that the total energy dissipation capacity of the reinforcement systems may need to be significantly greater than the ejection energy imposed at the start of the analysis due to the additional energy associated with the vertical loss of height when ejection is from the roof or the wall of an excavation.

1. INTRODUCTION

Reinforcement systems are used widely to maintain the stability of excavations formed in rock. The shapes and sizes of potential instabilities are usually defined by combinations of discontinuities within the rock mass and the free faces of the excavation. The discontinuities may exist prior to the creation of the excavation or may initiate and propagate due to stress redistribution subsequent to the formation of the excavation.

The forces causing detachment of potential instabilities may be classified as being imposed statically or dynamically. Consequently, the responses from reinforcement systems are required to be resistant to these different imposed forces. Also, the methods of analysis for reinforced blocks need to be different.
Early Detection of Fault Activation Using Surface Tilt Monitoring During a CO2 Injection Project
Authors: Salimzadeh S. , Kasperczyk D. , Chen Z. , Movassagh A. , Arjomand E. , Mow W. S. , Kear J.
Published: September 2022

Tracking the migration of injected CO2 is critical to understanding the performance of subsurface intervals selected for CO2 sequestration. Measuring and analyzing the shape and magnitude of the surface deformation caused by subsurface CO2 injection, provides confidence that sequestered CO2 will remain in the target formations. Surface deformation information for onshore injection operations can be obtained via InSAR satellite imagery or an array of high-resolution tiltmeters installed in shallow (10-15m deep) boreholes.

In this study, synthetic tilt data produced from a full 3D finite element model of the In Salah CO2 injection site, are used to determine the shape and location of the CO2 plume at the three injection wells. A strong tilt signal (>0.1μRad) can be observed well ahead of the equivalent detectable surface heave (>2 mm, typical InSAR limitation magnitude). The inverse analysis for an array of ten tiltmeters around each of three injection wells clearly shows whether the injected CO2 remained in the target layer or leaked into a nearby fault. Results show that tracking the tilt vectors on the top of the fault can inform the operators of the onset of the fault pressurization.

1. INTRODUCTION

Geological storage of carbon dioxide in the deep subsurface was identified as one of the key technologies for reducing the emission of greenhouse gases to the atmosphere and mitigating global warming and climate change (Baines & Worden, 2004; Hepple & Benson, 2005; Schrag, 2007). The types of geological reservoirs suitable for large-scale, long-term subsurface storage of CO2 mainly include deep saline aquifers, existing oil and gas fields, and deep unmineable coal seams. There are major concerns on the large-scale injection of CO2 into these geological formations and the long-term storage security such as injection induced reservoir stress and deformation, CO2 plume migration and leakage, well integrity, caprock sealing performance, potential fault reactivation and notable seismic events (Rutqvist et al., 2015). Effective modelling and monitoring of geological CO2 injection and storage are therefore essential to better understanding of the mechanisms of the storage process, to the development of reliable geological carbon storage technologies and their successful implementation (Adriana Paluszny et al., 2020; Saeed Salimzadeh et al., 2018).
Dynamic Fracture Conductivity Considering Stress-Deformation Behavior of Proppants in Shale Formation
Authors: Wang Qiqi , Chen Mian , Wang Su
Published: September 2022

Most of the artificial fractures formed by fracturing need to be injected an appropriate amount of proppant into the fractures to achieve long-term support and maintain a stable fracture opening. Under the action of closing stress, the fracture wall and proppant will undergo a dynamic deformation process. Based on the Hertz elastic contact theory, this study establishes a two-dimensional finite element model of the contact deformation between the proppant sphere and the fracture plane. The model uses the spring as the medium to define the contact pair between them, and analyzes the stress interference phenomenon between the proppants. The smaller the proppant particle size, the smaller the gap between adjacent spheres, the stronger the inter-particle interference, and the strained regions are connected, which is close to a semicircle. The larger the particle size, the easier the embedding of the proppant will lead to the uneven wave shape of the fracture surface. Whether a large amount of proppant is embedded globally or locally has a particular influence on the dynamic evolution of the stress field, it is necessary to consider whether such agglomeration effect occurs when studying subsequent fracture opening and effective propping.

