the effect of neat magnesium oxide (mgo) as …

THE EFFECT OF NEAT MAGNESIUM OXIDE (MgO) AS EXPANDING ADDITIVE ON CEMENT SHEAR BONDSTRENGTH AT HIGH TEMPERATURE UP TO 2500C

Aris Buntoro, UPN “Veteran” Yogyakarta; Rudi Rubiandini R.S., Bandung Institute of Technology

Key words : Thermal Cement

ABSTRACT

Good bonding between cement and casing andbetween cement and formation is essential for effectivezone isolation. Poor bonding will create problems inproduction operations both for oil wells and geothermalwells. Expanding additives, such as magnesium oxide(MgO) are used for improving primary cementing.

Based on the results of previous research, thereis a dangerous temperature transition at 1500C, becausethe lowest compressive strength as well as shear bondstrength of cement will be attained after 24 hours curingtime. The presence of mud film on casing wall and mudcake on formation will decrease shear bond strength, and,for the composition without expanding additive, it mayreach zero. However, by adding expanding additives,shear bond strength of cement will increase, because theimproved bonding is the result of mechanical resistance ortightening of the cement against the casing and formation.There is no chemical bonding.

I. INTRODUCTION

Bonding between cement and casing is affectedby temperature changes which cause the casing to expandand contract alternately. During the initial setting of thecement slurry, the casing is in an expanded state becauseof the heat generated from hydration in the cement.Subsequent internal temperature reduction, as the result ofmud circulation, can cause the casing to contract and thismay destroy the cement/pipe bond partially or wholly.

Expanding additives can alleviate this problembecause they tend to expand after the initial set, resultingin the development of a stress condition in the cement thathelps to maintain bonding during pressure andtemperature changes. The overall result is greatlyimproved zone isolation. Because the bonding betweencement and formation determines its bonding quality, theincreased shear bond strength caused by an expandingadditive becomes necessary.

In this research, neat magnesium oxide (MgO)with fineness 2000 - 3000 sqcm/gr. and burned up to14000C was used as expanding additive2,5). This additive

was selected based on the result of previous research.Laboratory testing was performed in a modified pressurecuring chamber simulator. It was designed to operate at atemperature up to 3500C and pressure up to 3000 psi.

The objective of this research is to determine theconcentration usage and the quality of neat magnesiumoxide (MgO) as an expanding additive of cement for bothoil and gas and geothermal wells.

This research was done by using API Class GPortland Cement HSR Type with silica flour (SiO2) 35 %by weight of cement (BWOC) and neat magnesium oxide(MgO) with the concentrations varying from 0 to 20 %BWOC. Fresh water was used as the curing chambermedium. The temperatures in the test vary from 100 to2500C with the pressure constant (1500 psi). The durationof the tests varies from 24 hours (1 day) to 7 days(1 week).

II. LITERATURE REVIEW

Cementing is an integral, necessary aspect ofdrilling both oil and gas wells and geothermal wells.Cement is used to secure the casing string and to isolatezones for production purposes as well as to solve varioushole problems, such as vibration, and to protect the casingfrom attack by corrosive fluids.

These compounds are formed in a kiln by a seriesof reactions at temperatures as high as 15000C betweenlime, silica, alumina and iron oxide. When hydrated, thesecompounds make a major contribution to the strength ofthe cement.

In the manufacturing process selected rawmaterials are ground to a fine powder, and proportioned insuch a way that the resulting mixture has the desiredchemical composition. The raw material consists of twotypes of materials which are needed to prepare a mixture,i.e. “calcareous” materials which contain lime, and“argillaceous” materials which contain alumina, silica andiron oxide. After blending, the raw material mixture is fedinto the kiln and converted to cement clinker. The clinkeris cooled, a small amount of gypsum (3% to 5%) is added,and the mixture is pulverized. The pulverized product isfinished Portland cement.

