CA1070236A - Process for cementing casing in a well - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process for reducing the waiting-on-cement (WOC) time assoc-iated with the cementing of a casing string in an oil well involving the specific improvement of pumping into the well a highly retarded cement slurry containing a dispersed encapsulated accelerator, such as calcium chloride or anhydrous sodium metasilicate, wherein the encapsulating material is specifically selected such that it will melt at a tempera-ture above the bottom-hole circulating temperature but below the bottom-hole static temperature of the well to be cemented. Petroleum-derived waxes and petroleum-derived waxes in combination with vinyl resins, such as wax-compatible copolymers of ethylene and vinyl acetate, are particu-larly suitable as the meltable encapsulating material exhibiting excel-lent strength and abrasion resistance as well as serving as a moisture barrier. The improved process is particularly useful in cementing deep hot wells.

Description

1~7~236 BAC~GROUND OF TE~3 INVENT~ON
During the course o drilling an oil well, it i~
necessary at various intervals to lower casing or plpe into the hole and cement it in place. The cementing operatlon i9 accomplished by pumping a slurry down the pipe and up the .
annulus between the casing and thelwell bore wall. Dependi~g o~
the particular ~ell and the associatet circumstansa~, it ls :~
quiee common to use one or more of a ~ariety of addltives to alleviate specific problem~ or achieve de~ired re~ult~. Thus, 0 it i3 known and well documented in the art that additlves can be used to either increase or decrease the density of the ~lurry; certain additives can he used to retard the th~ckening time, hence increasing the pumpability time; other additives can be used to accelerate the setting, and still others are used to expend the set cement during cure. Furthermore, various proces~es a~d methods of cementi~g have been per~ected to achieve ~pecific reRults, such as the proces~ dlsclo~ed in U.S. Patent 3,420~299 lssued January 7, 1969 to Cloud, wherein a~ expandi~g age~t i8 released at a ~ime corre~pondin~ to the o~set of ~he exotherm associated with the setting of cement. ~owever, prior to our inventlon, the use of certaln types of additlves haR been considered to be mutually exclu-sive in that their e~fects are oppo~ite in nature and their si~ultaneous use would be counter productive~ Thus, a cement accelerator additive is not ordinarily used concurrently with :~ :
a cement retarder. Rowever, in cementing deep hot well~
~uch as fou~d in the Gulf Coast area, ~he need for b~th type~
of additi~es become6 apparant. Because of the extreme depth~
and the high temperatures, the cement slurry mu t be highly retarded in orter to insure sufficient time to pump the slurry town the caslng and back up the annulus.

~1017();~36 As a con~equence of being high~y retarded, the time required for ~lurry to gel and ~et 19 prolonged. The longer the time that the cement slurry remains in an ungelled or unset ~tate after placement the greater the odds are th~t gas channeling will take place neces~itating a subsequen~ cement ~;
~queeze step. Furthermore, the presence of the retarder will al~o 310w down the hardening process ater ~etting. Since it i9 nPces~ary for the mechanical strength of the cement to increase to a minimum value before work in the well hole can be re~umed, the ~o-called "wait-on-cement tNOC)" tlme i9 in- ;
herently extsnded~ which, in practlce, mean~ prolonged down time and increased expense. It i8 also known that using a highly retarded ~lurry results in a lowering of the ultimate tensile strength of the cement. Thu~, in a number of deep hot wells where highly retarded cement slurries were used, WOC
times of up to 72 hours were required and unset or green cement has been found after several day~ following the cementing operation. ~ `
To alle~iate these problems, we have developed a process of lncorporati~g a cemen~ accelerator into a highly retarded slurry without i~fluencing pu~pability o the slurry, yet, shortly after ~he placement of the slurry at the bottom of a well hole, the accelera~or additive will essentially pro mote a flash set.
~U~-~r O~ 0 We have discovered an improved method of cementing casing in a deep ho~ oil well Comprising the steps of first pumplng a highly re~arded cement slurry containing a dispersed encapsulated accelerator down tbe caoing and up th~ annulus between the cas~ng and the well bore` wall. The encapsulating r~

