Gypsum (Gyp) Muds are water mud system which containing gypsum. Gyp mud can be used for drilling shales, but it is also well-suited for drilling gypsum, anhydrite and salt stringers. An advantage of gyp over lime muds is that the pH of gyp mud need not be so high because it contains more soluble Ca+2 to inhibit shale swelling. Gypsum, CaSO4·2H2O, content is measured by an API test, and more can be added as needed. A calcium tolerant clay deflocculant may be needed to control viscosity. Carboxymethylcellulose (CMC) and starch are used for fluid loss control along with a small amount of prehydrated bentonite.
- Gypsum fluids are maintained with higher filtrate calcium and lower alkalinity than lime fluids to increase their inhibiting affect on clays.
- Gypsum fluids are used when large sections of gypsum or anhydrite are to be drilled. Because of the limited solubility of CaSO4 in water, additional gypsum or anhydrite will not dissolve into the fluid system but will be carried as a solid.
- Gypsum fluids are more resistant to salt and salt water than lime fluids, as long as solids are kept in line.
- Gypsum fluids are less susceptible to high temperature solidification than other calcium fluids because the alkalinity is maintained at a lower value. A temperature of 350°F is usually considered to be the upper limit for well-treated gypsum fluids.
Factors Which Affect Gypsum Fluid Breakovers
- Since the degree of viscosity change during and after conversion to a gypsum fluid depends upon the total solids concentration of the fluid system, dilution with water is necessary prior to conversion. Good solids control equipment is strongly recommended. A centrifuge is recommended for weighted fluids.
- The type of clay solids present in the fluid influences the viscosity during conversion. Because MILGEL has a much higher Base Exchange capacity than most drilled clays, the viscosity increase is much more pronounced need when converting fluids that contain high concentrations of MILGEL.
- The type and amount of deflocculating agents added to the fluid prior to conversion will have a significant influence on viscosity during the conversion. When necessary, treatments should be made with small increments of UNI-CAL and caustic soda. If a phosphate has been used, the conversion will normally exhibit drastic viscosity humps. These are the result of the reaction between phosphate, lime, and solids all caught up in a mass ion impact while displacement and sequestering is all taking place simultaneously.
- Maximum agitation should be maintained in the surface active system.
- Pilot tests should be run prior to conversion to help determine necessary amounts of dilution and concentration of conversion chemicals.
- The pit volumes should be reduced to allow room for dilution.
- Necessary amount of gypsum and deflocculant should be stacked near the hopper, and caustic soda should be carried to the top of the suction (mixing) pit. Gypsum and deflocculant are added through the hopper. Caustic soda is added directly to the suction (mixing) pit near the point of maximum agitation.
- Accurate circulation time should be determined so that conversion materials can be added at timed intervals over a period of one or two complete circulations.
- Conversion to a gypsum fluid should be initiated shortly after a new bit has been placed on bottom, i.e. once the conversion starts it should not be interrupted until them mud system has stabilized. Best results are obtained when the conversion is made inside the casing prior to drilling out.
- The amount of material needed for conversion varies depending upon the desired properties and the condition of existing fluid. Most conversions can be made as follows.
- Drilling fluid systems low gravity solids content should be diluted to a minimum solids content of 3 to 5% by volume. If the system is weighted then the calculated dilution volume is to be added simultaneously with the below listed chemical additions. The biggest concern is after the break over the fluids viscosity could drop so low that the barite would fall out if a weighted system.
- 3 to 6 lbm/bbl UNI-CAL®
- 4 to 6 lbm/bbl gypsum (CaSO4 •2H20)
- 1.0 lbm/bbl caustic soda
- 0.5 to 1.5 lbm/bbl CMC, or
- 2 to 6 lbm/bbl MILSTARCH with preservative.
