--> Using Time-Dependent Borehole Failure to Understand Diffusion-Driven Weakening and Strain in Reservoir Rocks and Seals

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Using Time-Dependent Borehole Failure to Understand Diffusion-Driven Weakening and Strain in Reservoir Rocks and Seals

Abstract

Understanding the coupling between thermo-hydro-mechanical-chemical (THMC) processes in reservoir rocks and seals will enhance our ability to characterize and predict reservoir behaviour. The coupling between these processes inside a reservoir occurs at various length- and time-scales. However, the coupling is not equally tight at every scale and depends on lithology as well. At the borehole-scale coupled THMC processes manifest as time-dependent borehole failure. Although intrinsic mechanical processes (i.e. creep) can explain some time-dependent failure, it is widely recognised that pressure, temperature and chemical gradients introduced by the drilling mud into the formation play an important role. In these instances equilibration of the gradients is achieved via a diffusion process and the coupling to the mechanical behaviour of the rock either results from a strain (i.e. “swelling” and “shrinking”) or a change in rock strength. The problem is that the exact interaction between these thermodynamic gradients and their associated mechanical failure processes are not well known, because (a) the impact of thermodynamic gradients is not routinely considered in rock mechanical testing and (b) repeat or “time-lapse” runs of borehole imaging tools over the same formation interval are rare. In this contribution we present a fully coupled material model that allows for diffusion-driven weakening and strain. The material model is based on non-equilibrium thermodynamics and the material weakening is achieved via a damage tensor. This material model is implemented into a Finite Element simulation of near-well bore processes. Apart from improving borehole stability predictions (the forward problem), such numerical simulations enable us to study the influence of the individual coupling parameters. We therefore conducted a parameter study that explores the feasibility of using in-situ observations (e.g. time-dependent widening of borehole breakouts in repeat image logs) for the inversion of THMC coupling parameters. We will show that a complete inversion for the most general formulation is not possible due to non-uniqueness of the problem. However we can constrain the type and tightness of coupling mechanisms and thereby improve modelling of wellbore stability.