A METHOD AND APPARATUS FOR NMR MEASUREMENTS IN SMALL BOREHOLES AND IN UNDERBALANCED DRILLING

摘要

NMR spin echo signals are measured in a borehole during underbalanced drilling. A magnetic field gradient in the region of examination is selected to suppress an effect of a formation fluid flow on the produced signals, the fluid flow being caused by the excess of the formation pressure over the borehole fluid pressure. NMR measurements while drilling of small boreholes is done by combining signals from multiple regions of examination. By making measurements in a plurality of radially spaced regions, a radial profile of a formation property can be estimated. Additionally, a warning of a gas kick may be provided.