The hydration of Portland cement is a sequence ofoverlapping chemical reactions between clinker

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ARIS BUNTORO, RUDI RUBIANDINI R.S.

components, calcium sulfate and water, leading tocontinuos cement slurry thickening and hardening.Although the hydration of C3S is often used as a model forthe hydration of Portland cement, it must be kept in mindthat many additional parameters are involved. Portlandcement is essentially a calcium silicate material, the mostabundant components being tricalcium silicate (C3S) anddicalcium silicate (C2S). Upon addition of water, bothhydrate to form gelatinous calcium silicate hydrate called“C-S-H gel”, which is responsible for the strength anddimensional stability of the set cement at ordinarytemperatures. In addition to C-S-H gel, a substantialamount of calcium hydroxide (CH) is liberated.

C-S-H gel is the early hydration product even atelevated temperature and pressure, and is an excellentbinding material at well temperatures less than about2300F (1100C). At higher temperature, C-S-H gel issubject to metamorphosis, which usually results indecreased permeability of the set cement. Thisphenomenon, known as “strength retrogression”, was firstreported by SWAYZE (1954)7 in the petroleum literaturebecause of the growing trend towards deep wellcompletions.

C-S-H gel often converts to the phase called“alpha dicalcium silicate hydrate (α-C2SH)”. α-C2SH ishighly crystalline and much more dense than C-S-H gel.As a result, shrinkage occurs which is deleterious to theintegrity of the set cement. Significant loss of compressivestrength occurred within one month; however, the level towhich strength falls is sufficient to support casing in awell (SUMAN and ELLIS, 1977)4. The real problem liesin the severe permeability increase. To prevent interzonalcommunication, the water permeability of well cementsshould be no more than 0.1 md. Within one month, thewater permeabilities of the normal density Class G system(1,2) were 10 to 100 times higher than the recommendedlimit.

The strength retrogression problem can beprevented by reducing the bulk lime-to-silica ratio (C/Sratio) in the cement (MENZEL, 1935; KALOUSEK, 1952;CARTER and SMITH, 1958)4. To accomplish this, thePortland cement is partially replaced by ground quartz,usually as fine silica sand or silica flour. In some areas,special cements are available where quartz has beeninterground with Portland cement clinker(ITALCEMENTI, 1977; BERRA et al., 1988)4. C-S-H gelhas a variable C/S ratio, averaging about 1.5. Theconversion to α-C2SH at 2300F (1100C) can be preventedby the addition of 35% to 40% silica (BWOC), reducingthe C/S ratio to about 1.0. At this level, a mineral knownas tobermorite (C5S6H5) is formed; fortunately, highstrength and low permeability are preserved. As the curingtemperature increases to about 3000F (1500C), tobermoritenormally converts to xonotlite (C6S6H) and smalleramounts of gyrolite (C6S3H2) with minimal deterioration.Tobermorite sometimes persists to 4820F (2500C) in

Portland cement systems because of aluminum substitutionin the lattice structure (KALOUSEK and CHOW, 1976)4.

At 4800F (2500C) the phase truscottite (C7S12H3)begins to appear (LUKE and TAYLOR, 1984)4. As thecuring temperature approaches 7500F (4000C), bothxonolite and truscottite are near their maximum stabletemperatures, and dehydration of the residual CH to Coccurs. At higher temperatures the xonolite and truscottitedehydrate, resulting in the disintegration of the set cement.

In addition to the compounds cited above, otherphases such as pectolite (NC4S6H), scawtite (C7S6CH2),reyerite (KC14S24H5), kilchoanite (C3S2H approximately),and calcio-chondrodite (C5S2H approximately) may appearin Portland cement system cured at elevated temperatures.These phases can affect the performance of set cement,even when present in small quantities.

Cements containing significant amounts oftruscotite are usually characterized by low permeability(GALLUS et al., 1978)4. The formation of pectolite, asodium calcium silicate hydrate, is accompanied bycement expansion (NELSON and EILERS, 1982)4; inaddition, pectolite appears to render cements moreresistant to corrosion by highly saline brines (NELSONand KALOUSEK, 1977; NELSON et al., 1981)4. Scawtitehas been shown to enhance cement compressive strengthwhen present in minor amounts (EILERS et al., 1983)3. Ingeneral, set cements which consist predominantly ofcalcium silicate hydrates with C/S ratios less than or equalto 1.0 tend to have higher compressive strengths and lowerwater permeabilities.

