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    Oil based drilling mud

    Base fluid in Oil based mud is oil as an continues phase,it also contains some water but as dispersed phase.Base Oil in OBM could be diesel,kerosene,fuel oil or mineral oil.Water phase in OBM could be fresh water ,NACL solution and CACL solution. OBM were  primarily developed to minimize formation damage due to water entering pore spaces of reservoir rock.As outer and continuous phase is oil in OBM,it does not allows water to come in touch with formation which also helps by minimizing swelling clay (montmorilonite or smectite) as clay without coming in touch with water don’t swell.OBM also allows drilling high temperature deep wells where water based mud may get dehydrated and OBM also helps in drilling water soluble zones.Apart from Liquid phase ,OBM also contains solid phase ,Solid phase in OBM includes weighting materials ,fluid loss reducers and viscosifiers. Solid phase should be designed in such a way that it should always remain oil wet.If solid phase becomes water wet,the system is said to be flipped and that can create serious operational issues.

    Primary technical issues while drilling with OBM are :-

    • Oil shows on cuttings mixes up oil in OBM hence interpretation of fluorescence becomes tricky.
    • Oil contamination from OBM also creates issues with Geo-chemical analysis of cutting and cores and for TOC. Some extra steps are needed to remove oil coating from cuttings before they are describing them
    • Determination of native oil API .
    • Drilling with Oil based mud filtrate may cause some wireline sensors not work properly hence limiting formation evaluation suite to be run.

    Primary Non-technical issue with OBM are :-

    • High initial cost of OBM as  cost of oil fraction of a barrel of oil  is considerably higher than that of water-based mud.
    • Oil based  mud often results into  lower rate of penetration as compared to water-based mud.
    • OBM causes environmental damage by contaminating fresh water aquifers and cuttings needs to be disposed off properly to prevent any environmental contamination.
    • Cuttings produced by OBM may have to be cleaned up before dumping. Often regulatory agencies require cuttings be sent to a designated disposal area.

    The Primary  advantages of drilling with OBM  are:

    • Reduction of torque and drag problems: Since oil is the continuous phase, the borehole and the tubulars are wetted with  oil as lubricating fluid. This is a distinct advantage in deviated well-bores.
    • Thermal stability: Oil-based muds have shown stability in wells with high borehole temperature.
    • Shale control: In soft,sticky and swelling clays ,OBM helps borehole maintaining  stability and cuttings samples are generally intact.
    • Resistance to chemical contamination: OBM are also highly resistance to chemical contamination.