Steeply dipping fractures and horizontally oriented stylolites occur in carbonate reservoirs and lead to the formation of orthogonal fracture networks. Interpretation of elastic waves propagated through orthogonal fracture networks is complicated because of the potential existence of guided modes along and between fractures, multiple internal reflections, as well as scattering from fracture intersections. The existence of some or all of these potentially overlapping modes depends on the local stress field that can preferentially close or open either one or both sets of fractures. For this study, an acoustic wavefront imaging system was used to examine the effect of bi-axial loading conditions on acoustic wave propagation in isotropic media with two orthogonal fracture sets. From the experimental data, guided wave behavior was affected by fracture spacing and fracture specific stiffnesses. The fracture stiffness differed among the fractures in the two sets and also differed from the stiffness of the intersections. Interpretation of the properties of fractures or fracture sets must consider the existence of non-uniform fracture stiffness within and among fracture sets, as well as consider the effect of fracture intersections.
Natural fractures are ubiquitous and play key roles in ground-water hydrology [1-3], construction of subsurface facilities , hydrocarbon resources exploration and production [5-8], earthquake precaution [9, 10] and nuclear waste sequestration . In carbonate reservoirs, steeply dipping fractures are the most common type that are a result of imbalanced vertical and horizontal stresses . On the other hand, horizontally-oriented or strike-perpendicular fracture/joints are also frequently observed [12, 13]. The major causes of the formation of strike-perpendicular fracture/joints are either pressure dissolution of mineral material in rocks (which forms horizontally-oriented stylolites [14, 15]) or stress rotation mechanisms  that results in orthogonal cross-joints in many geological sites.
Schemeta, J.E. (Unocal Corporation) | Minner, W.B. (Unocal Corporation) | Hickman, R.G. (Unocal Corporation) | Johnston, P.A. (Unocal Corporation) | Wright, C.A. (Pinnacle Technologies Inc.) | Watchi, N. (Japan National Oil Corporation, Japan)
UNOCAL and Japan National Oil Corporation jointly carried out a hydraulic fracture operation in the Monterey Formation in the Newlove 110 well, located in the Santa Maria basin in central coastal California, USA. The treatment well was monitored by an array of tiltmeters to measure the surface deformation, and with a 3-component oriented geophone to monitor microseismic events during the stimulation. There is good agreement between the results of the two monitoring programs, each showed the procedure produced a vertical fracture with an azimuth of about N 60 E. This orientation also agrees well with the regional data from tectonic stress and fracture orientations.
In October, l992, as part of a multi-faceted cooperative research program focused on fractured reservoirs, UNOCAL and Japan National Oil Corporation pumped a hydraulic fracture into the Newlove 110 well located in the Orcutt oil field of central coastal California (Figure 1). This paper discusses the procedure used to place the proppant and the results of two geophysical monitoring techniques used to monitor the placement of the treatment. The objective was to determine the fracture geometry and the effect the natural fractures would have on the well stimulation.
Geology. The Orcutt oil field is located in the Santa Maria basin (Figure 1). The hydraulic fracture was pumped into the Monterey Formation, which consists of a variety of lithologies (clay, siliceous and phosphatic shales, porcelanites, dolomites, cherts, and sandstones) interbedded on a scale of inches. The hard, brittle lithologies such as chert and dolomite are more densely fractured than porcelanites or siliceous shales, which in turn are more fractured than associated clay or phosphatic shales.
The fracture density and orientation in the Monterey Formation (2030 to 2805 ft) in the Newlove 110 well of the Orcutt oil field was determined to be N 60 E from core, formation microscanner, and borehole televiewer logs.
The examination of the Monterey Formation oucrops regionally indicates that they are dominated by northeast-striking fractures that are at a high angle to bedding which form a very consistent pattern (Figure 2; Table 1). The vector mean of the regional outcrop fractures is N39.0 E 23.3.