Modeled Effects of Formate Mud on Density Logs
- Gordon L. Moake (Halliburton)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- April 2013
- Document Type
- Journal Paper
- 113 - 127
- 2013. Society of Petrophysicists & Well Log Analysts
- 2 in the last 30 days
- 168 since 2007
- Show more detail
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 in the future may well hinge on the industry’s ability 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 barite-weighted 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. Invasion from formate mud is different from other muds because the invading brine has the same high density as the mud, as opposed to just the water invading from a barite-weighted water-based mud.
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. Consequently, the only practical way to determine invasion effects is with computer modeling.
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 fine-tuned 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.
Using the calibrated model, the density changes caused by invasion were computed for the three muds in formations with porosities of 10, 25, and 40 p.u. using tool standoffs of zero, 0.25, and 0.50 in. The changes vary with standoff, although the trends are the same for all formation densities and standoffs studied. They can be as large as 0.5 g/cm3 in a 40 p.u. formation. Charts of the corrections required to compensate the density measurement for invasion are provided.
Unfortunately, determining the invasion depth to use with the charts is difficult. However, this paper illustrates how tool measurements can be used to compute formation density before invasion, depth of invasion, and standoff. The method computes values for the uninvaded density that are reasonably accurate for invasion depths of ≥2 in., depending on the porosity and mud weight. The computed depth of invasion can be used to correct measurements made with other tools; it is accurate for invasion depths up to 2 to 5 in., depending on the porosity.
|File Size||12 MB||Number of Pages||15|