Acoustic monitoring of hydraulic fracture growth

Hydraulic fracturing is a technique to improve the inflow performance of oil and gas wells by creating large fractures around the borehole. By doing this, a highly permeable path is created through which the hydrocarbons can flow to the well more easily. The process consists of two phases. In the first phase, a suitable fluid is pumped into the reservoir under great pressure, thereby rupturing the formation and creating fractures. In the second phase, a propping agent, usually a well-sorted sand, is added to the fluid, and after injecting this mixture for some time, the pumps are stopped. The remaining fluid in the fracture leaks away and the fracture closes on the proppant, thus providing the highly conductive path aimed for. The success of fracture treatments depends on our ability to predict and influence the fracture shape and orientation. In the Geometry of Hydraulic Fractures project, sponsored by a consortium of oil and service companies and the Dutch Technology Foundation, the physics of hydraulic fracture growth is being studied. To this end, scaled fracturing experiments are being carried out at the rock-mechanics laboratory at the faculty of Applied Earth Sciences. These hydraulic fracture experiments on model blocks are monitored by an acoustic scanning technique using active transducers. An artificial rock cube (made of cement or plaster) with edges of 0.3 m, is placed in a compression machine to apply an in-situ confining stress. A fracturing fluid is injected through a borehole assembly mounted in the block and eventually a fracture is created inside the block. The fracture grows with a rate of about 0.1 mm s71. After a certain time the pump is stopped (called shut-in) and the fracture is allowed to close again. The total duration of the experiment is in the order of hours. During all stages of fracture initiation, growth and closure, acoustic waves scan the complete block every 30 s. The recording time of a separate scan is in the order of one millisecond, so each one can be regarded as a still picture taken of the growing hydraulic fracture. With 48 transducers scanning the entire block, different combinations of sending and receiving transducers or records, reveal different aspects of the fracture, e.g. radial extent and width. Because we use both P- and S-waves, different features of the scattering of acoustic waves by fractures can be observed. Some aspects of these measurements have been discussed by Groenenboom & Romijn (1996) and Groenenboom et al. (1997a). These laboratory experiments closely resemble seismic monitoring surveys of hydraulic fracturing jobs as performed in the field (Wills et al. 1992; Meadows & Winterstein 1994).