The total porosity obtained from nuclear magnetic resonance (NMR) logging-while-drilling (LWD) data is typically not affected by lateral motion of the NMR-LWD tool; however, some other deliverables (e.g., bound water, movable fluid, permeability, viscosity) might be affected. This paper introduces a data-based lateral-motion correction (LMC) that uses a four-parameter function to quantify and correct potential lateral-motion effects.
The objective of the LMC is to improve the accuracy of the final T2 porosity distribution. The LMC extends the operational range of the NMR-LWD method and enables more advanced petrophysical NMR applications. In addition, the LMC approach can be used to quantify the lateral-motion effect and to mark intervals where the motion effect is too severe to be fully corrected.
The LMC was developed by analyzing the potential lateral-motion effect on numerous NMR-LWD data in combination with numerical simulations. By using drilling dynamic simulations of a complete bottomhole assembly (BHA) under realistic drilling conditions, motion paths of an NMR sensor were calculated, which were then used to simulate NMR signals. Actual data and the NMR simulation indicate that lateral-motion effects can be adequately described by a four-parametric function. Two parameters describe an exponential decay, while the other two parameters describe a periodic variation of the amplitude. The motion effect function was integrated into the forward matrix of the NMR joint inversion, and a non-linear optimization algorithm was used to determine the four motion parameters and, if present, to compensate for lateral-motion effects.
Although motion paths are typically complex, the motion periodic characteristic relates to the revolutions per minute (RPM) of the BHA. The amplitude of the motion mainly depends on the drilling regime (e.g., from "smooth" drilling to whirl), the gap between the BHA stabilizers and the borehole, and the borehole inclination. Numerical simulations show that the NMR motion effect is small-to-negligible if an NMR-LWD tool has a low magnetic field gradient and has adequate stabilization.
The correction method was tested on synthetic and real NMR-LWD data from more than 30 runs with different realizations of lateral motion. The approach is robust and works for all data sets. The magnitude of the lateral motion effect is reliably shown by the implemented multiple quality control indicator. Some examples of synthetic and actual NMR data with and without LMC are included.
The inversion that includes the LMC improves the quality of the T2 spectrum, which is important for standard and advanced NMR applications, such as accurate calculation of volumetrics (e.g., movable fluid, bound/irreducible fluids) and pore size distribution, as well as improved estimation of fluid viscosity and formation permeability.
