The Unconventional Unconventionals: Tectonically Influenced Regions, Stress States and Casing Failures

Abstract

Abstract Not all unconventional plays are created equal, in a substantial number of regions around the world the tectonic environment is quite different from the typically relaxed and more passive states found widely in most, if not all, of the US unconventional plays. This is merely a function of the relative proximity of such plays to distinct geological features characterized by active tectonic plates and with dynamic margins and recent activity. The Nazca plate associated with the Andes, the Arabian plate linked with the Al-Hajar mountains and the Indian plate connected with the Himalayan mountain range are just a few examples of tectonically influenced regions, where potential hydrocarbon traps are subject to complex states of stress generated by convergent plates, subduction zones and associated faulting. This scenario often translates into severe strike-slip and reverse fault stress states. Additionally, the presence of both multi-layered and laminated formation geology as well as the presence of overpressure and pressure differentials, typical of tight gas and shale gas, can exacerbate this situation even further. This situation can result in an extremely challenging environment for the successful execution of hydraulic fracturing and the associated development of unconventional resources. This paper will demonstrate, that such complex stress-states will directly affect well completions and hydraulic fracturing in a multitude of ways, but that some of the most impactful consequences are often severe casing failures, production-liner restrictions and complex fracture initiation scenarios. Casing failures are responsible for increased intervention costs as well as higher costs for the upgraded and strengthened wells. Equally, such issues can severely impair efficient execution of the completion plan and create a bottle-neck to subsequent well production. Horizontal, complex and pancake fractures will typically develop in strike-slip / reverse fault stress states, often resulting in fracture conductivity and connectivity loss that will greatly impair the eventual well performance. Layer interface slippage and natural fault re-activation are dominant mechanisms for hydraulic fracture induced casing failures. Examples of micro-fracs, micro-seismic and other diagnostics will be presented aiming to document the practical difficulties encountered while completing wells in these complex environments. This paper will demonstrate that unconventional development in such environments requires a renewed focus on all aspects of well design and construction, from directional drilling and lateral placement to casing selection and lower completion design. All these considerations are made with the goal of enabling the competent delivery of a highly effective and conductive fracture network, to efficiently access and produce the hydrocarbon resource.