THE EFFECT of NEAT MAGNESIUM OXIDE (Mgo) AS EXPANDING ADDITIVE on CEMENT SHEAR BOND STRENGTH at HIGH TEMPERATURE up to 2500C

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

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

Key words : Thermal Cement was selected based on the result of previous research. Laboratory testing was performed in a modified pressure ABSTRACT curing chamber simulator. It was designed to operate at a temperature up to 3500C and pressure up to 3000 psi. Good bonding between cement and casing and The objective of this research is to determine the between cement and formation is essential for effective concentration usage and the quality of neat magnesium zone isolation. Poor bonding will create problems in oxide (MgO) as an expanding additive of cement for both production operations both for oil wells and geothermal oil and gas and geothermal wells. wells. Expanding additives, such as magnesium oxide This research was done by using API Class G (MgO) are used for improving primary cementing. Portland Cement HSR Type with silica flour (SiO2) 35 % Based on the results of previous research, there by weight of cement (BWOC) and neat magnesium oxide is a dangerous temperature transition at 1500C, because (MgO) with the concentrations varying from 0 to 20 % the lowest compressive strength as well as shear bond BWOC. Fresh water was used as the curing chamber strength of cement will be attained after 24 hours curing medium. The temperatures in the test vary from 100 to time. The presence of mud film on casing wall and mud 2500C with the pressure constant (1500 psi). The duration cake on formation will decrease shear bond strength, and, of the tests varies from 24 hours (1 day) to 7 days for the composition without expanding additive, it may (1 week). reach zero. However, by adding expanding additives, shear bond strength of cement will increase, because the improved bonding is the result of mechanical resistance or II. LITERATURE REVIEW tightening of the cement against the casing and formation. There is no chemical bonding. Cementing is an integral, necessary aspect of drilling both oil and gas wells and geothermal wells. Cement is used to secure the casing string and to isolate I. INTRODUCTION zones for production purposes as well as to solve various hole problems, such as vibration, and to protect the casing Bonding between cement and casing is affected from attack by corrosive fluids. by temperature changes which cause the casing to expand These compounds are formed in a kiln by a series and contract alternately. During the initial setting of the of reactions at temperatures as high as 15000C between cement slurry, the casing is in an expanded state because lime, silica, alumina and iron oxide. When hydrated, these of the heat generated from hydration in the cement. compounds make a major contribution to the strength of Subsequent internal temperature reduction, as the result of the cement. mud circulation, can cause the casing to contract and this In the manufacturing process selected raw may destroy the cement/pipe bond partially or wholly. materials are ground to a fine powder, and proportioned in Expanding additives can alleviate this problem such a way that the resulting mixture has the desired because they tend to expand after the initial set, resulting chemical composition. The raw material consists of two in the development of a stress condition in the cement that types of materials which are needed to prepare a mixture, helps to maintain bonding during pressure and i.e. “calcareous” materials which contain lime, and temperature changes. The overall result is greatly “argillaceous” materials which contain alumina, silica and improved zone isolation. Because the bonding between iron oxide. After blending, the raw material mixture is fed cement and formation determines its bonding quality, the into the kiln and converted to cement clinker. The clinker increased shear bond strength caused by an expanding is cooled, a small amount of gypsum (3% to 5%) is added, additive becomes necessary. and the mixture is pulverized. The pulverized product is In this research, neat magnesium oxide (MgO) finished Portland cement. with fineness 2000 - 3000 sqcm/gr. and burned up to The hydration of Portland cement is a sequence of 14000C was used as expanding additive2,5). This additive overlapping chemical reactions between clinker

