A Fracture Toughness-Based Evaluation Method for Deep Shale Oil Reservoir Compressibility

Compressibility is a key parameter in shale volumetric fracturing design and effect evaluation, which reflects the ability of shale oil reservoirs to be pressed open and form complex network fractures of a certain scale under existing technology conditions. Existing compressibility index evaluation methods are mainly based on static data such as experimental data or logging evaluation, and the influencing factors of compressibility are different, resulting in unsatisfactory evaluation results. In order to solve the shortcoming of using brittleness index only to evaluate shale reservoir compressibility, this paper not only considers the influence of fracture toughness on compressibility, but also introduces another influencing parameter – critical mechanical energy release rate. The effect of type I fracture toughness on shale joint and fracture development and the average critical mechanical energy release rate during shale reservoir sliding are studied. Finally, combined with the sliding average critical mechanical energy release rate and brittleness index, a primary model of shale reservoir compressibility evaluation based on fracture development index is established. This method can provide ideas for accurately evaluating the compressibility of deep shale reservoirs.