Mud density will decrease with increasing temperature, and increase with increasing pressure. Theoretical models similar to those mentioned in SPE Paper 11118 predict that temperature is the major effect. In an attempt to validate the model, the density of a sample of field mud was measured downhole temperatures and pressures.
While drilling with 977pptf IOEM, a downhole pressure gauge was run in the well. Higher than expected mud weights were found between 2,500 and 5,000ft, and between 15,000 and 16,500ft. The increasing trend of mud weight between 15,000 - 16,500ft could be explained by a slug of heavy mud pumped for tripping.
Experimental.
It was intended that the mud density should be measured at 5 different downhole temperatures and pressures. Because of the high solids content of the mud the test method had to be modified, and an absolute mud density could not be determined. Instead the mud density at 500psi and 65deg F was assumed to be 977pptf, and all the other densities were calculated relative to this value. Because of the limitations of the apparatus the highest pressure that could be used was 15,000psi.
Results.
Regression analysis of the measurements showed the the main effect on downhole density is temperature. The laboratory measurements and theoretical densities from the model, follow the same general trend, but are on different curves. The difference in curves may be due to the differences in thermal expansion coefficients between the base oil (used in the field mud) and diesel (used in the model). No evidence of the higher than expected mud density between 2,500ft and 5,000ft could be found in the laboratory measurements. Since two different gauges showed the same trend it is unlikely that a faulty gauge is the cause of the higher onsite densities.
Finally the sensitivity of the model to changes in oil/water/solids content was checked. Altering the percentage of oil, water or solids by up to 5% altered predicted density by only 1pptf. The differences in density between the observed laboratory and predicted densities cannot therefore be explained in terms of alteration of the field mud during transfer to the test cell.
Conclusions.
Downhole densities when drilling with IOEM tend to be lower than surface measured densities. The exact variation will depend on the oil/water ratio of the mud, and the downhole temperature. However on the basis of the lab measurements and the model mentioned in the SPE Paper, as a very rough rule of thumb, on a high temperature high pressure well, downhole oil based mud density decreases by about 1pptf per thousand feet depth.