Previous research2,5) concerning shear bondstrength, which is the adhesion strength between cementand casing and between cement and formation, concludedthat the failure of cement was caused by poor bonding.Furthermore, they determined that the shear bond strengthminimum was 100 psi after 24 hours curing time. Thatresearch was also done to understand the compressivestrength phenomenon at elevated temperatures by addingsilica flour (SiO2) in varying concentrations, and shearbond strength phenomenon, which never increased atelevated temperatures by adding expanding additivescement, but they tend to expand after the initial set,resulting in the development of a stress condition thathelps to maintain the bonding during pressure andtemperatures changes.

DANJUSHEVSKY5 stated that increasing cementvolume means increasing the relative volume which isaffected by the increasing size hardened cement. Thisincrease in volume is caused by :

ë Liquid condition Increase is caused by chemicalcontraction, the other hydrate products are formedsuch as the formation of salt dissolved crystal atelevated temperature.

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THE EFFECT OF NEAT MAGNESIUM OXIDE (MgO) AS EXPANDING ADDITIVE ON CEMENT SHEAR BOND STRENGTH AT HIGHTEMPERATURE UP TO 2500C

ë In curing time condition Increase is caused byexpanding materials in cement matrix such as CaO,MgO and CaSO4.

ë After curing time condition Increase is caused by saltwater in the formation, as the electrolyte charges.

In cementing operations, the second point above is animportant condition in increasing cement shear bondstrength. DANJUSHEVSKY’s research, which was doneby adding CaO and MgO where those materials are burnedbefore use as expanding additive at certain temperature,shows that the expansion is more than 1%, even up to 15%in particular conditions. RUDI RUBIANDINI5, in his research concerningshear bond strength phenomenon, stated that by addingCaO and MgO as expanding additives, with the burningtemperatures up to 16000C and with certain finenesswhich was tested up to 1500C, caused increased shearbond strength that varies from 250% to 1000%. He alsostated that the silica flour concentration less than 30%BWOC did not improve compressive strength, because theC/S ratio was still more than 1.0. III. LABORATORY TEST 3.1. Simulator Preparation The apparatus for this research was a simulatorwith a modified pressure curing chamber which can beoperated at temperature up to 3500C with pressure up to3000 psi. The main parts of the simulator for conditioningcement slurry are : ë Cylindrical simulator, completed with heater and

thermocouple in the inside.ë Maximator pump, used to increase pressure from the

compressor up to 6500 psig.ë Safety valve or rupture diskë Reservoir liquid injection reserveë Temperature regulatorë Flowmeter for injection gasë Cooling drumë Manometer and valves to regulate flow distribution of

liquid and gas. 3.2. Curing Molds For testing both compressive strength and shearbond strength, three types of curing molds were used toplace cement slurry. These are :

ë First mold, a cubic form with the size 2x 2x 2in. Thismold is used for standard API compressive strengthtest.

ë Second mold, a cylindrical form with inside diameterand height of 1 inch and 2 inches respectively. It iscompleted with caps both at the top and the bottom.This mold is used to test shear bond strength betweencement and casing.

ë Third mold, a cylindrical core with the hole diameter1 inch and height 1.5 inches. This mold consists of sixholes to place the cement slurry. The specimens fromthis mold are used to test compressive strength, andcement permeability without casing .

These molds were designed to be placed into the simulatorsimultaneously before testing. 3.3. Specimens Test This test was done by using API Class G Portlandcement - HSR Type, with the following composition : ò Water Solids Ratio (WSR) = 44%ò Silica Flour (SiO2) = 35% BWOCò Neat Magnesium oxide (MgO) varies from 0 to 20%

BWOC

The medium used in the curing chamber was fresh water.The duration of the tests varied from 24 hours (1 day) to30 days (1 month).

