This study focuses on using borehole fracture traces to estimate the size and aspect ratio of long and narrow subsurface fractures in sedimentary rocks. The method is well suited to fractures with length greater than borehole diameter and width less than borehole diameter, or more specifically apparent width as projected into the plane perpendicular to the borehole axis. Rectangles are assumed to be the shape of fractures in sedimentary rocks. Fracture traces associated with long and narrow (elongate) fractures are often caused by fractures piercing through either one side (single piercing) or two sides (double piercing) of the borehole. In such cases, a fracture trace map or borehole image will contain characteristic traces. An approach is described to estimate the size and aspect ratio of fractures based on borehole diameter and the intersection counts of singly and doubly piercing fractures. The method is directly applicable to large diameter cylinders, such as shafts and tunnels, with a few modifications.
Fracture shape, size and orientation have a great impact on the permeability of fractured rock oil and gas fields and also affect the geomechanical properties of rock masses. Since fractures are embedded in an opaque rock mass, these parameters are commonly estimated from rock mass outcrops, boreholes, tunnels, or other exposures, or from geophysical imaging [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12]. Recently Wang et al.  proposed an approach to estimating fracture size in sedimentary rocks by using borehole or tunnel data, in the special case where all fractures are of constant size. In this paper we focus on estimating the size and shape of piercing-type fractures in sedimentary rocks. Here, the piercing-type fractures refer to long and narrow fractures with apparent length greater than borehole diameter and apparent width less than borehole diameter (Fig. 1).