~L0~236 ma~erial coating the cement aocelerator i8 selectèd such that it wlll me~t a~ a temperature below the eharacteristic botto~-hole static temperature (BHST) of the particular well bein8 cemented but will not melt at a temperature correspond~
ing to the bottom-hole circulating temperature (BHCT, also referred to as the bottom-hole cementi~g temp~rature). After placeme~t, the cement slurry i~ held in a static no~clrculating state such that the slurry ~emperature will approach the BHST. In doing so, the encap~ulating material ~oftens and/or melts ~ releasing the accelera~or and i~ducing a rapid set of the cement. In thls manner, the WOC time is significantly r~duced, resulti~g in more efficient u~e of the well drilling equipment.
... ..... ....... ..
I~ one ~mbodi~ent of our in~ention, a new process for cement-ing casing in a deep hot oil well is disclosed. In another embodiment o~ our invention, an improved cement slurry is used in cementing oil well casing.
The primary object of our invention is to reduce the WOC time when cementing casing in a deep hot oil or gas well. Another object is to create simultaneously in a single cement slurry the advantages of both the presence of a cement retarder and a cement accelerator, while minimizing their associated disadvantages. Specifically, one object is to have a cement slurry that has sufficient pumpability time (highly rctarded) such that it can be used in deep hot wells. Another object i9 to have that same cement slurry flash set shortly after placement, thus minimizing ~OC time and gas channeling. An associated object is to have the post-set curing process proceed rapidly, thus minimizing the green cement problem and assuring high ultimate strength. The mere addition of our encapsulated accelerator to cement slurries as insurance against the formation of green cement will frequently justify the additioaal , ' ~1~7~36 expense of the addltlve. Thi~ i9 particularly true in 9~ ~uations where the well ha~ been exposed to cement retarding-type additlve ~such 8S drilling muds contal~ing lIgnosulfonate and the like) whlch may later contaminate the ~ubsequent :
cement Job.
In one a~pect of this in~ention there i3 pro~lded a proces~ of cementing casing ln a deep hot well compri~ing the steps of~
(a) pumping 8 highly retarded cement slurry down said casln~ whereby said slurry will ~e ~orced up the annulus between ~aid casi~g a~d the ~ell bore wall, wherein said hi~hly `~

,.... . ,, , , , . ~ . .
retarded slurry contains a dlspersed accelerator, sa~d acceler~
ator being encapsulated with a material capable of melti~g at ;~
a temperature below the bottom-hole stat~c temperature of said well but not capable of melting at a temperature correspond- :
i~g to the bottom-hole circulating temperature, and (b) holding the cement slurry in a noncirculating statlc state at the desired level in said well ~uch that said encap- -~
sulating material will melt relea~ing the accelerator and ~ :
i~duce a flash se of ~aid c~ment.
In another aspect of thi~ invent~on there is provided an oil-well ceme~t slurry for cementing in a well containing an effective amount up to about 15 perce~t by weight of an encapsulated cement accelerator wherein the encapsulating material covers the e~tire exter~al surface of the accelerator, isolating ~aid cement accelerator from said oil~well cement slurry and wherein said encapsulating material is capable of , melting at a temperature below the bottom-hole static temper~
ature of a~ oil or gas well to be cemented but not capable of melting at a temperature corre~ponding to the bottom-hole . ~;

, .. ~,: , ......

~ 97~);236 clrculating temperature of said well, ~hereby after placement, the cement slurry i9 held in a ~tatic no~circulatl~g state such that the ~lurry temperature causes the encapsulating material to release the accelerator which induces a rapid set of the cement.
In a further a~pec~ of th~s i~entlon ~here i~
provided a method o~ preserving the desirable short wa~t-on~
cemant time characteristic of an accelerated cement ~lurry in cementing application~, wharein e~ernal retarder-type contami~ation ls ant1cipated 9 lnvolving the specific improvement of adding to the cement slurry, prior to placemen~, a~
encapsula~ed accelerator having ~ coating which will 90f ten and melt at the temperatures characteriatic of the cement placement J placing said cement slurry ~ith said encapsulated accelerator and holding said enczp~ulated accelerator in place such that said coati~g softens and melts, releasing said accelerator which promotes a rapid set of ~aid cement slurry.

