Anyone familiar with drill site operations is aware that many active mud systems tend to increase in surface volume. The excess must be disposed of by some means after being hauled away, or jetted out, etc. Examine Figure 1 with the following:

The subsurface mud system consists only of the hole. This subsurface system increases at the rate of drilling new hole, plus caving or sloughing rate. Drilling the hole does not change the level in the surface tanks, as every 100 barrels of hole drilled adds 100 barrels of cuttings and results in
100 barrels of hole to be filled with mud. The same is true of caved volume, and whether the formation might be bentonite or granite does not change this.

Figure 1 Mud system material balance

Filtration to the formation tends to reduce mud pit level at rates varying from rare cases of hundreds of gallons per minute drilling sands and silts with “clear water ,” to small losses using filtration control, or to no filtration loss in impermeable zones. Solids removal tends to reduce system level. The reduction of system level can be more than the drilling rate, even
though the net solids removal rate is always less than the drilling rate. This
is because solids are not removed nearly as dry as they exist in the formation, so the volume of solids removed plus the liquid necessarily removed with them may be much more than the hole drilling rate.

Drill joint addition slightly increases the level in the tanks, each 31 foot drill joint amounting only to approximately one cubit foot, or 7.5 gallons or 28 liters, of steel. During the drilling of that 31 feet of hole the cuttings formed amount to much more volume than the steel added (in a 77/8″ hole it is roughly 10 times as much as the steel volume).

The drilling of a gas sand displaces some liquid from the hole by expansion of the gas rising in the annulus. Liquid level in the surface tanks will increase temporarily. If a gas sand were continuously drilled, the level in the tanks would begin to decrease as the formation gas porosity was replaced with drilling fluid in the new hole .

The gas breaking out at the surface reduces total system content. Once the gas zone is past and the gas circulated out , the surface mud system level would return to a point lower than when the gas zone was first encountered.
The fact that the gas expansion in the annulus is a major temporary affect should not mask the underlying and continuing minor reducing effect on the surface volume and level.

There are other very minor effects that are sometimes confused. Solubility is a negligible factor causing a decreasing trend in surface level too small to be noticed in the field. Temperature expansion should increase a system in the early stages of shutdown, assuming the subsurface is hotter than ambient.

Reverse the statement if subsurface is cooler than ambient, but in any normal field case it would be very difficult to detect. Excluding entrained gas, already discussed, there is no liquid compressibility affecting the circulating system . The column of liquid changes only by drilling deeper and at a rate of absolute change that would preclude field observation . None of the effects in this paragraph have any significance other than theoretical, and will not be discussed further.

The natural trend of a surface mud system is to decrease in volume as the hole is drilled, formation fluid entry being the only natural factor capable
of reversing this trend. In normal overpressure drilling, with mud hydrostatic head greater than the formation (pore) pressure, any excess surface system volume that we must drain off, jet out , or vacuum truck away, can be caused only by factor No. 5 in Figure 1, “Addition of Liquid
and Commercial Solids Materials.” Even water is a cost item , and though
the scarcity varies more than most other materials it is a major item in some areas.

Since buying material to increase pit level is made doubly expensive by the problem of disposing of the excess, there should be powerful valid reasons for willfully adding so much to the problem and the cost. Before the advent of modern solids removal equipment, the valid reason for “dumping,” or “watering back ” ; was to control mud properties. Today it is usually a holdover in custom or the result of failure to understand: (I) the problem and (2) the solids removal equipment available.