1. INTRODUCTION

There were several constraints confronted in the field during the hydraulic fracturing process. One of the main issues faced was the closure of the newly fractured area, limiting the future extraction of gas (M.A.A Ahamed, 2019). Proppants were utilized in hydraulic fracturing applications as a countermeasure to fracture closure due to the high in-situ stress at deep unconventional rock deposits (C.T. Montgomery, 1985). Several studies evaluating the fracking process have shown that effective stress and several other factors can lead to proppant rearrangement, proppant diagenesis, and proppant embedment, which can significantly reduce the hydraulic conductivity (M. Mehdi Mollanouri Shamsi, 2016).

Many scholars conducted laboratory studies on proppant embedment. The effects of elastic modulus, closure pressure, water saturation, particle size and distribution, proppant concentration, and fluid viscosity were investigated (Volk et al., 1981; R. Hartley et al., 1985; Lacy, L.L. et al., 1997; Lacy, L.L. et al., 1998; Qingzhi Wen et al., 2007; 9. Zhang, Junjing, et al., 2015). However, the disadvantage of experimental research was that there were many rock types, complex reservoir environments, and specific use scenarios for proppant types, sizes, and concentrations, so it is difficult to obtain general conclusions. The shape of the proppant was generally spherical particles, and due to the difference in its size, the fracture face can be regarded as a semi-infinite plane. Therefore, theoretical research can be abstracted as the contact deformation problem of spherical particles and the semi-infinite plane.
The Conductivity of Proppant and Impacts of Proppant Flowback on Fracture Conductivity in Different Conditions
Authors: Wang Bin , Zan Jingge , Liang Tianbo , Pan Zhikun , Liu Sen , Zhou Fujian
Published: September 2022

Hydraulic fracturing can increase the fracture conductivity by providing high-conductivity channels for hydrocarbon flow. Proppants flow back from the propped fracture during the early stage of production in the hydraulically fractured tight reservoirs, during which the fracture conductivity decreased due to the proppant flowback. In this paper, the conductivity of the proppant is quantified at different concentrations and closure stresses as baselines. Then, flowback experiment is conducted to understand the impacts of flowback rates, fracturing fluids, closure pressures, and proppant placement methods. When the pressure is higher than 40 MPa, the fracture conductivity of paving with sand will approach zero due to the crushing of sand under the high closure pressure, while the ceramic proppant has high conductivity due to a low crushing rate. The conductivity gradually decreases with time, during which the decline rate is large at the initial stage, and decreases with time till reaching a plateau. When the slickwater is used, the daily production of 70— 200 m3/d in conventional horizontal wells can reduce sand production. When the guar is used, the critical flow rate is expected to be doubled due to the guar residues after gel breaking. Proppant size has a larger impact on proppant flowback than the fracturing fluid. Combined proppant can also decrease the proppant flowback. It is found both fracturing fluids residue and special proppant paving methods can reduce the proppant flowback, which can guide the choke sizes in field conditions.

1. INTRODUCTION

Hydraulic fracturing is used for shale reservoir stimulation to get more daily production to solve the problems of low natural productivity and difficult waterflood development due to the shale formation characteristics of small pore throat radius and low permeability. The proppant is pumped into the fractures to support artificial fractures in the hydraulic fracturing to create the hydrocarbon migration channel. The high conductivity of fractures is the guarantee of enhancing oil recovery in shale reservoirs. (Cohen et al., 2013; Gomaa et al., 2014)
An Experimental Study of CO2 Huff-N-Puff Enhanced Oil Recovery in Three Forks Formation, Williston Basin
Authors: Sennaoui B. , Pu H. , Rylander E. , Afari S. , Malki M. L.
Published: September 2022

