Numerical study of an electrode-based resistivity tool for fracture diagnostics in steel-cased wellbores

The efficiency of a hydraulic fracture treatment depends primarily on the dimensions and orientation of propped fractures. We have developed a novel electrode-based resistivity tool concept for mapping proppant distribution in hydraulic fractures in steel-cased wellbores. The proposed tool configuration is shown to overcome the severe limitations of induction tools for the detection and resolution of propped fracture geometries in such wellbores. The concept makes use of an array of electrically insulating gap subsections, which are installed and cemented as permanent parts of the casing string, separating the casing sections. By imposing voltages on the insulating gaps, the conductive casing is excited directly, thus avoiding through-casing signal degradation caused by its high electrical conductivity. This allows us to detect subsurface fractures propped with conductive proppant. The envisioned measurements are performed by running a bottom-hole assembly into the fractured zone on a coiled tubing to impose a voltage across each insulating gap at a time, before and after hydraulic fracture operations. For each excited insulating gap, the voltages across all other insulating gaps are recorded by the electronics embedded in the insulating gaps. To interpret the envisioned measurements, a forward model of the tool, based on a finite volume method, is developed, and the design's sensitivity to the fracture parameters is demonstrated via case studies. The results indicate that measurements made based on the proposed concept will be highly sensitive to a fracture's location, size, and angle, and less sensitive to a fracture's shape. Simulations also indicate that direct contact of the fracture with an excited casing section enables the differentiation of fractures of up to a 100 m radius. Fractures with angles greater than 30° or aspect ratios greater than two can also be distinguished from the ones orthogonal to the well or with an aspect ratio of one.