1. Field of the Invention
This invention broadly relates to the process of drilling boreholes which penetrate subsurface formations and, further, to aqueous compositions employed during the performance of the process. The invention still further relates to the process of rotary drilling and, particularly, to a method of, and a composition for, reducing the loss of liquid from aqueous compositions employed while rotary drilling.
2. Related Art and Problem Solved
It is well known in the art, that rotary drilling is a method useful to produce boreholes which penetrate subsurface formations which contain oil and/or gas, and that boreholes which do penetrate such formations are commonly referred to as oil and gas wells. It is also well known, that rotary drilling features the process of contacting an earthen formation with a bit, a tool having rigid cutters, i.e., "teeth," attached thereto, and causing the bit to penetrate the earthen formation, and subsequent formations, to produce a borehole which extends from the surface of the earth to some subsurface location. In broad terms, the bit is caused to penetrate an earthen formation by applying force against the formation with the bit while the bit rotates, whereby the bit cutters contact and abrade the formation.
In the performance of the process of rotary drilling, the bit is attached to the bottom of connected lengths of rigid, hollow pipe, called a drill string, wherein the diameter of the bit is greater than the outside diameter of the drill string. The drill string is suspended in the borehole by a surface structure, called a derrick. Machinery associated with the derrick operates to rotate the drill string in the borehole to, thus, cause the bit to rotate. As the bit rotates, some, or all, of the weight of the drill string above the bit is transferred to the formation to thereby generate the necessary cutting/abrading force against the formation. Accordingly, the cutters break small pieces of rock, called drill solids, from the formation which must be removed from the borehole in order to continue drilling. Aqueous compositions employed during the process operate, among other things, to remove the drill solids from the borehole.
Recall the earlier statement that the drill string is hollow. Accordingly, a water-based composition, called a drilling fluid, is pumped by equipment associated with the derrick from the surface down the hollow interior of the drill string to the bit. The drilling fluid exits the drill string through openings, or nozzles, in the bit and then returns to the surface by flowing in the annular space between the walls of the borehole and the exterior walls of the drill string. The process of pumping the drilling fluid from the surface down the interior of the drill string, through the bit and then back to the surface in the annulus is referred to as circulation.
The circulating drilling fluid functions to lubricate and cool the bit, to transport the previously mentioned drill solids from the borehole to the surface, and to seal the sides of the borehole to help prevent movement of water from the drilling fluid into the formations penetrated by the borehole. Upon being returned to the surface, the drilling fluid is processed, as is well known in the art, to remove drill solids therefrom and to add materials thereto. The added materials provide, or otherwise enhance, the ability of the drilling fluid to function as intended by contributing chemical and physical properties required by the drilling fluid to enable it to cope with and/or to resist environmental conditions encountered by the drilling fluid in the borehole.
An aqueous drilling fluid is a uniform liquid comprised of water, materials which are water soluble and materials which are not water soluble. The combination of components operates to produce properties essential to enable the drilling fluid to function as required. The water component may be fresh water, sea water or salt water. It is noted that "salt water" at least includes water that contains an appreciable quantity of sodium chloride dissolved therein, and water that contains an appreciable quantity of potassium chloride dissolved therein. The other components must be chemically and physically compatible with the water employed so as to produce a uniform fluid whose viscosity, plasticity and elasticity properties are sufficient to enable the fluid to suspend drill solids therein for transport to the surface for removal. While fluid viscosity is important, the fluid must not be so highly viscous as to require excessive pumping pressure to initiate and maintain circulation. The flow properties of a drilling fluid which involve elasticity, viscosity and plasticity are referred to in the art as rheological properties.
It is clear from the above, and well known in the art, that a drilling fluid is a carefully balanced combination of components which must operate together to stabilize the rheological properties of an aqueous drilling fluid. Rheological properties are, thus, influenced by many factors including, but not limited to: the chemical nature of the water itself; the component, or components, employed to alter fluid viscosity; materials employed to change the weight (density) of the fluid; materials employed to alter the acidity/alkalinity (pH) and salinity of the fluid; additives employed to prevent, control or at least diminish, the loss of water from the drilling fluid to formations penetrated by the borehole; drill solids produced during drilling; water produced by formations penetrated by the borehole; and the temperature and pressure of the borehole itself.