The primary oil recovery in the Bakken Petroleum System is still small, with a range of 5% to 12% of the original oil in place (OOIP). This study aims to investigate the potential and the performance of CO2 injection with HnP on the recovery factor (RF) in Upper Three Forks (UTF) and Middle Three Forks (MTF) formations, Sanish Field, Williston Basin, North Dakota (ND). In order to achieve this purpose, a series of laboratory experiments were applied where the performance of cyclic CO2-EOR was evaluated under different scenarios of injection with various operational conditions. Different parameters were considered to understand the mechanisms of HnP CO2-EOR, including the pressure of injection above and under the minimum miscibility pressure (MMP), the effect of soaking time on production, and several injection cycles to maximize the oil recovery. Then a comparison of the performance of CO2-EOR with ethane (C2) and propane (C3) under the same operating condition to evaluate the feasibility of different gases in the HnP approach. In addition, X-ray diffraction mineral analysis (XRD) was performed on four samples taken from UTF and MTF to determine the mineral composition in both formations. The results showed that the most recoverable formation was UTF with 56% of RF during the first cycle, while MTF showed only 37%. Increasing the number of cycling from one to four cycles showed that RF increased up to 88% and 75% in UTF and MTF, respectively. In addition, injection pressure seems to have a significant effect on RF in both formations, where the results showed that increasing the pressure from 800 Psi to 3600 Psi improved RF from 15% and 16% to 56% and 37% in UTF and MTF, respectively. Moreover, increasing the soaking time from 4 hrs to 48 hrs, RF increased from 37% and 25% to 72% and 39% in UTF and MTF, respectively. The results from comparison of different gases injections showed that, with injection pressures of 3600 Psi and soaking time of 24 hrs, ethane was the most effective gas in both formations, followed by propane and CO2, respectively. In conclusion, CO2 HnP has good potential in the lab and can be a promising method in unconventional reservoirs. This research work may provide an improved understanding of the CO2 HnP enhanced recovery strategy for unconventional reservoirs.
Molecular Simulation of Adsorption and Diffusion Behavior of CO2 in Bakken Nano-Porous Media for Enhanced Oil Recovery Assessment
Authors: Bellal Abdelmalek , Assady Ailin
Published: September 2022

The use of molecular simulation to quantify gas adsorption and diffusion is needed to study the CO2 Enhanced Oil Recovery (EOR) efficiency. Favoring CO2-Oil interaction over CO2-Rock interaction depends on the strength of the interaction bond between CO2 and nanopores surface. In this study, we explored the effect of pore size and reservoir temperature on CO2 adsorption and diffusion behavior in Bakken. This comprehensive study covers the characterization of Bakken CO2-EOR using grand canonical Monte Carlo and molecular dynamic simulations. The results indicated that increasing reservoir temperature would reduce the amount of adsorbed CO2 and the adsorption kinetics of CO2 into Bakken nanopores follows a first-order Langmuir model. CO2 adsorption in Bakken nanopores was strong at low temperatures and large pore sizes. Also, high CO2 injection pressures (e.g., 55MPa) led to strong diffusion of CO2 into the nanopores. This is undesirable as high CO2 adsorption and diffusion into the walls of the nanopores would cause molecular loss of the injected CO2, consequently lower EOR efficiency. The insights gained from this work might help improve theoretical understanding of the micro behaviors of CO2 in Bakken reservoir nanopores and give valuable recommendations in CO2-EOR.

1. INTRODUCTION AND BACKGROUND

In 2021, North Dakoda contributed to US oil production as second-ranked with production over 1MMbscf. Although using advanced technologies of hydraulic fracturing in unconventional reservoirs such as Bakken made higher oil production possible, the recovered production oil is only 2% of the Bakken original oil in place (OOIP). Also, maintaining production is quite challenging in unconventional reservoirs. Thus researchers are exploring the flow mechanisms to sustain oil production, including developing several enhanced oil recovery (EOR) methods such as gas injection and CO2 EOR for improving the recovery factors over the past few decades. Based on the experimental and numerical studies, one of the potential methods with a promising indication of oil incremental is CO2 EOR (Jia et al., 2018; Thakur, 2019). During a CO2 injection process, oil recovery factor can increase as a result of CO2-oil interaction. Injected gas can swell the oil and reduce viscosity leading to higher oil mobility. Also, the underground injection of captured CO2 from combustion stations has the benefits of reducing greenhouse gas emissions which makes CO2 injection a valuable process. (S. Wang, 2016). On the other hand, CO2 injection processes are not always in favor of increasing oil recovery due to the adsorption of CO2 in rock. Moreover, there is always a gap between field and simulation-predicted oil recovery (Jia et al., 2018). Therefore, it is important to investigate the effect of some common assumptions in tight reservoirs such as the ignorance of the diffusion flow mechanism and adsorption to improve oil recovery prediction.
Prediction of Shear Wave Velocity in the Williston Basin Using Big Data Analysis and Robust Machine Learning Algorithms
Authors: Laalam A. , Mouedden N. , Ouadi H. , Chemmakh A. , Merzoug A. , Boualam A. , Djezzar S. , Aihar A. , Berrehal B. E.
Published: September 2022

