Integrated Unconventional Gas Evaluation Workflow: from Anisotropic Geomechanical Modelling to Completion Design

Abstract

Abstract Emerging unconventional exploration targets in North Africa are gaining increasing attention. Experience gained over the past two decades on unconventional gas reservoirs (e.g., tight sands, gas shales) has demonstrated that viable economics for unconventional resource plays can only be met if reserves are proved (reservoir quality) and producible (completion quality). With the reservoir quality identified, improved productivity in ultra-low-permeability unconventional reservoirs requires extensive hydraulic fracturing treatment to maximize contact area and unlock resources. North Africa geological settings represent a challenging environment for effective hydraulic fracturing design. Deep reservoir targets, active tectonics and simultaneous occurrence of heterogeneous rock properties (including isotropic and anisotropic behavior) constitute a unique endeavor for a correct geomechanical modeling and a reliable assessment of closure pressures and fracture containment as input for hydraulic fracturing design. This paper describes the implementation of an integrated unconventional evaluation workflow adopted for the completion design and hydraulic fracturing of a vertical well in tight sands and gas shales. A data acquisition program for tight formations was implemented to ensure full integration of logs data and core data from laboratory testing. Anisotropic measurements from an advanced borehole sonic logging tool and laboratory geomechanical tests were used to characterize the mechanical behavior of the reservoir lithological units, showing high degree of mechanical anisotropy in Silurian shales and Ordovician clayey horizons. Since isotropic assumptions predict inaccurate stress magnitudes and lead to significant error in stress contrast, the use of anisotropic-based geomechanical modeling is emphasized with regard to wellbore stability analysis and completion design. The integrated workflow involving anisotropic stress modeling allowed addressing and solving open issues related to wellbore failure prediction and estimation of hydraulic fracture containment. For completion design purpose a comparison between different assumptions in terms of geomechanical input and hydraulic fracturing stimulation outputs is presented. The influence of the two different stress models (i.e. isotropic and anisotropic) is discussed. The effect on fracture geometry of the anisotropic clay layers, commonly treated as purely isotropic, is emphasized. It is shown that the hydraulic fracture in tight sands stays more contained in the reservoir target providing a better reservoir contact and optimum completion due to the higher minimum horizontal stress predicted by the anisotropic model in the clayey intervals.