BACKGROUND OF THE INVENTION
This invention relates to a method of drilling water sensitive shales. More particularly, it relates to reducing the cost of such an operation by utilizing a sequence of relatively low cost drilling fluids that cooperate to avoid borehole problems due to either shale instability or mechanical instability while maintaining a relatively high drilling rate.
The history of drilling water sensitive shales with conventional water-based drilling fluids has been plagued by borehole instability problems related to the welling and sloughing of shales due to their interaction with water. Such problems include stuck drill pipe, time lost in redrilling shale that has sloughed into the borehole, difficulty in running casing, and the like. Heretofore an avoidance of such problems required a use of dense viscous aqueous drilling fluids or the use of oil-base drilling fluids. However, such solutions are relatively expensive due to slow drilling rates and/or high drilling mud costs.
A primary object of the present invention is to reduce the cost of drilling water sensitive shales by reducing the drilling time without increasing the cost of the drilling fluid. A series of mutually compatible and/or interconvertible drilling fluids are used in a sequence that first maximizes the drilling rate and later, substantially as required by the earth formations being encountered, provides shale stabilization and enhanced cleaning.
SUMMARY OF THE INVENTION
This invention relates to drilling a borehole that encounters a water sensitive shale and is drilled by operating a drilling means while circulating a drilling fluid. Initially the rate of drilling is enhanced by circulating a relatively low density, fast drilling, substantially clear fluid. When the encountering of borehole instability due to water destabilization of water sensitive shale is at least imminent, a shale stabilization while drilling is effected by circulating a shale-stabilizing aqueous solution containing both a partially hydrolyzed polyacrylamide (which is from about 20 to 40 percent hydrolyzed, has a molecular weight above about 1 million, and is present in an amount at least sufficient to satisfy the minimum adsorption requirement of the shale) and an alkali metal halide (which is present in an amount at least sufficient to enhance the adsorption of the polymer on the shale). When the encountering of borehole instability due to sloughing of mechanically incompetent earth formations is at least imminent, the rate of borehole cleaning is enhanced by suspending a drilling fluid niscosifier such as prehydrated bentonite in said aqueous solution (with the amount of prehydrated bentonite being sufficient to enhance the solids lifting capacity of the fluid) and circulating said suspension while maintaining the fluid loss of the circulating suspension at a level sufficient to facilitate said shale stabilization by polymer adsorption.
As used herein the terms "a borehole instability due to water destabilization of water sensitive shale is at least imminent" refers to a situation in which a water sensitive shale has been or is about to be contacted by an aqueous fluid. That aqueous fluid may be a circulating air or gas drilling fluid containing water from a water-producing formation, a mist or foam drilling fluid, an aqueous drilling fluid, or the like. Similarly, "a borehole mechanical instability due to sloughing of mechanically incompetent earth formations is at least imminent" when the borehole has penetrated or is about to penetrate a mechanically incompetent zone such as a rubbled zone, a highly fractured zone, an unconsolidated sand or gravel, or the like. As known to those skilled in the art, the times when such events are about to occur are often known or are readily determinable from subterranean geological information (in the light of the rate of the borehole advance) and/or previous experience with wells drilled in similar regions. In addition, the encountering of such borehole instabilities are readily detectable from the behavior of the drill string and/or drilling fluid and/or rate of borehole advance, or the like signs and symtoms that are commonly looked for during the drilling of a well.
DESCRIPTION OF THE INVENTION
The shale destabilizing interaction of water and water-sensitive shale involves a long known and troublesome problem. It is known to be more complicated than the result of a single type of phenomena. Osmosis and osmotic pressure forces tend to induce transfer of water from a water-containing drilling fluid to the layers or interstices within the shale. Osmosis is known to be involved but to be less than a total explanation. For example in a U.S. Pat. No. 2,165,824 filed over 30 years ago, Garrison pointed out that such an interaction involves more than osmosis and, accordingly, a use of salts that provide a high osmotic pressure, i.e., an "inhibited" aqueous drilling fluid, will not prevent either the heaving or dispersing of the shale. That patent suggests using a relatively concentrated aqueous solution of an alkali metal silicate. But, such a solution is disadvantageously expensive to maintain as a circulating drilling fluid.
One aspect of the interaction between water and a water-sensitive clayey or shaly earth formation causes a dispersion of colloidal sized particles of clay in a circulating aqueous drilling fluid. Such a dispersion tends to increase the viscosity and density of the drilling fluid. It causes a problem of drilling fluid maintenance that may be expensive even in the drilling of clayey earth formations which are not sufficiently active to create a borehole instability problem. Numerous prior patents have described combinations of carboxyl group-containing polymeric materials that are designed to handle such clay dispersion problems. They relate to clear or low solids drilling fluids or clay-containing drilling muds in which the types and amounts of the polymeric materials are adapted to effect a beneficiation or dispersion of the viscosity-adjusting bentonitic type clays while tending to flocculate and enhance the removal of other clays, such as those encountered in drilling operations. As known to those skilled in the art such clay beneficiating and flocculating aqueous polymer systems are not suited for drilling in water sensitive shales and are generally no more effective than a simple aqueous clay mud with respect to stabilizing a borehole in a water sensitive shale. Such aqueous polymer systems are described in U.S. Pat. Nos. 2,775,557; 3,070,543; 3,081,260; 3,323,603; 3,558,545; 3,511,799; etc.
