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Deep shale has gradually become a focus point for unconventional natural gas exploration and development in China and even the world. The key to deep shale development is the fracability evaluation of a reservoir; conventional evaluation methods cannot provide accurate evaluations of deep shale fracability under high confining pressures. In this study, experimental methods are used to obtain the mechanical parameters, mineral composition, and stress–strain characteristics of shale. The fracture complexity coefficient based on the fractal dimension and fracture angle is defined by analyzing the fracture morphology of shale samples. Using the rock fracturing complexity as an index, the weight coefficients of different factors for the shale fracture morphology are obtained, thus establishing a new evaluation model for the deep shale brittleness index. The results show that the Young's modulus, dilatancy angle, and peak strain are the main factors affecting the complexity of shale fractures. The gray correlation theory is used to calculate the weights of various factors in the model. The weight coefficients of the Young's modulus, dilatancy angle, and peak strain on the rock brittleness index are 0.262, 0.353, and 0.385, respectively. Based on this, a novel method for evaluating the fracability of deep shale using the brittleness index is proposed. The fracability index of the horizontal section of the deep shale gas well in this area was calculated, and the location of the fracturing perforation cluster was optimized accordingly. This technology has been successfully applied in more than 10 deep shale gas wells in the southern Sichuan Basin, effectively reducing the fracturing pressure by 5–10 MPa, increasing the average daily production of a single well by 15.3 × 104 m3. This paper proposes a novel fracability evaluation method for deep shale based on the brittleness index, which provides a reference for optimizing deep shale fracturing design.

The development of shale gas is faced with low reservoir porosity, low reservoir permeability and high formation fracture pressure. In order to deal with these problems, this paper selected shale samples from the bottom of the Lower Silurian Longmaxi–Wufeng Formation of Upper Ordovician in the Weiyuan Block of the southern Sichuan Basin as the research objects. After acid dissolution experiments were carried out, the microstructures and the mechanical parameters of shale after acidizing treatment were investigated by means of X-ray diffraction, scanning electron microscopy and triaxial mechanical test. Then, the effect of acidizing treatment on the microstructures and the mechanical properties of shale were analyzed. And the following research results were obtained. First, after acidizing treatment, the carbonate mineral content of shale decreases and the number and size of pores increase. In the process of dissolution, micro-fractures occur, leading to the increase of shale porosity and permeability. Second, after acidizing treatment, the mechanical properties of shale change. Its deformation mode transforms gradually from elastic–brittle deformation to elastic–plastic deformation, and its fracture mode transits from brittle to semi-brittle and semi-ductile. Third, after shale is treated with the acid with the concentration of 15% for 240 min, its permeability is increased by 3.09 times. After 3 days, its porosity is increased by 1.65 times. And after 7 days, its compressive strength, Young's modulus and brittleness index are decreased by 50.1%, 58.1% and 32.8%, respectively. Fourth, the mechanical parameters of shale of the Longmaxi–Wufeng Formation in the Weiyuan Block is in a quadratic relationship with an average pore size and permeability and in a quadratic or linear relationship with porosity, and their correlation is strong. In conclusion, the research results provide technical support for the prediction of the mechanical parameters of shale in this block after acidizing treatment and for the design of acid fracturing scheme.

The pulse fracturing is widely used in unconventional reservoirs. It alternately pulse pumping the proppant slurry and clean fluid to form discontinuous placement proppant pillars in the artificial fractures and the pulse fracture conductivity is several orders of magnitude higher than conventional hydraulic fracture conductivity. However, the understanding of the deformation law of proppant pillar under the action of closure pressure and proppant normal stress is unclear, resulting in difficult to calculate the fracture conductivity and prefer proppant. Firstly, replacement construction and experimental displacement by Renault Similarity Criteria, three typical proppant pillars placement structures are extracted through the large-scale visualized flat plate device. The Young's modulus of the proppant pillars are calculated in modified API conductivity cell. Secondly, proppant pillars are dispersed into particles by the Smooth Particle Method (SPH). Using the parameters obtain from the above experiments, fracture-proppant pillar contact models are established to simulate the deformation process of proppant pillar and get normal stress of proppant particles. Thirdly, extracting the shape of stabilized proppant pillars, establish the fracture-proppant pillar flow model, calculate the fracture conductivity in different closure pressure. The simulation results show that as the closure pressure increases from 14MPa to 41MPa, the fracture width present an accelerated downward trend, The fracture width under the support of the initial radius of 9 mm proppant pillars are the largest, decreasing from 2.52mm to 1.72mm, the larger the radius of the proppant pillar, the greater the fracture width, the normal stress of three types of proppant pillar particles are both changed from 73MPa to 110MPa. The elliptical cylinder proppant pillar has the largest fracture conductivity. Its fracture conductivity is reduced from 12500D•cm to 3630D•cm. The larger the construction displacement and the pulse time of proppant slurry, the greater the fracture conductivity. The model in this article can calculate the normal stress of proppant particle and fracture conductivity in different closure pressure, which can significantly guide the choice of construction parameters and the type of proppant.