--> 3-D Geomechanics Workflow Applied to a Tight Gas Turbiditic Field, Colombia

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3-D Geomechanics Workflow Applied to a Tight Gas Turbiditic Field, Colombia

Abstract

The Lower Magdalena Basin, located in the northwest Colombia, contains gas and oil reserves. Recently, gas prospectivity was established in a shallow sand play (up to 7,000 ft) in the Miocene formation at the south of El Difícil field. The prospective succession is composed of interbedded, laminar and laterally discontinuous deep marine sands embedded in massive claystones. The sands show good-to-moderate porosity and poor permeability. This complex distribution of reservoir properties (porosity, permeability) and rock mechanical properties poses significant technical challenges for drilling and commercial exploitation of gas resources. This paper presents the geomechanics approach applied to well trajectory optimization as a key contribution to the development of tight gas turbiditic assets. A 3D geomechanical model was created through integration of seismic data, local geology, drilling experience, core and log data and geostatistics techniques. Natural fractures in the target formations were identified from electrical image logs, and their shear slippage tendency was assessed in the current in-situ stress field to determine the subset of natural fractures that will most likely be conductive with stimulation. A new workflow was developed to select optimal well trajectories based on: (a) interception of maximum number of sand geobodies identified through elastic inversion and mapped into the 3D grid, (b) best performance in relation to wellbore stability and (c) interception of optimally oriented natural fractures that are expected to undergo shear slippage during injection treatment. After the highest rank well trajectories were identified, optimization of the stage-fracturing design was conducted and fracture geometry, treatment pressures and fluid volumes were obtained. Results show that well deviations up to 65° are more favorable in terms of wellbore stability when anisotropic failure is considered. In addition, post-stimulation analysis of natural fractures indicated some fractures became critically-stressed, which means that larger reservoir volumes are being stimulated. An optimized well trajectory in terms of both wellbore stability and reservoir stimulation through an integrated geomechanics analysis provides value in economic and operational terms.