Drilling fluids are used to perform the following six functions:
1- To stabilize the wellbore while controlling subsurface pressure.
2- To suspend drilled particles and transport them to the surface.
3- To cool and lubricate the bit and drill string.
4- To assist in formation of valuation while minimizing formation damage.
5- To assist in supporting the suspended drill string.
6- To transmit hydraulic horse power to the bit.
Watch the video to look at each of these functions individually.
The normal fluid pressure gradient of the earth is about 0.465 psi per foot of depth which is equivalent to a column of fluid weighing about 8.9 pounds per gallon. In many drilling areas, the weight of water plus accumulated drilling solids is sufficient to balance this gradient and thus prevent fluid flow into the wellbore from permeable formations. However there are areas where abnormally high pressures requiring increase in the density of the drilling fluid. In such cases, the drilling fluid will be wetted up with a high specific gravity additive. Typically, the column of drilling fluid exerts a slightly higher pressure than that within the drilled formations, a condition which results in some degree of fluid flow into permeable rocks. The drilling fluid must be designed to form a filter cake of solid particles against the wellbore wall. Preventing significant fluid loss into the formation and thus stabilizing the hole.
The addition of specially formulated natural clays to the drilling fluids normally provides this capability. These clays also improve the drilling fluids ability to transport the drilled rock cuttings to the surface where they can be screened out and sampled for geological information. The fluids viscosity permitted to carry these particles in suspension and the clay increases this viscosity of the fluid. The clay also gives the fluid gel strength, a property which causes the mud to stiffen or gel when pumping stops but allows it to be liquefied again when pumping resumes. This property keeps the suspended cuttings from falling to the bottom of the hole when circulation is halted. With subsurface temperatures in the hundreds of degrees Fahrenheit and enormous pressures being applied to the drill bits face, the need to cool the rotating surfaces of the bits components becomes important. The fourth function of the drilling fluid is to add in the collection of geological information. The most obvious manner, in which this is accomplished, is by the transport of cuttings to the surface.
By calculating the depth of which these cuttings were drilled, a log of the hole can be developed. Coupled with detection of formation fluids in the returning mud, this information is the first glimpse of the subsurface available to the geologist. But the drilling fluid also provides important assistance to the logging of the drilled hole by permitting the shallow invasion of unknown salinity filtrate into potentially productive formations. By minimizing the amount of this filtrate the filter cake formed on the wellbore wall protects the formation from large amounts of foreign drilling fluids which could damage the permeability characteristics of the formation. With average well depth increasing, the weight needed to be supported by the drilling rig becomes an increasingly important factor.