BRI F DESCRIPTION OF T~E DRAWINGS
FIGURE 1 o the drawing is a plot of ~he bottom-hole static temperature and a plo~ o~ the bottom-hole circulating temperature of a deep hot oil well in degrees Fahrenheit as a function of the depth expre~sed in fee~. The ~uperimposed broken lines indicate that our encapsulated accelerstor, havi~g a coating that melts a~ 195F, would have an anticipated useful range ~rom about 8000 to about 13,500 feee.
. . . ~ .
FIG~ 2 and FI6URE 3 of the drawing show co~parative behav-iors of a highly retarded cement slurry with and without a wax-coated CaC12 accelerator.
FIGURE 4 of the drawing illustratés behaviors o-E a highly retarded cement slurry with and wi~hou~ a wax/ethylene vinylacetate coated Na2SiO3 accelerator. ~ -. ~ . , .

36 ~ ~:
: :
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FIGURE 5 of t~e drawi~g illu~trates a tes~ apparatus for ~ :
repeatedly circulating a cement slurry through a cement pump and simu- :~
lated well casing annulus.
DESCRIPTION OF T~E PREEERRED EMBODIMENTS ;~
The highly retarded cement slurries of our i~ve~tion co~sist :~
~s~entially of Portland cem~t, particularly those of class A through H, :
ic combin~tion with a minor amount of a chemical retarder, such as calcil~ lignosulfonates, organic acids, boric acid, sodium pho~phates, potas~ium tartrate, and/or their mixtures. The API class J cemen~s, ~ith or without addition of chemisal retarder, are also considered equivalent for the purposes of our invention. Various other additives, used for specific purposes, such as agents to either increase or de-crease density, prevent lost circulation, and compensate for shri~kage, ~ ~ .
ca~ also be present. The a~ount of retarder added can best be defined ; ~ :
for purposes of this invention in terms of the desired pumpability time.
Thus, for cementing deep hot wells, it is desirable to add sufficient retarder to the ceme~t ~lurry such that the pu~pabili~y time (the time prior to ~he cement siurry~~~èxc:eedi~:~:g I00 API~units of con~istency, measured at ~emperatures characteristic of the particular well) will be of sufficient durat~on to allow placement of the slurry in ~he particular wellr Depending on the circumstances, ~uch as temperature, pressure, tepth, :
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~07023G
and flow rates, this pumpability time ca~ be as low as two hours or as high as 6 or more hours. In the case of the preferred class of re-tarders (additives containing the ligno~ulfonates), a highly retarded cement slurry can be achieved by the addition of from about 0.1 to about

2.5 weight percent retarder.
The cement accelerators of our invention are common additives, --well known in the art, including such accelerators as calcium chloride, sodium chloride, sodium silicate and their mixtures. The preferred accelerator is calcium chloride and/or anhydrous sodium me~asilicate.
~G The quantity of the accelerator to be used corresponds closely ~;
to the k~own pumpability time as a function of concentration a~d tem-perature. Thus, for the calcium chloride embodiment, good results are observed at fractional weight percents with a near flash set occurri~g at about 4 to about 15 percent calcium chloride, depending on particle size and how well it is dispersed i~ the slurry. Similarly, for the anhydrous sodiwm metasilicate embodiment, signiicant acceleration occurs at concentration greater than 2% and flash set occurs at 5~ ~ ~
~a2SiO3 even when the cement slurry was intentionally selected as a ~ ;
highly retarded slurry having practically unlimited thickening time (greater than 24 hours at 230F) in the absence of accelerator. Since, ~;
iQ our invention, the accelerator is encapsulated and is released shortly after placement, the only limitation on th~ maximum amount used is the physical limitation on how much accelerator can be suspended in the slurry.
The encapsulating materials useful in our invention include a variety of materials, all of which possess certain characteristic prop-erties. In particular, they are capable of forming a thin moisture-proof barrier when coated onto the cement accelerator. Additionally, the coating must soften and/or melt when subjected to the static temp-erature o~ the well being cemented, thus releasing the accelerator to ~(:17~1:Z36 flash set the cement slurry. Also, the coati~g ~ust remain sufficiently intact during the pumping and circulating pxocess such that no signif-icant increase in consistency of slurry is induced. Thus, certain , information regarding the casing to be cemented must be considered in order to select the proper encapsulated additive. As illustrated in FIGURE 1 of the drawing, by plotting both the BHST and the ~HCT for ~he well of interest as a function of depth on a single graph, the desired melting te~perature of the coating can be determlned. This is accom-plished by requiring that the entire depth range to be cemented fall between the two curves.
The preferred encapsulating materials of our i~vention are categorically organic waxes, including the waxlike resinous materials.
These include the naturally occurring waxe3 composed of organic esters, higher fatty acids and alcohols, hydrocarbons, and their mixtures, the paraffin waxes either isolated from or derived from petroleum, the synthetic waxes lncluding acrylic and vinyl polymers, pol~olefins, and fractionated polyolefins, as well as acrylic, olefinic or vinyl modified natural waxes and their ~ixtures. The petroleum-derived paraffin waxes are of particular utility either by themselves or in combination with other wa~ co~patible polymers and copolymers forming compositions com-monly recogniæed as hot melt adhesives and paper coatings. In particu-lar, a petroleum derived wax in combination with vinyl resins, such as copolymers of ethylene and vinylacetate, are suitable as meltable encap- ~ ;
sulating material e~hibiting excellent fitren8th and abrasion resistance as well as serving as a moisture barrier.
The coating or encapsulating process can be any of the well known methods, such as solvent deposition, spray coating, electrostatic coating, condensation, and the like, providing the method results in a ~ ~
uniform layer coverin~ the entire external surface of the accelerator. ~ ;

