Characterization of productive fractures based on geomechanical study at Yufutsu fractured reservoir

Productive fractures in the basement fractured reservoir were characterized from a view point of geomechanics. “The critically stressed fracture model” was assumed in this study, which suggests the fracture under the critical stress state for shear slip is permeable because the apertures are sustained by shear dilatancy, and hence, those fractures are expected to contribute to the productivity of the well. In this paper, fractures in the Cretaceous granitic basement and the tight conglomerate of the Eocene Ishikari Group at depth of around 4,000 m in the Yufutsu oil/gas field, Hokkaido, Japan are discussed. Resistivity images obtained at reservoir section of all the wells in the Yufutsu field were fundamentally utilized to detect fractures. Temperature profiles during production were used to identify the productive zones along the well in the reservoir section. Super wide conductive sinusoids (over 5cm) on resistivity images, hereinafter called “mega fracture”, were found at 70% of the temperature anomaly zones. The number of the mega fractures was well correlated with the productivity of the wells. These facts suggest that the majority of gas inflow from the formation should be controlled by mega fractures, which have super wide apertures. From the geomechanical point of view, the identified mega fractures tend to align along the optimum orientation for shear failure under the critical stress state, corresponding with the critically stressed fracture model. We conclude that the critically stressed fracture model is applicable to the naturally fractured reservoir in the oil gas field.