Abstract
Tight glutenite reservoirs are typically characterized by highly variable lithology and permeability, low and complex porosity, and strong heterogeneity. Glutenite brittleness is an essential indicator for screening fracture targets, selecting technological parameters, and predicting the hydraulic fracturing effect of tight glutenite reservoir exploitation. Glutenite formations with high brittleness are more likely to be effectively fractured and form complex fractures. Accurate evaluation of glutenite brittleness facilitates the recovery of oil and gas in a tight glutenite reservoir. Accordingly, two brittleness indexes are proposed in this paper based on energy balance and damage evolution analysis of complete stress–strain curves to evaluate the brittleness of glutenite. Uniaxial and triaxial compression tests of glutenite specimens were carried out and the brittleness indexes were verified by comparison with other existing indexes. The relationships between the mechanical properties and brittleness of glutenite under confining pressure were analyzed based on experimental results and the effects of mechanical and structural parameters on glutenite brittleness are investigated with a numerical approach. The brittleness of glutenite increases with the increase of gravel size and/or volume content. During hydraulic fracturing design, attention should be paid to the brittleness of the matrix and the size and content of gravel. This paper provides a new perspective for glutenite brittleness evaluation from the perspectives of energy dissipation and damage evolution. Our results provide guidance for fracturing layer selection and may also facilitate field operations of tight glutenite fracturing.
Highlights
The exploration and development of tight glutenite reservoirs have attracted increased attention in recent years
Two novel brittleness indexes are proposed based on the energy balance and damage evolution analysis of the complete stress–strain curve to evaluate the brittleness of glutenite reservoirs
The theoretical method provides a new perspective for glutenite brittleness evaluation that considers energy dissipation and damage evolution during the entire process of glutenite failure; a perspective which is innovative for its connection between brittleness and damage evolution
Summary
The exploration and development of tight glutenite reservoirs have attracted increased attention in recent years. Tight glutenite reservoirs are formed by rapid sedimentation near the provenance and are characterized by highly variable lithology, low porosity, poor permeability, and strong heterogeneity [1]. Glutenite cores with fine gravel (Figure 1b) and coarse gravel (Figure 1c) are observed. Conglomerate, psephitic sandstone and pebbled sandstone are observed, demonstrating that gravel size varies at different vertical depths and the gravel size and content
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