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Karst fracture-cavity carbonate reservoirs, in which natural cavities are connected by natural fractures to form cavity clusters in many circumstances, have become significant fields of oil and gas exploration and exploitation. Proppant fracturing is considered as the best method for exploiting carbonate reservoirs; however, previous studies primarily focused on the effects of individual types of geological formations, such as natural fractures or cavities, on fracture propagation. In this study, true-triaxial physical simulation experiments were systematically performed under four types of stress difference conditions after the accurate prefabrication of four types of different fracture-cavity distributions in artificial samples. Subsequently, the interaction mechanism between the hydraulic fractures and fracture-cavity structures was systematically analyzed in combination with the stress distribution, cross-sectional morphology of the main propagation path, and three-dimensional visualization of the overall fracture network. It was found that the propagation of hydraulic fractures near the cavity was inhibited by the stress concentration surrounding the cavity. In contrast, a natural fracture with a smaller approach angle (0° and 30°) around the cavity can alleviate the stress concentration and significantly facilitate the connection with the cavity. In addition, the hydraulic fracture crossed the natural fracture at the 45° approach angle and bypassed the cavity under higher stress difference conditions. A new stimulation effectiveness evaluation index was established based on the stimulated reservoir area (SRA), tortuosity of the hydraulic fractures (T), and connectivity index (CI) of the cavities. These findings provide new insights into the fracturing design of carbonate reservoirs.

The high temperature and high in-situ stress geological environment can significantly affect the mechanical properties, failure modes, and deformation characteristics of deep shale reservoirs. In this study, real-time high temperature triaxial compressive tests simulating the deep shale formation environment (temperature: 25–150 °C, confining pressure: 0–100 MPa) are carried out. The GSI-strength degradation and constitutive models are derived based on the Hoek–Brown criterion. The results show that in low confining pressure conditions, the mechanical behavior of shale is greatly influenced by temperature. Compared with shale at 25 °C, the compressive strength of shale at 150 °C decreases by up to 13.7%, and the elastic modulus decreases by up to 36.9%. The peak strain was increased by a factor of up to 1.4, and the yield stress level was advanced by as much as 7.4%. However, in high confining pressure conditions, the shale plasticity characteristics are significantly enhanced and the failure mode is relatively single. The GSI-strength degradation model can well characterize the variation law of shale strength with confining pressure under high temperature conditions. The statistical damage constitutive model matches the actual stress–strain curve very well, and it can fully reflect the deformation and failure characteristics of deep shale. The findings of this study can help us better understand the variation of mechanical properties of deep shale.

Abstract: Rocks in many high-temperature underground engineering applications experience complex stress disturbances, which greatly affect their performance. However, the influence of combining the temperature and fatigue loading on the deformation characteristics of granite has rarely been investigated. In this study, multi-level multi cyclic loading tests were carried out on granite samples over the temperature range of 25–600 °C, revealing the influence of temperature, stress level and number of cycles on the deformation characteristics and energy conversion characteristics. The results show that the deformation behavior of granite under fatigue loading at high temperature is different from that at room temperature. It can be divided into three stages, hardening stage (Stage I), transition stage (Stage II) and softening stage (Stage III), which may be applicable to evaluate the deformation of deep rocks under complex geological conditions. A linear energy storage law that is independent of temperature and loading history was observed. A new calculation method for elastic energy at the peak stress was introduced to modify the traditional method based on the assumption of equivalent elastic modulus at the loading and unloading stage, which can provide a more reasonable and reliable estimate of rock brittleness. The experimental results of this study have important theoretical significance and engineering value for long-term stability evaluation and disaster prediction. Article Highlights:

Influence of temperature and damage-controlled cyclic loading on the deformation of rocks are experimentally studied.

A conceptual model of rock deformation is proposed, which may be more suitable for the evaluation of rock deformation under complex geological conditions.

A new method for accurately calculating the pre-peak brittleness index is introduced, which can provide a more reliable estimate of rock brittleness.