Preferably, the accelerator should be of particulate 5r granl~lar form - 8 - ~;

70Z3~

capable of pQ~Si~g ~hrough a 20~ to 40- mesh screen and retained on a 60-me~h screen. However, powder~ cap able of pa~sing through a 60-me~h scree~ are consldered equlvalent for purposes of thi~ invention. For ea e of coating, a spher~cal shaped particle i~ prefarred. Whe~
costing calcium chloride, because of its hygro~copic nature, dry conditio~s must be ma~ntai~edO The Wurster hot air encapsula~ing process ~s particularly useful.
Although our invention will find lts greatest utility in cementing deep hot oil well~ having B~ST of from about 260F to about 400F, the baslc proce~s is generally applicable to other types of well~ includi~g water wells, geothermal wells, and the like. Generally, all of the ad~antages of the proces~ will be real~zed in cementing sny well having a B~ST of 200P, or greater.
Thu~, as ~een in FIGURE I, this 200F B~ST will correspond to c~a~ing materials ha~lng melting point~ or softening poi~ts of appro~imately 125F, or greater, and be useful at depths from about 8,000 feet to about 13,500 feet or deeper. Yet, FIGURE l, further indicates that the dva~tages of our invention can also be achleved at a _~ _ . . .. ...... .. . . . . . . . . .. . .
B~ST as low as about 138F when?coatings meltable at about 98F are employed at a depth of about 4,000 feet. In the case of oil and gas wells, static temperatures as high as 450F have bee~ e~countered, whlle some geo-thermal wells have bee~ reported as high a3 7955F. In the case of these ultra hot well~, the coating materials having oorrespondingly higher m lti~g a~d sof tening poluts are selected. -'. ' ' .

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, . . . .
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~7~)2~6 ~a~ing thus descri~ed the details of ~ :~
the preferred embodiment5 the ~ollowlng examples plù8 controls are prese~ted a~ comparative data illu3trating the improved be.havior of our invention - ~
under co~rolled laboratory co~ditions, and a~ . :
~uch should ~ot be int~rpreted as being unduly llmiting.

