Hydraulic fracturing is a widely used technique for production enhancement. One critical property of a hydraulic fracture is the fracture conductivity, which has a direct impact on the productivity and hence, the economics of the stimulated well. Several laboratory techniques have been developed to characterize the materials that are placed in the fracture and influence its conductivity. Conductivity laboratory measurements have become a routine experiment with well-established procedures and setups aimed at simulating downhole conditions. However, correlating the performance of materials in a conventional laboratory setup with their performance in real conditions is not straightforward. This problem occurs because conventional laboratory setups are of limited length scale and fail to capture processes and variations that occur at a scale of the order of, or exceeding, the dimensions of the conductivity cell. Also, even if conventional setups provide accurate and reproducible measurements, they fail to provide fundamental, in-depth understanding of the actual processes that occur at the pore scale, occur in the fracture, and influence the fracture conductivity.
A unique, large-scale X-ray computer tomography (CT) setup improves understanding of the factors influencing fracture conductivity. The state-of-the-art setup allows taking high-resolution scans of a proppant pack under conditions similar to an actual fracture. The setup provides unique insight into static and dynamic phenomena occurring at the proppant and the pore scale. The use of imaging software then provides an accurate 3D image over the entire length of the proppant pack. Computer models of flow in porous media can then be used to predict the flow and its parameters in the propped fracture and its porous network specified by the high-resolution images. Moreover, the setup scale (1 m long) is much larger than that of a conventional conductivity cell (20 cm long). The larger dimensions are adequate to capture heterogeneous processes such as fracture cleanup.
The performance of the new setup was validated using baseline proppant pack conductivity and non-Darcy flow coefficients experiments. An example was also shown where the unique X-ray CT setup was used for investigating fracture cleanup of new, elongated rod-shaped nondeformable high-strength proppant compared with conventional spherical proppant. The CT images show how the new proppant provides a better fracture cleanup compared with spherical proppant. For both cases, images showing the distribution of residual gel saturation highlight how pore volume geometry affects fracture cleanup.