Formate Mud Effects On Density Logs

Although drilling with formate muds provides many advantages that appeal to drillers, these muds pose serious challenges to those attempting to evaluate formations using standard nuclear-logging tools. Significant use of formate muds may well hinge on the industry learning to address these challenges. This paper focuses on the challenges and solutions to providing meaningful gamma-gamma density logs. Formate muds affect density logs in three ways. First, potassium in the mud generates gamma rays that increase count rates, but that effect is small. Second, gamma-scattering and gamma-absorption properties of the mud differ significantly from those of bariteweighted mud; consequently standoff-correction schemes designed for barite-weighted systems do not adequately compensate for standoff in formate muds. Third and most important, invasion of the mud has a very large effect on the measured density and Pe, as well as on the porosity derived from the density measurement. As shown in the paper, the second effect can be addressed by using corrections devised specifically for formate muds, and the corrections can be determined experimentally. However, it is extremely difficult to devise experimental measurements to measure invasion effects, because the apparatus used to separate the different borehole fluids will greatly distort the measurements. In addition, so many data points are required that the task would become monumental. The computer program MCNP was used to model two logging-while-drilling density tools of different sizes in three different formate-mud systems. One mud was primarily potassium formate brine, one was primarily cesium formate brine, and one contained substantial quantities of both. The modeled tool response was calibrated to measurements in water-filled formations. In addition, the theoretical mud properties were tweaked so that modeled count rates for the tool placed in a vat of mud agreed with actual measurements. Finally, the model was verified with standoff measurements in formate mud.