1. Object of the Invention
This invention relates to a method for primary cementing a well using a composition of matter containing a polymeric compound which is useful as a drilling mud and may be converted to a cement upon irradiation with a proper source.
2. Field of the Invention
The process of searching for oil and gas is fraught with risk. Approximately three out of every four wells drilled in the United States are dry holes. Even in the instance when a well is found to have penetrated a subterranean formation capable of producing an economic amount of hydrocarbon, the well must be carefully completed after drilling has ended or less than the maximum amount of hydrocarbon will be produced. One problem caused by the improper well completion step of cementing is subterranean movement of gas from a high pressure formation to another formation of lower pressure. The gas lost in this way may never be recovered. This invention solves many of the problems associated with poor cementing procedures by converting the fluid known as "drilling mud" directly into a hardened cement. Drilling mud is the fluid typically used during the drilling of a well to lubricate and cool the bit as well as remove rock cuttings from the borehole. Drilling mud is usually displaced in a discrete step by a cement slurry after the borehole is lined with steel casing.
The process of drilling a well followed by the steps of casing and cementing it are described below.
a. Drilling the Well
In conventional rotary drilling, a borehole is advanced down from the surface of the earth (or bottom of the sea) by rotating a drill string having a drill bit at its lower end. Sections of hollow drill pipe, usually about 30 feet long, are added to the top of the drill string, one at a time, as the borehole is advanced in increments.
In its path downward, the drill bit may pass through a number of strata before the well reaches the desired depth. Each of these subsurface strata has associated with it physical parameters, e.g., fluid content, hardness, porosity, pressure, inclination, etc., which make the drilling process a constant challenge. Drilling through a stratum produces significant amounts of rubble and frictional heat; each of which must be removed if efficient drilling is to be maintained. In typical rotary drilling operations, heat and rock chips are removed by the use of a liquid known as drilling mud. As noted above, drill strings are usually made up of sections of hollow pipe terminated by a drill bit. Drilling mud is circulated down through the drill string, out through orifices in the drill bit where the mud picks up rock chips and heat, and returns up the annular space between the drill string and the borehole wall to the surface. The mud is sieved on the surface, reconstituted, and pumped back down the drill string.
Drilling mud may be as simple in composition as clear water, but more likely it is a complicated mixture of clays, thickeners, and weighting agents. The characteristics of the drilled geologic strata and, to some extent, the nature of the drilling apparatus determine the physical parameters of the drilling fluid. For instance, the drilling mud must be capable of carrying the rock chips to the surface from the drilling site. Shale-like rocks often produce chips which are flat. Sandstones are not quite so likely to produce a flat chip. The drilling fluid must be capable of removing either type of chip. Conversely, the mud must have a viscosity which will permit it to be circulated at high rates without excessive mud pump pressures.
In the instance where a high pressure layer, e.g., a gas formation, is penetrated, the density of the drilling mud must be increased to the point such that the hydrostatic or hydraulic head of the mud is greater than the downhole (or "formation") pressure. This prevents gas leakage out into the annular space surrounding the drill pipe and lowers the chances for the phenomenon known as "blowout" in which the drilling mud is blown from the well by the formation gas. Finely ground barite (barium sulfate) is the additive most widely used to increase the specific gravity of drilling mud; although, in special circumstances, iron ore, lead sulfide ferrous oxide, or titanium dioxide may also be added.
In strata which are very porous or are naturally fractured and which have formation pressures comparatively lower than the local pressure of the drilling mud, another problem occurs. The drilling fluid, because of its higher hydrostatic head, will migrate out into the porous layer rather than completing its circuit to the surface. This phenomenon is known as "lost circulation". A common solution to this problem is to add a lost circulation additive such as gilsonite.