~A~R~
Granular calcium chloride capable o ~ :
pa~ through a 30-mesh screen wa~ coated to the axtent of 10% by weight wlth a petroleum-deri~ed paraffinic ~ax ~Petrolite C~1035~, having a melting point of 195F. The Wurster hot air proces~ for coatlng material was employed. Careful testing determined that the coating did ha~e some holidays -~
(les~ than 100% encapsulation), which would Iead to some premature reaction of the calcium chloride.
795 grams of a class G Portland cement (sold a~
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1~0 ~36 Permenete G), 2.4 grams of a lignosulfonate retarder (sold as additive HR12), and 350 ml of water were placed in a Waring blender and mixed for 35 seconds. After the blending, 79.5 grams of the encapsulated CaC12 were stirred into the slurry by use of a spatula. The resulting cement slurry was tested according to the API RPlOB procedure in a ~hickening ~ -time tester. The recorded consistency, as a f~nction of time, is pre-sented in FIGURE 2 of the drawing. I~ a similar manner, a sample of the ce~ent slurry without the accelerator was tested and thP results are shown in FIGURE 3. E~ch test was performed by st~rti~g a~ 70F and 1~ ra~idly increasing the ~emperature to 200F under constant stirring.
The tempera~ure was maintained at the 200F level until the consistency of the slurry reached 100 API units, at which time the slurry was con-sidered no~ pumpable and the test terminated. As indicated in the data, the cement slurry without the accelerator has a normal thickening time of 4 hours at 200F. In contrast, the pumpability limit was reached in 77 minutes in the case of loZ encapsulated calcium chloride. Further-more, shortly after reaching the 200F temperature, the consistency appears to asymptotically approach infinity, indicating a flash set of the cement, thus establishing that an encapsulated accelerator would in fact shorten the WOC time after placement.
EXAMPLE II
Granulated anhydrous sodium metasilicate (~a2SiO3), capable of passing through a 20-mesh scxeen and being trapped on a 60-mesh screen, was uniformly coated with a mixture of 8 petroleum-derived paraffinic wax and ethylene vinylacetate copolymer by the Wurster coating process;
see U.S. Patents Nos. 2,648,609 and 2,799,241. The coating process involved the use of a coating solution made by dissolving 90 grams of a hard microcrystalline wax (Petrolite's Barecd~ C-1035 having a melting point of 199.5~F, ASTM D-127) and 60 grams of an ethylene vi~ylacetate copolymer (Du Pont's Elvax3 420 being approximately 17% to 19~ vinyl-acetate with a ~oftPning poi~t, rin8 and ball, of 210F, ASTN ~-28) in '''''`.

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each liter of trichloroethylene solvent. Solvent removal was accom- -plished by performing the coating proce~s at 140F t~ 150F, which resulted in a uniform layer of the wax/ethylene vinylacetate m~xture being deposited on the gran~les. This coating constituted 30% by weight of the resulting encapsulated accelerator and e~hibited a softening and melting point of lB8F to 190F.
A highly retarded API class H cement slurry was prepared using the encapsulated accelerator by irst dry blending 7.14 parts by weight (5 parts Na2SiO3~ coated accelerator and 1 part by weight HR-12 ligno-sulfonate retarder per 100 parts of dry class H (~onestar) ce~en~. To the resulting mixture was added 38 parts by weight water per 100 parts cement. To determine the effectiveness of the coated accelerator in the cement slurry~ three thickenin~ time tests (API R~-lOB, "Testing Oilwell Cements and Cement Additives," January 1974) were performed using ap-proximately 500 ml samples of the cement slurry. The tests were de-signed and carried out to simulate an API 10,000-foot casing schedule at a bot~om-hole circulating te~perature of 144F. The consistency of each -~
run is plotted as a fu~ction of time in FIGURE 4 of the drawi~g. The three slurries were maintained at 144F for approximately 1 hour, 5 hours, and 10 hours, respectively, at which times the temperatures were intentionally raised to 200F, indicated as X' on FIGURE 4, causing the coating to soften and melt. Upon reaching 200F, the sodium silicate was exposed to the slurries, cau~iug the cement to set in about 10 minutes. For comparison, a sample of the cement slurry less the sodium silicate accelerator was tested in an identical fashion with a temper-ature rise from 144F to 200F after approximately 5 hours with no detectable change in consistency. Even after 24 hours at 230F, the -~
control slurry failed to set, indicative of its highly retarded state.
Additional thickening time tests were conducted o~ the retarded slurry containing 5% coated accelerator at other temperatures. The results of these tests are summarized in the following Table I.

