Abstract
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.
1. INTRODUCTION
Natural fractures are ubiquitous and play key roles in ground-water hydrology [1-3], construction of subsurface facilities [4], hydrocarbon resources exploration and production [5-8], earthquake precaution [9, 10] and nuclear waste sequestration [11]. In carbonate reservoirs, steeply dipping fractures are the most common type that are a result of imbalanced vertical and horizontal stresses [12]. 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 [16] that results in orthogonal cross-joints in many geological sites.