A "coherred inhibited mud system" that is an aqueous fluid for drilling a water sensitive shale has been described by R. L. Sperry, in Netherlands Pat. No. 6,414,645 . In that system a carboxyl group-containing polymeric material and a water-soluble inorganic salt are dissolved in water in a ratio of polymer to salt that ranges from about one to 25 to three to one with the stipulation that no more than about 6 pounds per barrel (ppb) of the inorganic salt be present in the solution. The patent teaches that the specified proportions of the polymer and salt are critical and should be adapted to cause the surfaces of clay particles to be slightly swollen, without being dispersed in the water, so that the swelling action seals the clay against further water intake or interaction.
The present invention is at least in part based on a discovery that, contra to such prior teachings, in a low cost aqueous drilling fluid that is readily convertible from a fast drilling clear fluid to a solids-suspending mud when enhanced cleaning is needed, a stabilization of a water sensitive shale is advantageously effected by an aqueous solution that contains only a relatively small amount of polymer in the presence of a relatively large amount of salt. In such a solution the polymer must be a polyacrylamide of the specified degree of hydrolysis and molecular weight while the salt is either a sodium or potassium halide, and is preferably the latter. The shale stabilizing mechanism of this solution appears to be distinctly different from that described in the Netherlands patent. The shale stabilization requires at least about seven pounds per barrel of the salt -- and is better when more than 10 pounds per barrel are present.
Fast drilling clear fluids suitable for use in the present invention can vary in type of fluid phase as well as in composition as long as the fluids are adapted to enhance the rate of drilling within at least an upper portion of a borehole. Such a fluid can be an essentially gaseous fluid such as air, nitrogen, hydrocarbon gases, flue gases or the like, an aqueous liquid containing essentially gaseous fluid such as mist or foam or the like, or a substantially clear aqueous liquid such as water, brine, or the like, or a solution of partially hydrolyzed polyacrylamide and an alkali metal halide in proportions that are effective or ineffective for shale stabilization, or the like, as long as the fluid has a density and viscosity at least substantially as low as those of water. Such a fast drilling fluid is preferably a relatively low cost fluid such as air or brine.
Partially hydrolyzed polyacrylamides suitable for use in the present invention can comprise such polymers formed by polymerizing and subsequently hydrolyzing acrylamide (or a lower homolog of acrylamide) or copolymerizing acrylamide with an acrylate, or the like. The molecular weight of the polyacrylamide should be at least greater than about one million and is preferably greater than about three million. The range of hydrolysis (and/or the proportion of the amide groups of the polyacrylamide that are carboxyl groups or have been hydrolyzed to form carboxyl groups) should be from about 20 to 50 percent and extents of hydrolysis of from about 30 to 40 percent are particularly suitable. Examples of such polymers that are commercially available include Separan AP-273 (30 percent hydrolyzed molecular weight greater than three million, Dow Chemical Co.), Separan AP-30 (30 percent hydrolyzed, molcular weight greater than two million, Dow Chemical Co.), Pusher 500 (40 percent hydrolyzed, molecular weight 3.4 million, Dow Chemical Co.), Pusher 700 (40 percent hydrolyzed, molecular weight about seven million, Dow Chemical Co.), Pusher 1000 (40 percent hydrolyzed, molecular weight about seven million, Dow Chemical Co.), RC-326 (20-30 percent hydrolyzed, molecular weight greater than 12 million, American Cynamide), RC-334 (greater than 30 percent hydrolyzed, molecular weight greater than 12 million, American Cynamide), RC-304 A (30 percent hydrolyzed molecular weight greater than three million, American Cynamide), Reagent S-3595 (30 percent hydrolyzed, molecular weight 10 million, Cynamide of Canada), Percol 155 (30 percent hydrolyzed, molecular weight 12-15 million, Allied Colloids), Percol 156 (40 percent hydrolyzed, molecular weight 12-14 million, Allied Colloids), etc. As known to those skilled in the art such polymers are usually marketed in form of their sodium salts. As used herein (in reference to the weights or concentrations by weight of such polymers) the amounts of such polymers refer to amounts of the sodium salt or the equivalent amount of polymer that is needed to form the sodium salt. If desirable, the present solutions can be made by dissolving the carboxylic acid forms of the polymers in an aqueous lquid and converting them to the salts in solution.
Aqueous liquids suitable for use in the present invention can comprise water or substantially any aqueous solution that is free of interferring solutes. Examples of suitable aqueous liquids include natural waters or brines or sea waters and/or waters softened or otherwise treated by means of ion exchange resins, flocculating agents, etc. comprising aqueous solutions that are free of significant proportions of solutes that cause chemical changes in polyacrylamide polymer molecules, e.g., degredation, cross-linking, etc. or compete with polyacrylamide polymer molecules for adsorption sites on clay particles (e.g., solutes such as chromium or the like polymer reaction catalytic materials, carbonate, phosphate, sulfide or the like anions that compete for adsorption sites, etc.). Relatively soft sodium or preferably potassium brines are particularly suitable.
