Fracture Conductivity Behavior in Shale Formations

Abstract

The objective of this study was to further the understanding of fracture conductivity and its dependence on rock mechanical properties, mineralogy, and fracture surface attributes, as well as its impairment due to flowback, based on a systematic experimental study on several mudstone formations. As part of this objective, a practical workflow was developed to extend the measured conductivity behavior to downhole stress conditions. The experimental measurements conducted in this study used mudstone samples obtained from outcrop and downhole core segments, from the Eagle Ford, the Marcellus, the Mahantango, the Middle Bakken, and the Three Forks formations. Fracture conductivity experiments were conducted utilizing a Modified API Fracture Conductivity Cell by flowing dry nitrogen and/or saline solutions representative of flowback water. As such, undamaged and water-damaged fracture conductivity was measured. Simulated fracture conditions included unpropped and propped fractures. Natural sand proppant, with a commonly used mesh size, was used with a variety of areal concentrations. Additional experiments consisted of evaluating rock mechanical properties, mineralogy, and fracture surface attributes. A large degree of heterogeneity in properties was observed for each formation. Experimental measurements showed that fracture conductivity under increasing closure stress generally follows an exponential decline rate, which is manifested as a linear relationship on a semi-log plot that is condition-specific. These functions can be representative of the formation fracture conductivity behavior, and can be extended to additional simulated fracture conditions. Furthermore, these laboratory-based measurements can be related to downhole stress conditions predicated on poroelasticity theory. This estimated in-situ fracture conductivity can be used during the performance evaluation of a hydraulically fractured horizontal well, or during the design of a new treatment. This study provides an insight into fracture conductivity and its dependence on formation physical properties. Additionally, this study presents a practical application to the measurement of fracture conductivity in mudstones and its importance in evaluating well performance.