716 Subsurface Geologic Methods 



ground condition to at least an equal volume of water along with a small 

 amount of colloidal material to keep the solids in suspension, a mud having 

 a density of 20 lb. per gal. or 150 lb. per cu. ft. may be produced without 

 excessive viscosity. 



Several instruments are available for measuring fluid density. One 

 such device is the Mudwate hydrometer. The hydrometer consists of a 

 float spindle with calibrated stem, a detachable bakelite cup, and a metal 

 carrying case. The cup is filled with the fluid to be tested and replaced 

 on the spindle. The carrying case is filled with water and the spindle 

 floated therein. The density of the fluid determines the depth to which 

 the spindle sinks, and the stem is calibrated to indicate fluid density in 

 suitable units. 



Another convenient and entirely dependable instrument for measur- 

 ing fluid density is the Baroid Mud Balance. It consists essentially of a 

 base and graduated beam with cup, lid, knife-edge, rider, and counter- 

 weight. The cup is filled with the fluid to be tested, and the lid is placed in . 

 position to insure constant volume. The balance arm is placed on its base 

 with the knife-edge resting on the fulcrum. The rider, adjusted on the 

 beam until a balance is obtained, indicates the fluid density. 



Colloidity 



The colloidal properties of a drilling fluid determine its ability to 

 form a suitable cake on the wall of the well, to seal the pores of the wall 

 formations, and to lubricate the drill pipe. The wall-building properties 

 of the drilling fluid are of great importance. As the clay-laden fluid is 

 circulated over the walls of the hole, there is a tendency for the fluid either 

 to enter the pores of the formation or, if the pores are too small, to permit 

 the solid particles to enter. There is also a tendency for the liquid phase 

 of the mud to be squeezed from the fluid into the surrounding formation 

 and to leave the solid matter deposited as a cake. The thickness and perme- 

 ability of the cake may exercise considerable influence over the drilling of 

 the well. If the formation and cake permeabilities are both high, a thick 

 wall cake will be developed. This cake may become so thick that it will 

 interfere seriously with movement of the drill pipe and may even result 

 in sticking the pipe. 



The fluid which enters the formation may encounter shales and clays 

 susceptible to hydration, and the resulting swelling may cause heaving or 

 slipping of the shale or clay into the hole. The hydration or swelling of 

 clays in producing sands may impair production through the loss of 

 permeability. 



There is no unit of colloidity, and the colloidal value of a drilling 

 fluid cannot be measured quantitatively. Relative colloidal values and 

 their effect on the properties of drilling fluids are indicated by a wall- 

 building tester. This instrument provides an observation of the ability 

 of the fluid to form a cake under conditions similar to those deep in the 

 well. The wall-building tester consists principally of a filter cell and a 



