Estimation of fracture height growth in layered tight/shale gas reservoirs using flowback gas rates and compositions – Part I: Model development

Abstract Gas production from low permeability (‘tight’) and shale formations has become a major source of hydrocarbon supply in North America over the past two decades. This shift in supply from conventional to unconventional sources was enabled by the combination of hydraulic fracturing and horizontal wells which allowed production rates to reach economically viable levels. Thus, the design and implementation of hydraulic fracturing (‘HF’) treatments occupies a central role in reservoir management of unconventional plays. A primary goal of an HF treatment is to create a high-conductivity fracture or fractures with sufficient reservoir contact. However, efforts must be undertaken to monitor and evaluate the extent of HF growth into the target reservoir and bounding horizons for the purposes of not only optimizing the treatment, but also for mitigating the risk of groundwater contamination. In this work, it is demonstrated for the first time that fracture height growth in layered geologic systems with variable and distinct fluid compositions by layer maybe assessed. For this purpose, an analytical model was developed to predict the degree of fracture penetration of a hydraulic fracture in a 2-layer tight/shale reservoir. The model uses gas production rates and compositions during flowback to assess height growth, assuming that layer-specific gas compositions are accurately known. These layer fluid compositions may be assessed using mudgas and gas samples extracted from the headspace of isojars containing collected cuttings samples during drilling. The theoretical development of the new analytical model is described in this work, and its accuracy determined through datasets generated with a numerical simulator. The practical application of this new model to an actual field dataset will be provided in Part 2. The importance of this study is that it provides a practical and independent method for establishing what portions of a layered system that the HF is in hydraulic communication with, which is not provided with other surveillance methods.