Hydraulic Fracture Behavior in Soft Rocks under Confining Stresses Observed by Using X-ray CT Method

摘要

Hydraulic fracturing has become applied to the enhancement of oil and gas production not only inrnhard rocks but also soft rocks such as unconsolidated sands. However, in the latter case, there stillrnremain many uncertainties for the fracture behavior in the soft rocks. We present here the laboratoryrnexperiments in which hydraulic fracturing was carried out in the unconsolidated-sand specimens andrnthe fracture behavior was monitored by the X-ray CT method. For the experiments, a test vessel wasrndeveloped to hold the soft specimen in a cylindrical shape and to apply confining stresses to thernspecimen. A part of the vessel is made from a Carbon Fiber Reinforced Plastic, CFRP, as X-rayrnpasses easily without interruption. The test vessel is so compact to be placed on a bed of the X-ray CTrnscanner. The transverse multi-slices of the cylindrical specimen were successfully obtained as the CTrnimages, while a viscous oil was being injected to induce fractures inside the specimen. In the CTrnimages, there appeared clearly the induced fracture and the region into which the fracturing fluid, i.e.rnthe viscous oil, permeated from the fracture surfaces. The CT images were analyzed to estimate thernfracture width. The results show that (ⅰ) the injection pressure necessary for fracture propagation wasrnclose to but larger than the compressive stress acting normally to the fracture, (ⅱ) the fracturing fluidrninvaded ahead of the fracture tip during the fracture propagation, and (ⅲ) the fracture width was a fewrntimes larger than the estimation based on the linear elastic fracture mechanics.