An Innovative Approach To Correct Density Measurements While Drilling For Hole Size Effect

Formation density measurements made while drilling can be adversely affected if the hole becomes enlarged before the density measurement is made. This can occur even if a stabilizer is used to exclude mud from in front of the source and sensor windows. The position of the rotating tool in the borehole is unknown when a measurement is recorded. During several rotations, the mud standoff may vary from none to any amount. The density measurement is therefore, an average reading of the formation density and a combination of formation and mud standoff densities. To correct the density measurement for standoff, a technique has been developed that is based on the distribution of many very short measurements during a longer sample period. It allows the computation of the maximum and minimum densities encountered by the tool during the sample period. The maximum or minimum density is the real formation density, depending on the contrast between the mud and formation densities. A caliper is also derived from the measurement. Both computations require that the standoff size be less than the depth of investigation of the measurement, which is generally the case while drilling. The density variation that the tool encounters around the borehole is computed using a statistical approach. The density count rate is sampled very rapidly and the standard deviation of the many samples is computed. This is compared with the expected standard deviation, which is computed from the average count rate for the same period. In a hole that is in gauge, the measured and predicted standard deviations will be about equal. In a hole that has become washed out or oval, the measured deviation will be larger than expected. The difference is used to calculate the maximum and minimum densities that the tool has encountered. When the drilling mud is less dense than the formation, the maximum computed density will be that of the formation. Heavy, barite-weighted muds produce the opposite effect. The density contrast that is measured is a function of the contrast between mud and formation densities and the amount of hole enlargement. Lab data is used to calibrate this contrast so that the amount of enlargement can be calculated. The result is a differential caliper (hole size minus bit size) of the borehole at the time of the density measurement. Log examples and lab data are shown to support the technique. Although the borehole is generally near gauge while drilling, the technique is useful when the density sensor is far in time or distance from the bit, for washdown logs and for log quality control.