3.3.1. Calculation of Compressive Strength

The specimens of cement are removed from themolds, then placed in a hydraulic press where increasinguniaxial pressure is exerted on each until failure. Thecompressive strength is then calculated from the pressureat which failure occurred referenced to the cross-sectionalarea of the specimen. The calculation of compressivestrength can be calculated by using the equation below :

CS kPAA

=��

�1

2 (1)

where ;CS = compressive strength, psi.P = maximum load, psi.A1 = block bearing cross section area at the

hydraulics mortar, sqin.A2 = specimen cross section area, sqin.k = correction constant, is a function of the

height/diameter (t/d) ratio.

3.3.2. Calculation of Shear Bond Strength

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In this test, it is assumed that the casing is realclean. The shear bond strength test is done in the samemanner as the compressive strength test, but with adriving stick and cylindrical holding. The pressure atwhich initial shear occurred is the maximum shear stress.Shear bond strength can be calculated by using theequation below :

SBS PAdh

=��

1

π (2)

where ;SBS = shear bond strength, psi.P = maximum shear stress, psi.A = block bearing area at the hydraulic

mortar, sqin.d = inside diameter of casing, in.h = the height of specimen in cylindrical

mold, in.

IV. RESULTS OF THE LABORATORY TESTS AND DISCUSSION

4.1. Results of The Laboratory Tests

The results of the laboratory tests can be seen indata plots for each specimen test. Thus, the concentrationof neat magnesium oxide (MgO) which provides the bestshear bond strength can be selected.

Figures-1, 2 and 3 depict the effect on shear bondstrength of neat magnesium oxide (MgO) as an expandingadditive at temperatures of 1000C, 1500C, 2000C and2500C; at constant pressure (1500 psi); with durations ofcuring of 24 hours (1 day), 3 days and 7 days. Figures-4, 5and 6 depict the effect on compressive strength ofmagnesium oxide (MgO) as an expanding additive attemperatures of 1000C, 1500C, 2000C and 2500C; atconstant pressure (1500 psi); with durations of curing of24 hours (1 day), 3 days and 7 days.

4.2. Effect of Temperature

Figure-1 (24 hours curing time) shows that theshear bond strength significantly decreases to 1500C,increases sharply at 2000C, then at 2500C increasesslightly. However, for the 20% BWOC concentration ofthe neat magnesium oxide, its bonding decreases from2000C to 2500C. Figure-2 (3 days curing time) indicatesthe inverse trend to 24 hours curing time, with the shearbond strength increasing sharply at 1500C, then increasesslightly at temperature of 2000C and 2500C. However, for3 and 20% BWOC concentration of the neat magnesiumoxide, bonding decreases at temperatures of 1500C and2000C, then at 2500C increases slightly. Figure-3 (7 dayscuring time) shows that the curve at 1500C is similar to

the curve in Figure-2. However, the shear bond strengthfrom 1000C to 1500C increases slightly, and also from1500C to 2000C and from 2000C to 2500C.

The effect of temperature on compressive strengthfor 24 hours curing time is illustrated in Figure-4. Thecompressive strength significantly decreases at 1500C,increases sharply at 2000C, but sharply decreases from2000C to 2500C. However, for 3 - 5% BWOCconcentration neat magnesium oxide, the strengthincreases significantly. The behavior of the curves inFigure-5 (3 days curing time) is inverse to that in Figure-4, with the compressive strength significantly increasing at1500C to 2000C, but decreasing at 2500C. However, for 3 -5% concentration neat magnesium oxide, values sharplyincrease at 1500C, but sharply decrease at 2000C, andslightly increase at 2500C. Figure-6 (7 days curing time)shows a trend that is similar to Figure-4, but at 1500C thecompressive strength decreases, and slightly increases at200 to 2500C. However, for 0% BWOC concentrationMgO (by adding silica flour only), the compressivestrength increases for temperatures of 100 to 2000C, butdecreases at 2500C.

