At about the time that lime muds came into use in Gulf Coast drilling, oil emulsion mud was recognized as a method of "making a good mud better." Much earlier, oil had been used to loosen stuck drill pipe and to "slick up the hole" before running casing25. Drillers in the Oklahoma City field (1934 -1936) added crude oil to mud to reduce sticking of pipe, and in 1937 a drilling contractor in Pottawatamie County, Oklahoma, noted a faster drilling rate after adding oil.66
By 1950 numerous reports of favorable field experiences with oil emulsion muds'1 "8 indicated such interest that the API Southwestern District Study Committee on Drilling Fluids surveyed the subject69. In brief, the conclusion was that the emulsification of either refined or crude oil improved the performance of water muds as evidenced by an increase in drilling rate and bit life, and by a reduction in hole problems. Reduction of torque, less sticking of pipe and balling of bits, and less hole enlargement were cited as major advantages of oil emulsion mud. There were no difficulties in the preparation and maintenance of the oil emulsion that were not common to the basic mud. The interpretation of electric logs and cores was not adversely affected. There was some evidence of improved productivity; none of impairment. Emulsification of oil was effected by substances already present in the mud, such as lignosulfonates, lignitic compounds, starch, CMC. or bentonite. or by the addition of surfactants, such as soaps. Similar favorable experiences with the use of oil in all types of clay-water muds was reported bv the API Mid Continent Study Committee on Drilling Fluids.00
Muds treated with lime had definite advantages over the earlier compositions. They were less expensive to maintain while drilling thick shale sections and less affected by the common contaminants: salt, anhydrite, and cement.. These benefits were attributed to the conversion of sodium clays to calcium clays by the calcium hydroxide (lime). As well-depth increased, a serious problem developed in deep, hot holes, especially those requiring heavy mud. The mud in the lower portion of the hole became excessively-thick when not being circulated, and actual solidification took place on prolonged heating. Studies showed that this solidification resulted from the reaction of lime with the silicious constituents of the mud. " 0 To minimize the problems associated with the effects of high temperatures on the strongly alkaline lime muds, compositions were proposed that were less alkaline and. consequently, less affected by high temperatures.71 72
Shale control mud was introduced by Texaco in about 1956. The objective was to gain stabilization of shale through maintenance of a high calcium ion concentration and a controlled alkalinity in the mud filtrate.73 A premixed product composed of calcium chloride, lime and calcium lignosulfonate furnished the desired chemical environment.
A different approach to shale stabilization was developed by Mobil research."4 Calcium surfactant mud was designed to overcome the temperature limitations of lime-treated muds and to minimize the swelling and dispersion of clays by the adsorption of a nonionic surfactant (a 30-mol ethylene oxide adduct of phenol). The aggregation of the clays by the surfactant was supplemented by the addition of gypsum. Filtration was reduced by carboxymethylcellulose. If the temperature became so high as to make CMC uneconomical, the calcium ion concentration was reduced; salt was added; the system became a sodium surfactant mud, and polyacrylates were used to control filtration. An aq ueous solution of the surfactant blended with a defoamant is sold under the trademark DMS.H DMS® has been found to be a distinctly useful constituent of muds for high-temperature drilling. 75 76
While lime muds were finding widespread application in drilling thick shale sections and high-pressure formations, gypsum-treated mud was introduced in Western Canada as a means of drilling anhydrite. " This gyp mud was prepared by adding calcium sulfate to bentonite dispersed in fresh water. Starch or CMC was added to reduce filtration rate. Properties of the gyp mud were affected only slightly by anhydrite or salt, but because of its rapid gel development, the mud was not suited to shale drilling nor 10 high densities. Dilution with water was the only practical means of thinning [he gyp mud. Calcium lignosulfonate and the tannins required an increase in pH. and, in effect, conversion to lime mud.
