Interpretation of Asymmetrically Invaded Formations with Azimuthal and Radial LWD Data

摘要

In permeable, dipping formations, invasion of drilling fluid is often asymmetric because of gravity slumping of the filtrate. This effect can be observed within less than an hour of the bit penetrating a highly permeable, gas-bearing formation. Accurate interpretation of log data in such environments requires a technique that accounts for both azimuthal and radial distribution of filtrate. Logging-while-drilling (LWD) measurements, when rotated through the zone of interest, offer the data necessary to evaluate such formations. The interpretation process used combines azimuthal nuclear data with azimuthal and radial resistivity data to compute accurate values of porosity, water saturation and mineralogy.rnFirst, resistivity data are inverted for R_(x0) (flushed zone resistivity), R_t (true resistivity) and D_l (diameter of invasion) in four directions. Next, the different values of D_i are used to compute individual invasion scalars for density and neutron log data based on their radial response functions. This is possible because the resistivity measurements used are similar in radial response to the density and neutron radial responses. Then, the log data and invasion scalars are entered into a petrophysical solver for the final results. The invasion scalars assist the solver in determining the magnitude of the hydrocarbon correction required, which is especially important in gas zones. Full correction of log data provides results that are in close agreement with core data.rnUnderstanding the effects of gas in three dimensions helps explain "lazy" neutron curves. In gas zones, the density log is highly affected by varying invasion, where as the neutron has an almost constant gas effect that is relatively independent of invasion as predicted by modeling.