Unconventional Gas Resources Research Articles

Surface analysis of liquid adsorption onto shale

Shale rocks are one of the world's most important unconventional gas resources today, thanks to technical advancements. Fluid adsorption in tight rocks like shale is critical for designing fracturing and treatment fluids. However, adsorption of fluids in shale is not fully understood, and quantifying it remains difficult. In addition, the complicated pore structure of shale rocks makes characterisation challenging. Wettability can be used to understand the affinity of a solid surface to adhere certain fluid. Shales present several basic problems when employing standard techniques because of their small grain size, low permeability, and reactive components. We assessed and compare the wettability of shale using contact angle and spontaneous imbibition methods in two shale samples. The findings showed no correlation between contact angle and imbibition curves. Such behaviour is due heterogeneity of shale surface. Contact angle produces local wetting characteristics, but shale sample is rather complex and contact angle is therefore not representative. Imbibition results might be more reliable since fluids contacts with the whole sample.

Study on Effect of Perforation Orientation on Hydraulic Fracturing of Shale

The core technology to realize the development of unconventional oil and gas resources is the large-scale volume transformation of shale reservoirs, but volume fracturing is a complex physical and mechanical process, and its mechanism remains to be further studied. In this paper, starting from the current research status of shale volumetric fracturing, the perforation orientation of volumetric fracturing is numerically simulated by using the finite element software. The following conclusions are drawn: ① when the ground stress conditions are equal, with the increase of perforation angle, the maximum principal stress at the front of perforation decreases gradually; that is, when the perforation is along the horizontal minimum principal stress, the maximum principal stress at the front of perforation is the largest, which is most conducive to fracturing of shale reservoirs. ② When the perforation is along the direction of the horizontal minimum principal stress and the horizontal stress difference is zero, the smaller the horizontal stress, the greater the maximum principal stress at the front of the perforation. Therefore, the smaller the horizontal stress, the more conducive to the rupture of shale reservoirs. ③ When the perforation is along the horizontal minimum principal stress, the maximum principal stress at the front of the perforation increases with the increase of the horizontal stress difference. Therefore, the larger the horizontal stress difference, the more favorable for the fracture of shale reservoirs.

A Semi-Analytical Mathematical Model for Off-Center Fractured Vertical Wells With Asymmetrical Fractures in Columnar Coalbed Methane

Coalbed methane (CBM) is one of the important unconventional oil and gas resources. Since the permeability of the CBM reservoir is very low, CBM is usually developed by hydraulic fracturing technology to enhance gas productivity. In fact, most of the wells are not at the center of the columnar CBM reservoirs, which leads to different wellbore pressure responses. It is meaningful to analyze the wellbore pressure response of the off-center fractured vertical wells with asymmetrical fracture. The adsorption-desorption, fracture asymmetry are considered and a semi-analytical mathematical model is established. The pressure wellbore solution of the Laplace domain can be obtained by Laplace transformation, model coupling, and Gauss elimination. Then, the Stehfest numerical inversion is employed to obtain the pressure solution and rate distribution in the real-time domain. This semi-analytical solution is verified; the results show that the wellbore pressure includes six flow regimes, which include bilinear flow, linear flow, elliptic flow, radial flow, arc boundary reflection, and boundary dominated flow regime. The influence of fracture angle, dimensionless conductivity, and off-center distance on wellbore pressure and rate distribution are discussed in detail.

Biogeochemical Assessment of the Coalbed Methane Source, Migration, and Fate: A Case Study of the Shizhuangnan Block, Southern Qinshui Basin.

