US3499491A

US3499491A - Method and composition for cementing oil well casing

Info

Publication number: US3499491A
Application number: US741198A
Authority: US
Inventors: Reece E Wyant; Orien Van Dyke
Original assignee: Dresser Industries Inc
Current assignee: MI Drilling Fluids Co
Priority date: 1968-06-28
Filing date: 1968-06-28
Publication date: 1970-03-10
Anticipated expiration: 1987-03-10
Also published as: DE1909919C3; NL166752B; NL166752C; GB1241886A; CA934772A; DE1909919A1; DE1909919B2; FR2011745A1; NL6905727A

Description

auu"' LMI'I Ll hll United States Patent 0 f ABSTRACT OF THE DISCLOSURE A process of cementing a string of pipe in a borehole by gombining a properly treated aqueous drilling fluid with a cemefititious mmre comprising hy iraul c ement and powdered sodiugi g licateifia ssl'to forma mud concrTe."'lli incr'asEd temperature of the borehole triggers the setting reaction of the mud concrete= BACKGROUND The general procedure for drilling an oil well includes setting a relatively short string of casing in the borehole soon after the start of the operation. This casing is emplaced either by drilling the hole and running the casing or driving the pipe. This first string of casing is normally referred to a conductor pipe, and theannulus between the outside of the pipe and the inside of the borehole is filled with a mixture of portland cement and water. A slightly smaller hole is then drilled below the conductor pipe, and another string of casing is run and cemented. This is referred to as surface casing. It is usual practice after running the casing to pump the mixture of cement and water down the inside of the casing and up around the outside displacing drilling fluid in the annulus between the casing and the hole. In the Gulf Coast oil producing area of the United States, it is usual to set surface casing at about 3,000 ft. when starting relatively deep holes. In other areas, surface casing may be set as shallow as 500 ft. or as deep as 6,000 ft.

The next string of casing that is set in the hole isv usually referred to as an intermediate string or a protection string if it is in a deep hole. The next string might be to case the hole to its total depth, in which case it would be referred to as the production string. This invention relates primarily to cementing intermediate and production strings.

The principal objective in cementing surface casing is to secure the casing in the borehole so that the vibration and hammering of the whipping drill pipe will not damage the casing. Occasionally, the bottom joints of pipe break free and drop, obstructing further drilling. Another reason for properly cementing the surface casing is to seal otf communication of the borehole with upper water sands preventing a loss of heavier drilling fluids that might be required for drilling the deeper portion of a hole. When running the intermediate and production string into a hole, a good primary cement job is extremely important to the proper drilling and completion of a well. Basically, the purpose of cementing at the lower levels is to secure the casing and prevent communication between water, oil and gas-producing zones and other porous zones.

Failure of a well to produce may be a result of a faulty cement job, which is not known to be faulty. It is only recently that surveys such as bond logs have been available to give indications as to the quality of a primary cement job. When it is evident that the primary cement job after running casing is not good, as indicated by communication between water, gas or oil zones or by failure of the hole to hold pressure, or by other testing devices, such as the bond log, temperature log, etc., it is necessary to attempt some type of remedial work (secondary 3,499,491 Patented Mar. 10, 1970 cementing). In the case of deep protection casing or the production string, it may be necessary to perforate the casing and squeeze cement under high pressure through the perforations into the annulus hoping that it will fill the zones that were not properly cemented during the primary operation. Sometimes squeeze cementing is successful, and sometimes many attempts to correct a faulty primary cement job by squeeze cementing have resulted in failure which eventually led to abandoning of the hole.

