The present invention relates to a composition which is capable of imparting a high degree of fluid loss controlling properties to aqueous systems under elevated temperature conditions. More particularly, the present invention relates to the formation of an improved water-based drilling fluid containing the subject composition and to an improved method of drilling bore holes into subterranean formations wherein high temperature conditions are encountered.
In normal well drilling operations in which a well is drilled by a rotary method, the well bore hole is generally filled with a drilling fluid or mud which is circulated therein. Drilling fluids are usually pumped down through the drill stem of the rotary rig, circulated around the drill bit and returned to the surface through the annular passage between the drill stem and well wall. These drilling fluids perform a number of functions including lubricating the drill stem and bit, cooling the bit, carrying the cuttings from the bit up the bore hole to the surface where the cuttings may be separated and disposed of, and providing a hydrostatic head against the walls of the well to contain downhole geopressure.
A primary requisite of a satisfactory drilling fluid is its ability to readily circulate and flow, that is, to have low viscosity, under the high shear conditions which are present in the environs of the drill bit while, at the same time, being capable of having and maintaining sufficient viscosity to be capable of efficiently carrying the bit cuttings to the surface and maintaining in suspension, any other solid components of the drilling fluid.
One of the primary requisites of a satisfactory drilling fluid is its ability to inhibit the amount of fluid, normally water, which is lost into the porous strata through which the bore hole traverses. The loss of fluid causes the formation and build-up of a cake deposit which, after a period of time, can cause sticking of the drill pipe and stoppage of the drilling operation. The drilling fluid must, therefore, be of a nature which permits minimum loss into the porous strata. Agents which impart such property are conventionally termed "water loss controllers" or "fluid loss controllers".
The drilling fluid components must also be stable and functional after subjection to elevated temperature conditions. In addition to the heat generated by the frictional forces of the drill bit, it is well known that the temperatures encountered in the bore hole are substantially above that found at the earth's surface. The deeper the bore hole is, the higher the temperature encountered. Drilling to greater depths has become common in today's guest for discovering new reserves. A general rule of thumb indicates that for each 10.degree. F. to 20.degree. F. increase in temperature stability one can use the same fluid to drill an additional 1000 ft. It is, therefore, desired to produce a composition capable of exhibiting stability and desired functionality at the elevated temperatures commonly encountered in deep drilling operations.
The viscosity of a drilling fluid has been relied upon as a mode of aiding in fluid loss control with little success especially when drilling into and through porous substrates. Various agents have been added to enhance the fluid loss properties of the mud. For example, U.S. Pat. No. 3,032,498 describes a cynaoethylated starch as a water loss controller to be used in combination with a clay-based mud. U.S. Pat. No. 3,988,246 describes an esterified or etherified starch as a water loss controlling agent which is compatible with a xanthan gum based drilling mud. Other agents as well as the starches mentioned above which have been employed in clay-free muds to control fluid loss are generally found to be unstable to the temperature conditions encountered in deep well drilling.
Bentonite clays which have sodium as the major exchangeable ion, such as Wyoming bentonite, have been used to impart fluid loss controlling properties to drilling fluids. These bentonites must be used in large dosages to approach practical fluid loss properties and usually impart fluid loss control which is still less than desired by the industry. Bentonite clays which have calcium or magnesium as the major exchangeable ion are not capable of imparting fluid loss properties.
The various agents which have been used to aid in controlling fluid loss have, generally been found to be unstable at elevated temperatures encountered in deep drilling presently done.
There is a general need for a composition which is capable of imparting and water loss controlling properties to aqueous compositions while being stable to varying conditions and elevated temperatures commonly encountered in deep drilling operations.