The exploration and exploitation of coalbed methane (CBM), an essential unconventional gas resource, have received much attention. In terms of shallow groundwater assessment during CBM production, biogenic methane natural formation in situ and methane migration from deep sources into shallow aquifers need to be of most concern. This study analyzes geochemical surveys including ions, isotopes, and dissolved methane concentrations in 75 CBM coproduced water samples in the southern Qinshui Basin. Most of these water samples are weakly alkaline. Some samples’ negative oxidation/reduction potential (ORP) values reveal that the CBM reservoir water samples are mainly produced from reductive groundwater environments. Cl–, Na+, and HCO3– are the dominant ionic constituents of the water samples, which are usually associated with dissolved methane concentrations. The biogeochemical parameters and isotopic features provide an opportunity to assess the origin, migration, and oxidation of biogenic or thermogenic methane. Some water samples suggest biogenic methane formation in situ characterized by negligible SO42– and NO3– concentrations and low δ13CCH4. Only a few water samples indicate the migration of biogenic methane into shallow aquifers without oxidation based on elevated SO42–, NO3–, and δ13CDIC and low δ13CCH4. A few cases characterized by elevated δ13CCH4, negative δ13CDIC values, and negligible SO42– and methane concentrations suggest the oxidation of biogenic methane rather than the migration of thermogenic methane. A significant number of cases mean methane migration to shallow aquifers. Partial oxidation of thermogenic or mixed methane is evaluated by negligible SO42–, NO3–, and methane concentrations and elevated δ13CCH4. Dissolved methane isotopic compositions and aqueous biogeochemical features help study methane formation and potential migration in shallow groundwater.

Development and performance evaluation of epoxy resin system for repairing agent

AbstractIn the development of unconventional oil and gas resources, such as offshore oil, high pressure oil and sour wells, casing‐casing annulus pressure is a common problem. To prevent casing‐casing annulus pressure, bisphenol F epoxy resin, dicyandiamide hardener SH‐500A and urea accelerator SH‐A100 are added to develop an epoxy resin system. The thickening time and the compressive strength at 90°C are used as the main evaluation indexes. The optimal formulation was found to be as follows: bisphenol F epoxy resin NPEF‐170 with 20 wt% of diluent benzyl glycidyl ether, 9.0 wt% of hardener dicyandiamide and 0.5–0.4 wt% of urea accelerator SH‐A100. The experimental results showed that the initial consistency of the epoxy resin system is less than 30 Bc, the thickening time is 3–4 h, and the compressive strength at 90°C is above 27 MPa, which meet the requirements of on‐site construction.

The Influence of Tensile Strength Difference with Variable Gravel Sizes on the Hydraulic Fracture Propagation in the Conglomerate Reservoir

Conglomerate reservoir is a representative reservoir of unconventional oil and gas resources, and hydraulic fracturing is also used to increase its production. However, the tensile strength of conglomerates with different sizes is various, which has an important impact on the hydraulic fracture propagation in the conglomerate reservoir. To study the tensile strength difference caused by the size of conglomerate gravel, some Brazilian splitting tests were conducted. To find the influence of tensile strength difference on the hydraulic fracture propagation, some true triaxial hydraulic fracturing experiments for layered formations were carried out. The concept of the fracture element in conglomerate formations was proposed. The results showed the following: (i) the tensile strength of conglomerate rock decreased with the increase of the gravel size, and the average tensile strengths of specimens with gravel diameters of 2.0–6.0 mm and 6.0–20.0 mm were 65.5% and 43.6% of that with 0.1–2.0 mm diameter, respectively; (ii) hydraulic fractures easily deflected at the interface of fine-grained conglomerate and medium-grained conglomerate; (iii) hydraulic fractures were prone to propagate to the layers with larger gravel sizes, and the smaller the gravel size, the lower the penetration probability. It is helpful to increase the understanding of the hydraulic fracture propagation under different gravel sizes in the conglomerate reservoir, and it can provide a reference for well distribution and hydraulic fracturing design.

Corrosion Resistance Mechanism of Mica-Graphene/Epoxy Composite Coating in CO2-Cl- System.