The water employed to make a drilling fluid may be fresh water, sea water or salt water. Fluid viscosity can be increased by use of materials which swell or thicken upon contact with the water. Such materials, referred to as viscosifiers, are known to include clays, such as bentonite, sepiolite and attapulgite, and polymers, such as cellulose derivatives. Fluid viscosity can be stabilized at desired values by use of materials, called thinners, which interfere with the ability of viscosifiers to increase viscosity. Examples of thinners include phosphates, polyphosphates, lignins and lignosulphonates. Weighting agents are, preferably, water insoluble materials, such as, barite. Materials employed to adjust pH and salinity, respectively include, alkali metal hydroxides, such as sodium hydroxide and alkali metal halides, such as, potassium and sodium chloride. It is known that drill solids can change the viscosity of a drilling fluid, and the effect of temperature and pressure on fluid viscosity is a well known phenomenon.
Water is a major, if not the principal, volume component of an aqueous drilling fluid. Consequently, loss of water from the drilling fluid, referred to as fluid loss and filtration loss, is a problem, long recognized by persons skilled in the art, to be considered when faced with the task of stabilizing the rheological properties of a drilling fluid. Accordingly, additives, referred to in the art as fluid loss and filtration control additives, employed to prevent, control or at least to diminish, the loss of water from drilling fluid to formations penetrated by the borehole, have been, and continue to be, the subject of extensive research and product development.
It has long been an object of research to develop compositions which operate in both fresh and salt water drilling fluids to control fluid loss at temperatures and pressures likely to be encountered in a borehole, wherein the compositions are compatible with other components of the drilling fluid and do not adversely effect the rheological properties of the drilling fluid. The following United States Patents, and items cited therein, propose solutions to problems associated with rotary drilling and drilling fluids and to various chemical compositions employed in achieving those solutions: U.S. Pat. Nos. 4,293,427; 4,547,299; 4,555,558; 4,782,120; 5,032,295; and 5,099,930. Thus, U.S. Pat. No. 5,032,295 discloses, among other things, that copolymers of sulfonated styreneitaconic acid improve the rheological properties of aqueous drilling fluids. Also, fluid loss additives consisting of copolymers of sulfonic methylacrylamido-alkyl acid and methylacrylamide and terpolymers of the sodium salt of 2-acrylamido-2-methylpropane sulphonic acid, N,N-dimethylacrylamide and acrylonitrile are disclosed in U.S. Pat. Nos. 4,782,120 and 4,555,558, respectively. Fluid loss additives are discussed in U.S. Pat. No. 4,547,299, one of which is a copolymer of a (meth)acrylamido alkyl sulfonic acid or alkali metal salts thereof and N,N-dialkyl(meth)acrylamide, wherein the copolymer can be cross-linked. The same patent also discloses the use of water soluble sulfonated polystyrene derivatives for filtration control. In addition, Japanese Patent document HEI 01[1989]-313584 discloses a terpolymer of 2-acrylamido-2-methylpropane sulfonate, N,N-dimethylacrylamide and acryloylmorpholine. The terpolymer is said to be useful as a drilling fluid conditioning agent which is stable under high temperature brine systems.
It is thus an object of this invention to provide a composition and a method of using the composition to control the loss of fluid from an aqueous drilling fluid which is exposed to a broad range of temperatures and pressures in a borehole.
It is another object of this invention to provide a composition useful to control fluid loss from an aqueous drilling fluid which is made with fresh water, sea water or salt water.
It is still another object of this invention to provide a fluid loss composition which will not operate to materially change the Theological properties of an aqueous drilling fluid and which is compatible with other components ordinarily present therein.