The problem of obtaining a good cementing job is a problem of displacing substantially all the drilling fluid or mud in the annulus with cement. Unfortunately, drilling fluids and oil well cements are not compatible with one another. A slurry of cement will flocculate and thicken most water base muds. Furthermore, most water base muds, when added to cement, will either cause flash setting or act as a retarder that will greatly decrease the strength of the cement and substantially increase the setting time. Hydrated shale and clay, such as most water muds contain when mixed with cement, greatly decrease its strength. Muds used in deeper holes not only contain colloidal clays and shale that decrease the strength of cement, but they also contain organic chemicals such as quebracho, starch, carboxymethyl cellulose, calcium lignosulfonates, chrome lignosulfonates, or other organic materials. These chemicals in small quantities are considered retarders that substantially increase the setting time of cements, but in larger quantities are cement-destroying retarders. One solution to this very problem is proposed in U.S. Patent 3,190,356, entitled Methods of Cementing Wells, issued to H. I. Beach. Lignosulfonates, which are currently considered to be one of the most useful and most widely used treating chemicals for drilling mud, are' one of the prominent members of the group of organic compounds that are considered in small quantities (e.g. 0.5% based upon the weight of the cement) to be a powerful retarder, or in larger quantities a cement-destroying retarder. A drilling fluid that is both physically and chemically stable is required for the .drilling of deep holes in most areas, and chemicals such as lignosulfonate are required in concentrations of 6 to 20 lbs./bbl. to provide a fluid with the desired properties.

One of the principal causes of a faulty cement job is the comingling of the incompatible oil well cementing slurry and the drilling fluid in the hole and especially the fluid that has formed a filter cake on the sides of the hole in permeable zones. Numerous attempts have been made to use special wash fluids to displace the mud and wall cake, but these have given very limited success. Sometimes relatively large quantities of water are used as a separating slug between the cement and the mud that is being displaced by the cement slurry. Other times a special chemical solution, or sometimes a viscous slurry is used. To the best of our knowledge there is no separating slurry that has given consistent results of improving primary cement jobs. The diflicult displacement problem is complicated when the cement slurry and drilling fluid are antagonistic; that is, the cement slurry flocculates and gells the drilling fluids and the drilling fluid inhibits or prevents the set of the cement. It is easy to visualize that as the cement is pumped out of the bottom of the casing and starts to rise in the annulus, it may break flow channels through relatively large sections of flocculated drilling mud.

Experts in the oil drilling industry consider a cement with a compressive strength of between 400 and 500 lbs. in 24 hours as being sufliciently strong to serve any requirement that might be placed upon it. Some authorities state that strength even lower than these values would be satisfactory; however, there are some who favor higher strengths.

In view of the incompatibility of drilling fluids and cements, it is indeed surprising that we have invented a cementitious material for securing oil well casings which incorporates drilling muds available at the drilling site as an aggregate.

BRIEF SUMMARY OF THE INVENTION A process of cementing a string of pipe in a borehole comprising an aqueous drilling fluid treated with dispersants such as lignite, tannins, lignosulfonates, and mixtures thereof, adding to the drilling fluid a cementitious combination of hydraulic cement and powdered sodium silicate glass, pumping the resulting mixture into the annular space between the string of pipe and the walls of the borehole, and allowing the temperature of the borehole to harden the mixture. Also, a process of treating aqueous drilling fluids to make same cementitious in the presence of increased temperature comprising adding to the drill' ing fluid a dispersant as above defined and also a cementitious combination of hydraulic cement and powdered sodiuin silicate glass. Further, a cementitious mixture for addition to equeous drilling fluids which have been treated with a dispersant as above defined said mixture comprising hydraulic cement and powdered sodium silicate glass.

It is an object of this invention to provide an oil well cement which incorporates drilling fluid as an aggregate, thereby saving cement costs. It is a further object of this invention to provide a cementitious material which is .compatible with highly treated drilling mud. It is yet another object of this invention to provide a cementitious material incorporating drilling fluids that has suflicient thickening time to enable it to be pumped down a string of pipe and up the annular space between the string of pipe and the walls of a borehole. It is yet another object to provide a cementitious material which has little tendency to thicken or set until it is placed in the borehole and subjected to the higher temperatures present therein It is yet another object to provide a cementitious material which has adequate strength after about 24 hours. It is yet another object of this invention to provide a cementitious material which has reduced tendency toward retrogression of strength at elevated temperatures. It is a further object of this invention to provide a method of cementing strings of casing in boreholes. It is an object of this in vention to convert water base drilling fluids into cementitious materials suitable for cementing oil well casing.

