Drilling shales

A number of cationic muds have been developed and used. These ate formulated around quaternary amines or positively charged polymers (108,109). The polymer in some iastances may be a cationic polyacrylamide. Poly(dimethylarnine-fi9-epichloiohydrin) is another material that has been used successfiiUy for drilling shale formations (110,111). Some of these additives may requite a salt such as sodium or potassium chloride for best results. 

Select a tungsten carbide insert with the greatest amount of offset and the longest chisel crested inserts when drilling shale and soft limestone. 

Invert emulsion drilling fluids are commonly selected for their temperature stability and their ability to prevent the wellbore stability problems associated with the hydration of clays in shale formations. The thermodynamic activity aw of the water in the aqueous (dispersed) phase is controlled by the addition of a salt (usually calcium chloride) to ensure that it is equal to or less than the activity of the water in the drilled shale formations. The emulsified layer around the water droplets is claimed to act as a semipermeable membrane that allows the transport of water into and out of the shale but not the transport of ions (61). When the activities (or, more strictly, the chemical potentials) of the water in the shale and invert emulsion are equal, then no net transport of water into or out of the shale occurs (i.e., the drilling fluid does not hydrate or dehydrate the shale). This equality of water activity has lead to the development of so-called balanced activity oil-based drilling fluids. 

The rate at which the drill bit penetrates the formation gives qualitative information about the lithology being drilled. For example, in a hard shale the rate of penetration ROP) will be slower than in a porous sandstone. 

Earlier on when we described the cutting action of the drill bit we learned about the drilling fluid or mud. The mud cools the bit and also removes the cuttings by carrying them up the hole outside the drill pipe. At the surface the mud runs over a number of moving screens, the shale shakers (Fig. 3.11) which remove the cutting for disposal. The fine particles which pass through the screens are then removed by desanders and desilters, usually hydrocyclones. 

Considerable effort will be made to predict the onset of overpressures ahead of the drill bit. The most reliable indioations are gas readings, porosity - depth trends, rate of penetration and shale density measurements. 

Although numerous mud additives aid in obtaining the desired drilling fluid properties, water-based muds have three basic components water, reactive soHds, and inert soHds. The water forming the continuous phase may be fresh water, seawater, or salt water. The reactive soHds are composed of commercial clays, incorporated hydratable clays and shales from drilled formations, and polymeric materials, which may be suspended or dissolved in the water phase. SoHds, such as barite and hematite, are chemically inactive in most mud systems. Oil and synthetic muds contain, in addition, an organic Hquid as the continuous phase plus water as the discontinuous phase. 

High initial cost and environmental restrictions prevent use of oil and synthetic muds in many cases where shale problems are expected. It is necessary then to treat a water-base mud to minimize the destabilizing effect of the drilling fluid. Salts, polymers, and other organic materials are added to the mud to reduce the water sensitivity of the shale, shale sweUing, and weakening arising from mud contact, or the rate of water uptake by the shale. 

Sodium chloride has long been used as a shale stabilizer because of low cost, wide availabiUty, and its presence in many subsurface formations. The inhibitive nature of salt muds increases as the salt content increases from seawater to saturated sodium chloride. In addition to the sodium chloride consumed aimuaHy for drilling fluid, considerable quantities are incorporated while drilling salt zones. This material has been used more for minimizing washouts in salt zones than for stabilizing shales. High salt levels have found appHcation in deep water drilling (7). 

The method of action of the polymers is thought to be encapsulation of drill cuttings and exposed shales on the borehole wall by the nonionic materials, and selective adsorption of anionic polymers on positively charged sites of exposed clays which limits the extent of possible swelling. The latter method appears to be tme particularly for certain anionic polymers because of the low concentrations that can be used to achieve shale protection (8). 

The methylene blue test can also be used to determine cation exchange capacity of clays and shales. In the test a weighed amount of clay is dispersed into water by a high-speed stirrer. Titration is carried out as for drilling muds, except that hydrogen peroxide is not added. The cation exchange capacity of clays is expressed as milliequivalents of methylene blue per 100 g of clay. 

Gypsum-treated muds have proved useful for drilling anhydride and gypsum, especially where these formations are interbedded with salt and shale. The treatment consists of conditioning the base mud with plaster (commercial calcium sulfate) before the anhydride or gypsum formation is penetrated. By... 

KCl-polymer (potassium chloride-polymer) muds can be classified as low solids-polymer muds or as inhibitive muds, due to their application to drilling in water-sensitive, sloughing shales. The use of polymers and the concentration of potassium chloride provide inhibition of shales and clays for maximum hole stability. The inverted flow properties (high yield point, low plastic viscosity) achieved with polymers and prehydrated bentonite provide good hole cleaning with minimum hole erosion. 

In onshore drilling there is no need for chlorides above these background levels. Potassium chloride has been added to some drilling fluids as an aid to controlling problem shale formations drilled. Potassium acetate or potassium carbonate are acceptable substitutes in most of these situations. 

Undesirable solids are drilled cuttings and those solids sloughed into the borehole. They usually occur in all size ranges from colloidal to coarse. The specific gravity of commonly encountered drilled solids ranges from 2.35 (shale), through 2.65 (sand), 2.69 (limestone), to 2.85 (dolomite) see Table 4-57 [29]. 

Drilled solids include active drilled solids and inactive drilled solids. Clays and shales are considered to be active drilled solids they disperse into colloidal size readily and become detrimental to drilling by increasing the apparent viscosity and gel strength of the mud. Inactive drilled solids are sand, dolomite, limestone, etc. if they occur in colloidal size, these solids may increase plastic viscosity of the drilling mud. 

Select a tungsten carbide bit with chisel crest inserts when drilling a formation that is predominantly shale. Use bit type 4-2, 5-2, 6-1 or 6-2. 

Select a tungsten carbide insert bit with a medium offset and long chisel crested inserts when drilling sandy shale with limestone and dolomite. Use bits type 4-1 to 5-3. 

Select a tungsten carbide insert bit with a minimum offset and projectile or conical inserts when drilling limestone, brittle shale, nonporous dolomite and broken formations. Use bit type 6-3 to 7-3. 

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