IMAGING FRACTURE NETWORKS USING JOINT SEISMIC AND ELECTRICAL CHANGE DETECTION TECHNIQUES

Imaging fractures is a notoriously difficult task. Generally speaking this difficulty is attributed to the tortuous nature and fine structure in fractured systems. These features are often challenging to resolve in field settings due to temporal and/or spatial constraints. In an effort to highlight the advancements in geophysical imaging of fractures, as well as the topics where the most gain could be realized from targeted research, this SubTER team undertook a number of extensive near field fracture imaging experiments. During the initially phase of this SubTER project, Sandia National Labs (SNL) conducted a series of high resolution seismic imaging campaigns designed to characterize induced fractures. Fractures were emplaced using a novel explosive source that limits damage to the borehole. In the next phase of the project, SNL and its collaborators (LBNL, LLNL, and PNNL) developed and demonstrated emerging seismic and electrical geophysical imaging technologies that characterized 1) the 3D extent and distribution of fractures stimulated from the explosive source, 2) 3D fluid transport within the stimulated fracture network through use of a particulate tracer, and 3) fracture attributes through advanced data analysis. The project consisted of two phases. The objective of the first phase was to collect a comprehensive set of 4D crosshole seismic and electrical data to image the fracture network generated from a novel explosive source. In addition, autonomous seismic and electrical resistance tomography (ERT) data were collected to image the migration of a tracer designed to enhance the electrical conductivity contrast of the fracture network. Near real-time 4D ERT imaging was tested and demonstrated during this phase. The objective of the second phase was to use data collected during the first phase to 1) develop methods of estimating fracture attributes from seismic data, 2) develop methods of assimilating disparate and transient data sets to improve fracture network imaging resolution, and 3) advance capabilities for near real-time inversion of cross-hole tomographic data. Advancements in these areas are relevant to all situations where fracture stimulation is used for reservoir stimulation (e.g. Enhanced Geothermal Systems (EGS) and tight shale gases).