BRIEF DESCRIPTION OF THE INVENTION According to a broad aspect of this invention, there is provided a process of cementing a string of pipe in a borehole by combing a properly treated aqueous drilling fluid with a cementitious mixture comprising hydraulic cement and powdered sodium silicate glass. The dfilling ui witha e cementi 10115 mi after referred to as mud concrete,,is pumped down the string of pipe and up the annular space between the pipe and the walls of the borehole. The increased temperature of the borehole triggers the setting reaction of the mud concrete.

Most aqueous drilling fluids can be made suitable for use according to this invention. Additions of oil in the mud do not affect the suitability of the mud for the purposes of this invention. Light mud such as those having a density less than about 10 lbs/gal. are not considered suitable for direct additions of cementitious mixture. For example, light muds such as those having a density of less than 9.6 lbs./ gal. may require solids to be added, as well as chemical conditioning. By properly treated, it is meant that the mud must be treated with organic dispersants such as lignites, tannins, lignosulfonates, or mixtures theieof. The drilling fluid must contain sufficient organic dispersants to make it resistant to the gelling effect of cement additions. Most muds can be properly conditioned with any or a mixture of any of the dispersants mentioned above.

4 Lignites used as dispersants are humates or derivatives of humic acid. One especially effective lignite is a chrome reacted potassium humate sold under the trade name XP-ZO by Dresser Industries, Inc. Another lignite product is sold under the trade name TannAthin by Dresser Industries, Inc. The lignosulfonates which can be used to treat drilling fluids are well known. They include the lime neutralized lignosulfonates described in US. Patent 2,491,437 issued to Barnes and the heavy metal oxide or heavy metal lignosulfonates described in US. Patent 3,126,291 entitled Hydraulic Cement Composition issued to E. G. King et a1. Tannins are derived from the bark and wood of certain trees, The most notable tannin used as a drilling fluid treating agent is quebracho.

According to this invention, properly treated drilling fluids are converted to a mud concrete by addition of approximately 100 to 200 lbs. of the cementitious mixture per barrel (42 gallons). Sodium silicate suitable must have a soda (Na O) to silica (SiO weight ratio between 1:1.6 and 1:4.5. By glass, it is meant that the sodium silicate contains less than about 5% water. The hydraulic cement and powdered sodium silicate should be mixed in a ratio between 6:1 and 1:1. By powdered, it is meant that the sodium silicate is crushed or ground to substantially all pass 65 mesh Tyler.

By hydraulic cement, this invention intends to include all compositions of lime, silica and alumina or of lime and magnesia, silica and alumina and iron oxide as are commonly known as hydraulic cements. Hydraulic cements include hydraulic limes, grappier cements, pozzolan cements, high alumina and portland cements. Car-i, tain materials, such as volcanic ash, fly ash and some? C113 s have pozzolanic properties and are commonly used' in cements. Because of its wide availability and superio strength, portland cement is preferred. It is also preferable to include 10 to by weight of the cementitious mixture of a pozzolanic material.

The thickening time of cementitious fluids, according to this invention, varies with the ratio of sodium silicate to cement. Also, sodium silicates having a lower silica to soda ratio have a greater tendency to accelerate gelling and shorten thicking times. In some boreholes, the pressure and temperature conditions may necessitate the addition of a retarding agent to cementitious fluids, according to this invention. We have found that certain alkaline materials including caustic soda, hydrated lime, soda ash, lithium carbonate, lithium hydroxide, and lithium chloride are effective in retarding the set without appreciably reducing set strength. These materials are especially effective when combined. For example, soda ash and lithium hydroxide together are very effective retarding agents.

