Borehole logging method for fracture detection and evaluation

Abstract

In a method for locating fractures in subsurface earth formations traversed by a borehole a first signal is generated representative of low frequency Stoneley waves (that is tube waves) propagating along the borehole from a source mounted in a logging tool. A second signal is generated by an array of detectors on the tool, representative of low frequency Stoneley waves propagating along the borehole and arising from reflection from fractures of the waves propagating from the source. An indication of the presence of fractures is derived by deconvolving the second signal with the first signal and identifying the time and magnitude of the peak value of the envelope of the deconvolved signal. The variation that would be expected in the presence of fracture and the actual variation are compared and a fracture detected when the actual and expected variations match. Alternatively the first and second signals can be cross-correlated and the time of the peak negative value of the cross-covariance determined. The width of a fracture can also be found, by estimating the fracture reflectivity R' for a range of possible fracture widths w according to the equation ##EQU1## where H.sub.n.sup.(1) is the nth order outgoing Hankel function; F is Stoneley wave frequency; S is Stoneley wave slowness; a is the borehole radius; and a is a term related to borehole fluid viscosity (typically set equal to unity). The value of w yielding an estimated value of reflectivity closest to the value derived from the magnitude of the envelope of the deconvolved signal is taken as being indicative of the fracture width.