A Novel Approach for Studying Hydraulic Fracturing Success Factors Beyond Brittleness Indices (original) (raw)

2018

Abstract

1. BACKGROUND United States has benefited economically from the boom in horizontal drilling and hydraulic fracturing technology, which rendered the development of domestic unconventional energy resources possible. US production of liquid fuels surpassed the Middle East in 2013 [Yo and Neff, 2014], adding 169,000 jobs between 2010 and 2012 [Brown and Yucel, 2013]. Reducing a country's dependence on the imported energy helps mitigate economic losses caused by foreign oil supply disruptions [Brown and Huntington, 2009]. The success of investment decisions pertaining to the exploitation of unconventional resources depends strongly on the reliability of models making predictions of post-stimulation performance. However, due to a lack of models based on accurate knowledge of the reservoir and rigorous understanding of the governing physics, there is a technology gap between the current models of stimulation and the field observations in the E&P industry. A major drawback associated with common hydraulic fracturing simulation methods is that they need prior knowledge on the fracturing path, meaning the outcome of the stimulation job should be fed as input to the model, rather than obtained as output. In addition, prevalent approaches for modeling performance of hydraulic fracturing jobs often fail to quantify the job results realistically, as linear elasticity and rock brittleness are the main underlying assumptions of most models. It has been shown, however, that there are a number of influence factors that need to be accounted for in prediction models. Brittle materials demonstrate a shorter period of ductile deformation before failure, which does not necessarily guarantee easier fracturing at lower ultimate rock strength values. Bai, 2016 states that, in fact, certain ductile formations may break at lower downhole pressures based on field measurements. Papanastasiou, 1997 incorporated the effect of plasticity in hydraulic fracturing using a cohesive crack model, showing that ductile rock behavior can lead to higher resulting fracture width values, while creating fractures with a smaller length. These observations suggest that limiting our target rocks and prediction models to linear elastic materials leads to inaccurate conclusions, since both mechanisms of brittle and ductile fracturing need to be considered for better modeling purposes. ABSTRACT: The success of hydraulic fracturing jobs is often related to rock brittleness indices, which are taken as the sole impact factor determining fracturing results. Indeed, hydraulic fractures play a principal role in producing from low-permeability reservoirs; however, brittleness is not the only parameter contributing to productivity of unconventional resources. Under a variety of circumstances, brittleness indices are insufficient to explain rock fracability and permeability enhancement during hydraulic stimulation. For better prediction and design, it is imperative to identify and understand other factors affecting fracture creation and propagation, and to build models that include the effect of these factors on flow enhancement. To numerically model permeability enhancement after injection, we can regard fractured rock as a damaged continuum, which allows simulation of the deformation and fracturing response of the reservoir using material constitutive laws for brittle and ductile regions. We outline a coupled flow-geomechanical simulation framework that fits into available reservoir simulation platforms and does not require pre-specified fracture paths. We develop the fracture growth mechanisms for the coupled simulation framework by analyzing the effect of rock properties and in-situ stresses on the fracture length at different injection pressures. Based on these mechanisms, we propose factors that quantify the success of hydraulic fracturing jobs beyond the simplified rock brittleness indices.

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