Fracture toughness investigation of an indigenous limestone rock formation
Khaqan Khan , Naser Al-Shayea
كلية الهندسة-جامعة الملك فهد للبترول والمعادن · السعودية
Hydrofracturing is a well known technique used to create fractures in a rock formation in order to enhance the oil or gas recovery from a reservoir of low permeability. Results of various analytical approaches to understand this technique are very sensitive to the fracture toughness of the rock material. Fracture toughness is a strength parameter in terms of ease of initiation and propagation of a preexisting rack subjected to some externally applied stress field. Due to high cost involved in field scale testing, laboratory based testing under reservoir simulated conditions of temperature and pressure using small core specimens remains the only alternative to have a representative value of fracture toughness of a rock formation. Nevertheless, poor quality in some cases and most of the time limited amount of core based specimens from a deep-seated formation still remains a big hurdle in a comprehensive experimental investigation. However, the problem could be addressed if the outcrop specimens, instead of reservoir, from the same geological formation could be used for fracture toughness investigation. An experimental fracture toughness investigation was carried out using specimens collected from Khuff formation in the Ghawar region, the largest oilfield in the world producing oil and gas from multi-reservoir zones. The study was focused on reservoir specimens form a depth of 3.5 km as well as form outcrop under both ambient and reservoir simulated temperature and pressure. Notched Brazilian disk specimens under diametral compression and notched semicircular disk and circular rod specimens under three point bending were used for this purpose. The results reveal that outcrop specimens can be successfully used to predict the fracture behavior at in-situ conditions. Furthermore, the effect of confining pressure on fracture toughness was more pronounced than the temperature. The fracture toughness showed an increase of 100 to 250% over the ambient value under effective reservoir confining pressure of 28 Mpa (4000 psi); whereas an increase of only 20 to 50% under reservoir temperature of 116 oC was observed. Experimental results were in good agreement with the theoretical fracture criterion based on maximum tangential stress. Moreover, it was observed that the experimental crack propagation trajectories follow a path which is high in tensile stresses.