1~7~2;~6 TABLE I
ICKENING TIME TEST

Time Required to Temperature Achieve 100 Uc 173-175F 10 hours 40 minutes 180-182F 6 hours 185-187F 3 hours 30 minutes These data indicate that the coating appare~tly softens at temperature~ near 175F to 180~F making it susceptible to removal by physical abrasio~.
1~ In order to detenmine the durability of the coating under 3im- ,~
ulated field conditions~, three circulation tests involvi~g two ce~ent slurries containing the coated accelerator and one without accelerator were performed. The tests were conducted w in~ a~ apparatus illustrated in FIGURE S of the drawing. An API Class G ce~ent slurry containing 1 part of HR-12 retarder a~d 44 parts water per 100 parts by weight dry cement was used. 7.14 parts by weight of the above-described coated Na2SiO3 was added to tests 1 and 2. Each of the first ~wo æamples wa~
continuously circulated through the apparatus of FIGURE S until a con-sistency of 100 Uc was achieved. As lllustrated, a double-acti~g duplex ~ steam pump whi~h was equipped with poppet valves with steel-to-steel seats was used to pump the ce~ent slurry through a 20-foot long 1.5 inch pipe to the inlet of a simulated oilwell annulus involving an inner 1.5 inch pipe and an outer concentrically positioned 4 inch pipe, 5 feet lo~g. The entire aDnulus model was immersed in an oil bath which was m3intained at 144~. ~owever, the slurry te~perature during circulation . .
ran8ed from 97F to 122F. The cement slurry being tested flowed down the inner pipe o the annulus model and then back up the outer aDnulus ~ -formed by the concentrically placed outer pipe. The cement slurry was tken returned to the inlet of the pump for further recirculation of the same slurry. The third sample (no accelerator) used t~e apparatus of :, `.';' :.' - 13 - ~
. ~'. ' ~ 7~236 FIGURE 5 except no a~nulus model was present. In each case, the flow rate through the apparatus was held betwee~ 2 and 2.25 BPM with 15 p~i backpressure on the pump discharge. Table II presents the results of the three circulation tests.
TABLE II
PllMP CIRCUI ATION TEST
Coucentration of Coated Circulation Time No. of Passes Sample Accelerator, %~ to 100 U&, hours throu~h Pump 1 5 1.75 798 2 5 2.0 927

3 0 3.67 1672 *Net weight ~ of anhydrous sodium metasilicate It can be concluded from Table II that the coating on the i encapsulated accelerator is sufficiently durable to be used in a com~
~ercial cement job. This is particularly obvious in that a commercial ;
- placement of ceme~t in a well usually involves a single pass through a pump while the above test was much more severe. It should also be noted ~ ~ ;
that the accelerator-free sa~ple had a circulating time of 3.67 hours in this test but exhibits a thickening tLme (API RP-lOB) greater than 24 hours at 230F. Therefo~e, the accelerating effects illustrated in Table II are probably due to a combination of fsctors~ failure of some o the coating re~easing the accelerator, and (2) the shearing action of the pump directly on the slurry. ;-To iso1ate the efect of the pump shearing action from the failure of the coating, periodic samples of the slurry containi~g the coated accelerator were withdrawn from ~he pump suction tank during the ~;
circulation test. These sa~ples were subjected to the standard thicken~
ing time test on the at~ospheric pressure consistometer. Table III pre~

sents the thickening times remaining at 144F for the four samples withdrawn during the pump circulation test.
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~ 7~Z36 TABLs III

THICKE~ING TIMES ~URING PUMP CIRCULATION TEST
~. . .. .
Circulation Time Thickenin~ Time No. of Passesthrough Pump, to 100 Uc at through P~ minutes 144F, hours 1 10 ~s~c) 2S+
198 25 26+
397 52 ~6 The data suggest that no significant amount of accelerator has 1~ been exposed to the slurry, otherwise much shorter thickening times would have been observed. The accelerating effect noted in t~e pump circulation test can be attributed almost entirely to the severe shear-ing action of the pump. The durabiIity of the coated accelerator is sufficient for commercial applications.
To further evaluate the durability of the coating on the accelerator, a sample of the slurry after one pas~ through the pump circulation test was subjected to a long-term abrasion test. A cylinder 3 inches ID by 4.75 inches long was filled with 300 cc of slurry and rolled at 52 rpm for 48 hours at room temperature. At the e~d of this time, the sample was still a slurry of low consistency. During the 48 hours, the slurry had contacted 46,000 square feet of surface, which is equivalent to cbntacting 34,000 feet of 5.5 inch cssing.
In order to *est the shear bond strength developed between the set cement aud the well casing when ~ur encapsulated accelerator is used, the second sample employed in the pump circulation test was al-lowed to set up in the annulus between the two concentric pipes at the end of the second circulation test. In preparation for this me~sure-ment, the inner pipe o~ the annulus model was sandblasted prior to the pu p circulation test. At the end of the second pump circulation test, the aDnulus model was shut-in snd the temperature of the slurry in the .:

