The interaction between drilling fluid and shale has a significant impact on wellbore stability during shale oil and gas drilling operations. This paper investigates the effects of the drilling fluid activity on the surface and osmotic hydration characteristics of shale. Experiments were conducted to measure the influence of drilling fluid activity on surface wettability by monitoring the evolution of fluid-shale contact angles. The relationship between drilling fluid activity and shale swelling ratio was determined to investigate the osmotic hydration behavior. The results indicate that, with increasing drilling fluid activity, the fluid–shale contact angles gradually increase—the higher the activity, the faster the adsorption rate; and the stronger the inhibition ability, the weaker the surface hydration action. The surface adsorption rate of the shale with a KCl drilling fluid was found to be the highest. Regarding the osmotic hydration action on the shale, the negative extreme swelling ratio (b) of the shale was found to be: b_KCl < b_CTAB < b_SDBS. Moreover, based on the relationship between the shale swelling ratio and drilling fluid activity, shale hydration can be divided into complete dehydration, weak dehydration, surface hydration, and osmotic hydration, which contributes to the choice of drilling fluids to improve wellbore stability.

Bentonite cement paste (BCP) is among the grouting materials used widely in large-pore grouting and karst cave treatment. The mechanical properties of bentonite cement paste (BCP) will be improved by additional basalt fibers (BF). In this study, the effects of basalt fiber (BF) contents and their lengths on the rheological and mechanical properties of bentonite cement paste (BCP) have been examined. Yield stress (YS), plastic viscosity (PV), unconfined compressive strength (UCS), and splitting tensile strength (STS) were used to evaluate the rheological and mechanical properties of basalt fiber-reinforced bentonite cement paste (BFBCP). Scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS) characterize microstructure development. The results indicate that the Bingham model can provide the rheological behavior of basalt fibers and bentonite cement paste (BFBCP). The yield stress (YS) and plastic viscosity (PV) increase as the content and length of basalt fiber (BF) increase. The effect of fiber content on yield stress (YS) and plastic viscosity (PV) is greater than that of fiber length. The addition of basalt fiber (BF) enhanced the unconfined compressive strength (UCS) and splitting tensile strength (STS) of basalt fiber-reinforced bentonite cement paste (BFBCP) at the optimum basalt fiber (BF) content of 0.6%. The optimum basalt fiber (BF) content tends to increase as curing age increases. The basalt fiber length of 9 mm is the most effective for improving unconfined compressive strength (UCS) and splitting tensile strength (STS). The large increments in unconfined compressive strength (UCS) and splitting tensile strength (STS) were 19.17% and 28.21% for the basalt fiber-reinforced bentonite cement paste (BFBCP), with a basalt fiber length of 9 mm and content of 0.6%. Scanning electron microscopy (SEM) shows that the randomly distributed basalt fiber (BF) forms a spatial network structure in basalt fiber-reinforced bentonite cement paste (BFBCP), which composes a stress system under the action of cementation. Basalt fibers (BF) used in crack generation processes slow down the flow through bridging and occur in the substrate to improve the mechanical properties of basalt fiber-reinforced bentonite cement paste (BFBCP).

The marine shale in the Lower Cambrian Niutitang Formation is the dominant shale-gas reservoir in northwestern Hunan, which accounts for more than 70% of unconventional energy in Hunan province. Accurately evaluating the shale-gas exploitation potential is a key to determining whether commercial exploitation standards can be met. In the literature, most existing studies have focused on evaluating the shale-gas exploitation potential based on either accumulation conditions or the shale fracability, which will lead to a gap between the real production and proven gas reserves due to the characteristics of the shale’s low permeability and low porosity. Therefore, further studies are needed to evaluate the effective shale-gas exploitation potential. To address this need, the outcrop shale samples in the Niutitang Formation were collected from the target regions, and the geological characteristics, mechanics behavior, and microstructure performance were discussed via both field data and laboratory tests. The results revealed that the shale-gas exploitation potential in the Niutitang Formation was indicated to be comparable to that of five validated shale-gas exploitation regions in the United States. To further illustrate the effective shale-gas exploitation potential, this study suggested using a comprehensive evaluation framework for this purpose, in which both accumulation condition and the shale fracability are simultaneously considered. Therefore, the shale gas reservoir in the Niutitang Formation has highly effective shale-gas exploitation potential by considering both the accumulation conditions and the shale fracability.

Ion stabilizers can enhance the reinforcement slurry effect on the wall and stabilize the wall actively in slurry shields. This paper presents different cation exchange capacities obtained by changing the content of the ion stabilizer (1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, and 6.0%) in slurry associated with the basis of the existing slurry formula through the laboratory tests. In order to study the effect of the cation exchange capacity on the performance of slurry properties, the rheological properties of the slurry are analyzed and evaluated by the power law model. Results indicate that the cation exchange capacity of the slurry decreases first and then increases with the increase of the ion stabilizer content. When the content of the ion stabilizer is maintained at 3.50%, the cation exchange capacity reaches the minimum value of 2.92. The filtration volumes, pH values, and rheological parameters of the slurry also indicate an obvious linear change with the change of the cation exchange capacity. The minimum filtration volume is 9.70 mL/30 min when the ion stabilizer content reaches 3.50%. However, the pH value reaches the maximum, that is, 11.34 which is changed from 10, and the change could be considered as a constant value in the field work. When the cation exchange capacity increases, the continuity of polymer structure in the slurry decreases first and then increases, the flow index and consistency coefficient are located within a reasonable range, and the rheological properties of the slurry meet the design requirements of the standards.