Active and passive seismic imaging of a hydraulic fracture in diatomite

A comprehensive set of experiments was used to monitor the growth of a hydraulic fracture in the Belridge diatomite, including remote and treatment well microseismic monitoring, interwell shear wave shadowing, and surface tiltmeter arrays. To obtain high accuracy measurements, an extensive subsurface network of geophones was cemented spanning the diatomite formation in three closely spaced observation wells around the well to be fracture treated. Analysis of the different data sets indicates that the mini and main hydraulic stimulations resulted in a nearly vertical fracture zone, striking N 26°E. This fracture zone was vertically segregated into two separate elements, the uppermost of which grew 60 ft above the perforated interval. The interwell seismic effects are consistent with a wide process zone of reduced shear velocity, which remote well microseismic data independently suggest may be as wide as 40 ft. Together, the experiments indicate complicated processes occurring during hydraulic fracturing which have significant implications for stimulation, waterflooding, infill drilling, and enhanced oil recovery. These processes are not well understood and are not included in current hydraulic fracture models.