The use of “dewatering units,” as a regular part of the drilling fluid treatment process on drilling rigs, continues to increase. A dewatering unit, the final phase of a solids control program, is designed to discard no free liquid. A solids control program in which a dewatering unit is used is referred to as a “closed-loop system” (CLS), “sump-less system,” or “chemically enhanced centrifugation” (CEC).
These units can process drilling fluid from the active mud system or underflow from the solids removal equipment (shale shakers, desanders, desilters, and centrifuges). Chemical pH adjustments are made to the excess mud: flocculants and coagulants are added to the input stream of the dewatering unit. The flocculated solids are usually removed by a high-speed (high-gravity) decanter centrifuge. Colloidal-rich discards typically contain approximately 50% by volume water but can have as little as 10% by volume solids and still be handled as a solid. These solids can be handled with a frontend loader and hauled on dump trucks, therefore, disposal is typically not a problem. The recovered liquid is usually treated and returned to the active mud system. Treating chemicals, including acids, polymers, strong bases, and clarifying agents, should be carefully selected and additions carefully monitored in order to prevent personnel injury, overtreatment, and excessive treatment cost.
Dewatering of drilling fluids while drilling is the final step when removing solids from excess fluid. All solids are removed from the drilling fluid, related sludge, and waste water. The recovered water is normally reused as treatment water in the active mud system. This process is now relatively common in environmentally sensitive areas. Although the process is expensive, it can be cost effective in areas where disposal costs are excessive.
In many areas, the cost of drilling fluid disposal may greatly exceed the preparation cost. These fluids may be classified as “hazardous” and, therefore, require disposal in hazardous disposal sites. The combined cost of transportation and disposal can run into the hundreds of dollars per barrel. The chemicals in the fluid, such as caustic soda, oil, or salt, are usually major constituents of the drilling fluid. By removing the solids from the fluid and adjusting the chemistry, the remaining water can be reused in the drilling fluid.
A variety of shale shakers, pumps, centrifuges, tanks, mixers, and chemicals are used in the dewatering process. The solids are removed from the waste fluid to minimize the volume of waste and reduce the cost of disposal. Although the final result may be approximately the same, the solids removal cost and techniques used may vary considerably.
The fluid from which the solids are to be removed may be any type of drilling fluid, including oil mud. The fluid may be excess native mud from the active mud system or may contain concentrated solids from the underflow of mud cleaners, drying shakers, desilters, or centrifuges. The mud type may be native mud, low solids non-dispersed mud, highly treated lignosulfonate mud, salt mud, or even oil mud. Generally, the more highly treated the mud system, the more complicated and expensive it is to dewater and return a clean, usable water.
Water-based muds vary from lightly treated “spud mud” to highly treated, saturated salt mud. The lightly treated muds are simple to dewater and the solids can often be disposed of without restrictions. Poly-acryl-amide treated muds are usually the simplest to treat because they have a low pH and may contain the same type poly-acryl-amide that is used as the flocculants. The process becomes more difficult and costly as the chemical content of the mud increases. Saturated salt mud is more expensive to process because the removed solids contain excessive amounts of salt, and the water cannot be disposed of because of the high salt content. The only use for this water is in the mud system.
The first step in the dewatering process is to form units of the aggregated solids that are large enough to be removed by a decanting centrifuge. After these aggregated units are removed, the free water is sent to a holding tank where a “clarifying” chemical is added and the pH increased to neutral (Figure 13-1). In the holding tank the finer solids are allowed to settle by gravity. How clean and clear the water needs to be depends on how the recovered water is to be used.
Adding an acid to the drilling fluids is necessary since these fluids usually have a pH of 8 or higher. The charges on the clay particles are usually negative. For flocculation to work properly the charges need to be positive. Adding an acid increases the positive charges on the solids. In order to obtain adequate positive charges on the clays, the pH must be reduced to about 4. The acid used may be hydrochloric, sulfuric, phosphoric, or buffered phosphoric. The use of sulfuric acid is discouraged because it is so dangerous to handle. Although more expensive, buffered phosphoric is the most desirable because it is relatively safe. Next, a flocculant, such as a poly-acryl-amide or alum, is added to increase the size of the floes. This allows the solids to be brought together in large floes, which can then be removed by a centrifuge. These additions may be made in batches in a large pit—approximately 150 barrels—or in a continuous flow, computer-controlled dispenser.
After the fluid is flocculated, it is pumped to a high-speed, 3000 to 3400 rpm, decanting centrifuge where the flocked solids are removed. Although the underflow solids from the centrifuge appears very dry and can be handled with a frontend loader, the water content is approximately 50% by volume. It may be possible to use the solids as dike building materials depending on local environmental regulations, the chemical content of the solids, and the water that remains in the solids.
The recovered water is pumped to a holding tank for use as mud treatment water. The pH is increased to Between 9 to 10.5, and the turbidity value is tested prior to adding the water to the mud system. Neutralization of the acid coagulants is commonly accomplished using caustic soda, magnesium hydroxide, or sodium carbonate. Of these neutralizing agents, magnesium hydroxide works at the lowest level of concentration. Using magnesium hydroxide in conjunction with phosphoric acid produces only water and an insoluble phosphate precipitant. Treatments with the other neutralizers, such as caustic soda and soda ash, produce significant amounts of water soluble salts. Continued use of these products will create an unacceptable buildup of the salts and increased turbidity levels. Chlorine, called “ever-clear,” may be added to the recovered water to precipitate the colloidal solids and increase the pH.
Oil muds are more difficult to dewater than water-based muds. The cuttings travel from the shaker to a wash tank containing a detergent and an agitator where the oil is removed from the cuttings. The water/oil mixture is then put into a pit where the oil is removed by a “skimmer,” or it is processed through a small ID-high pressure hydrocyclone.
Dewatering of oil-based muds follows approximately the same procedure as that of water-based muds. First, the emulsion must be broken down, which is accomplished by adding an acid and additional water. The oil separates and rises to the surface where it can be removed by a skimmer. The remaining solids are then sent to the processing tank where they are treated in the same manner as water-based mud. Additional washing may be required if the remaining oil content in the sludge exceeds acceptable environmental levels.
Vacuum filtration is another method of removing solids from the liquid. Chemical flocculation may be used to aggregate the solids, or the mud can be processed through a filter depending on the properties of the filter. The solids can then be dried to further reduce any residual water. Chemical flocculation and removal by a decanter centrifuge has proven the least expensive of the two methods, however, the filtration method is continuously being improved and should be given consideration.
In comparison field studies the most effective and economical treatment method has been chemical treatment, settling, and centrifuging of the mud. In these field tests, mud from the active system was treated with anionic poly-acryl-amide polymers, flocculants, and phosphoric acid coagulants. The flocculated mud was processed through a high-speed (1500 to 2000 times the force of gravity) centrifuge. The “dry” solids removed contained approximately 50% solids by volume. The recovered water, or filtrate, was placed in a holding tank where the pH was neutralized with magnesium hydroxide. After settling, the water was pumped into another holding tank where it was given a final treatment with chlorine prior to returning to the active mud system.
This technique removes dry solids and returns the recovered water to the active mud system, which has no suspended solids, a minimum of dissolved solids, and a pH in the 7 to 10 range.