Shale stabilizing aqueous solutions of partially hydrolyzed polyacrylamides and alkali metal salts used in the present invention are relatively low cost solutions adapted for use as drilling fluids that stabilize the shale while the maximizing drilling rate and/or borehole cleaning effects. The shale protection appears to be the result of polymer adsorption at specific critical sites on the shale. The critical sites appear to be the positively charged edges of the packets of clay platelets. On them the adsorption of the specified polymer molecules sufficiently reduces the inhibition of water between the clay platelets to prevent the shale destabilization. In addition to containing at least enough polymer to satisfy those adsorption sites, the solution should contain at least enough alkali metal halide salt to enhance the rate of polymer adsorption on the clay. In addition the solution preferably has an alkaline pH in a range that tends to enhance the thermal stability of the polymer. It is also important that such a solution be free of interferring solutes that tend to cause chemical changes in the polymer molecules or to compete with them for adsorption of the critical adsorption sites of the clay. In general, suitable concentrations range from about 0.2 to 0.75 ppb of polymer (preferably 0.3 to 0.6), from about seven to 70 ppb of salt (preferably from about 10 to 35) in water solution having a pH of from about nine to 11 (preferably from 9.5 to 10). Such solutions should be kept substantially free of polymer stability and/or adsorption-interferring solutes that tend to cause chemical changes in the polymer or compete with the polymer molecules for adsorption sites on clay. Where such solutions are gelled (by a suspension of prehydrated bentonite) in order to increase their hole-cleaning capabilities, they should contain from about six to 20 ppb prehydrated bentonite (preferably from about 10 to 20 ppb) and be maintained as moderatly high filter loss muds having an API 30 minute filter loss of from about eight to 30 ml.
Field tests have indicated that as clear drilling fluids, the above solutions provide the advantages of clear water drilling without the attendant hole instability problems that result from using fresh water in water-sensitive shales. Penetration rate improvements in the order of 50 to 100 percent compared with the gelled muds (i.e., approaching clear water drilling rates) have been experienced. In field preparations of such solutions, it is advantaous to use about 1/4 to 1/2 pound per barrel Separan AP-273 (or equivalent polymer) partially hydrolyzed polyacrylamide, about 10 to 11 pounds per barrel muriate of potash (potassium chloride) and enough sodium hydroxide or metasilicate to provide a pH from about 9.5 to 10. It is advantageous to mix the muriate of potash first and then the Separan. Mixing the Separan in fresh water tends to produce high fluid viscosities that are difficult to handle and can result in the plugging of the hoppers, pumps, etc. "Gunning" the mixing tanks during the mixing period is advantageous. Such an aqueous solution can be circulated and maintained as a substantially clear drilling fluid by simply using no desitter and allowing all solids to settle in tanks or preferably in a reserve pit through which the solution is circulated.
Test Examples 1-11 (Table 1) on Effects of: Percent Hydrolysis, Molecular
Weight, and Salt Concentration
In the test examples described herein, various properties of fluids were measured by means of the triaxial shale tester described in Journal of Petroleum Technology, Volume 21, pages 883-892, July 1969. This tester simulates field conditions in the behavior of cylindrical shaped specimens (about 2 inches in order diameter and 1 inch high) which are placed in a rubber jacket. Isotropic loading is obtained by applying approximately equal vertical and radial pressures simultaneously and/or independently. Fluids being tested are circulated through a hole drilled in such specimens in a manner permitting a filtrate penetration of the specimens. The specimens may be natural cores or shales that were reconstituted by compacting drill cuttings or fragments, or the like.
Table 1 shows of circulating the specified aqueous polymer and salt solutions through Atoka shale specimens under equivalent conditions of temperature, isostatic loading, etc. Tests 1, 3 and 7 compared the extent of shale protection by substantially equivalent polymers having extends of hydrolysis of, respectively, less than one, five and 30 percent -- and show that the less hydrolyzed polymers were less than 10 percent as effective. Tests 4 and 9 and tests 10 and 11 indicate that molecular weight increases (of from two million to three million and from 3.4 million to seven million) provide significant increases in the extent of protection. As known to those skilled in the art, when a solution of polymer is circulated through a drilling assembly, the polymer is subjected to relatively severe shearing forces which tend to degrade its molecules from high moleclar weights to lower molecular weights. Such a degradation is reflected by Test 9 in which the fresh polymer provided 4,885 minutes of stabilization but a retesting of a previously used sample (on a fresh shale specimen) provided only 540 minutes of protection, due to the degradation of the polymer. Tests 4 and 5 show protection effects of salt concentrations of zero and one percent and are comparable with tests 6, 7 and 8 in which the salt concentrations were two, three and 25 percent. The relative effectiveness indicates that substantially no shale protection is provided by salt concentrations of less than about two percent (about 7 ppb). In addition, tests 6, 7 and 8 show that the effectiveness of the shale protection with increase in salt concentration peaks within the range between two and 25 percent with the amount of protection provided by about three percent being substantially twice that provided by amounts near the upper and lower eds of the range.