The working environment for tubing in oil and gas fields is becoming more and more serious due to the exploration of unconventional oil and gas resources, leading to the increasing need for a protective internal coating to be used in tubing. Therefore, a new mica–graphene/epoxy composite coating with different graphene contents (0.0, 0.2, 0.5, 0.7, and 1.0 wt.%) was prepared to improve the tubing resistance to a corrosive medium, an autoclave was used to simulate the working environment, and an electrochemical workstation assisted by three-electrodes was used to study the electrochemical characteristics of the coating. The results showed that the addition of a certain amount of graphene into the mica/epoxy coating significantly improved the corrosion resistance of the composite coating, and when the graphene content increased, the corrosion resistance of the mica/epoxy coating first increased and then decreased when the corrosion current density of a 35 wt.% 800# mica/epoxy coating with a 0.7 wt.% graphene content was the lowest (7.11 × 10−13 A·cm−2), the corrosion potential was the highest (292 mV), the polarization resistance was the largest (3.463 × 109 Ω·cm2), and the corrosion resistance was improved by 89.3% compared to the coating without graphene. Furthermore, the adhesion of the coating with 0.7 wt.% graphene was also the largest (8.81 MPa, increased by 3.4%) and had the smallest diffusion coefficient (1.566 × 107 cm2·s−1, decreased by 76.1%), and the thermal stability improved by 18.6%. Finally, the corrosion resistance mechanism of the composite coating with different graphene contents at different soaking times was revealed based on the electrochemistry and morphology characteristics other than water absorption and contact angle.

Methane adsorption capacity measurement in shale matrix nanopores at high pressure by low-field NMR and molecular simulation

Shale gas is an important unconventional gas resource because of its rich storage ability. The quantitative measurement of adsorption capacity is essential to shale gas exploration. However, traditional measurements, such as gravimetric and volumetric method, are influenced by excess adsorption effect and need to be corrected at high pressures. In this work, methane adsorption curves without excess adsorption effect were obtained through two ways: a low-field nuclear magnetic resonance (NMR) experimental measurement and the adsorption curves modification using density correction method by molecular simulation. Shale powder samples from the Longmaxi Formation, China, are used as representative sample in this work. In NMR experiments, the gas in packed beds, free spaces and nanopores is identified based on the multi-peaks in the NMR transverse relaxation spectrum, which guarantees the inexistence of excess adsorption effect. In simulations, considering the complex composition of the shale matrix and complicated pore shapes, the average density of methane in organic and inorganic nanopores with various size and shape was obtained using Grand Canonical Monte Carlo (GCMC) simulation, and the excess adsorption curve measured by gravimetric and volumetric method is corrected based on such density. Through comparing the NMR measured adsorption curve and corrected excess adsorption curve, it indicates that the adsorption curve measured via NMR was in good agreement with the modified adsorption curves by density correction, which validates the NMR experiment for measuring adsorption capacity of nanoporous media with complex structure, which is suitable for shale gas and hydrogen storage, as well as other adsorption processes.

Microbial communities reveal impacts of unconventional oil and gas development on headwater streams

The demand for natural gas has led to the development of techniques used to access unconventional oil and natural gas (UOG) resources, due to the novelty of UOG, the potential impacts to freshwater ecosystems are not fully understood. We used a dual pronged approach to study the effects of UOG development on microbial biodiversity and function via a laboratory microcosm experiment and a survey study of streams with and without UOG development within their watersheds. The microcosm experiment simulated stream contamination with produced water, a byproduct of UOG operations, using sediment collected from one high water-quality stream and two low water-quality streams. For the survey study, biofilm and sediment samples were collected from streams experiencing varying levels of UOG development. In the microcosm experiment, produced water decreased microbial aerobic and anaerobic CO2 production in the high water-quality stream sediment but had a positive effect on this microbial activity in the lower water-quality stream sediments, suggesting habitat degradation alters the response of microbes to contaminants. Results from the stream survey indicate UOG development alters stream water temperature, chemistry, sediment aerobic and anaerobic CO2 production, and microbial community biodiversity in both sediments and biofilms. Correlations among UOG associated land use, environmental, and microbial variables suggest increases in light availability and sediment delivery to streams, due to deforestation and land disturbance, impact stream microbial communities and their function. Consistent changes in the relative abundance of bacterial taxa suggest microorganisms may be good indicators of the environmental changes associated with UOG development. The observed impacts of UOG development on microbial community composition and carbon cycling could have cascading effects on stream health and broader ecosystem function.

