Solids Control In A Drilling Fluid
Authors: Marshall William H. , Brandt Louis K. 
Published: December 2007

The higher costs of drilling today as well as the increased depths required are forcing operators and contractors around the world to become knowledgeable of the benefits, techniques, and equipment available to control solids effectively in a drilling fluid. Through the use of shale shakers, desilters, mud cleaners, and centrifuges, solids concentrations can be effectively controlled. This discussion will not include the detrimental effect of drill solids on the formation itself, but will cover the "whys" and "hows" of controlling drill solids using this equipment.

WHAT IS THE PURPOSE OF A DRILLING MUD?

The purpose of a drilling mud can be broken into nine basic functions:Cool and lubricate the bitCarry cuttings out of the well boreSuspend cuttings when circulation is stoppedRelease cuttings once they reach the surfacePrevent the walls from cavingEliminate adverse effect on the formationControl subsurface pressuresAllow maximum well gathering informationHelp support the drill string

WHAT IS IN A DRILLING FLUID?

The majority of drilling fluids in use today employ water as the liquid phase and are thus classified as water-base muds. The remainder of today's drilling fluids use oil as the continuous liquid phase and are classified as oil-base muds. The solid phase of drilling muds may be broken into two basic types:Commercial solidsDrill solids

Commercial solids are those solids that are purchased from the mud company and placed in the mud to obtain desired properties to perform required functions. All commercial perform required functions. All commercial solids have a particle size of less than one micron with the exception of barite and LCM (lost circulation material). Barite particle size distribution runs from 1 to 74 microns as is indicated in Figure 1. Lost circulation material varies in its size; however, LCM is considerably larger than barite.

Drill solids are those solids that enter the mud in the form of cuttings or cavings from the formations being drilled. Drill solids vary in their particle size from less than one micron to 1500 microns and larger. Drill solids are necessarily in the mud; however, their time in the mud should be limited to the time it takes to circulate from the bit back to surface. The composition of drill solids is, of course, the same as the composition of the formation being drilled or having been drilled earlier in the well. One of the most important objectives in drilling any well is to control the concentration, size, and type of solids in the drilling mud. Meeting this objective can eliminate many related problems through the duration of a well. This problems through the duration of a well. This task is the responsibility of the people engineering the well, the operations personnel the drilling contractors, and the mud company. The cooperation and understanding of all of the above individuals is necessary to meet this objective.

The actual dollar value of today's drilling fluids is quite often a significant direct cost factor of a well. The typical mud system contains between 1000 and 2000 barrels.