A new LWD porosity logging system has been developed that addresses specific issues related to the repeatability and accuracy of porosity measurements while drilling. This system provides density and neutron porosity measurements at penetration rates up to 400 ft/hr with the precision and accuracy of an equivalent wireline tool. Designed to operate in the offshore deepwater and ultra-deepwater environment, system specifications include operating pressures up to 30,000 psi. The system is designed to reduce standoff effects on the density measurement and correct the neutron and density measurements for the effects of standoff and borehole size while drilling.
Standoff compensation of the density measurement is achieved using a new technique called "Self Binning". This technique relies on measurements made by the density tool itself to correct for tool standoff. The count rate of the near detector is sampled at a very high rate and combined in such a way as to divide the borehole into segments of fairly constant standoff. A standoff correction is applied to each segment and an average corrected density is calculated after removing data with excessive standoff.
A similar technique is used to segment the borehole according to formation density. The corrected densities sampled at a high rate are sorted according to formation density to provide a continuous profile of the azimuthal formation density around the borehole circumference.
Additionally, estimates of tool standoff and borehole diameter are determined while the tool is rotating. This information is applied to the neutron measurement to obtain a neutron porosity measurement corrected for borehole and standoff effects.
Details of these new algorithms as well as modeling and experimental data used to optimize and test these algorithms are presented. Examples are presented that show the accuracy and repeatability of the new toolstring.