Pore Structures of the Lower Permian Taiyuan Shale and Limestone in the Ordos Basin and the Significance to Unconventional Natural Gas Generation and Storage

In the Ordos Basin, multiple sets of coal seams, organic-rich shale, and limestone are well developed in the Permian Taiyuan Formation, which are favorable targets for collaborative exploration of various types of unconventional natural gas resources, including coalbed methane, shale gas, and tight gas. In this study, core samples from the Permian Taiyuan Formation in the eastern margin of the Ordos Basin were used to carry out a series of testing and analysis, such as the organic matter characteristics, the mineral composition, and the pore development characteristics. In the shale of the Taiyuan Formation, the total organic carbon (TOC) content is relatively high, with an average of 5.38%. A thin layer of black shale is developed on the top of the Taiyuan Formation, which is relatively high in TOC content, with an average of 9.72%. The limestone in the Taiyuan Formation is also relatively high in organic matter abundance, with an average of 1.36%, reaching the lower limit of effective source rocks (>1%), being good source rocks. In the shale of the Taiyuan Formation, various types of pores are well developed, with relatively high overall pore volume and pore-specific surface area, averaging 0.028 ml/g and 13.28 m2/g, respectively. The pore types are mainly mineral intergranular pores and clay mineral interlayer fractures, while organic matter-hosted pores are poorly developed. The limestone of the Taiyuan Formation is relatively tight, with lower pore volume and pore-specific surface area than those of shale, averaging 0.0106 ml/g and 2.72 m2/g, respectively. There are mainly two types of pores, namely, organic matter-hosted pores and carbonate mineral dissolution pores, with a high surface pore rate. The organic matter in the limestone belongs to the oil-generation kerogen. During thermal evolution, the organic matter has gone through the oil-generation window, generating a large number of liquid hydrocarbons, which were cracked into a large number of gaseous hydrocarbons at the higher mature stage. As a result, a large number of organic matter-hosted pores were generated. The study results show that in the Ordos Basin, the shale and limestone of the Permian Taiyuan Formation have great potential in terms of unconventional natural gas resources, providing a good geological basis for the collaborative development of coal-bearing shale gas and tight limestone gas in the Taiyuan Formation.

Experimental Investigation of Seepage Mechanism on Oil-Water Two-Phase Displacement in Fractured Tight Reservoir

As a type of unconventional oil and gas resources, tight sandstone reservoir has low permeability and porosity properties and thus is commonly necessary to develop through hydraulic fracturing treatment. Due to the coexistence of natural fractures and induced hydraulic fractures, the heterogeneity of reservoir permeability becomes severe and therefore results in complicated fluid seepage mechanism. It is of significance to investigate the oil-water two-phase seepage mechanics before and after the hydraulic fracturing stimulation with the aim of supporting the actual production and development of oilfield. This paper experimentally investigated the influences of fracture system on seepage characteristics of two-phase displacement in sample cores of fractured tight sandstones. In details, the changes of injection rate, cumulative production rate, recovery ratio, and water content were analyzed before and after the hydraulic fracturing treatments. To further analyze the displacement characteristics of the sample core, the displacement indices of four rock samples in different displacement stages were investigated. The sensitivity of sample core displacement indices to many key factors, including injection time, oil production rate, oil recovery factor and injection multiple factor, and moisture (i.e., water content was 95%, 98%, and 99.5%, respectively), before and after the hydraulic fracturing treatments were obtained synthetically. Besides, the relationship between recovery difference and contribution of fracture to permeability was explored at different water contents. The experimental results reveal that the fracture system shortens the water-free production period and hence reduces the recovery rate. The greater the contribution of fractures to permeability, the lower the recovery of water during this period.