The shear velocity is one of the most critical parameters in determining the mechanical rock elastic properties, which serve as inputs for different studies such as wellbore stability, mechanical earth modeling, hydraulic fracturing, and reservoir characterization. However, the sonic log is not acquired in every drilled well. We analyzed the log data of more than 35000 wells in the Williston Basin, and we found that only very few wells had sonic logs. For this reason, several studies attempted to correlate the shear velocity (or slowness) to other easily accessible properties; these will be presented in the literature review, with their pros and cons.

The focus of this paper is to apply machine learning algorithms to synthesize the shear slowness log. Our models are trained and tested with log data from 27 wells drilled in the Bakken petroleum system, Williston Basin. Logging data include Gamma Ray, Deep Resistivity, Density, Neutron Porosity, and Shear Slowness. Five different algorithms were developed and tested against blind data including Xtreme Gradient Booster, Random Forest Regressor, Linear Regression, Ada Boost Regression, and Bayesian Ridge Regression. Overall, the R2-score varied from 0.55 to 0.92, with the XGBoost outperforming the other algorithms.

1. INTRODUCTION

With more than a century of oil and gas production, companies have generated a tremendous amount of data with every drilled well and field operation. These data banks are either unused or poorly analyzed. While the proper technical analysis of this could improve the performance of the oil and gas industries significantly.

In the past, the focus of researchers and engineers was to develop efficient tools and methods to acquire necessary data. With the advances in computational power, companies realized that they could take advantage of the data more efficiently. The application of artificial intelligence (AI) and specifically machine learning (ML) along with big data analytics have known an enormous surge.
Characterizing Rock Fractures and Physical Properties for Experiment 2 of the EGS Collab Project, Sanford Underground Research Facility
Authors: Ulrich C. , Dobson P. F. , Kneafsey T. J. , Roggenthen W. M. , Uzunlar N. , Doe T. W. , Neupane G. , Artz T. , Dobler K. , Schwering P. , Smith M. , Burghardt J. A.
Published: September 2022

The EGS Collab project is focused on understanding and predicting permeability enhancement and evolution in crystalline rocks to support geothermal energy production. To support this effort, the project is creating a suite of intermediate-scale test beds coupled with stimulation and interwell flow tests that will provide a basis to better understand the fracture geometries and processes that control heat transfer between rock and stimulated fractures. Therefore, high level site characterization is paramount for building models to estimate potential interwell flow rates and heat exchanges. For the second experimental testbed, the EGS Collab team visually mapped the distribution, orientation, and nature of open and healed fractures exposed along the drift wall in the Sanford Underground Research Facility 4100 feet below ground surface, and within the eleven boreholes drilled for this test bed. Continuous core logging of each borehole and a suite of geophysical wireline logs was collected to characterize the spatial variability of rock properties and fracture orientations. All of the data were then compiled into multiple 3D visualization software packages for interpretation and further analyzed for slip and dilation tendencies that will be incorporated into coupled-process geomechanical flow and transport models to better constrain the planned flow and tracer tests.

1. INTRODUCTION

Following the completion of Experiment 1 at 4850 feet below ground surface (the 4850 level), the EGS Collab team initiated the development of a second testbed for Experiment 2 (E2) in the Sanford Underground Research Facility (SURF and former Homestake Gold Mine) in Lead, South Dakota, USA. The overarching goal of the EGS Collab project is to create field test beds to host stimulation and interwell flow tests that will provide improved understanding of fracture stimulation methods, resulting fracture geometries, and processes that control heat transfer between rock and stimulated fractures for Enhanced Geothermal Systems (EGS). This experiment will contrast from Experiment 1 in that its goal is to create a series of hydro shear fractures for interwell flow tests in support of the EGS Collab project (Kneafsey et al., 2018) at the intermediate-scale (~10-20 m). In the winter-spring of 2021, the EGS Collab team started to develop the E2 testbed on the 4100 level (1.25 km below the ground surface).
Evaluation of NATM Tunnel Cutting Face in Japan Using Machine Learning
Authors: Halim K. D. , Yun Y. , Kusumi H.
Published: September 2022

