In oil drilling operations, a drilling fluid is circulated downwardly through a drill string to cool and lubricate the drill string, suspend the cutting removed from the well bore and to keep out formation fluids. The drilling fluid containing the suspending cuttings are further circulated upwardly through the annulus between the drill string and wall of the well bore to the surface, where the cutting are separated and the recycled drilling fluid is circulated down the bore. Drilling fluids, also known as drilling muds, may be oil- or water-based. Both water-based and oil-based drilling fluid systems are known. The more economical water-based systems are used when practicable with oil-based systems being used where increased lubricity at the drilling head is desirable or when traversing formations which would be adversely affected by a water-based system, such as water soluble shale formations.
A conventional oil-based drilling fluid (mud) generally comprises an oil fluid vehicle, such as a diesel oil, emulsifiying agents, such as alkaline soaps of fatty acids, wetting agents or surfactants, such as dodecylbenzene sulfonate, water, generally as a NaCl or CaCl2 brine, and a viscosifying agent, such as an amine treated clay. Oil-base fluids may have an aromatic or aliphatic oil, or a mixture of oils, as the continuous phase. These oils may include diesel, mineral or synthetic (PAO, esters, ether) oil. They may be comprised entirely of oil or, more commonly, may contain water ranging from 5% to upwards of 50-60%. In the latter case, water becomes the internal phase, is emulsified into the oil as a heterogeneous fine dispersion, and the resulting system is referred to as an oil-based or oil-invert emulsion fluid.
A water-based drilling fluid comprises a viscosifying agent, generally a clay such as a solid phase bentonite attapulgite or sepiolite, and a water fluid vehicle. In addition, salt or salt water can be added to the components of the drilling fluid to prepare a salt water based drilling fluid. Numerous different additives to this drilling fluid are also employed to control viscosity, yield point, gel strength (thixotropic properties), pH, fluid loss, tolerance to contaminants such as salt and calcium carbonate, lubricating properties, filter caking properties, cooling and heat transfer properties, and tolerance to inactive solids such as sand and silt or active native mud making clays such as smectites, illites, kaolinites, chlorites, etc. Clays are not usually used as the sole viscosifying agent and typically organic water-soluble polymers such as starch, carboxymethylcellulose, natural gums or synthetic resins are used in conjunction with clays. These polymers also aid the clay component of the drilling fluid to serve as a filtration aid to prevent or retard the drilling fluid from being lost into the formation.
A number of drilling fluid formulations have been described. For example, U.S. Pat. No. 3,726,850 discloses a lignin dispersing agent for dispersing clays, and the like. The lignin dispersing agent is reported to have utility in both alkaline and acidic media. A relatively low viscosity aqueous silicate solution is disclosed in U.S. Pat. No. 3,746,109, and is reported to be particularly useful in drilling through shale formations. U.S. Pat. No. 4,799,549 discloses a stable, gel-forming microemulsion comprising an aqueous solution of an alkali metal silicate, a gelling reagent, and a surface-active agent (surfactant). This composition is reported to be useful for permanent or reversible plugging or clogging of subterranean formations. Also, U.S. Pat. No. 5,374,361 discloses a composition for cleaning out cased wellbores, and the like, using a fluid that includes a caustic alkyl polyglycoside surfactant formulation. This formulation is reported to be more biodegradable than previous detergent systems. A further additive encountered in aqueous drilling fluids is a metal compound, such as that described in U.S. Pat. No. 5,399,548, or a derivative of a metal compound such as a hydroxy-aluminum compound provided in a polymer, such as disclosed in U.S. Pat. No. 4,045,357. U.S. Pat. No. 5,333,698 also discloses a drilling fluid additive in combination with a white non-toxic mineral oil.
Although oil- and water-based drilling fluids are widely used, they require large, complex pumps to circulate the fluid down the drill string and up the annulus of the well bore. As the drill is operated, the resulting cuttings from the drill bit are suspended in the drilling fluid, thereby increasing the density and further increasing the pumping costs. In offshore well the hydrostatic pressure put additional strain on the pumping equipment and further increase the pumping costs. These cost associated with use and maintenance of these pumps contribute significantly to the costs of oil drilling operations. Further, the increased pressures and loads on the pumps make it difficult to maintain the pressure of the drilling fluid in the optimal range; between that of the pore pressure and the fracture pressure.
Several methods have been proposed to reduce the costs and overcome the problems associated with pumping drilling fluids. Shell E&P has introduced the Shell Subsea Pumping System (SSPS) whereby the drilling fluid is processed, cuttings removed and discharged at the seafloor, and gas separated prior to being pumped back to the surface. Conoco has developed a dual gradient system called Subsea Mudlift in which the drilling mud is removed from the riser with triplex pumps at the seafloor, and is then filled with seawater to reduce the riser load. Another approach called DeepVision, by Baker-Hughes and Transocean Sedco Forex uses centrifugal pumps to separate the mud at the seafloor and send it to the surface.
Some well operators have used a gas injection system to reduce the density of the drilling mud. In this system a gas such as nitrogen is added to the drilling fluid, which is circulated in the conventional manner. However due to the compressible nature of gas, large volumes and high pressures are required to maintain a gas phase in the mud, increasing the complexity and cost of the system and maintaining the appropriate pressures in the well bore. Mud/gas systems have shown a tendency to foam at the reduced pressures encountered as the mud/gas system ascends the well bore or riser causing fluid handling problems. In addition, small amounts of oxygen in the injected gas have led to corrosion problems.
To overcome the problems associated with gas injections systems, the use of hollow microspheres has been proposed. Hollow microspheres, being relatively incompressible, do not require the high pressures and associated pumps necessary with gas injection and the addition of microspheres will not lead to the foaming problems. However, improperly handled, and the size shape, density and particle size distribution can provide a nuisance dusty environment. Further, the microspheres can be difficult to efficiently separate and recycle from the drilling fluid, adding cost and complexity to their use.