Shale gas resources across the world: A future source of energy

Unconventional natural gas resources are very important for ensuring energy security. It is a concern that conventional natural gas resources will be unable to meet growing supply requirements. This concern has prompted a scramble to secure long-term natural gas supplies. In a high oil and gas price environment, unconventional resources are viewed as important and economically attractive components of future natural gas supplies. Shale gas is natural gas produced from shale formations. Gas shales are organic-rich shale formations. Shale gas is a dry gas composed of methane. Various factors which have contributed to its rapid development are mainly advancements in horizontal drilling, hydraulic fracturing, and, perhaps most importantly, the rapid increase in natural gas prices in the last several years as a result of significant variation in supply and demand. India has a high potential for shale reserves. According to the available data, a comprehensive shale gas pilot project carried out in the Damodar Valley Basin has made an initial gasin- place estimate of 300 to 2,100 trillion cubic feet (TCF) in Indian shale gas basins, which is around 300 times higher than the Krishna Godavari Dhirubhai 6 (KG-D6) basin, by far the largest gas field in the country. In this paper, a brief review of shale gas reserves across the world and their government policies has been explained.

The experimental methods and elastic properties of shale bedding planes materials state-of-the-art review

The identification of physical properties and fracture behavior of shale or hydrocarbon materials has grown to have substantial importance in the last four decades to industry and investigators alike due to its importance in unconventional oil and gas resources. This interest deviates from shale being a transversely isotropic material formed by bedding layers with different orientations and isotropic properties. In this paper, the experimental setup and the mechanical properties of shale materials have been reviewed. The investigator shows that the properties of shale are not unique, and it is highly dependent on, volume, loading type, and geolocation. Furthermore, the investigator utilized different experimental setup methods to identify the elastic properties of shale and the most common methods are Transducers Strain Detection, Strain gauges, Ultrasonic, and Digital image correlation that is shown in detail.

Potential Analysis and Technical Conception of Exploitation and Utilization of Tar-Rich Coal in Western China

The resources of tar-rich coal in Western China are abundant and are available for large-scale development. Understanding the oil and gas resource properties of tar-rich coal and promoting the development and transformation of tar-rich coal to produce oil and gas are crucial for increasing domestic oil and gas supply, relieving external dependence on oil and gas, and ensuring national energy security of China. This study estimated the potential of tar-rich coal resources in Western China based on the current exploration results of coal resources, reviewed the current status and existing problems, and proposed the basic concept and technical conception for the development and utilization of tar-rich coal in Western China considering the new requirements for carbon reduction, intelligent green development, and compensation for oil and gas shortage. The study concluded that the development and utilization of tar-rich coal needs to focus on key technologies, including high-precision comprehensive exploration, high-recovery-rate mining, underground in-situ pyrolysis, pyrolysis and gasification integration, and in-situ pyrolysis char CO₂ storage. Furthermore, we suggest that the tar-rich coal should be included in the management of unconventional oil and gas resources, scientific and technological research on the development and utilization of tar-rich coal should be strengthened, a national demonstration area should be established, and development of new energy and tar-rich coal in Western China should be coordinated, so as to maximize the special advantages of tar-rich coal as coal-based oil and gas resources and realize the low-carbon and high-value utilization of coal resources.

Capturing dynamic behavior of propped and unpropped fractures during flowback and early-time production of shale gas wells using a novel flow-geomechanics coupled model

Hydraulic fracturing has been widely used to exploit unconventional oil and gas resources for decades. The fracture closure during flowback and early production may be characterized by fluid dehydration and pressure drop in oil and gas production. In this paper, a fully coupled flow and geomechanics model is proposed to capture the dynamic behavior of key fracture parameters for flowback and early-time production. In this coupled model, the controlled volume method is used to numerically simulate the fracture flow, which can consider the geometry and conductivity distribution of the propped and unpropped fractures. For the fracture geomechanics, the joint-closure relationship is introduced to describe the fracture aperture of unpropped fracture. The empirical formula of effective normal stress and proppant parameters is applied to characterize fracture conductivity. The fracture aperture can be calculated coupled with the discontinuous displacement method (DDM) and the matrix transient linear flow. The coupled geomechanically model can consider the flow of the propped and unpropped fracture system, which is easy model-setup and convenient for practical application due to its excellent computational performance. Detailed flow behavior analysis shows that the aperture of the unpropped fracture is relatively slow during the fracture closure process. Compared with the fracture section opening under higher normal stress condition, the fracture section opening under lower normal stress condition has larger initial aperture and faster attenuation of aperture. The crucial fracture parameters, including fracture permeability and fracture length, can be well interpreted by matching flowback and production data. Furthermore, the relationship between fracture conductivity and effective stress for both propped and unpropped fractures can be explored. These new findings can also be applied to interpret reservoir/fracture properties from ChangNing shale fractured wells in China during flowback and early production.

