Using oil based mud can impact logging operation and you need different set up of logging tool in order to get accurate data. The impacts of oil based mud on logging are mud invasion, conductivity of electricity, oil density and water salinity.
Firstly, It is mud invasion. Oil based mud invasion will effect value of resistivity and it may lead to wrong interpretation. You need a good logging tool which can read several feet from the well bore in order to get the correct resistivity figure. Nowadays, several service companies offer series of resistivity measure. It means that a tool can read several depth of investigation from few inches to several feet from wellbore. This is real benefit to all people to understand mud invasion, improve accuracy of logging interpretation, etc. Tool may cost more money but it is worthy to invest from my opinion. Moreover, you need to have the mud that creates good mud cake across formation because good mud cake prevents formation damage due to mud invasion.
Secondly, oil based mud does not conduct electricity therefore you are not be able to use the Laterolog log to determine formation resistivity. Laterolog log has a basic circuitry of emitting and measuring electrodes across which a potential drop in measured to give rock resistivity. It is suitable for conductive drilling fluid as water-base mud. You must use induction logs to get the resistivity values when you drill with the oil based mud.
Thirdly, the oil based mud has different fluid density and modulus therefore traveling speed of sound will be different when compared to speed of sound in water based mud. A log interpreter specialist must ensure that correct type of mud is used for logging interpretation.
Finally, the water salinity is also an important figure that must be taken into considerations while processing the log because it is used to correct the measurement of nuclear particles.
source : http://www.drilling-mud.org/logging-and-oil-based-mud/