DETAILED DESCRIPTION Further features and other objects and advantages of this invention will become clear to those skilled in the art by a careful study of the following examples and detailed description. In the specification and appended claims, all percentages and ratios and parts are by weight, unless indicated otherwise. The designations used to describe some of the sodium silicates are trade names of commercial products sold by Philadelphia Quartz Co.

A standardv mud was prepared for the following laboratory tests which is typical of the highly treated muds used for drilling for the Gulf Coast area. It had a density of approximately 16 lbs/gal. and a composition as follows:

STANDARD MUD The ingredients combined provided one oil field barrel (42 gallons) of mud. This mud is a properly treated mud within the concepts of this invention as it is resistant to the gelling effects caused by additions of cements. n the other hand, as it is highly treated with organic additives,

of our invention where arelatively short thickening time is desired at 190 F. Example F was similar to the other examples of Table I, except that the sodium silicate used as part of the cementitious mixture had a soda to silica ratio of 1:2 and was ground to all pass 200 mesh. In

it prevents the set of cement with which it became mixed 5 Example G, the cementitious mixture contained a sodium in relatively small quantities. silicate having a soda to silica ratio of 1:4.5. It should To this standard mud we added various cementitious be noticed that the standard mud with addition of this mixtures. Table I contains the composition of the cementicementitious mixture did not thicken in the laboratory tious additive to exemplary mixes A through I. The consistometer at 190' F. in 5.5 hours. Hence, this mixture efiects of this cement on the standard mud are recorded would only be suitable for use in very deep wells where in the table. the temperature of the borehole exceeded about 200 F.

TABLE I Example A B o D F o H J Drilling fluid standard mud, barrel 1 l -1 1 1 v 1 1 1 Cementitious mixture, pounds 130 130 130 130 120 130 130 120 Cement, percent 60 70 80 6B 76 68 65 75 Sodium Silicate:

SS 65, percent 17 7 20 SS 0-200, per n 20 SS Ratio 4.5, permnt l7 l0 Pozzolan, percent"... 16 16 Soda Ash, percent-. 3 Kaolin, percent 5 5 Relative breakover viscosity None None No'rle None None None None None Apparent viscosity, after mixing centipoise 180 163 210 53 ltB Thickening time, minutes to 100 poise:

(130 F.) N/I. F 248 (172 43 (190 F). 40 (206 Compressivestrength, after aging, p.s.i.:

(20 hours) (136 F.)

1 N .1. means not tested. 1 None at 5-hours.

3 Never thickened.

4 Approximately 200.

The cementitious mixtures were combined with the standard mud using high shear blenders, which was analogous to the mixing provided by the pumps adjacent to the mud pits 'in oil fields. The tendency, if any, of cement to gel the mud is apparent at this time and .is referred to in this application as relative breakover viscosity. After the cementitious mixture and drilling fluid were combined and mixed for 10 minutes, the apparent viscosity of the resulting mud concrete was measured with a direct indicating rotational type viscometer. The thickening time of the mud concrete was determined with the use of a Halliburton consistometer. The time for the mud concrete to reach a viscosity of 100 poises at a selected testing temperature is referred to as thickening time. Other portions of the mud concrete being tested wei'e placed into molds and aged for noted periods of time at selected test temperatures prior to testing for crushing strength.

