On the Mechanisms of Shale Microfracture Propagation

Abstract

Abstract Economical production rates in shale reservoirs are achieved solely by hydraulic fracturing the pay zones on horizontal wells with multiple stages, thus this technique is an essential tool for the development of enormous reserves of oil and gas in the United States. However, the processes that dominate hydraulic fracturing in shale reservoirs are poorly understood, and the variability of the properties from one shale play to another makes this learning process intricate. In this paper, we investigate the evidence of microfracture propagation on ductile shales and the key factors that constrain its slippage. First of all, we present evidence from the study of two reservoirs, the Eagle Ford formation in South Texas and the Vaca Muerta formation in Argentina. Secondly, we analyze the key parameters that affect flow through microfractures in organic-rich mudstones. We identify the key variables used to characterize these reservoirs through integration of geomechanics laboratory experiments with microseismic monitoring, mineralogy, organic carbon content, fabric, log, core, geological and field data analysis. Our ultimate goal is to identify the main parameters that influence fracture flow from very tight shales with the ultimate goal of optimizing the completion design in these complex formations. With this objective in mind, we examine the relatively slow slip process of tensile fracture growth in ductile, high clay and organic content shales, which results in the reactivation of preexisting horizontal fractures, producing aseismic energy. We then compare this mechanism against the fast shearing deformation that generates microseismicity in brittle shales. By observing the differences between the macro-scale (microseismic, anisotropy of the acoustic waves, geomechanics experiments), the meso-scale (fabric-thin section) and the micro- and nano-scales (clay particles) we suggest that the mechanisms of fracturing on ductile versus brittle rock are seen at any scale; that the anisotropy of the fabric of the rock within the interface between clay and non-clay minerals that differ in size and strength, the anisotropy of the horizontal stresses, the overpressure conditions, the friction angle of kerogen, the orientation of the pre-existing microfracture network with respect to the current stress state and –to a lesser extent- the elastic moduli of the rock matrix, are the most significant parameters that dominate microfracture propagation mechanisms on organically-rich rocks. The systematic integration of data from two analogous shale reservoirs and its comparison against existing theories on the mechanisms of fracture propagation has the benefit of approaching the process of fracturing from a predictive perspective, with potential benefits on stimulation optimization.