Active and Passive Seismic Imaging of a Hydraulic Fracture in Diatomite
Authors: Vinegar H.J. , Wills P.B. , DeMartini D.C. , Shlyapobersky J. , Deeg W.F.J. , Adair R.G. , Woerpel J.C. , Fix J.E. , Sorrells G.G. 
Published: December 2007

Summary
A comprehensive set of experiments including remote- and treatment-well microseismic monitoring, interwell shear-wave shadowing, and surface tiltmeter arrays, was used to monitor the growth of a hydraulic fracture in the Belridge diatomite. To obtain accurate 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.
Data analysis indicates that the minifracture and main hydraulic fracture stimulations resulted in a nearly vertical fracture zone (striking N26E) 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, process zone of reduced shear velocity, which remote-well microseismic data independently suggest may be as wide as 40 ft. The experiments indicate complicated processes occurring during hydraulic fracturing that have significant implications for stimulation, waterflooding, infill drilling, and EOR. These processes are neither well understood nor included in current hydraulic fracture models.

Introduction
At the very close well spacings in the Belridge diatomite, reservoir performance is dominated by the fracture system, both natural and hydraulically induced. Fracture azimuth, height, extent, and possible existence of a wide process zone surrounding the main crack are all critical because, if neighboring fractures link up either laterally or vertically, undesirable bypassing will occur during waterflooding and other enhanced recovery operations. Thus, understanding the hydraulic fracture process in Belridge is essential for optimizing diatomite reservoir performance.
Existing hydraulic fracture models depict a single, narrow (less than 1-in. thick), vertical, planar fracture with two symmetric wings radiating from the wellbore with an orientation orthogonal to the direction of least principal stress. With these models, leakoff of fracture fluids into the formation would suggest a leakoff-affected zone of several feet for the diatomite formation.
This perception of the fracturing process may be too simplistic in light of many laboratory and field observations. laboratory experiments on manmade materials indicate that a process zone is created around the main crack. The details of the process zone vary from material to material. Depending on the conditions under which the fracture is created, the process zone would contain tensile and/or shear cracks, as well as other transformations of the original material. The growth of the main crack is accompanied by the evolution of the process zone that often controls the fracturing process. The process zone induced during process. The process zone induced during hydraulic fracturing manifests itself indirectly by "abnormal" net fracture treatment pressures much larger than can be simulated wiih classic single hydraulic fracture models.
Field studies have provided evidence of multiply fractured, hydraulically communicating process zones in almost every case, especially when the material is heterogeneous. Examples include the Gas Research Inst. Second Staged Field Experiment, mineback and coring experiments after a hydraulic fracture, and geological analogs such as microfracturing around dikes. Green et al. using treatment-well vertical seismic profiles (VSP's) and interwell seismic surveys, reported an overlay between a low-velocity anomaly and the microseismic cloud in a hot, dry rock geothermal system.