Drilling fluid circulating systems are employed during downhole drilling operations to provide a number of functions. These functions include providing cooling to the drill bit, removing drilled cuttings to the surface and providing a hydrostatic head to the formation to prevent the escape of oil and gas from the well.
As is well known, in almost all drilling operations, drilling fluid is continuously lost to the formation during the drilling process due to seepage of the drilling fluid into the formations. Seepage will occur at varying rates depending on the relative porosity or fractures of the formation and the hydrostatic head pressure. While some seepage will always occur, generally it is necessary to control drilling fluid loss in order to ensure effective drilling fluid circulation and to reduce drilling fluid costs. Notwithstanding efforts to prevent drilling fluid losses, tens of thousands of cubic meters of these fluids are lost each year.
There are generally two classes of drilling fluids namely water-based and oil-based drilling fluids. Water-based drilling fluids are generally less expensive than oil-based drilling fluids but cannot be used effectively in all formations and may result in operational problems in certain formations such as hydratable shales, silts or clays. In these formations, there is a tendency for the hydratable materials to destabilize the wellbore as well as disperse within the drilling fluid. The dispersion effect causes substantial increases in the solids content of the drilling fluid and leads to various problems including solids separation difficulties at the surface and detrimental increases to the viscosity of the fluid.
In order to overcome these difficulties associated with water-based drilling fluids, drilling fluids comprised of oil or water-in-oil (a.k.a invert emulsions) are often used. Generally, emulsion compositions in which oil remains the predominant or continuous phase are preferred. In order to maintain effective and stable emulsions, the compositions will require the use of emulsifiers to stabilize the emulsion. Such emulsions are effective in stabilizing hydratable shale or clay formations.
It is also well known that seepage losses can be mitigated through the addition of fibrous materials into the drilling fluid such as sawdust, fabrics, nut hulls, seed hulls and minerals such as ground limestone. These materials can bridge the various loss zones where fluids are being lost by mixing with drilled cuttings, and “viscosifiers” to create a “filter-cake” that may significantly reduce fluid losses to the formation. Examples of the use of various plant derived components as drilling fluid additives include U.S. Pat. No. 6,323,158, U.S. Pat. No. 5,763,367, U.S. Pat. No. 5,599,776, U.S. Pat. No. 2,691,629, U.S. Pat. No. 6,750,179, U.S. Pat. No. 4,439,328, U.S. Pat. No. 5,076,944, U.S. Pat. No. 5,861,362, United States Patent 2004/0014609 and United States Patent Applications 2004/0063587 and 2004/00232816. In the past, any plant materials have generally been preprocessed to remove any oils and gums associated with the plant material.
Typically, these materials also have a detrimental effect on the drilling fluid and may significantly change the properties of the fluid by increasing viscosity to unmanageable levels or de-stabilizing the emulsion. These effects directly impact the efficiency of surface pumping and solids removal equipment as well as causing wellbore destabilization. In the particular case of fibrous and granular materials including but not limited to cardboard, nut shells, calcium carbonate, and sawdust, within invert emulsions, these materials will be broken down over time to smaller sizes resulting in an increase in the relative surface area of these materials within the emulsion. The net effect of this size breakdown and the hydrophilic nature of these materials will contribute to the breakdown of the emulsion. As a result, over time, additional emulsifiers and oil-wetting agents must be added to the emulsions to maintain the emulsion properties, which again contribute to the cost of the drilling fluid. In the specific case of adding limestone or other mineral based seepage control agents to the drilling fluid, the specific gravity of the drilling fluid is also increased which increases the hydrostatic pressure within the wellbore which may directly increase the seepage losses and also requires stronger pumping equipment.
Other functional requirements of drilling fluids, problems that may be encountered with the use of drilling fluids and past solutions are briefly discussed below:
Drilling fluid systems also require the use of torque reducing and drag reducing agents to relieve both rotational twist that builds up in the drilling pipe during rotation and the frictional forces required to lift the drill string from the borehole through the drilling fluid. In the past, the use of spherical media such as glass beads, Teflon beads, styrene-divinylbenzene copolymer beads, walnut hulls, and oil based additives (petroleum or vegetative) within the drilling fluid have demonstrated the ability of these additives to relieve rotational twist and reduce drag.
Drilling fluids also preferably require materials to disperse entrained drilled solids that build up in the drilling fluid system during the excavation process. In many systems, the materials used are surfactants and phenolic compounds including lignins and tannins.
Filtration control agents may be added to both water and oil-based drilling fluids to reduce the penetration of the drilling fluid into formation rock media. The addition of colloidal, polymeric and colloid dispersing chemicals like surfactants, emulsifiers, lignite and/or lignosulphonate materials may be used.
In both oil and water based drilling fluids, emulsifiers may be added to aid the emulsification of oils added to water based fluids and water added to oil-based fluids. Emulsifiers also act as surfactants in both water based and oil based drilling fluids. Typically this effect is observed as a reduction in the viscosity of the treated fluid. The effect is caused by the adsorption of the emulsifier onto the surface of an entrained particle or emulsified phase component which enhances steric hindrance to flocculation between dispersed molecules. Generally, the addition of emulsifiers is dictated by the requirements of the fluid for the level of dispersion of secondary phase and/or entrained solids in the fluid system.
In the past, emulsifiers have been prepared using crude tall oil fatty acids (CTOFA) derived from Kraft pulp-making processes. CTOFA's are often used as the source of fatty acids for creating soaps/emulsifiers in oil-based drilling fluids. CTOFAs are normally considered to be a waste product and as a result, the price of CTOFAs is relatively low (in the range of $310/metric ton). The chemical composition of CTOFAs is primarily C18 oleic and linoleic acids in addition to relatively high concentrations of resins including abietic, dehydroabietic, palustric, isopalustric, isopimaric, neoabietic and pimaric acids. CTOFAs are very viscous and must typically be diluted with solvents in order to be efficiently mixed within a drilling fluid.
Some invert emulsions are blended with purified phospholipids to assist in oil wetting and dispersion of hydrophilic materials (drilled cuttings, barite and the like) which become entrained in the fluid. Purified phospholipids are used sparingly due to the high cost (typically in the range of $1000/ton).
In comparison, oil seeds are characterized by a much wider range of fatty acids with chain lengths that typically range from C14-C24 and naturally contain phospholipids. Different oilseeds contain very different specific compositions of fatty acids and phospholipids. From a cost perspective whole or crushed oilseeds are also in the same range as CTOFAs. However, any purified oilseed fatty acid is expensive (in the range of $1000+/ton).
While many solutions to the requirements of drilling fluids have been provided, there continues to be a need for enhanced drilling fluid compositions, while maintaining functionality to effectively bridge fractures and other loss zones. In particular, there has been a need for drilling fluid compositions having lipophilic/oleophilic characteristics to assist in dispersing the drilling fluid as the drilling fluid is broken down. Still further, there has been a need for drilling fluid compositions with increased stability where the stability of the drilling fluid is enhanced through delayed release of emulsifying agents.