Accurate Logging In Large Boreholes

The oil industry has historically depended upon quality log data to evaluate the potential of wells. In certain areas this requirement is in conflict with mechanical considerations of the well (casing programs, etc.), which require large boreholes near the surface. Most logging tools are designed for optimum tool response in an 8-in. borehole, and modeling and careful experiments have extended the range of reliable tool operation or correctability to hole sizes up to 12 1/4 in. hole sizes in excess of 12 1/4 in. sometimes result in potential sources of error such as cycle skipping on sonic logs, borehole effect on induction tools, standoff on neutron tools, and pad contact problems on density devices. As a result of the problems in large holes, it is usual practice either to not log the large holes or to drill a smaller hole for logging and underream to final size. Modified tool designs and new modeling techniques now allow quality logging results in hole sizes up to 24 in. Modeling has expanded the range of borehole corrections for induction, and mechanical or operational changes allow the sonic and nuclear tools to make logs consistent in quality with those recorded in smaller boreholes. Tests from a well which was drilled at 8 1/2 in., then widened to 12 1/4 in., widened again to 17 1/2 in., and finally underreamed to 23 in. demonstrate the consistency of the logs over the range of borehole sizes.