Drilling operations typically account for the largest percent of the expenses incurred in the exploration and development of hydrocarbon reservoirs. Modern well placement and real-time navigation methods take into account uncertainties in a predefined “earth model” to reach and produce hydrocarbon-bearing rocks in an efficient manner. Because the construction of the initial earth model is commonly based on low-spatial-resolution data such as seismic-amplitude measurements or is from extrapolation of conditions encountered in nearby wells, the well trajectory needs to be adjusted continuously as the drill bit encounters rocks formations, geometric conditions, and rock properties unaccounted for by the initial earth model. Most modern drilling operations are accompanied not only by measurements while drilling (e.g., rate of penetration, weight on bit, torque, and so forth) but also by logging-while-drilling (LWD) measurements such as electrical resistivity, density, neutron-porosity, acoustic velocities, and magnetic resonance, among others. The latter measurements are placed in close proximity to the drill bit and hence provide useful information to modify the well trajectory in real time to maximize drilling efficiency and to steer the well toward the best storage and production zones.

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