- UNI-CAL should be started through the hopper while caustic soda is added to the pit. After five or six sacks of deflocculant and two or three sacks of caustic soda have been added, the gypsum additions should then be mixed through the hopper along with the UNI-CAL. The gypsum addition will cause a viscosity hump but will “break over” when agitated and become quite fluid. If the viscosity becomes excessive, the rate of addition of deflocculant and caustic soda should be increased along with an increase in water dilution.
- API Filtrate is normally controlled with CMC and/or MILSTARCH. Periodic additions of MILGEL in moderate quantities will help also help. MILGEL is usually accompanied by treatments of UNI-CAL.
- After all chemicals have been added to the fluid system, a check should be made of fluid properties. Minor adjustments can then be made on subsequent circulations with additional gypsum, UNI-CAL, caustic soda, and filtration control agents.
- The addition of gypsum and CMC simultaneously through the hopper should be avoided because of drastic viscosity increases.
- Typical fluid properties after conversion and before weight-up are as described below.
- Occasionally, it is necessary to convert weighted fluids when dilution is not practical. In this case, the fluid can be converted in the surface pits and displaced down the hole in stages.
- If a high pH fluid is being converted to a gypsum fluid, more water dilution and higher concentrations of UNI-CAL and gypsum are necessary to obtain desired properties.
- Abnormal foaming may occur during and immediately after conversion. This foaming is usually confined to the surface of the pits and can be reduced by adding a defoamer such as LD-8 to the pits. Mud guns should be turned off if excessive foaming persists.
Table 1. Typical Fluid Properties after Conversion and Before Weight-up
|Fluid Weight||10.0 lbm/gal|
|Funnel Viscosity||40 to 44 sec/qt|
|Plastic Viscosity||16 cP|
|Yield Point||6 lbf /100 ft2|
|Gels||2/10 lbf /100 ft2|
|API Filtrate||4 to 8 cc/30 min|
|Calcium||600 to 1200 mg/L|
|pH||9.5 to 10.5|
|Pf||0.2 – to 0.7|
|Excess Gypsum||2 to 4 lbm/bbl|
Maintenance of Gypsum Fluids
- Treatments of gypsum, caustic soda, UNI-CAL, and a filtration control agent are normally required each tour to maintain desirable properties. The amounts added depend upon volume, rate of penetration, amount and type of formation being drilled, and degree of dilution.
- The pH of a gypsum fluid is normally controlled between 9.0 and l0.5. Often the pH is slightly higher just after a conversion but can be allowed to drift back to the desired range as drilling progresses. The Pf should be in the 0.2 to 0.6 range.
- Gypsum is added to maintain filtrate calcium between 600 and 1200 mg/L. The solubility of the calcium greatly depends upon the alkalinity and salinity of the filtrate.
- Excess gypsum should be maintained above 2 lbm/bbl. This can be determined by titration for total calcium sulfate.
- CMC, and/or MILSTARCH with a preservative or additional deflocculant can be added as required for desired filtration control.
- If starch is used for filtration control, it will be necessary to add proportional amounts of a preservative.
- Although gypsum fluids tolerate a higher concentration of colloidal solids than conventional fluids, solids must still be controlled at a reasonable level to obtain optimum rheological and filtration values. Mechanical solids control equipment such as fine screen shakers, hydrocyclones (low-density fluids), and centrifuges (high-density fluids) should be used to minimize dilution requirements. MILGEL should be added daily to provide proper particle size distribution.
- Gypsum fluids normally possess high gel strengths, but they are fragile gels and should not be considered alarming. Gels can be decreased by reducing total solids content and increasing concentration of UNI-CAL. Gypsum fluids are considered in good rheological condition if initial gel is less than 5 lbf /100 ft2 and ten-minute gel is less than 15 lbf /100 ft2.
Treatment of Contamination in Gypsum Fluids
Salt – A properly conditioned gypsum fluid can be expected to tolerate up to 100,000 mg/L salt. Since an increase in salt increases the solubility of the calcium ion, the filtrate alkalinity should be increased with caustic soda to limit the level of soluble calcium. UNI-CAL should be added as needed to control the rheological properties.