Phase-Field Modeling of a Single Horizontal Fluid-Driven Fracture Propagation in Spatially Variable Rock Mass

Hydraulic fracture propagation directly affects the recovery rate of resources when hydraulic fracturing techniques are applied to exploiting unconventional oil and gas resources. Rock mass is the main engineering medium of hydraulic fractures and is generally considered to be of considerable spatial variability in physical and mechanical properties. Understanding the irregular propagation mechanism of hydraulic fracture in spatial heterogeneity rock mass is essential and beneficial to assess the recovery rate of oil or gas resources. This work develops a random phase-field method (RPFM) to simulate the irregular propagation of hydraulic fracture in spatially variable rock mass. The spatial variability of elastic modulus is characterized by the random field theory. Utilizing the advantages in modeling complex crack patterns and crack kinematics, the phase-field method (PFM) is used to predict the fracture propagation. Various anisotropic random fields of elastic modulus with different coefficients of variance and scales of fluctuation are generated via the Cholesky decomposition method. The random fields are subsequently implemented into COMSOL Multiphysics and combined with the PFM to investigate the hydraulic fracture propagation. This study investigates the influence of spatial variability of elastic modulus on the peak fluid pressure, fracture length, fracture area and fracture shape. It reveals that the spatial variability of elastic modulus has a significant influence on the propagation of hydraulic fractures, and the results provide a preliminary reference for hydraulic fracturing design with consideration of spatial variability of rock mass.

Evaluation of transfer learning in data-driven methods in the assessment of unconventional resources

In a novel application of data-driven analytics to the assessment of unconventional oil and gas resources, we present a Transfer Learning framework based on a combination of unsupervised and supervised machine learning techniques on a relatively large dataset from one of the main unconventional plays in the United States. To improve the generalizability of our work, instead of focusing on building one model with the highest predictive accuracy, we focus on quantifying the changes in predicted productivity of multiple sub-basin models that are trained on data from different sub-basins in the same shale play. We present the main steps of developing a data-driven solution in the unconventional oil and gas domain and discuss the importance of taking into account the domain knowledge of petroleum engineering and geology during the process. In an unsupervised clustering problem, we show the impact of clustering variable choice guided by domain expertise. We investigate the variation of model's accuracy when trained and tested on different parts of the unconventional play. The proposed Transfer Learning framework allows quantification of the change in predictive performance of the model of one part of the unconventional play as the amount of training data taken from another part is varied. We show how a careful selection of data types can balance between three objectives: reduce overfitting of the model to training data, augment model transferability across a very large shale play, and enhance accuracy in a part or sub-basin of the play. We show that selecting the clusters based on geologic variables such as average depth of the formation and thermal maturity of the source rock instead of geographical coordinates of the wells significantly increases the predictive performance of the models. In addition, we demonstrate the increase in model accuracy after adding production data to our predictors. We also discuss the limitations associated with accessing production data in a real-world Transfer Learning workflow. The variable importance ranking interpretation of monthly production rates also supports the significance of production data. We used statistical analysis of large samples of decline curves to find optimum duration of production history for training. To address the hyperparameter fine-tuning for our machine learning algorithms, we identified optimal ranges of Random Forest hyperparameters to achieve lowest test-to-train error ratio. Finally, we demonstrate a successful application of the proposed workflow on a real dataset from the Eagle Ford basin in South Texas in a few examples of improved Transfer Learning under the domain experts' guidance. Our findings show that by using models that have been trained on a large group of wells in a mature field in a transfer learning framework we can achieve better predictive performance in a greenfield with very few wells.

