The use of “Closed” mud systems reduces the environmental impact of oil-well drilling to an absolute minimum. The strict definition of a closed mud system is one from which the only waste is damp cuttings. This requires the removal of all drilled solids (and caving) and the reuse of the liquid discharged by the solids removal equipment. Reuse of the liquid requires the separation of colloids and ultrafine solids particles before it is returned to the system. This can only be accomplished with water based mud, and requires dewatering. The personnel, chemicals, and equipment required make it a costly process which is difficult to justify except under very special circumstances, such as drilling in an urban environment, or in areas in which governmental regulations prohibit the discharge of drilling fluid wastes. Historically, the cost of eliminating onsite waste disposal has been found to be about 20% of total well cost.
Closed systems have also been more loosely defined as those which do not require (on location) reserve pits. The option of transporting cuttings and liquid waste to an off-location disposal site is always available but can be very expensive if large volumes of liquids are involved. Transportation costs are to a very great extent a function of the dilution required and, therefore, of the efficiency of the solids removal operation.
In most drilling operations it is very difficult to justify trucking waste materials. In many cases it amounts to little more than spending large quantities of money to move a problem from one site to another.
Attempts to close a mud system without clarifying the liquid phase before returning it to the mud system, or to operate a closed system without disposing of liquid mud can be successful only when the interval to be drilled is short and the operator does not feel that quality drilling fluid is required.
Degradation of re-circulated larger particles, And the colloids and fines generated at the bit gradually increase the colloid and ultrafine solids content of drilling mud; the increase is most rapid while drilling clays and soft shale with dispersed (dispersive) drilling mud. Failure to control this increase through dilution centrifuging, or dewatering permits a progressive deterioration in mud quality, and increases drilling cost.
Some of the methods presently being attempted to run closed systems without dewatering increase the rate at which drilling fluid quality deteriorates. An example is the use of a second centrifuge to process the overflow from a “barite recovery” centrifuge. When processing drilling mud weighing 14.0 lb/gal or more, the cut points on barite for low speed and high speed centrifuges working with water based mud can be estimated at 8 and 4 microns respectively. It is interesting and informative to analyze what these figures mean in terms of solids removal and the effects of the process on mud quality.
In the two-centrifuge application, the first (low speed) unit does not discard anything. At the assumed cut point, it returns the barite larger than 8 microns and the low gravity solids larger than 12 microns to the mud system. The liquid fraction and the solids finer than these cut points are then processed by the second centrifuge. This unit discards the barite between 4 and 8 microns and the low gravity solids between 6 and 12 microns, and returns the liquid and finer solids to the system.
The primary problem with the process is that it permits the gradual increase of the concentration of colloidal and near colloidal solids, none of which are removed. Additionally, most of the solids discarded will be barite, which must –of course- be replaced. The barite which is discarded, which can be expected to be 10 – 15% of that which is processed, is in the desirable particle size range, while seven or eight percent of the fresh barite with which it must be replaced will be colloids.
In this example, the net result of the complete process; double centrifuging followed by restoring the density of the fluid returned to the system by the second centrifuge is the following:
- The removal of 10 -15% of the better barite.
- The removal of the low gravity solids in the 6 – 12 micron range. (This is beneficial but these solids are far less damaging than those which are returned to the system).
- Replacement of the barite and low gravity solids removed by the second stage centrifuge with fresh barite.
- An increase in the colloidal solids content of the mud arising from the addition of fresh barite.
It is an expensive process which does more harm than good.