One of the most vital elements of the drilling process is the fluids.Without a circulating drilling fluid, rotary drilling would have been very difficult.Drilling fluids were simple in the early days as whatever water was used.Recently,drilling fluid has become complex because of the development of special type of fluids channelled to resolve all sorts of downhole challenges.
Drilling fluids, also called drilling mud, are composed of clay, bentonite, plus other chemicals and water (although there are organic based fluids, as will be discussed later) which are pumped down the drill pipe to lubricate and cool the drilling.Drilling fluids are necessary for drilling oil and gas wells, satisfying the various operational demands and challenges has made it complex.
In general,drilling fluid can be broadly classified as water based mud,oil based mud and synthetic based mud.Water based mud has water or brine as its base fluid.This makes it environmentally compliant as the drill cuttings can be disposed of easily without harming the marine and human life.Oil based mud consist of oil as it base fluid.Its adequate lubricating nature makes it the preferred choice over water based although environmentally,it is not compliant.Synthetic based mud comprises of synthetic material as its base fluid.It low aromatic content makes it less toxic and more environmentally preferable.
A Coal bed methane reservoir development can be heterogeneous,but the use of a bridging drilling fluid which is either polymer based or bentonite based helps to maintain constant pressure and eases borehole instability.Also for a High Pressure High Temperature(HPHT) well,an oil based drilling fluid composed of organophilic clay which has a high value of low shear rate viscosity (LSRV) is used because it can withstand the sagging of particles as a result of reduced viscosity [3].
In this thesis recently drilled wells were examined both for oil and unconventional gas to determine the properties and composition of the drilling fluids used and their influence on the rate of penetration of the drill bit.
