Hydraulic fracturing has transformed the international energy landscape by becoming the go-to method for the exploitation of natural gas from unconventional shale reservoirs. However, in the recent years, the search for an alternative method of shale-gas exploration has intensified, because of various problems (e.g., contamination of ground and surface water, overexploitation of precious water resources, air pollution, etc.) associated with the usage of water-based fracturing techniques. The use of CO2 for shale gas exploitation has emerged as a better alternative to aqueous-based gas exploration techniques. CO2 when injected into deep shale reservoirs, transitions into supercritical CO2 (SC-CO2) when temperature and pressure condition exceeds the critical point, i.e., 31.1 °C and 7.38 MPa. In this paper, we comprehensively review the impact of SC-CO2 on shale gas reservoirs during the different stages of shale-gas exploration, i.e., (i) drilling, which involves the superiority of SC-CO2 over water-based drilling fluids, in terms of achieving under-balanced well condition, higher rates of penetration, and resistance to formation damage; (ii) fracturing, which involves factors affecting the tortuosity of fractures created by SC-CO2 fracturing, breakdown pressure, and proppant-carrying capacity; and (iii) injection, which involves the twin-headed benefit of enhanced recovery due to CO2/CH4 competitive adsorption and geological sequestration, CO2 vs CH4 excess sorption as a function of pressure, etc. Several research works have indicated discrepancies on how SC-CO2 impacts different shale properties. Some studies show low-pressure N2-gas-adsorption-derived surface area and total pore volume to be increasing with SC-CO2 imbibition, while others show a decreasing trend for the same. Similarly, for some shales, the quartz content, along with the clay mineral contents, decreased as the exposure to SC-CO2 increased, while in some other studies, with similar long-term exposure to SC-CO2, the quartz content was observed to increase along with the decrease in clay content and vice versa. Essentially, the increased exposure to SC-CO2 results in the dissolution of primary porous structures and fractures, and reformation of newer porous structure and conduits in shales. Nonetheless, these changes in the mineralogy weaken the microstructure of the rock bringing significant changes in the mechanical properties of the shales with implications on the wellbore stability and fracturing efficiency. The mechanical properties such as uniaxial compressive strength (UCS), Young’s modulus, and tensile strength decrease as the SC-CO2 saturation period increases. However, some studies have shown factors like bedding angle and phase-state of CO2 having varying effect on the strength behavior of the shales. Moreover, changes in the structure of shales caused by the creation of fractures and the reduction of their strength can also pose major risks, because of potential leakage of CO2 through these created pathways. How these processes would interact at field scale would control the sealing capacity, especially at field-scale for addressing long-term seepage of CO2. 

Impact of Supercritical CO2 on Shale Reservoirs and Its Implication for CO2 Sequestration

Prediction of the shale gas permeability: A data mining approach

Hydraulic fracturing with low and high viscous injection mediums to investigate net fracture pressure and fracture network in shale of different brittleness index

With the continuous advancement of oil and gas field development technologies, there has been an increasing focus on the exploration of unconventional oil and gas resources, such as those with low permeability and density. Among these, Hydraulic fracturing (HF) has emerged as an effective technology to enhance the recovery of unconventional oil and gas resources. Scholars have made significant progress in understanding fracture initiation and propagation through laboratory tests. Building upon this foundation, this paper comprehensively analyzes and summarizes the research on fracture propagation and extension during the HF process. It provides a summary of the laboratory simulation experimental setup, materials used, and experimental procedures, while also comparing the rock fracture pressures under various experimental conditions. Furthermore, the fracture propagation behavior of different rocks under diverse influencing factors is thoroughly examined. The experimental results are systematically differentiated and integrated to investigate the influence of various factors on fracture propagation. The findings of this research offer valuable guidance for experimental design and serve as a reference for future fracturing operations. 

