| Title | Calibrating wellbore stress-strength models from borehole geometry and stress measurements |
|---|---|
| Authors | Peter Meier, Andres Alcolea, Asmae Dahrabou, Benoît Valley, Philip Brunner |
| Year | 2023 |
| Conference | World Geothermal Congress |
| Keywords | Borehole failure, Stress state-Strength models, Calibration, Enhanced geothermal system |
| Abstract | Borehole stability is a major concern in the drilling and operational phases of a deep geothermal project. The shape, and especially the direction, of the borehole must be controlled during drilling to ensure (1) stability to avoid expensive losses of rig time, (2) a borehole trajectory intersecting enough structures (e.g., fractures or fault zones, initially not water conductive) for the posterior engineering of the reservoir, and (3) a sufficiently smooth borehole wall to facilitate borehole completion (e.g., the installation of packers/multi-packers for a multi-stage hydraulic stimulation). During the operational phase, the partial or total collapse of the borehole and the inflow/circulation of solid particles must be prevented to avoid damage in the injection/production pumps. Borehole stability is controlled by the complex interactions between the state of stress and the strength of the surrounding rock. Thus, a proper characterization of these parameters together is crucial to the success of a deep geothermal project. Typically, the magnitudes of the vertical and minimum horizontal principal stresses (?v and Shmin, respectively) can be estimated from measured densities or by standard interpretation of, e.g., mini-frac tests. Instead, the maximum principal stress, ?Hmax is difficult to constrain and, often, only rough estimates are obtained after imposing frictional equilibrium. In this work, we present a systematic methodology to jointly evaluate the non-linear heterogeneous distributions along the borehole profile, of the principal components of the stress tensor, their corresponding orientations and the rock strength properties (e.g. cohesion, friction). Model parameters are estimated from measurements available during or shortly after drilling, i.e., breakout width, breakout extent/depth of penetration, breakout orientation and drilling induced tensile fractures. Additionally, measurements of estimated parameters or suitable transformations of them can be considered as prior information in the parameter calibration process. The parameter estimates, obtained via inverse problem, are non-unique, which leads to a broad range of equally plausible stress-strength solutions (e.g., small stress with enhanced rock strength or vice versa). We demonstrate that the inherent parametric uncertainties are drastically reduced by conditioning the stress-strength model to hard measurements (e.g., ?hmin at a given depth interpreted from a XLOT). The calibrated model (or models) can be used for determining the optimum trajectory of the deviated section of a borehole, or the optimum completion scheme. For illustration purposes, the suggested methodology is applied to the extensive borehole data set along the crystalline section of the borehole BS-1, in Basel (Switzerland). The methodology allows us (1) to derive plausible sets of stress and strength parameters reproducing the complex distribution of breakouts along BS-1, (2) to demonstrate the benefit of conditioning parameter estimates to hard measurements, and (3) to unveil the paradox of having no borehole breakouts at sections with high density of natural fractures. |