A robust approach for determining borehole shape from six-arm caliper logs

We have developed a robust and reliable method for determining borehole shape from six-arm caliper logs. Four-and six-arm calipers are the common caliper tools used in open-hole logging. They provide information about wellbore geometry that is important in petrophysical and geomechanical analyses. The analysis procedure for four-arm caliper logs is well-established, but the analysis for six-arm caliper logs can be troublesome in complex hole environments containing breakouts or keyseats when the tool is off center. The challenge with the six-arm caliper is how to remove the effect of tool decentralization, which cannot be handled properly by using conventional correction methods, which are developed based on the assumption of circular or elliptical boreholes. To resolve this issue, we have developed a new approach for tool decentralization correction. The new method is based on an assumption that the true borehole center should be the center of a circle that fits the caliper pad positions in the least-squares sense subjected to the restriction that the circle is confined within the six pads. We first compare the new method with the conventional chord method and ellipse-fitting method through numerical modeling. We numerically investigate the general performance of these three methods by using a Monte Carlo approach to generate a large number of simulations that mimic caliper logging run in boreholes with a variety of different wellbore geometry. We then study the applicability of the new method in field data analysis by applying it to a field logging data set acquired in a well that contains the breakout and the keyseat. The modeling and field-data results indicate that the new method can yield more reliable estimates of the virgin borehole center than the other two methods and thus can give clearer delineation of the borehole shape.