Referring now to the examples in Table I, which are all according to this invention, it should be noted that no example had a breakover viscosity. In other words, because of the proper treatment of the drilling fluids prior to the addition of cementitious mixture, the fluids were not highly gelled by the mixture. All examples after mixing had apparent viscosities within an acceptable range for pumping them down the casing string and up through the annulus between the outside of the casing and sides of the borehole. In Examples A, B, and C, the cementitious mixture comprised only cement and sodium silicate glass having a soda to silica ratio of 1:3.22. The glass was ground to all pass 65 mesh. Example D is similar to Examples A, B, and C, except that the cementitious mixture included 17% of a pozzolan material of the volcanic ash type. The suitability of a cementitious mixture, according to this invention, depends on the particular borehole being cased and the cementing practice employed. Example D is a suitable mixture for the practice In Example H, the cementitious mixture comprised two types of sodium silicate and a small addition of soda ash which applicants have found to be an effective retarder for the cementitious mixture. Example H had an ample working time at a temperature of 206 F. Example I demonstrates that the setting reaction of cementitious mixtures according to this invention must be triggered by an appropriate temperature. Example J,showed no tendency to thicken in the consistometer after 5 hours at 136 F. However, when the temperature was raised to 190 F., the mixture thickened to poises in 52 minutes.

The examples in Table II are not according to the teachings of this invention as they did not develop adequate strength.

N.T. means not tested. 1 Soft gel, no set. Firm gel, no set.

Examples L and M contain no sodium silicate glass. The data established that the highly treated drilling mud destroys the ability of the cement to set up in the absence of sodium silicate. Examples N and O establish that sodium silicate without cement, however, does not develop adequate strength.

The examples in Table III are not according to the teachings of this invention. They are included to show that the mud aggregate must be properly treated and ater, gallons Special ud A (14 lbs/gal), barrels SpecialMud B (11.51bs./gal.), barrels...

8 having a soda to silica of 113.22 and comprising 37.6% solids. Examples S, T, and U establish that even with the properly treated standard mud, the metso anhydrous, D brand, and N brand sodium silicate were not suitable for use in this invention because they react too fast.

that only sodium silicate glasses having a silica to soda Examples V and W establish that G brand powder, ratio t n 2 and 5 are Suitable g nts to v ra hydrated sodium silicate having a soda to silica ratio come the effects of the chemical introduced by the highly of 1:322 and containing 19% water and GD brand treated muds Two special muds of the gyp-type were powder, a sodium silicate having a soda to silica ratio prepared having the following composition: of 1:2 and containing 18% water are also unsuitable for use in this invention as they also react too fast. It should be noted that because of the very rapid thickening of most of the examples in Table III, it was difficult to distinguish high breakover viscosity from Spwal MudA special MudB 15 rapid cement reaction. Both are equally bad, however. Density, gal 14.5 11.5 Drilling muds present at all drilling sites are not gfg g g fbg gf'gm $5 fl ffig highly treated. Therefore, to make them suitable for High Yield Clny,lbs-- 41.5 30.0 the purposes of this invention, it may be-necessary to i'fiilosl aarsamadman 321 treat the muds prior to the addition of the cementitious cellulose), lbs .t 0.2 0.2 20 mixture. Examples X and Y, described m Table IV, are 6 4 prepared from the special gyp-type muds (special muds Ca t c o g A and B) already described. Example Z was prepared $3,832, 53; i gg gy-- 5 using a special untreated mud having the following comlbs 6 position:

SPECIAL MUD 0 Density 14.1 lbs/gal. Water 0.76 bbl. (266 lbs). These special muds and the standard mud described Wyoming bentonite 21.5 lbs.

above were used in the examples of Table III. Barite 275.0 lbs.

TABLE III Example I Q R s '1 U V w Drilling fluid:

Standard mud (16 lbs./gal.), barrels.-. 1 1 1 1 1 Cementitious mixture, cement, lbs 470 470 8A 5 84. 5 84. 5 84. 5 84. 5

S um Silicate:

Metso Anhydrous, lb D Brand Sodium Silicate 2.5 gal. 0 10 bbl N Brand Sodium Silicate 8.1 gal. 0 l2 bbl. Pozzolan, lbs 19. 5 19. 5 19. 5 19. 5 19. 5 Soda Ash, lbs 3. 9 3. 9 3. 9 3. 9 3. 9 Relative breakover viscosity None None None None N.T. Agparent viscosity, after mixing cent 0 200 N .I. N.'I. 183 N.'l T ickening time, minutes to 100 poise: F.) N.T. N.T. N.'l,. N.T. N.TJ N.'l.

