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Water based drilling mud
Water Base Mud (WBM) are generally less expensive and less difficult to maintain as compared to Oil Based Mud (OBM).Although WBM is less expensive but they sometime cause issues related to formation damage and borehole stability as water tend to invade the formation and change the mechanical properties of formation.In order to avoid this type of issues inhibited WBM can be used however it should be noted that inhibited WBM can minimize damaging effect of WBM to formation but it cannot completely prevent water wetting of pores of rock.
Base fluid in WBM is water,WBM also contains clay and other chemicals to achieve appropriate viscosity and density. Clays and other chemicals are mixed up and sold to drilling companies as additives ,Most commonly used additive in WBM system is bentonite,it is also referred to as gel as when it is pumped ,it can be very thin and free flowing but once pumping stops it builds up a gel structure which resists flow.When again adequate force with pumping is applied,gel breaks down and it comes back to its free flowing state. Barite is also used in mud to increase density of mud if needed.Other many more chemicals are use depending upon purpose like:- Cooling and lubricating the drill string,Enhanced Rate of penetration,achieving required viscosity and well bore stability.
Water-based muds are characterised as dispersed and non-dispersed muds:-
- Dispersed Muds: Dispersed mud contains chemical dispersant which deflocculate mud solids.Lignite and lignosulphonate are commonly used dispersants in WBM, both chemicals are acidic in nature hence require alkaline environment to work properly.Use of diepersants permit sufficient amount of clay in system to control fluid loss.In dispersed WBM Caustic soda (NaOH) is used to control pH ,mud density is controlled using weighting material and bentonite clay is used as viscosifier.
Dispersed WBM can be further divided into two categories as calcium-based and seawater based muds.
- Calcium-Based Mud:Calcium based mud maintain desired concentration of calcium by mixing gypsum or lime in mud.Calcium based mud are inhibitive and can tolerate anhydrite and cement contamination more effectively than fresh water dispersed WBM.
- Seawater Mud:In offshore environment using native sea water instead of transporting fresh water from outside can save some cost.As for seawater mud ,the upper limit of conventional dispersed fluid to function properly is 20 kppm of chloride,cost of this system is slightly higher but ready availability of seawater in offshore environment helps in offsetting that cost.
- Non–dispersed Mud: By not having dispersants ,Non-dispersed mud don’t require high pH.Non-dispersed muds are less tolerant to contamination and presence of solids.Fluid loss control and viscosity in non-dispersed WBM is maintained by polymers.
- Synthetic Based Mud (SBM):- Base fluid in SBM is synthetic oil.The best things about SBM is that it has all the properties of OBM but environment contaminant are as way less as compared to OBM. Due to this reason SBM is mostly used in offshore environment when drilling crew work in confined space in offshore rig environment. SBM creates same interpretation problems as we have with OBM.
- Water:– Top section of hole where no hydrocarbon is expected is sometime drilled with water only.
- Air:-Sometimes compressed air is also used to drill top section,this is done when formation drilled have little or no permeability to water.Although its called air drilling but various types of gasses are also used.This is a preferred method for fast drilling in dry,hard rock conditions.
- Air/Water or Mist :– Combination of Air/Water is also used for drilling top section.This method is useful when drilling wet formation with minimal water influx.
- Polymer:-Polymer is specially created chemical,added to water and air to create required conditions,like foaming agent.This is preffered method when drilling stable formation with moderate influx of water.