The trends in Figures-1, 2 , 3, 4, 5 and 6 indicatethat the values of shear bond strength and compressivestrength significantly change at 1500C for allconcentrations with 24 hours, 3 days, and 7 days curingtime. Thus, the temperature of 1500C is called adangerous transition temperature. It agrees with theresults of RUDI RUBIANDINI.5

The trends in the curves of Figures-1, 2, 3, 4, 5and 6 can be evaluated to determine the best concentrationof neat magnesium oxide as an expanding additive in wellcementing, especially at high temperature up to 2500C.

Figure-1 illustrates that after 24 hours curingtime the 3 - 5% BWOC concentration of neat magnesiumoxide as an expanding additive provides the highest valueof shear bond strength, even though at 1500C the shearbond strength decreases sharply, but the minimum shearbond value is more than 100 psi.4 In this case, theminimum shear bond strength value at 1500C is 628 psi.From Figure-2 it is seen that after 3 days curing time the 3- 5% BWOC concentration of neat magnesium oxide alsoprovides the highest shear bond strength. Figure-3 (after 7days curing time) also shows the same trend. However, atthe transition temperature (1500C) the shear bond strengthvalues are less than those for the 10 - 20% BWOC MgOconcentration.

The addition of 3 - 5% BWOC concentration ofneat magnesium oxide provides the highest value ofcompressive strength at 2500C after 24 hours curing time,although at 1500C its value is slightly lower, but this valueis more than 1000 psi, i.e. 1153 psi, as seen in Figure-4.Figure-5 also indicates that the 3 - 5% BWOCconcentration of neat magnesium oxide provides the bestvalue of compressive strength. From Figure-6 it is seenthat after 7 days curing time the 3 - 5 % BWOC neat

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magnesium oxide concentration gives fair compressivestrength, although its value is less than for the 0 - 1.5%BWOC MgO concentration.

From these evaluations, it can be recommendedthat the 3 - 5% BWOC concentration of neat magnesiumoxide as expanding additive which is burnt up to 14000Cprovides the best shear bond strength and faircompressive strength for geothermal and oil wellcementing at high temperature of up to 2500C.

4.3. Effect of Time

Figures-7 and 8 depict the effect of time on shearbond strength and compressive strength respectively.Figure-7 and 8 show that for curing time up to 3 days,there is a significant effect of time on shear bond strengthand compressive strength. After 7 days curing time thereis a slightly lower rate of increase in shear bond strengthand compressive strength .

Two days of curing will yield high shear bondstrength and compressive strength, thus 2 days waiting oncement will improve safety on drilling forces for the nexttrajectory.

4.4. Effect of Neat Magnesium Oxide as Expanding Additive

Addition of neat magnesium oxide as anexpanding additive will increase the CaO/SiO2 (C/S) ratiomore than 1.0. The C/S ratio value significantly effectsthe compressive strength at high temperature.

Figures-4 to 6 indicate that there is a decrease ofcompressive strength value with the addition of neatmagnesium oxide as an expanding additive into cementslurry. In these figures it is seen that cement slurry withoutneat magnesium oxide (by adding silica flour only, 0%BWOC MgO) provides the highest compressive strengthvalues. However, Figure-1 to 3 illustrate the increase inshear bond strength with the addition of MgO as anexpanding additive.

V. CONCLUSIONS

We have reached the following conclusions fromthis laboratory study on the effect of neat magnesiumoxide (MgO) as an expanding additive on cement shearbond strength at high temperature of up to 2500C :

1. The temperature of 1500C is a dangerous transitiontemperature, because there is no specific composition

able to increase both shear bond strength andcompressive strength after 24 hours curing time.

2. The concentration of neat magnesium oxide asexpanding (MgO) additive from 3 to 5% BWOC willprovide excellent shear bond strength and faircompressive strength in geothermal and oil wellcements at high temperature of up to 2500C.

3. For a curing time of up to 3 days there is a significanteffect on both shear bond strength and compressivestrength, but after 7 days curing time, the increase inboth shear bond strength and compressive strength isless.

4. Addition of neat magnesium oxide (MgO) to cementslurries will decrease compressive strength andincrease shear bond strength.