Compressive stren th, alter aging p.s.i 20 hours) (18o F.) (44 hours) (180 F.)

1 Slurry thickened too fast to obtain test. 2 Too thick to measure.

a Very in h. 4 Never tiickened. 5 Firm ge no set.

Examples P, Q and R demonstrate that lightly treated gyp-type muds gel immediately upon mixing or else have a very high breakover viscosity when combined with the cementitions mixture comprising metso anhydrous silicate, D brand sodium silicate, or N brand sodium silicate. Metso anhydrous sodium silicate has a soda to silica ratio of 1:1. D brand is a liquid sodium silicate having a soda to silica ratio of 1:2 and comprising 44% silicate solids. N brand is a liquid sodium silicate Table IV establishes that even with applicants cementitious mixture comprising a powdered sodium silicate glass having a soda to silica ratio between 1:2 and 1:45, muds that are not highly treated are not suitable (Examples X, Y and Z). On addition of 4 lbs/bbl. of quebracho and 2 lbs/bbl. of caustic soda to special mud A, it became a properly treated mud (Example XA). Addition of 4 lbs./ bbl. of Uni-Cal (a chromium lignosulfonate) and 1 lb./ bbl. of caustic soda also made mud A a properly treated TABLE IV Example X XA XB XC Y YA YB YC Z ZA Z8 Z ZD ZE Drilling Fluid:

Spec. Mud A (14.5#/gal. Gyp

Mu 1 l 1 l S .Mud B (11.5#/gal. Gyp

u 1 1 1 1 StfieMud C (MM/gal. Untreated ud) bbl 1 1 1 1 l 1 Additional Treatment Quebraeho, lbs. 8 6

14 14 14 14 14 14 14 14 14 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 None None None None None None None 27 30 45 130 158 150 188 210 N.T.' NIP. Nil) N.T. N31. N.T. N.l. N.T. NIP.

Kembreak, lbs.

TannAthin', lbs

Caustic, lbs 2 1 0. CementitiousMixture, Total ffbbl. 130 130 130 130 130 Cement, percent 65. 5 65. 5 65 65 65. 5 88-65, Sodium Silicate, percent 17 17 17 17 17 Pozzolan, percent 14 14 14 14 Soda Ash, percent 3. 5 3. 5 3. 5 3. 5 3. 5 Relative Breakover Vis None None None g Agpt. Vise. Alter Mixing, C p 178 165 165 T ickening Time,Minutes- Nil! NIP. NIP. Nil. N.T. Compressive Strength Tests:

Aging Temp, F 190 190 190 Strength, p.s.i. 725 560 802 Aging Time, hours... 20 44 V. High.

1 Too thick.

1 N .T. means not tested.

4 Too thick to test.

mud (Example XB). It should be noted that this mud originally contained 6 lbs./bbl. of TannAthin (lignite) and 6 lbs./bb1. of Spersene (a chrome lignosulfonate). It was found that special mud B could be made a properly treated mud by addition of 8 lbs./bbl. of Kembreak (calcium lignosulfonate), 8 lbs./bbl. of TannAthin, or 8 lbs./ bbl. of quebracho and a small addition of caustic soda (Ex- TABLE V Example AA AB AC AD AE AF A G AH AI Drilling fluid, standard mud barrels... 1 1 1 1 1 1 1 1 1 Cementitious mixture; pounds..... 120 190 190 130 130 130 130 Cement; reent 75 70 78 63 68 68 68 65 65 Sodium S lcate; 58-65; percent. 20 20 17 16.5 17 17 17 17 17 Pozzolan; percent ":5. i5 14. 5 15 l5 15 15 15 6Y8 61 i Relative breakover ty... None None None None None N one None None None Agparent viscosity, after mixing centipoise 2,10 148 203 200 183 200 '1 ickening time, minutes to 100 poise F.) 21 173 38 90 36 85 255 114 Compressive strength, after aging, p.s.i.:

(20 hours) (190 F.) (40 hours) (190 F.) (68 hours) (190 F.) (90 hours) (190 F.)

