**2. Background**

The drilling fluid provides the hydrostatic pressure necessary to balance the formation or pore pressure to control influx into the wellbore or prevent kick and blowout. This is achieved by using a densifier additive. The densifier also controls the buoyancy force for partial drillstring suspension in order to reduce the derrick load. Also, for effective wellbore cleaning and cleaning beneath the drill bit, the drilling fluid must possess viscosity as a property in addition to the required fluid hydraulics objective functions. Such objectives or combinations include hydraulic horsepower, jet velocity, Reynolds number and jet impact force. The fluid should also be able to suspend cuttings in the annulus when drilling is interrupted, and release the cuttings in the solids control equipment; the thixotropic property. Similarly, the fluid should have high heat capacity to be able to cool the drillstring and bit. It can also serve as a lubricant in the system. It is also designed to reduce or prevent formation damage and seal permeable formations by application of its fluid loss property. It must also control wellbore stability by preventing hydration of hydratable clays and retard other factors. Corrosion is also prevented by addition of agents that control the pH of the fluid. It must also be able to transmit hydraulic horsepower to the bit by maintaining its desired phase at every point. Formation evaluation must not also be interrupted. Nonetheless, in oil-based and water-based drilling fluids, it is the filtrate invasion that causes interruption of formation evaluation [3]. It is the fluid loss property that controls it. Zones of filtrate invasion and filter cake deposition are (i) the invaded zone into the formation (ii) the external cake on the wall of the wellbore and (iii) the internal cake that extends inches into the formation. An example is in the determination of petrophysical property known as resistivity. Low resistivity salty water-filled rocks are distinguishable from high resistivity hydrocarbon-filled rocks [4]. Even deep reading resistivity tools may only provide investigation data at depth not beyond the invaded zone. This leads to corrections to get the true formation resistivity that might introduce errors. This may be prevented by deposition of a suitable filter cake thickness with other qualities such as slickness, toughness and permeability. The high quality cake would prevent not only deep filtrate invasion but also formation damage.

Fluid systems have been developed to furnish and control the functions outlined. Based on the base fluid and materials deliberately added to the mud, such systems include; (i) air, gas, foam, mist (ii) calcium treated (iii) dispersed (iv) low lipids (v) non-dispersed (vi) oil/synthetic (vii) polymer (viii) saturated salt and (ix) workover fluid [5]. Cost, downhole conditions, type of well and environmental consideration are some factors that control the choice of drilling fluid type for application.

The traditional fluid systems have limitations that impede complete performance of the functions outlined. For instance, water based mud systems have the tendency to dissolve salts that might lead to density increase. Similarly, interference with oil and gas flow and clay dispersion has been potential problems. Moreso, oil based mud are not cost effective and pose environmental challenges. Gas based fluids have the tendency to cause explosion and cannot be used through water bearing formation. The stability observed under atmospheric conditions might not exist under high temperature and high pressure observed at increased depths both in marine and land environments. In deep offshore drilling, formation of hydrate also poses challenge. Though additives added to drilling fluids are expected to prevent or limit the adverse effects the conventional drilling fluids might experience, oilfield experience shows that problems still occur. New technologies are often sought to mitigate these

problems. The field of nanoscience, where nanoparticles with desirable attributes are evaluated for application, provides an alternative.
