Filter cake is formed when the insoluble solid portion of fluid slurry becomes deposited on a permeable material as the slurry makes contact with that material under pressure. This is how filter cake forms: Initially when the filter cake is being deposited on the surface of the permeable material, the material firstly serves as a filter and allows the liquid portions (filtrate) to pass through and trapping the insoluble solid portion as a cake.
Over time, enough filter cake gathers on the surface of the permeable material, allowing little or no further liquid invasion. In the oil and gas industry, this permeable material can be porous underground formations and the fluid slurry is the drilling fluid.
One major property of the drilling fluid is the formation of a filter cake. This filter cake will only be deposited on the porous rocks under overbalance pressure conditions. Luckily, due to drilling fluid circulation, filter cake will not continue forming indefinitely; there is always a point of equilibrium.
The circulating fluid will keep shearing and dislodging particles away from the filter cake until equilibrium is reached. At equilibrium, filter cake thickness will neither increase nor decrease; as some solids are being deposited on the walls of the porous formation others are being dislodged by the shearing action of the circulating fluid.
Good and Bad Filter Cake
The formation of filter cake prevents further loss of drilling fluid into the formation and helps minimize solid invasion as well. In other words, filter cake helps prevent loss circulation and formation damage through fines and filtrate invasion into reservoir rocks.
Thin, low permeability cakes are desirable when drilling a well. The thicker the filter cake formed, the greater the potential for the drillpipe to become stuck when it comes in contact with the filter cake under pressure. A good filter cake should also have very low permeability; a permeable filter cake will allow filtrates and fines to migrate into the porous formation ultimately damaging the formation.
Thinners are additives added to the drilling fluid that can help control the thickness of the filter cake. The presence of salt in a drilling mud system destroys the filter cake structure. Salt contaminated mud will form thick, permeable filter cake which is highly undesirable. Apart from rock properties and salt, if the overbalance pressure is too high, there is tendency for the filter cake formed to be too thick.
Filter Cake Structure for Oil Base Mud
The structure of the filter cake formed for oil base mud is different from that formed from water base mud. For oil based mud, in addition to the insoluble solid deposition, water droplets which are present in dispersed phase in the oil will also form on the pore throats of the filter cake.
But if the overbalance pressure of the drilling fluid becomes higher than the capillary pressure needed to force the water droplets through the pores, then the water droplets will be forced through the pores of the filter cake leaving only the solid layer.