Decreasing fracture effectiveness due to conductivity decay is a strong contributor to the steep production decline commonly observed in shale plays. The conductivity of a fracture is determined experimentally by measuring the pressure drop of a fluid flowing through a uniformly distributed proppant bed in a core with fixed length and height. Fracture conductivity degradation results from damage mechanisms and fluid interactions that occur during hydraulic fracturing operations. Rock softening and proppant embedment are some of these damage mechanisms. The impact of these interactions can be observed by measuring fracture conductivity in the laboratory under stress states similar to field conditions.
This study is based on experiments performed on fractured and propped Niobrara core plugs. The samples were characterized using X-ray Diffraction (XRD), and X-ray Fluorescence (XRF), and helical CT-scans. The experiments were performed on a triaxial stress test assembly to monitor the chemical and mechanical alterations in the formation, proppant, and fluid under reservoir conditions. To achieve this, fluid chemical composition, dynamic and static moduli, and conductivity were obtained. The setup was used for the simultaneous acquisition of stress, ultrasonic compressional and shear wave velocities, flow data and fluid sampling.
The results from this study indicate that stress-dependent, long-term fracture conductivity shows the sharpest decline in the early stages of the experiment. The associated fluid sample analysis indicates that the highest physicochemical dissolution of most of the elements is happening at the early contact of the fluid with the rock and is later enhanced by the pressure increase in the system. A comparison with the conductivity measurements performed on Vaca Muerta samples shows a similar behavior, yet a steeper initial decay than that observed in the Niobrara samples. The difference observed between the two samples is related to the mineralogy of the formation and the high proppant embedment observed in the Vaca Muerta samples. Although higher softening occurred in the Niobrara samples, larger embedment was observed in the Vaca Muerta sample. This experimental observation is an indication that the conductivity damage varies not only with the mineralogical content of the formation, but also with the distribution of minerals along the fracture face.
Geomechanical, geochemical, and flow data integration provided a better understanding of proppant embedment and mineral distribution of the rock. It is the conclusion of this study that even if the intact core sample contains an average mineralogical composition, the heterogeneity caused by variations in the mineralogy at where the fracture is induced has the biggest impact on embedment.