REFERENCES

1. ADAMS, N.J. : “Drilling Engineering, A completeWell Planning Approach”, PennWell PublishingCompany, Tulsa, Oklahoma, 1985.

2. CARTER, et al : “Expanding Cement for PrimaryCementing”, Petroleum Technology, 1966.

3. EILER, L.H., NELSON, E.B. AND MORAN, L.K. :“High Temperature Cement Compositions-Pectolite, Scawtite, Truscottite, or Xonotlite :Which Do You Want ?”, J. Petroleum Engineering,Vol. 35, pp. 1373-1377, July 1983.

4. NELSON, E.B., “Well Cementing”, SchlumbergerEducational Services, Houston, Texas, 1990.

5. RUDI RUBIANDINI R.S. : “Perubahan Sifat FisikSemen Pada Temperatur dan Tekanan Tinggi,Serta Kandungan CO2 dan H2S Tinggi”, JurnalTeknologi Mineral, No. 1, Vol.II/1995.

6. RUDI RUBIANDINI R.S., R.-Ing. Ir. : “TeknikPemboran II”, Jurusan Teknik Perminyakan, UPN“Veteran” Yogyakarta, 1993.

7. SWAYZE, M.A. : “Effect of High Pressure andTemperature on Strength of Oil-Well Cement”,International Petroleum Association, Oklahoma, April1954.

8. .......... : “Dowell Schlumberger CementingTechnology”, Published by Nova CommunicationsLtd. London, England for Dowell Schlumberger,1984.

9. .......... : “Studi Penyemenan Sumur Panasbumi”,Laporan Akhir, Proyek Kerjasama PERTAMINA Div.Panasbumi dengan Lembaga Penelitian-ITB,Bandung, 1994.

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Neat cement 0 % 1 .5 % 3 %

5 % 1 0 % 2 0 %

Figure-1Effect of temperature on the shear bond strength of API Class G Portlandcement containing silica flour (SiO2) 35 %BWOC and various amount of neat MgO (0 - 20%BWOC) after 24 hours curing time at 1500 psi

Neat cement 0 % 1 .5 % 3 % 5 %

1 0 % 2 0 %

Figure-2Effect of temperature on the shear bond strength of API Class G Portlandcement containing silica flour (SiO2) 35 %BWOC and various amount

of neat MgO (0 - 20%BWOC) after 3 days curing time at 1500 psi

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Neat cement 0 % 1 .5 % 3 %

5 % 1 0 % 2 0 %

Figure-3Effect of temperature on the shear bond strength of API Class G Portlandcement containing silica flour (SiO2) 35 %BWOC and various amount

of neat MgO (0 - 20%BWOC) after 7 days curing time at 1500 psi

Neat cement 0 % 1 .5 % 3 %

5 % 1 0 % 2 0 %

Figure-4Effect of temperature on the compressive strength of API Class G Portland

cement containing silica flour (SiO2) 35 %BWOC and various amount of neat MgO (0 - 20%BWOC) after 24 hours curing time at 1500 psi

2331


Neat cement 0 % 1 .5 % 3 %

5 % 1 0 % 2 0 %


cement containing silica flour (SiO2) 35 %BWOC and various amount of neat MgO (0 - 20%BWOC) after 3 days curing time at 1500 psi

Neat cement 0 % 1 .5 % 3 %

5 % 1 0 % 2 0 %


cement containing silica flour (SiO2) 35 %BWOC and various amount of neat MgO (0 - 20%BWOC) after 7 days curing time at 1500 psi

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1 0 0 C

1 5 0 C

2 0 0 C

2 5 0 C

Figure-7The effect of time on shear bond strength behavior of Portland cement system containing

silica flour 35% and 3% BWOC of neat magnesite at various temperature

1 0 0 C

1 5 0 C

2 0 0 C

2 5 0 C

Figure-8The effect of time on compressive strength behavior of Portland cement system containing

silica flour 35% and 3% BWOC of neat magnesite at various temperature

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the effect of neat magnesium oxide (mgo) as …

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