Fluid loss control additives may be included such as one containing either bentonite clay (which in turn contains sodium montmorillonite) or attapulgite, commonly known as salt gel. If these clays are added to the drilling mud in a proper manner, they will circulate down through the drill string, out the drill bit nozzles, and to the site on the borehole wall where liquid from the mud is migrating into the porous formation. Once there, the clays, which are microscopically plate-like in form, form a filter cake on the borehole wall. Polymeric fluid control agents are also well known. As long as the filter cake is intact, very little liquid will be lost into the formation.
The properties required in drilling mud constantly vary as the borehole progresses downward into the earth. In addition to the various materials already noted, such substances as tannin-containing compounds (to decrease the mud's viscosity), walnut shells (to increase the lubricity of the mud between the drillstring and the borehole wall), colloidal dispersions, e.g., starch, gums, carboxy-methyl-cellulose (to decrease the tendency of the mud to form excessively thick filter cakes on the wall o: the borehole), and caustic soda (to adjust the pH of the mud) are added as the need arises.
The fluid used as drilling mud is a complicated mixture tailored to do a number of highly specific jobs.
Once the hole is drilled to the desired depth, the well must be prepared for production. The drill string is removed from the borehole and the process of casing and cementing begins.
b. Casing and Cementing the Well
It should be apparent that a well that is several thousand feet long may pass though several different hydrocarbon producing formations as well as a number of water producing formations. The borehole may penetrate sandy or other unstable strata. It is important that in the completion of a well each producing formation be isolated from each of the others as well as from fresh water formations and the surface. Proper completion of the well should stabilize the borehole for a long time. Zonal isolation and borehole stabilization are also necessary in other types of wells, e.g., storage wells, injection wells, geothermal wells, and water wells. This is typically done, no matter what the type of well, by installing metallic tubulars in the wellbore. These tubulars known as "casing", are often joined by threaded connections and cemented in place.
The process for cementing the casing in the wellbore is known as "primary cementing". In an oil or gas well, installation of casing begins after the drill string is "tripped" out of the well. The wellbore will still be filled with drilling mud. Assembly of the casing is begun by inserting a single piece of casing into the borehole until only a few feet remain above the surface. Another piece of casing is screwed onto the piece projecting from the hole and the resulting assembly is lowered into the hole until only a few feet remain above the surface. The process is repeated until the well is sufficiently filled with casing.
A movable plug, often having compliant wipers on its exterior, is then inserted into the top of the casing and a cement slurry is pumped into the casing behind the plug. The starting point for a number of well cements used in that slurry is the very same composition first patented by Joseph Aspdin, a builder from Leeds, England, in 1824. That cement, commonly called Portland cement is generally made up of:
______________________________________ 50% Tricalcium silicate 25% Dicalcium silicate 10% Tricalcium aluminate 10% Tetracalcium aluminoferrite 5% Other oxides. ______________________________________
API Class A, B, C, G and H cements are all examples of Portland cements used in well applications. Neat cement slurries may be used in certain circumstances; however, if special physical parameters are required, a number of additions may be included in the slurry.
As more cement is pumped in, the drilling fluid is displaced up the annular space between the casing and the borehole wall and out at the surface. When the movable plug reaches a point at or near the bottom of the casing, it is then ruptured and cement pumped through the plug and into the space between the casing and the borehole wall. Additional cement slurry is pumped into the casing with the intent that it displace the drilling mud in the annular space. When the cement cures, each producing formation should be permanently isolated thereby preventing fluid communication from one formation to another. The cemented casing may then be selectively perforated to produce fluids from particular strata.
However, the displacement of mud by the cement slurry from the annular space is rarely complete. This is true for a number of reasons. The first may be intuitively apparent. The borehole wall is not smooth but instead has many crevices and notches. Drilling mud will remain in those indentations as the cement slurry passes by. Furthermore, as noted above, clays may be added to the drilling mud to form filter cakes on porous formations. The fact that a cement slurry flows by the filter cake does not assure that the filter cake will be displaced by the slurry. The differential pressure existing between the borehole fluid and the formation will tend to keep the cake in place. Finally, because of the compositions of both the drilling mud and the cement slurry, the existence of non-Newtonian flow is to be expected. The drilling mud may additionally possess thixotropic properties, i.e., its gel strength increases when allowed to stand quietly and the gel strength then decreases when agitated. The combination of these effects creates boundary layer conditions which hinder the complete displacement of the drilling mud from the annular space.
