Question

1) Why is the resistivity of the filter cake (the solid) higher than resistivity of filtrate, for after doing the filtration drilling fluids?

2) Should drilling fluids be high in resistivity or low? and why?

Answer 1

Ans. 1 This is the case of solid liquid filtration and seperation. By the Poiseulle's Law the rate of filtration is given as:

,

Here, the filtration rate is  ;

Here are the resitances to flow,

and is Broth's viscosity or viscosity of fluid.

The cake resistance is directly proportional to resistivity of filter cake. Hence in general by Poiseulle's Law the reistivity of cake must be lower than resistivity of filtrate. But in case of using the drilling fluids there is a large pressure drop. In using the drillling fluid, filtrate is the liquid which will passes through the medium. So the rate of filteration is largely affected due to high pressure drop.

Since pressure drop is high, hence the rate of filtration will be affected. With high pressure drop the filtration rate will become very low.

When the cake resistivity, will be very high compared to the resitivity of filtrate, it will directly effect the cake resistance , and filtrate resitance , as resistivity is directly proportional to resistance. Due to high (Cake resistance), the effect of high pressure drop, will be neglected and the filtration rate  ,  will be higher. As the filtration rate   is directly proportional to pressure drop.

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2. The drilling fluids must be high in resistivity. The resistivity must be high enough to provide sufficient hydrostatic head. This will prevent influx of fluids. The drilling fluids are of variable resistivity. The high resitivity drilling fluid perform very good function in filtration and other functions such as well bore stability, minimizing damage formation, cooling and lubrication. The main function of drilling fluid is transporting drilled cuttings to the surface. In this the high resitivity of drilling fluids gives high support, and the rate becomes high. The high resistivity of drilling fluids directly effect the rate of filteration. If the resistivity of drilling fluid will be low, it will disintegrate into ultrafineparticles, which will damage the filtration process and the filtration efficiency will largely decrease. The high density and resistivity of drilling fluid provide best filtration.

The flow of drilling filtrate through a drilling cake is given by Darcy's Law. The rate of filtration given as:

;

Here, the filtration rate is  ;

Here are the resitances to flow,

and is Broth's viscosity or viscosity of fluid.

Here in filtration through drilling there is another important parameter that is included is:   permeability of cake. The permeability of cake is directly related with the resitivity of drilling fluids. Hence for high rate of filtration the resistivity of drilling fluids must be high.

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