The problem of controlling the flow properties of gyp muds was solved by Gray King and Carl Adolphson,"78 who developed methods for preparing iignosulfonates of iron, chromium, aluminium, and copper from spent sulfite liquor. A ferrochrome lignosulfonate called Q-BROXIN had the unusual property of thinning gyp muds and salty muds. Roy Dawson introduced Q-BROXJNh to oil-field drilling in 1955. In June 1956, the first gyp-chrome lignosuljonate mud was used successfully in West Hackberr> Field, Louisiana. " The distinct advantages shown by the gyp-chrome lignosulfonate mud led to its rapid replacement of lime muds as the preferred drilling fluid in the Gulf Coast.80 In offshore drilling, sea water/chrome lignosulfonate muds found favor not only because sea water was readily available but also because sea water contains calcium and magnesium salts, which contribute to shale stabilization.81
Studies showed that chrome lignosulfonate was an effective defloccuiant, it afforded adequate filtration control and supplemented the action of the electrolytes in inhibiting disintegration and dispersion of shale cuttings.4 2 Improvements in hole stability and reduction in break up of cuttings in shale drilling were attributed to the sealing action of the lignosulfonate. T he formation on the clay surface of a multi-layered adsorption film of lignosulfonate retarded penetration of water, thereby opposing the tendency of the hole walls and shale cuttings to disintegrate.83
As previously mentioned, oxidized lignite (leonardite) was applied in lime muds, particularly for counteracting high-temperature solidification. " The superior heat stability of lignite was utilized also in sodium surfactant mud 1 After prolonged exposure to high temperatures, the flow properties of mud containing ferrochrome lignosulfonate, lignite and DMSI: were improved by the addition of sodium chromate.*-
Alkali-solubilized lignite and sodium chromate were combined in a single package to provide a product that reduced filtration and gel development at the temperatures met in deep wells in the Gulf Coast area.80 87 Chrome lignite (CL) along with chrome lignosulfonate (CLS) afforded a relatively simple chemical system that was widely applicable. The CL-CLS system provided control of both filtration and flow properties over a wide range in pH, salinity and solids content. Overtreatment usually produced no ill effects on properties. These features made such a program of mud treatment especially popular in foreign operations.
The term low-solids muds does not apply to any specific composition; rather, it has been applied to a number of compositions that utilize chemical and mechanical methods to maintain the minimum practical solids content.
Numerous field observations, particularly in hard rock areas, showed a decrease in drilling rate when mud replaced water as the drilling fluid. A reduction in rate had been noted also as the density of mud was raised. In spite of the fact that field experience did not distinguish between density and solids effects, Wheless and Howe88 concluded from observations of drilling times on field development wells in the Ark-La-Tex Area that accumulation of drill solids in the mud slowed rate of penetration. The same conclusion was reached from an examination of drilling records in DeWitt County, Texas, which showed one-third less time spent in drilling from 5,000 feet to 8,000 feet (1,500 to 2,500 m) with muds averaging 13% solids by volume as compared with muds averaging 18% solids.89
In the characteristic "clear water drilling" of West Texas, the cuttings were usually too small for satisfactory examination by the geologist. Often the hole was "mudded up" simply to obtain larger cuttings. Drilling rate then decreased."0 H.E. Mallory91 introduced a solution to this problem with milk emulsion. Milk emulsion consisted of water to which was added approximately 5% by volume of diesel oil and about 0.03% of an emulsifier, a non-ionic surfactant (a polyoxyethylene sorbitan tall-oil ester). By increasing the amount of emulsifier, oil could be emulsified in hard or salty waters. Adequate cuttings were recovered while retaining the drilling advantages of water.
Other methods in use to control the solid content were aerated mud, hydrocyclones and centrifuges, flocculants, and substitution of CMC for bentonite in the control of viscosity and filtration properties. CMC largely replaced bentonite in Phillips Petroleum Company's deep wells in Pecos County, Texas.92 One of these wells, the University E No. 1, in 1958 set a depth record of 25,340 feet (7,724 m) which was not broken until 1970.
Guar gum,93 and guar gum and starch,94 provided carrying capacity for cuttings and adequate filtration control in clay-free salt-water systems.
The use of flocculants to remove small cuttings from water was studied by Pan American Petroleum Corporation in the laboratory and in the field.In West Texas drilling, a 1% solution of an acrylamide-carboxylic acid copolymer was added to the discharge from the well just below the shale shaker. Circulation through earthen reserve pits allowed time for the flocculated solids to settle before the water was returned to the suction pit. While drilling permeable formations, polymer solution was injected at the pump suction to reduce water loss. Penetration rate and bit life were substantially increased by use of the polymers. Less sloughing of the "red bed" shale was observed
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