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if efficient drilling is to be maintained. In typical rotary METHOD FOR PRIMARY CEMENTING A WELL drilling operations, heat and rock chips are removed by WITH A DRILLING MUD WHICH MAY BE the use of a liquid known as drilling mud. As noted
CONVERTED TO CEMENT USING CHEMICAL above, drill strings are usually made up of sections of INITIATORS WITH OR WITHOUT ADDITIONAL 5 hollow pipe terminated by a drill bit. Drilling mud is IRRADIATION circulated down through the drill string, out through
orifices in the drill bit where the mud picks up, rock RELATED APPLICATIONS chips and heat, and returns up the annular space be
This application is related to the following applica- tween the drill string and the borehole wall to the surtions filed the same date as is this one: 10 face. The mud is sieved on the surface, reconstituted,
"Primary Cementing Technique" by Novak P.M. Ser. and pumped back down the drill string.
No. 463,200—2/2/83. Drilling mud may be as simple in composition as clear
"A Settable Drilling Fluid" by Novak P.M. Ser. No. water, but more likely it is a complicated mixture of 462,838—2/2/83. clays, thickeners, and weighting agents. The character
"Novel Drilling Mud Composition" by Novak and 15 istics of the drilled geologic strata and, to some extent, Talukder P.M. Ser. No. 462,833—2/2/83. the nature of the drilling apparatus determine the physi
"Alkadiene - Acrylic . Vinyl Compound Copolymers" Cal parameters of the drilling fluid. For instance, the by Novak, Talukder, and Sinclair P.M. Ser. No. drilling mud must be capable of carrying the rock chips 463,199—2/2/83. to the surface from the drilling site. Shale-like rocks
BACKGROUND OF THE INVENTION 2° °^ten Pr°duce cnips which are flat. Sandstones are not
quite so likely to produce a flat chip. The drilling fluid
1. Object of the Invention must t,e capable of removing either type of chip. ConThis invention relates to a method of primary ce- versely, the mud must have a viscosity which will per
mentmg a well with a drilling mud containing a poly- mit it to be ... at high rates without excessive meric compound which may be converted to a cement 25 mud ... pressures
with chemical initiators either with or without addi- In the mstance where a high pressure layerj e g; a gas tional irradiation. formation, is penetrated, the density of the drilling mud
2. Field of the Invention . must be increased to the point such that the hydrostatic The process of searching for oil and gas is fraught Qr h ... head of the^ud is ter than (he down.
with risk. Approxnnately three out of every four wells 30 <.iomaiion") pressure. This prevents gas leak
drilled m the United States are dry holes. Even m the V . . , \. ~ .„ .
instance when a well is found to have penetrated a a^ ?ut mt°ihe "PTM1" space surrounding the drill pipe subterranean formation capable of producing an eco- £"? ... ■the ?hTMT Phenomenon known as
nomic amount of hydrocarbon, the well must be care- bl°wout' m wmch the dr*m? mud 18 bl°wn fTMmthe fully completed after drilling has ended or less than the 35 we" bv the formation gas. Finely ground bante (barium maximum amount of hydrocarbon will be produced. sulfat«) 18 the additive most widely used to increase the One problem caused by the improper well completion sPeciflc 8TM"*. of drilling mud; although, in special step of cementing is subterranean movement of gas from circumstances, iron ore, lead sulfide, ferrous oxide, or a high pressure formation to another formation of lower titanium dioxide may also be added, pressure. The gas lost in this way may never be recov- 40 In strata wmch are very porous or are naturally fracered. This invention solves many of the problems asso- tured and which have formation pressures comparaciated with poor cementing procedures by converting tivelylower than the local pressure of the drilling mud, the fluid known as "drilling mud" directly into a hard- another problem occurs. The drilling fluid, because of ened cement. Drilling mud is the fluid typically used its higher hydrostatic head, will migrate out into the during the drilling of a well to lubricate and cool the bit 45 porous layer rather than completing its circuit to the as well as remove rock cuttings from the borehole. surface. This phenomenon is known as "lost circulaDrilling mud is usually displaced in a discrete step by a tion". A common solution to this problem is to add a cement slurry after the borehole is lined with steel cas- l°st circulation additive such as gilsonite. ing. Fluid loss control additives may be included such as
The process of drilling a well followed by the steps of 50 one containing either bentonite clay (which in turn casing and cementing it are described below. contains sodium montmorillonite) or attapulgite, com
monly known as salt gel. If these clays are added to the a. Drilling the Well drilling mud in a proper manner, they will circulate
In conventional rotary drilling, a borehole is ad- down through the drill string, out the drill bit nozzles, vanced down from the surface of the earth (or bottom 55 and to the site on the borehole wall where liquid from of the sea) by rotating a drill string having a drill bit at the mud is migrating into the porous formation. Once its lower end. Sections of hollow drill pipe, usually there, the clays, which are microscopically plate-like in about 30 feet long, are added to the top of the drill form, form a filter cake on the borehole wall. Polymeric string, one at a time, as the borehole is advanced in fluid control additives are also well known. As long as increments. 60 the filter cake is intact, very little liquid will be lost into
In its path downward, the drill bit may pass through the formation, a number of strata before the well reaches the desired The properties required in drilling mud constantly depth. Each of these subsurface strata has associated vary as the borehole progresses downward into the with it physical Parameters, e.g., fluid content, hard- earth. In addition to the various materials already noted, ness, porosity, pressure, inclination, etc., which make 65 such substances as tannin-containing compounds (to the drilling process a constant challenge. Drilling decrease the mud's viscosity), walnut shells (to increase through a stratum produces significant amounts of rub- the lubricity of the mud between the drillstring and the ble and frictional heat; each of which must be removed 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 of 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 mix- 5 ture 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. 10
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 15 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 20 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 25 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, 30 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 35 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. 40
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 such a slurry is the very same composition first 45 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 50
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 ce- 55 ments may be used in certain circumstances; however, if special physical parameters are required, additives may be included to the cement slurry.
As the cement is pumped in, the drilling fluid is displaced up the annular space between the casing and the 60 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 into the annular space between the casing of the borehole wall. Additional cement slurry is pumped into the 65 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. 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 em- 5 ployed". 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 10 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 15 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. 20
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 25 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 ox- 30 ide. 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 35 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 40 cement composition containing a setting retardant known as a heptolactone, preferably D-gluco-Dguloheptolactone. 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 CaCh, is 45 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 50 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 carbon- 55 ate 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 dril- 60 ling fluid from the well.
The inventive process disclosed herein requires the addition of a chemical initiator to the disclosed drilling mud prior to the time for cementing. The mud may be circulated to mix the mud in the well. The mud may 65 then, if desired, be irradiated. The cross-linkable polymers contained In the drilling mud 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 liquidresin 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).
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 3,972,629 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.
U.S. Pat. No. 3,635,289, to Van Dyk, discloses a process for sealing a cavity in the earth by using a water-inoil emulsion having a continuous oil phase having therein an unsaturated polyester and a solvent of a polymerizable compound and a discontinuous water phase containing a catalyst. The catalyst is prevented from polymerizing the emulsion for various lengths of time by the addition of specific amounts of inhibitor. Van Dyk does not disclose the use of the emulsion as a drilling fluid.
U.S. Pat. No. 3,199,589, to Boyd et al, discloses a process of polymerizing a composition containing a monomeric alkylidene bisacrylamide and a copolymerizable monomer with an oxidizing catalyst in a well by triggering the reaction with a composition of ascorbic acid and a water-soluble ferrous compound. The composition becomes solid shortly after introduction of that triggering compound. Boyd et al does not disclose use of the composition as a drilling fluid.