Most of the drilling fluids currently used for deep and super deep drilling arevery complex in nature and chemistry due to severe conditions encountered atthese depths. The mud, while circulated in the hole, experiences a gradualincrease in temperature and exponential increase in shear race. These factorswill substantially alter the rheological properties if the fluid composition isnot properly formulated and instantaneouslyconditioned.
An experimental see-up has been developed to measure and monitor the dynamicbehaviour of the fluid rheology. The equipment simulates the drill string andthe drilled hole parameters such as pressure, temperature of the penetratedformation, circulation rate, and the shear rates. Pressure drop and the flowrates data measured across the pipe viscometer are used to profile the flowdiagrams and consistency curves of each batch.
A series of data is obtained for power law constants, consistency factor K, non-Newtonian behaviour index N, and effective viscosity, µe, as afunction of temperature and shear rate for two fluids. Extreme value of shearrate was 12,000 sec−1 and temperature was 425 °F. The effectiveviscosity is reduced to a minimum as shear rate increased to maximum. Becauseof this, n values increased to reach a value of one, which describes aNewtonian fluid behaviour instead of non-Newtonian at the beginning of the testperiod. More than that, the non-Newtonian behaviour index, n, changes, in facepointing out that under the effect of the wide range of shear rates existing inthe hole, the mud rheological behaviour could become pseudo plastic, Newtonianand dilatent depending on the value of n.
There are two basic factors chat together determine the flow properties of adrilling fluid. These are the composition and the environmental factors.Compositionally, a drilling fluid can be broken into three main components:liquid fraction, electrically charged or active solid fraction, andnon-reactive solids. In addition to that, chemical additives are used in somecases co perform a special function in the mud. Environmental factors orcontaminants are the temperature, shear rate and pressure.
In deep and super-deep wells, the temperature and shear rates substantiallyaffect and would certainly, alter the flow behaviour which could possiblyterminate the drilling operation. As the mud circulates in the hole, it willexperience a gradual increase in temperature and, inside the drill stringitself, an exponential increase in shear race and both will reach a maximum atthe bottom hole enough to reduce the fluid viscosity to almost water.
The temperature may reach 380 °F at a depth of 20,000 ft, with a geothermalgradient of about 1.6 ºF/100ft and shear rates ranging from 20,000–100,000sec−1. Conventional methods available to measure therheological properties are certainly unable to reach these conditions. For thisreason a dynamic flow loop is developed to simulate high shear rate and hightemperature in the field.