-~15 -'~

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annulus was raised to 230F and held there for 24 hours. The model was then allowed to cool five days, at which time it was cut into eight 6-inch sections. Each section was tested by holding the outer pipe stationary while applying an increasing force to the inner pipe until movement occurs. This force divided by the contact are between pipe and cement is termed the mechanical shear bond in psi and is recorded in -Table IV. The resulting range of 177 to 493 psi are considered excel-le~t values. The higher values at the bottom of the pipe are probably due to higher curing temperature at the bott~m. As illustrated in FIGURE 5, the 12-inch heaters are placed near the bottom while the -temperature was controlled with a ther~ocouple placed near the top of the oil bath.
T ~ LE IV
SHEAR BOND TESTS
Sample Shear Bond, psi 1 "Top" 177

4 307 `
381 `

8 "Bottom" 493 Average 326 In order to determine the WOC time associated with the accel-erated slurries, an API Class N cement slurry containing 1 part by weight HR-12 retarder and 38 parts by weight wat~r per 100 parts by weight dry cement was prepared and divided into three portions. To one portion was added 5 parts by weight uncoated Na2SiO3 per 100 parts dry cement. To another portion was added 7.14 parts by weight of the previously described ~-'` ' ' ' - - , . , , . ~ , , ~ , ~7~1Z;~ ~
coated Na2SiO3 per 100 parts by weight dry cement. The third portion was used with~ut any accelexator. A serieæ of tensile strength speci-were prepared using the various cement slurries. Each specimen was prepared by pouring the respective slurry into a mold shape much like a "dog bone," being 1 inch thick and 3 inches long with a l-inch cross-secti~al area. The molds containing the slurry were cuxed in a water bath which begsn at room temperature and was raised to 2~0F at 300~ psi in 4 hours (API RP-lOB) to simulate downhole conditions. After curing for 8, 24, and 4~ hours, respec~ive specimens were removed and subjected to increasing tension until ailure ~ccurred. The average ma~imum force for three specimens of each slurry is recorded in Table V as the tensile strength of the resulting cement.
TABLE V
STRENGTlI DEVEL(:)PMENT
_ _ _ Tensile Strength, PSi Age, 0% 5% Coated 5% Uncoated ~oursNa2Si03 Na2Si03 Na~Si03 _ 8 0 ~03 lgO

~ 294Not determined Not determined The slurry containing the coated accelera~or ha~ almost as much tensile strength in eight hours as the retarded slurry has in 40 hours. This represents a major improvement in shortening WOC time, yet this slurry retains the unlimited pumpability charscter of a retarded slurry withoct ~ccelerator.

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Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An oil-well cement slurry for cementing in a well containing an effective amount up to about 15 percent by weight of an encapsulated cement accelerator wherein the encapsulating material covers the entire external surface of the accelerator, isolating said cement accelerator from said oil-well cement slurry and wherein said encapsulating material is capable of melting at a temperature below the bottom-hole static temperature of an oil or gas well to be cemented but not capable of melting at a temperature corresponding to the bottom-hole circulating temperature of said well, whereby, after placement, the cement slurry is held in a static noncirculating state such that the slurry temperature causes the encapsulating material to release the accelerator which induces a rapid set of the cement.

2. A cement slurry of Claim 1 wherein said slurry is highly retarded.

3. A cement slurry of Claim 2 wherein said encapsulated accelerator is calcium chloride coated with an organic wax.

4. A cement slurry of Claim 2 wherein said encapsulated accelerator is anhydrous sodium metasilicate coated with an organic wax.

5. A cement slurry of Claim 4 wherein said organic wax is comprised of a petroleum-derived paraffinic wax and an ethylene vinylacetate copolymer.

6. A cement slurry comprising from about 2 parts by weight to about 15 parts by weight anhydrous sodium meta-silicate granules per 100 parts by weight dry cement, wherein said anhydrous sodium metasilicate granules are isolated from the slurry by an external coating of organic wax.