Sensitivity Analysis of Factors Affecting Fracture Height and Aperture

Abstract

• The in-situ stress ratio, not the individual stress values, is more influential on fracture geometry; thus, stresses in both directions (not just σ h m i n ) are considered influential. Because making small changes in viscosity has caused extreme changes in fracture height and aperture, the injected fluid viscosity is considered influential as well. This is an expected result. • For fracture height, the stresses (and their ratio), viscosity, Poisson's ratio, elastic modulus, and fluid injection rate are more influential than other variables. • For a fracture's maximum aperture, the stresses (and their ratio), viscosity, the elastic modulus, and fluid injection rate are more influential than other variables. • We have shown that the interaction of Geometry and Geology is fundamental to understanding induced fracture geometry. Apart from the fluid viscosity and injection rate, influential parameters are related to the geology (and geomechanics) of the rock. • This research can be useful for researchers and decision makers. Hydraulic Fracture Stimulations (HFS) are designed to improve well production while minimalizing environmental and geomechanical stability issues such as unintended “frack hits”, excessive height growth, and unintended break-through to thief zones or water production zones. Planning a fracture geometry with the optimal height, aperture, and length is the goal, and factors affecting the geometry include geological and geomechanical properties (natural fractures, bedding fabric, stresses, geomechanical properties, permeability, etc.) play significant roles. These properties are usually predetermined and are considered as design inputs; other parameters such as the pumping rate, fluid viscosity and density, and proppant concentration and schedule are determined (designed) when proposing a stimulation. The proper HFS design should take into account all technical and environmental aspects, meaning that the design (operational) parameters are chosen based on geological factors and the designer's experience to target a desired fracture geometry. HFS design is therefore the interaction of G eometry and G eology, the G&G interaction . A commercial two-dimensional coupled discrete element software, UDEC TM , is used to study geometry outcomes from ranges of geology inputs and designed operational parameters. The sensitivity analysis methodology employs the Morris technique to assess which geological and operational parameters have greater impacts the geometry of a single vertical fracture. Emphasizing parameter ranges more applicable to shallow formations ensures that the results can help assess fracture height outcomes near the surface and groundwater, where variability in fracture height is of environmental concern. Geomechanics; Hydraulic Fracture Stimulation; Sensitivity Analysis; Fracture geometry; Fracture Height