In Japan, the determination of support patterns (CI, CII, DI, DII) in NATM tunnels plays an important role in the construction of mountain tunnels. Conventional methods of rock mass evaluation include grading the rock by experienced experts with a point-based system in 9 criteria as regulated by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT). However, discrepancy in judgement by these experts based on field experiences and subjective standards means that the current method of evaluation is ambiguous. Therefore, this research proposes a standardized evaluation method by verifying the applicability of machine learning to rock surface evaluation. Support vector machine (SVM) is implemented to build the model for the classification of four support patterns based on data collected from different tunnel construction sites. As a result, the proposed model has shown high accuracy. Furthermore, by taking into account the current system of evaluation of the tunnel cutting face, the classification accuracy significantly improved. Thus, the proposed model has shown strong candidacy as a standardized evaluation method.

1. INTRODUCTION

At mountain tunnels constructed using NATM (New Austrian Tunnel Method), the property of rocks found in mountains such as the type, strength, etc. of the rock mass determines the class of the rock itself. During the tunnel’s design stage, support patterns are determined in accordance with the classified rocks to prevent the mountain from collapsing. The tunnel cutting face is then observed and support patterns are changed accordingly in the construction stage to maximize safety and minimize costs. Furthermore, additional construction methods may be required when the initial support constructions are deemed insufficient based on these evaluations.

The former method of evaluating the rock of the tunnel cutting face is by observing and measuring the strength, discontinuity, etc. of the rock surface. Based on these observations, the tunnel cutting face is graded using a point-based system. However, the criteria for these evaluations were subjective and evaluation methods differed by engineer even if it was the same tunnel cutting face that was observed.
Numerical Simulation of Rock Breaking Mechanism Analysis of Drill Bit with Central Stinger in Axial Percussive Drilling
Authors: Wang Xinrui , Zhang Hui , Ma Xusheng , Zhou Yuting , Li Youyi , Lu Zongyu
Published: September 2022

The drill bit with central stinger element assisted by axial vibration has achieved a good rate of penetration in granite and diorite formation in China, but the study on the theoretical rock breaking mechanism of this bit is still insufficient. To better understand the rock-breaking mechanism of drill bit with central stinger and provide a theoretical basis for practical engineering, this paper presents numerical modelling with the finite element method of dynamic cutting processes under 50-200Hz loads. The rock breaking model with three 16mm plain cutters and a 16mm central stinger cutter is established, the cutters break a certain depth of granite at the fixed weight on bit and rotate speed. The rock failure mode, cutting force, mechanical specific energy (MSE) is analyzed and then compared with the steady load breaking condition. Also, the influence of cutter’s location has been analyzed and results indicate that, for 16mm cutter, when the cutting radius of the first plane cutter is less than 13.1mm, there is almost no cutting force on stinger cutter. When the radius is greater than 13.1mm, the force on stinger cutter always increases with the increase of the radius and exists a maximum value. Compared with the steady load, the average cutting force and MSE of stinger cutter under 100Hz load decrease by 11%. Meanwhile the fluctuation of the cutting force of stinger and other three cutters are significantly smaller, the maximum cutting force is reduced by over 19%, which reduces the occurrence of stick-slip. The findings contribute to understand theoretical rock breaking mechanism of central stinger bit, also optimize the bit selection and percussive drilling parameters.

1. INTRODUCTION

In drilling operation of petroleum engineering, the Polycrystalline Diamond Compact (PDC) bits are becoming much more widely used and important tools for directional wells, extended reach wells, deep wells and even the ultra-deep wells due to its good efficiency and economy of being able to repair. Nowadays the PDC bits have dominated almost all the footages in drilling operation of oil and gas wells, the efficiency of PDC bits has become a key part of the economics of the drilling operation (Cheng et al., 2018, 2019; Zhang et al., 2020). Therefore, the methods to design efficient and durable PDC drill bits have become the most important direction in drill bit development.