REAL-TIME INTERPRETATION OF FORMATION STRUCTURE FROM DIRECTIONAL EM MEASUREMENTS

摘要

A new Electro-Magnetic (EM) Logging-While-Drilling (LWD) tool provides a deep directional response for well placement and for high-angle and horizontal well formation evaluation applications. In order to use the measurements from this tool for real-time steering or other drilling decisions, a real-time data interpretation method had to be provided. To this end, we have developed a model-based parametric inversion technique, which computes the distance to bed boundaries as well as formation resistivity from the raw directional EM measurements. The inversion can be run in a fully automatic mode that requires no user intervention for a quick “first-look” interpretation as well as in an interactive mode in which manual constraints can be used to refine the inversion. In the automatic-interpretation mode, all available realtime measurements are inverted for a simple three-layer model. The inversion runs a fast approximate model to compute response to two boundaries, giving satisfactory results for thick beds. The algorithm inverts multiple models, from simple to complex, with the most complex model using up to six parameters, including distance to two shoulder beds, anisotropic bed resistivity, and two shoulder-bed resistivity values. An Akaike information criterion that favors simpler models is then combined along with physics-based constraints to select the simplest model that fits the data. To refine the interpretation in expert mode, data can be interpreted segment-by-segment. The model may be built point-by-point, or all data may be inverted for a common layered structure to determine the resistivities and thicknesses of all layers. This simultaneous inversion reduces the uncertainty due to any random measurement or to environmental errors. It also provides a way to estimate the apparent dip, which is essential for reservoir characterization and for highangle and horizontal well applications. Validation of the interpretation method will be shown first through synthetic examples and then through interpretation of field data from the EM directional tool. Inversion results from the field data demonstrate consistent structure interpretation over long sections. The field data also reveal that lateral heterogeneities of the reservoirs are common, thus underscoring that it is not sufficient to rely on 3D seismic surveys alone for accurate well placement.