A growing body of evidence reveals that in many fractured reservoirs, the most productive fractures are those that are optimally aligned in the current stress field to fail in shear. Thus, it is necessary both to obtain knowledge of the stress magnitudes and orientations and of the distribution of natural fractures to determine the optimal orientations for wells to maximize their productivity. After taking into account the contributions to flow of all fractures in proportion to their flow properties, the optimal well orientation is oblique to the orientations of all three principal stresses. However, the parameters required to predict the relative benefit of modifying a well trajectory to take advantage of this knowledge are uncertain. Similarly, the pressure required to stimulate natural fracture systems by triggering shear failure is also uncertain. Utilizing quantitative risk assessment (QRA) and realistic uncertainties in the critical parameters, it is possible to estimate the uncertainty in predictions of optimal well trajectories and of stimulation pressures to enhance natural fractures. The results indicate that the critical parameters are not always those with the most uncertainty.
1. INTRODUCTION
The magnitudes and orientations of earth stresses cannot be measured precisely. Furthermore, they vary with depth and lateral position. Analyses of the uncertainties associated with pore pressure and stress estimation [1], of the mud weight required to maintain stability during drilling ([2,3] and of sanding risk [4] that propagate these uncertainties through the analysis using quantitative risk assessment based on Monte Carlo methods have been previously published. However, published examples of the application of these methods to assessment of the optimal orientations in which to drill wells in a fractured reservoir, and the optimal pressures required to enhance the productivity of natural fractures are rare (e.g., [5]). This paper investigates the influence of uncertainties in the parameters that control fracture permeability and of the pore pressure and in situ stress magnitudes and orientations on predictions of the optimal orientation in which to drill a well through a naturally fractured reservoir, of the relative benefit of choosing a particular orientation, and of the effect of stimulation to trigger shear slip and enhance the permeability of fractures that are close to shear failure under ambient stress conditions. To model the effects of shear and normal stresses on fracture aperture we use a relation from [6] along with the assumption that relative fluid flow properties are controlled by a simple parallel-plate power-law model. Fracture slip is assumed to be governed by a linear Mohr frictional failure law. First, the models are presented. Next we discuss the results of calculations of relative well productivity under ambient conditions and also after stimulation. Finally, we use Monte Carlo analyses to assess the uncertainties in the predictions given reasonable uncertainties in the values of the model parameters and of the stresses and pore pressure.
2. FRACTURE MODEL
Fracture fluid flow properties are often parameterized in terms of a fracture "aperture" of an ideal parallel plate model with the equivalent properties of the actual fracture.
