Drilling Fluid: Wall Cake A drilling fluid will deposit a filter cake on the wall of the well bore. This wall cake helps protect the formation by retarding the passage of mud filtrate into it. The higher the permeability of a formation, the greater its ability to accept and receive large volumes of mud filtrate. Therefore, the nature of this filter cake will have a direct effect on such problems as formation damage, sloughing and caving, tight hole and stuck pipe. The type of wall cake is determined by the quantity and quality of particles in the mud system. It would normally be regarded as the thinner the better. The bentonite particles should be kept in the dispersed or deflocculated state and not allowed to flocculate. Bentonite in the flocculated state will increase the water loss and have a thick, soft filter cake. It is important to control the filtrate loss of the mud so that the water will not destabilize the formation.  A drilling mud with a low filtrate water loss will form a thin, tough filter cake. Specially formulated, bentonite product can provides filtrate control and a thin, tough filter cake.  Any contaminates that flocculate the bentonite should be treated out immediately. To minimize erosion, avoid any unnecessary reaming or circulating opposite unconsolidated formations. Bentonite, polymers and starch are use to control the water loss  It is essential that products be mixed in the correct order. Some products have an adverse effect on other products when they are mixed in an incorrect order. For example, if PHPA polymers are mixed before the bentonite then the bentonite will not yield properly. The following mixing guidelines should be adhered to when mixing products. If there is a water quality problem, treat the water before you add any products. Add the required amount of bentonite and mix for 5-10 minutes. This should be adequate if the mixing system has sufficient shear. I would recommend mixing longer if the mixing system shear is not adequate. The fluid should look smooth and not have any large lumps floating around. In general, the longer you mix any fluid products the better they will work. After the bentonite is hydrated, add any polymer additives that you might be using. Here again, the mixing time should be extended to allow the polymers to hydrate and shear. The last addition should be lubricants or drilling detergent. Only mix long enough to make sure the additives are homogeneous in the fluid. Over mixing may cause a foaming problem, especially in the case of drilling detergent. Caution: If you are using a PHPA polymer in the fluid make sure you rinse out the mixing tank before you mix another batch of fluid. PHPA polymer is an effective flocculent in dilute solution. If there is a residue of polymer left in the tank it will flocculate the bentonite that you try to mix. Rinsing out the tank is a good practice to implement and takes very little time.