Assessing Borehole-Position Uncertainty From Real-Time Measurements in an Earth Model

摘要

There has been much interest in improving uncertainty estimates of borehole position. Such estimates are useful in estimating the probability of intersecting a reservoir target, in estimating the probability of colliding with an adjacent borehole, and in avoiding high-risk zones. We describe here a new geometrical model for borehole-position uncertainty that encompasses both systematic and random errors in the borehole-position survey. Furthermore we present a new technology that refines the borehole-uncertainty estimate in real time during the drilling of the well through the use of logging-while-drilling (LWD) measurements (e.g., measurements of formation depth, dip, and azimuth from image logs). This technology uses a Bayesian formulation to update the uncertainty of both the borehole trajectory and the earth model to be consistent with these measurements. The final uncertainty model couples borehole uncertainty with earthmodel uncertainty. We use a geosteering field example to demonstrate how both earth-model and borehole-trajectory uncertainties are reduced through this use of real-time measurements.