1 Did not thicken in 400 minutes.

amples YA, YB and YC). Special mud C, which is an untreated mud, could be made properly treated by the addition of as little as 4 lbs./bbl. of Spersene plus 1 lb./ bbl. of caustic soda or 4 lbs./bbl. of Kembreak plus 6 lbs./bbl of caustic soda (Examples ZA and Z8). Example ZE was treated with additional quebracho, but still had a very high breakover viscosity. This particular mud could be made suitable, however, by a small addition of water. Those skilled in the art of working with drilling muds can determine suitable treatments.

In order for additions of lignite, tannins, lignosulfonates, etc, to be effective, it is necessary to add a proper amount of caustic soda. As an example, we normally add one pound of caustic soda for each four pounds of Spersene added. When using a tannin like qnebracho, it is usual practice to add about one pound of caustic soda for every two pounds of quebracho. In the case of TannAthin, we normally add about one pound of caustic soda for each six pounds of TannAthin.

The examples in Table V are according to the teachings of this invention and demonstrate the effectiveness of certain compounds for lengthening the thickening time of cementitious mixtures and drilling fluid. The retarding agents would be used in deep wells having higher tem- Examples AA and AB demonstrate the effectiveness of hydrated lime as a retarding agent. Examples AC and AD demonstrate the effectiveness of soda ash as a retarding agent. Examples AE and AF demonstrate the effectiveness of lithium carbonate as a retarding agent. Examples AE and AG demonstrate the effectiveness of lithium hydroxide as a retarding agent. Examples AH and AI demonstrate the effectiveness of a combination of soda ash and lithium hydroxide as retarding agents. With the exception of large additions of lithium hydroxide, the re-' tarding agents extend the thickening time without greatly reducing the compressive strength after 20 hours. Summarizing, we have found that materials including caustic soda, hydrated lime, soda ash, lithium carbonate, lithium hydroxide and lithium chloride are effective in retarding the set of our cementitious mixtures. These materials are especially eifective when combined. For example, soda ash and lithium hydroxide together are very effective retarding agents.

Cementitious mixtures, according to this invention, have reduced tendency to lose strength after extended times at elevated temperatures. For example, mix AD in Table V would have a comprissive strength of about 1623 p.s.i. after aging 500 hours at 190 F. If the aging temperature 1 1 were raised 350 F., the strength would be reduced to 750 psi. It is usual for conventional oil well cements containing bentonite to show a retrogression in strength of between 75 and 90% after long periods of aging at high temperatures.

We have found that the following well known set retarders were either ineffective or acted as accelerators in our cementitious mixtures: sugar, NaCl, CMC, sodium gluconate, tartaric acid, borax, gallic acid, maleic acid, pyrogallic acid, sodium phosphates.

It should be understood that mud concretes according to the teachings of this invention are suitable for use not only in primary cementing but in secondary cementing, plugging and other oil and gas well applications.

Having thus described the invention in detail, and with sufiicient particularity as to enable those skilled in the art to practice it, what is desired to have protected by Letters Patent is set forth in the following claims.

We claim:

1. A process of cementing a string of pipe in a borehole comprising:

( 1) preparing an aqueous drilling fluid comprising clay minerals treated with alkali and organic dispersants selected from the group consisting of lignites, tannins, lignosulfonates, and mixtures thereof, there being suflicient dispersants to make the fluid resistant to the gelling effect of cement additions, said fluid having a density of at least 10 lbs./gal.,

(2) adding to said drilling fluid to form a mud concrete 100 to 200 lbs./bbl. of a cementitious combination compirising hydraulic cement and powdered sodium silicate glass in a ratio between 6:1 and 2: 1, said sodium silicate glass having an Na O: SiO ratio from 121.6 to 1:4.5,

(3) pumping the mud concrete into the annular space between the string of pipe and the walls of the borehole, and

(4) allowing the mud concrete to set due to the increased temperature of the borehole.