Several remedial and preventative steps may be taken to assist in removal of drilling mud from the annular space. Long wire bristles or "scratchers" may be placed at intervals along the casing string as it is inserted into the hole. These devices have the direct beneficial effect of removing filter cakes from the borehole wall and providing an improved bonding site for the set cement. However, because of the flow characteristics of the cement slurry and the drilling mud, the scratchers are not completely effective in causing the mud to displace.
Similarly, a device known as a "centralizer" can be added to the casing string to centralize the casing and improve the flow around the string. Although centralizers are helpful in preventing quiescent areas, the borehole does not have a perfect interior surface and dead spots will occur in, e.g., dog-legs in the hole.
Other methods of aiding in the displacement operation, have been attempted and each has its own benefits and detriments. These methods include preflushes, spacers, additives to reduce drilling mud viscosity, abrasive materials to erode the filter cake, and high apparent viscosity cement to displace drilling mud in a piston-like motion. None of the known methods is completely effective in removing mud from the borehole wall.
One goal of the art has been to dispense with the necessity of displacing the drilling mud by utilizing but a single fluid capable of performing the functions of both the mud and the cement. The benefits of such a multifunctional fluid are apparent. The requirements that the filter cake be removed from the borehole wall and that the mud be taken from the imperfections in the wall are therefore obviated. A few patents disclose methods for converting drilling mud to cement and are discussed below. However, none of these disclosures suggest a drilling mud which is used per se as the well cement. The disclosures normally teach the addition of some other material to the mud prior to its use as a cement. None of the references shows the conversion of mud to cement by irradiation in the well.
A method of converting drilling mud to a cement slurry is disclosed in U.S. Pat. No. 3,168,139, issued to Kennedy et al on Feb. 2, 1965. Kennedy et al teaches the straightforward step of adding a hydraulic cement, preferably a Portland cement, to the drilling mud to form a cement slurry. Kennedy et al does not suggest using the cement slurry so-formed as a drilling fluid. Kennedy et al also notes at Column 13, line 32 et seq that "less channeling through the set cement occurs than when conventional cementing slurries are employed". The efficiency of the Kennedy et al process is therefore not assured.
The patent to Cunningham et al, U.S. Pat. No. 3,409,093, issued Nov. 5, 1968, teaches the use of a known cement slurry as the drilling fluid. This process is said simultaneously to produce an impenetrable filter cake on the borehole wall and a strong cement sheath on the walls capable of stabilizing the borehole wall. It should be apparent that close control of setting rate by retarder concentration and water content (via inclusion of water and water-loss additives) must be maintained using this process lest the cement slurry set and seize the drill string. It should also be apparent that the drilling fluid disclosed by Cunningham et al will be useful as a drilling fluid only for a short time.
Tragesser, U.S. Pat. No. 3,557,876, issued Jan. 26, 1971, teaches a drilling mud which can be converted, when desired, to a cementitious material by the addition of an alkaline earth oxide such as calcium, strontium or barium oxide. The particular drilling mud involved comprises water, collodial clay, various conventional additives, and a substance known as pozzolan. Pozzolan is a siliceous material (generally about 50 percent silicon oxide) containing various percentages of other oxides such as magnesium oxide, aluminum oxide, or iron oxide. These materials are said to form a cementitious material when reacted with an alkaline earth oxide in the presence of water at the temperatures found downhole in a well. There is no assurance that adequate mixing between the disclosed drilling mud and the alkaline earth oxide will occur in the well. Without proper mixing, quiescent volumes may remain within the well and prevent attainment of an acceptable cement job.
