Imaging Hydraulic Fractures: Source Location Uncertainty Analysis At The UPRC Carthage Test Site

Abstract

Hydraulic fracturing is a useful tool for enhancing gas and oil production. High-resolution seismic imaging of the fracture geometry and fracture growth process is the key in determining optimal spacing and location of wells and in improving reservoir performance for increased production rate. In this paper, we address how accurately the sources along a fracture zone at different depths can be determined for given velocity models, geophone array geometry configurations, and location of monitor wells. We apply a theory of uncertainty analysis to estimate microearthquake location uncertainties in both relative and absolute senses. To estimate the location uncertainties, we used the velocity models, two geophone arrays in two monitor wells, and the location of the fracture well, and an assumed fracture orientation of an upcoming hydraulic fracturing experiment by Union Pacific Resources Company (UPRC) and its partners at Carthage Field, Panola, Texas. We calculated the 95% confidence regions, in both absolute and relative senses, for five hypothetical sources along an assumed strike of a target fracture zone at three different depths. The semimajor and semiminor axes of the relative error ellipses for these epicenters are typically estimated to be 12 and 5 ft, respectively, and the relative depth uncertainty is derived at about 6 ft. The absolute location uncertainties are at least 3 to 10 times larger than the relative location uncertainties. The high-precision relative source locations result in a relative measurement error about 4-15% in measuring the fracture length. The location ambiguity from two-station locations is discussed and arrival azimuthals is proposed to to be used for removing such location ambiguity. The location uncertainty analysis is expected to be generalized as a practical tool in optimal designing of a two-well seismic monitoring system for high-resolution imaging of hydraulic fractures.