2. A process according to claim 1 in which there is added an amount of alkaline material suflicient to retard the set of the drilling fluid with added cementitious mixture while it is pumped into the annular space between the string of pipe and the walls of the borehole.

3. The process according to claim 1 in which a suflicient amount of a mixture of at least one of the materials in the group consisting of calcium hydroxide, lithium chlomentitious mixture includes up to about pozzolan material.

5. The process according to claim 1 in which the cementitious mixture comprises 65% portland cement, 17% powdered sodium silicate glass having a soda to silica ratio of 123.22, 15% ground volcanic ash with pozzolanic properties and 3% soda ash.

6. The process according to claim 1 in which the cementitious mixture comprises 65 percent portland cement, 17 percent powdered sodium silicate glass having a soda to silica ratio of 1:2, 15 percent ground volcanic ash with pozzolanic properties and 3 percent soda ash.

7 A process of treating aqueous drilling fluids containing clay minerals to make said fluids cementitious in the presence of increased temperature comprising the steps of:

(1) adding suflicient caustic and dispersant selected from the group consisting of lignites, lignosulfonates, tannins and mixtures thereof suflicient to make the fluid resistant to the gelling effect of cement additions,

-(2) adding to said drilling fluid 100 to 200 lbs./bbl. of

a cementitious combination comprising hydraulic cement and wdered 'icate lass a ratio ml and 1:1, said sodium s1 icate g ss having a soda to silica ratio from 1:16 to 1:4.

8. The method of claim 7 in which the fluid is treated with between 4 and 20 lbs./bbl. of dispersant.

9. A cementitious mixture for addition to aqueous drilling fluids containing clay minerals which fluids have been sufliciently treated with organic dispersants selected from the group consisting of lignites, tannins, lignosulfonates, and mixtures thereof to make the fluids resistant to the gelling eflect of cement additions ,comprisi'ng h draulic cement and uered sodim sicate lass in a ratio between :1 an 1:1, sai sodium s1 icae gass having a soda to silica ratio from 1:16 to 114.5.

10. A mixture according to claim 9 wherein said mixture includes an amount of alkaline material suflicient to retard the set of the drilling fluid with added cementitious mixture while it is being pumped down the string of pipe and up the annular space between the string of pipe and the walls of the borehole.

11. The mixture according to claim 9 wherein said mixture includes a suflicient amount of a mixture of at least one of the materials in the group consisting of calcium hydroxide, lithium chloride, lithium hydroxide, lithium carbonate, sodium hydroxide and soda ash to prevent set while the drilling fluid with cementitious mixture is being pumped down the string of pipe and up the annular space between the string of pipe and the walls of the borehole.

12. The mixture according to claim 9 comprising up to about 25% pozzolan material.

13. The mixture according to claim 9 in which the cementitious mixture comprises percent portland cement, 17 percent powdered sodium silicate glass having a soda to silica ratio of 1:322, 15 percent ground volcanic ash with pozzolanic properties and 3 percent soda ash.

14. The process according to claim 9 in which the cementitious mixture comprises 65 percent portland cement, 17 percent powdered sodium silicate glass having a soda to silica ratio of 1:2, 15 percent ground volcanic ash with pozzolanic properties and 3 percent soda ash.

References Cited UNITED STATES PATENTS 2,188,767 1/ 1940 Cannon et al. 166-31 2,287,411 6/ 1942 Boller et al 106--84 2,646,360 7/1953 Lea 166--3l X 2,705,050 3/ 1955 Davis et al. 166-31 2,786,531 3/1957 Mangold et al 166-29 X 3,168,139 2/1965 Kennedy et al. 166-29 CHARLES E. OCONNELL, Primary Examiner I. A. CALVERT, Assistant Examiner U.S. c1. X.R. 106--76,