Quantification and Reduction of Uncertainties in 3D Stress Models

<p>The undisturbed stress state of a potential site for nuclear waste disposal is of key importance for the assessment of long-term stability of the geotechnical installations and for seismic hazard assessment. In particular, the stability of pre-existing faults within and near a repository can only be evaluated with the knowledge of the initial stress state. Information on stress magnitudes is rare and unevenly distributed. Thus, 3D geomechanical-numerical modelling is used to estimate the stress state in an area of interest. However, due to the limitation of available data, the modelled stress state has a large uncertainty which has not been rigorously quantified yet. We present an approach to quantify the uncertainties in a 3D geomechanical-numerical modelled stress field. We combine the available S<sub>Hmax</sub> and S<sub>hmin</sub> data records to pairs. For each pair we compute an individual model scenario. At each location in the model each scenario contains the full stress tensor. Then, from all model scenarios we compute an average value and a standard deviation for each component of the full stress tensor at each location within the model. This provides a comprehensive assessment of the stress state and its uncertainties.</p><p>Furthermore, we present an approach to reduce the previously quantified uncertainties in the model results: We use additional borehole observables (Formation Integrity Tests) and observed seismicity and - if available - its focal mechanisms. These observables cannot provide any data records on the stress state. Yet, the information that can be extracted is valuable as it contains upper boundaries for the magnitudes of the minimum principal stress (Formation Integrity Tests) and the maximum principal stress/differential stress (seismicity), respectively. These boundaries are compared to the stress states in the individual model scenarios. Then, each scenario is assigned a weight based on its agreement with the additional data. This allows computing a weighted average and a standard deviation. The resulting standard deviation is clearly smaller compared to the unweighted approach and small changes in the average stress state are observed. Thus, even with only limited data record availability, a quantification and even a significant reduction of uncertainties in the modelling results is possible which increases the significance and value of the model.</p>

Coupling geomechanical modeling with seismic pressure prediction

We couple geomechanical modeling with seismic velocity to enhance the prediction of pressure and stresses in complex geologic settings. In these settings, pressure is controlled by mean and shear stresses rather than by only the vertical (overburden) stress. We estimate total mean and shear stresses from a geomechanical model. Effective mean and shear stresses are calculated from velocity using a relationship that we develop between velocity and these stresses. The pressure prediction process is iterated to attain convergence between the predicted pressure field and the one input in the geomechanical model. We also explicitly predict the full stress tensor. We apply our method along with the standard, vertical-effective-stress method to a salt basin beneath the Sigsbee Escarpment in the Mad Dog field, Gulf of Mexico. The methods are constrained to the same pressure data along a calibration well and are then used to predict pressure and stresses across the basin. We find that salt and basin bathymetry substantially perturb the stress field. The pressures predicted by the two methods differ the least at the calibration well and the most in areas where the total mean and shear stresses are the most different from those at the same burial depth at the calibration well. Our method is shown to predict pressures measured along a subsalt well better than the standard, vertical method. We calculate minimum stress and the drilling window along a vertical profile near salt and find that they significantly differ from the ones predicted by the standard, vertical method.

An approach to constrain maximum horizontal stress magnitude using wellbore Failure observations from image logs

The magnitude of the maximum horizontal stress is generally the most challenging term in estimating the full stress tensor. In this paper, an approach to constrain the magnitude of the maximum horizontal stress using the frictional limits to stress and wellbore failure observations (drilling-induced tensile fractures DITFs and/or breakouts BOs) from image logs is presented. This approach is applied to constrain the magnitude of the maximum horizontal stress at some interest depths (3900 m, 4100 m, 4300 m and 4500 m) of the basement reservoirs at the White Tiger field, Cuu Long basin, Vietnam from the program STRESS POLYGON. The occurrence of DITFs and/or BOs proved to be useful in estimating stresses around the wellbore, especially the maximum horizontal stress magnitude.