Experimental simulation of fracture propagation and extension in hydraulic fracturing: A state-of-the-art review

Hydraulic fracturing has completely revolutionized how shale resources are exploited to extracthydrocarbons. However, sustainability and environmental issues have fueled the desire for alternate fracturing technologies. Supercritical carbon dioxide (Sc-CO2) fracturing is a new method that uses high-pressure, high-temperature CO2 in its supercritical state (7.38 MPa and 31.1 °C) to generate fractures in shale formations, thereby increasing reservoir permeability. This Review thoroughly analyzes the effects of Sc-CO2 on shale reservoirs during the fracturing process, including the factors that affect fracture breakdown pressure and the complexity and roughness of fracture induced by Sc-CO2. Sc-CO2 can fracture rock at a fracturing pressure lower than that of slick water. CO2, in its supercritical state, shows strong permeability, low viscosity, and extremely low surface tension, like gas, because of which it can infiltrate any space larger than its kinetic diameter (0.330 nm). Many research investigations illustrate how Sc-CO2 affects shales with diverse pore structures, mineral compositions, and mechanical properties when exposed to Sc-CO2 for several hours to days. It is challenging to effectively store CO2 in shale gas reservoirs due to their low permeability and limited storage capacity. To improve the effectiveness of CO2 storage, shale can be fractured to increase the pore space and surface area in reservoirs. Thereby, a certain amount of the CO2 pumped for shale gas production can be securely stored in shale formations, reducing carbon emissions and allowing for a zero-carbon footprint. This paper discusses the pathway and different chemical reactions involved in the storage of CO2 after fracturing over a period. However, fully understanding the interactions between Sc-CO2 and shale rock and the possibility of long-term storage of CO2 in shale formations is quite challenging. It is because of a lack of research and limited knowledge in the above field. Hence, more investigation and development are required in the research area of Sc-CO2 fracturing and the interaction of CO2 with shale rock. 

Supercritical Carbon Dioxide Utilization for Hydraulic Fracturing of Shale Reservoir, and Geo-Storage: A Review

Experimental Assessment of Fracture Initiation and Wettability in Low and High Brittle Shales by CO2 Foam Fracturing Fluids

Tarbela dam: resolution of seepage.

Antimycoplasma drugs paved the way for the treatment of mycoplasma infections even before the vaccines were developed. Macrolides, tetracyclines, or aminoglycosides were being used for long, incorporating quinolones in the newer regimes. Former drug classes target protein synthesis, whereas the latter one targets DNA synthesis. These drugs proved quite effective and helped minimize severity and loss to farmers. However, prolonged use of antibiotics may have serious concerns of drug resistance, side effects, and drug residues in food animal products. In addition, despite treating mycoplasma infection, some drugs create a carrier state of infection that can spread the infection without being recognized. Hence, novel alternate therapeutics need to be explored to overcome these limitations whilst checking the antibiotic resistance with regular antibiotic sensitivity tests, which need to be standardized for different mycoplasma. Future therapeutics are needed under the prevailing situations of emerging and transboundary spread of mycoplasma infections. 

A mini-review on antimycoplasma antibiotics

Studied drill core of the Sungai Perlis Beds in the Terengganu State, Peninsular Malaysia, consists of a diverse suite of organic rich shale lithofacies at various subsurface depths. Lithofacies were delineated on the basis bedding thickness, mineralogy, and organic richness carried out by X-ray diffraction and (XRD) total organic content (TOC) analysis. Field-emission scanning electron microscopy (FE-SEM) analysis was carried out to identify the pore structure, range of pore sizes and mineral arrangement within each lithofacies unit Elastic geomechanical properties (Young’s modulus and Poisson’s ratio) were measured on core samples from each lithofacies unit using static and dynamic approaches. The identified lithofacies include moderate TOC massive argillaceous shale (MTOCMAS), high TOC massive argillaceous shale (HTOCMAS) and high TOC massive siliceous shale (HTOCMSS). FE-SEM analysis shows that pores in all the lithofacies are mostly intergranular, associated with phyllosilicate minerals, and ranges from <1 micrometer to approximately 8 micrometer. Static and dynamic elastic properties measurement shows that shale lithofacies having a high content of phyllosilicate minerals has a lower Young’s modulus and a high Poisson’s ratio value while lithofacies rich in brittle minerals has a higher Young’s modulus and a low Poisson’s ratio value. A good correlation exists between mineralogical brittleness index of the lithofacies and their plane strain modulus. The plane strain modulus is found higher in lithofacies having abundance of brittle minerals in the matrix suggesting that the high TOC massive siliceous shale (HTOCMSS) would preferably allow formation of dense hydraulic fractures, as observed in the CT scan analysis of triaxially fractured specimen, compared to argillaceous shale lithofacies.

Elastic properties and pore structure characterization of selected shales in the sungai Perlis beds, Terengganu, Peninsular Malaysia

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