The disclosure in Harrison et al, U.S. Pat. No. 3,605,898 issued Sept. 20, 1971, relates to a hydraulic cement composition containing a setting retardant known as a heptolactone, preferably D-gluco-D-guloheptolactone. The composition is said to be usable as a drilling mud as long as the retardent is effective. A water soluble polyvalent metal salt, preferably CaCl.sub.2, is added to the mud to effect a conversion into cement.
The teachings in Miller et al, U.S. Pat. No. 3,887,009, issued June 3, 1975, relate to a clay-free magnesium-salt drilling fluid. Such fluids are said to typically contain 15 to 60 pounds of magnesium sulfate per barrel of drilling mud, from 20 to 70 pounds per barrel dolomite, about 3 to 15 pounds of calcium oxide per barrel, and 4 to 10 pounds of gypsum per barrel. In order to form a cement from this drilling mud, sufficient magnesium oxide, magnesium sulfate and dolomite or magnesium carbonate are added to produce a magnesium oxysulfate cement. Again, control of the concentration of the added material appears to be critical.
None of the disclosures of Tragesser, Harrison et al or Miller et al eliminate the step of displacing the drilling fluid from the well.
The process disclosed herein does not require the sequential addition of various materials as the time for cementing approaches, although it is acceptable to do so. The step which initiates the conversion of the disclosed composition from drilling mud to cement is the irradiation of that composition in the well. The cross-linkable polymers contained in the polymeric composition thereafter crosslink forming a strong set cement.
There are, of course, other known compositions containing polymerizable compounds which have been placed in wells for a variety of reasons.
A disclosure of one such composition is found in Perry et al, U.S. Pat. No. 3,114,419 issued Dec. 17, 1963. Perry et al suggests a "method for polymerizing liquid-resin forming materials suitable for use in well bores penetrating permeable subterranean formations". The preferred method uses radiation to copolymerize an alkylidene bisacrylamide with an ethylenic monomer. Perry et al teaches that the polymeric composition is made up so as to have a specific gravity between 1.07 and 1.18 (Column 4, lines 48 et seq). The specific gravity may be adjusted by the addition of non-ionizing organic weighting agents such as sugar or glycerol. The composition should retain some water solubility to be effective (Column 3, lines 43 et seq). The composition is pumped to the formation to be plugged by first pumping fresh water down the casing followed by the polymeric composition (Column 5, line 65 et seq). An amount of salt water is then pumped in until the composition is placed at the site of the porous formation. Because the specific gravity of the composition is between that of the overlying fresh water and the underlying salt water, it will stay in place. A radioactive source is then inserted into the well, to effect copolymerization and seal the permeable formation. Perry et al additionally discloses the use of similar processes to seal permeable formations when using a gas as the drilling fluid (Column 6, line 46 et seq and Example V) and to plug a permeable formation in a water flood injection well (Example VI).
Perry et al does not disclose or suggest the use of drilling muds containing polymeric materials which can be converted to cement by irradiation in the well.
Other disclosures relating to the use of irradiated polymers in well liquids can be found in U.S. Pat. Nos. 3,830,298, 3,872,923, 3,877,522, and 3,973,629 each issued to Knight et al. Another disclosure of radiation induced polymerization may be found in Canadian Pat. No. 1,063,336 to Ressaine et al. Each of these disclosures relates to a particular use of a polymerized "acrylamide and/or methacrylamide and acrylic acid, methacrylic acid, and/or alkali metal salts thereof" in a number of different ways, e.g., as a water loss additive in cement, as a plugging medium in a porous formation, etc. However, each of the disclosures deals with a polymeric composition which is irradiated prior to being placed in a well.
c. Completing the Well
After the casing is cemented in place, a well is prepared for hydrocarbon production using a number of separate steps. A perforation tool is commonly lowered within the cemented casing to the region of a producing formation. The perforation tool is a device which often is constructed of a number of guns which produce holes through the casing and its enclosing cement and into the producing formation. The interior of the casing is thereafter in open communication with the formation.
Other completion steps may involve fracturing to increase well productivity, installing screens to exclude sand from the well bore, and installing production tubing between the producing formation and the surface.