Langmuir adsorption isotherm in unconventional resources: Applicability and limitations

Accurate hydrocarbon reserve estimation is a crucial step for successful field development. Unlike for conventional reservoirs, however, reserve estimation for unconventional reservoirs is challenging due to the multiscale transport and multiphysics storage mechanisms involved. In this paper, we investigate the applicability and the limitations of Langmuir adsorption isotherm for the major unconventional gas resources, namely, shale-gas and coalbed methane (CBM) reservoirs, respectively. In general, reserve estimation methods for both shale-gas and CBM rely on Langmuir isotherm to model the sorbed gas capacity. Thus, we provide a detailed discussion on the characteristics of unconventional reserves and elucidate the applicability of the Langmuir model for estimating gas storage volumes. To add to the discourse on storage capacity modeling, molecular simulation studies of organic materials (kerogen) with various degree of heterogeneity were conducted. The adsorption behavior of multicomponent mixtures was also investigated. Simulations suggest that increased heterogeneity of the organic constituents and the presence of more than one component curtail the predictive power of the Langmuir framework. Furthermore, we observed that kerogen storage capacity is not only governed by its chemical composition but also by the particular kerogen type. Observed discrepancies with respect to reserve estimates for different evaluation models hint at a lack of understanding the underlying dynamics of unconventional reservoir gas storage phenomena.

Groundwater sources for the Mataranka Springs (Northern Territory, Australia)

The Mataranka Springs Complex is the headwater of the iconic Roper River of northern Australia. Using environmental tracers measured in springs and nearby boreholes, the origin of groundwater contributing to the springs was evaluated to help assess the impact of proposed groundwater extraction in the Cambrian Limestone Aquifer (CLA) for irrigation agriculture and for hydraulic fracturing in the Beetaloo Sub-basin (an anticipated world-class unconventional gas reserve). Major ions, Sr, 87Sr/86Sr, δ18O-H2O, δ2H-H2O, 3H, 14C-DIC were consistent with regional groundwater from the Daly and Georgina basins of the CLA as the sources of water sustaining the major springs (Rainbow and Bitter) and one of the minor springs (Warloch Pond). However, 3H = 0.34 TU in another minor spring (Fig Tree) indicated an additional contribution from a young (probably local) source. High concentrations of radiogenic 4He (> 10–7 cm3 STP g–1) at Rainbow Spring, Bitter Spring and in nearby groundwater also indicated an input of deeper, older groundwater. The presence of older groundwater within the CLA demonstrates the need for an appropriate baseline characterisation of the vertical exchange of groundwater in Beetaloo Sub-basin ahead of unconventional gas resource development.

Settling behavior of spherical particles in vertical annulus: Experimental study and model development

Coiled tubing (CT) drilling technology offers significant advantages in terms of cost and efficiency for exploitations of unconventional oil and gas resources. However, the development of CT drilling technology is restricted by cuttings accumulation in the wellbore due to non-rotation of the drill string and limited circulating capacity. Cuttings cleaning becomes more difficult with the wall resistance of pipe-wellbore annulus on the cutting transport. Accurate description of particle transport process in the pipe-wellbore annulus is, therefore, important for improving the wellbore cleanliness. In this study, high-speed camera is used to record and analyze the settling process of particles in the transparent annulus filled with power-law fluids. A total of 540 tests were carried out, involving dimensionless diameters of 0.10–0.95 and particle Reynolds Numbers of 0.01–12.97, revealing the effect of the dimensionless diameter and particle Reynolds number on the annulus wall effect, and the wall factor model with an average relative error of 2.75% was established. In addition, a dimensionless parameter, Archimedes number, independent of the settling velocity, was introduced to establish an explicit model of the settling velocity of spherical particles in the vertical annulus, with the average relative error of 7.89%. Finally, a calculation example was provided to show how to use the explicit model of settling velocity in annulus. The results of this study are expected to provide guidance for field engineers to improve the wellbore cleanliness of coiled tubing drilling.