A tri-axial electromagnetic induction tool for hydraulic fracture diagnostics

The monitoring and diagnostics of induced fractures are important for the real-time performance evaluation of hydraulic fracturing operations. Previous electromagnetic (EM) based studies show that single backbone tri-axial induction logging tools are promising candidates for the real-time monitoring and diagnosis of fractures in uncased wells. To support the development of field deployable tools, the concept must be tested in experiments, in a controllable environment, before it is tested under field-like conditions. To this end, we have developed numerical tools which can simulate any wellbore environment while logging hydraulic fractures with the induction tool. We have designed and built a prototype induction tool and performed two sets of tests to compare with numerical simulation results. The computational and experimental setup consists of tri-axial transmitter and receiver coils in co-axial, co-planar and cross-polarized configurations. Both lab and shallow earth measurements are shown to be in good agreement with simulations for all examined cases. The average relative and maximum discrepancies of the measured signals from the simulated ones were lower than 3% and 10%, respectively. With the prototype tool, strong signals sensitive to the fracture’s surface area and dip-angle were measured in the co-axial coil configuration, while weaker signals sensitive to the fracture’s aspect ratio were observed in the co-planar configuration. Cross-polarized signals are also shown to be strong and sensitive to the fracture’s dip. Lastly, we resolved the detectable components of the measured signal tensor to obtain parameters for simplified fracture geometries. The inversion algorithm, a derivative free directional search model, uses an objective function defined as a combination of co-axial and cross-polarized signals from different tool spacing, and the function provides a well behaved global minimum. The robustness of the inversion algorithm is tested on synthetic data for single cluster fractures in a homogeneous and heterogeneous background electrical conductivity. All the effective model parameters for different cases, electrical conductivity, size and dip-angle, are shown to be recovered with good accuracy. We also evaluated the effect of neighboring fractures and suggested a multi-cluster inversion path which can recover the proppant distribution in a stage very accurately. Based on the numerical and experimental results we suggest a tool with specifications that can effectively recover far-field proppant distribution in the fractures.