2325 ARIS BUNTORO, RUDI RUBIANDINI R.S. components, calcium sulfate and water, leading to Portland cement systems because of aluminum substitution continuos cement slurry thickening and hardening. in the lattice structure (KALOUSEK and CHOW, 1976)4. 0 0 Although the hydration of C3S is often used as a model for At 480 F (250 C) the phase truscottite (C7S12H3) the hydration of Portland cement, it must be kept in mind begins to appear (LUKE and TAYLOR, 1984)4. As the that many additional parameters are involved. Portland curing temperature approaches 7500F (4000C), both cement is essentially a calcium silicate material, the most xonolite and truscottite are near their maximum stable abundant components being tricalcium silicate (C3S) and temperatures, and dehydration of the residual CH to C dicalcium silicate (C2S). Upon addition of water, both occurs. At higher temperatures the xonolite and truscottite hydrate to form gelatinous calcium silicate hydrate called dehydrate, resulting in the disintegration of the set cement. “C-S-H gel”, which is responsible for the strength and In addition to the compounds cited above, other dimensional stability of the set cement at ordinary phases such as pectolite (NC4S6H), scawtite (C7S6CH2), temperatures. In addition to C-S-H gel, a substantial reyerite (KC14S24H5), kilchoanite (C3S2H approximately), amount of calcium hydroxide (CH) is liberated. and calcio-chondrodite (C5S2H approximately) may appear C-S-H gel is the early hydration product even at in Portland cement system cured at elevated temperatures. elevated temperature and pressure, and is an excellent These phases can affect the performance of set cement, binding material at well temperatures less than about even when present in small quantities. 2300F (1100C). At higher temperature, C-S-H gel is Cements containing significant amounts of subject to metamorphosis, which usually results in truscotite are usually characterized by low permeability decreased permeability of the set cement. This (GALLUS et al., 1978)4. The formation of pectolite, a phenomenon, known as “strength retrogression”, was first sodium calcium silicate hydrate, is accompanied by reported by SWAYZE (1954)7 in the petroleum literature cement expansion (NELSON and EILERS, 1982)4; in because of the growing trend towards deep well addition, pectolite appears to render cements more completions. resistant to corrosion by highly saline brines (NELSON C-S-H gel often converts to the phase called and KALOUSEK, 1977; NELSON et al., 1981)4. Scawtite

“alpha dicalcium silicate hydrate (a-C2SH)”. a-C2SH is has been shown to enhance cement compressive strength highly crystalline and much more dense than C-S-H gel. when present in minor amounts (EILERS et al., 1983)3. In As a result, shrinkage occurs which is deleterious to the general, set cements which consist predominantly of integrity of the set cement. Significant loss of compressive calcium silicate hydrates with C/S ratios less than or equal strength occurred within one month; however, the level to to 1.0 tend to have higher compressive strengths and lower which strength falls is sufficient to support casing in a water permeabilities. well (SUMAN and ELLIS, 1977)4. The real problem lies Previous research2,5) concerning shear bond in the severe permeability increase. To prevent interzonal strength, which is the adhesion strength between cement communication, the water permeability of well cements and casing and between cement and formation, concluded should be no more than 0.1 md. Within one month, the that the failure of cement was caused by poor bonding. water permeabilities of the normal density Class G system Furthermore, they determined that the shear bond strength (1,2) were 10 to 100 times higher than the recommended minimum was 100 psi after 24 hours curing time. That limit. research was also done to understand the compressive The strength retrogression problem can be strength phenomenon at elevated temperatures by adding prevented by reducing the bulk lime-to-silica ratio (C/S silica flour (SiO2) in varying concentrations, and shear ratio) in the cement (MENZEL, 1935; KALOUSEK, 1952; bond strength phenomenon, which never increased at CARTER and SMITH, 1958)4. To accomplish this, the elevated temperatures by adding expanding additives Portland cement is partially replaced by ground quartz, cement, but they tend to expand after the initial set, usually as fine silica sand or silica flour. In some areas, resulting in the development of a stress condition that special cements are available where quartz has been helps to maintain the bonding during pressure and interground with Portland cement clinker temperatures changes. (ITALCEMENTI, 1977; BERRA et al., 1988)4. C-S-H gel DANJUSHEVSKY5 stated that increasing cement has a variable C/S ratio, averaging about 1.5. The volume means increasing the relative volume which is 0 0 conversion to a-C2SH at 230 F (110 C) can be prevented affected by the increasing size hardened cement. This by the addition of 35% to 40% silica (BWOC), reducing increase in volume is caused by : the C/S ratio to about 1.0. At this level, a mineral known as tobermorite (C5S6H5) is formed; fortunately, high ë Liquid condition Increase is caused by chemical strength and low permeability are preserved. As the curing contraction, the other hydrate products are formed temperature increases to about 3000F (1500C), tobermorite such as the formation of salt dissolved crystal at normally converts to xonotlite (C6S6H) and smaller elevated temperature. amounts of gyrolite (C6S3H2) with minimal deterioration. Tobermorite sometimes persists to 4820F (2500C) in

2326 THE EFFECT OF NEAT MAGNESIUM OXIDE (MgO) AS EXPANDING ADDITIVE ON CEMENT SHEAR BOND STRENGTH AT HIGH TEMPERATURE UP TO 2500C

ë In curing time condition Increase is caused by ë First mold, a cubic form with the size 2x 2x 2in. This expanding materials in cement matrix such as CaO, mold is used for standard API compressive strength MgO and CaSO4. test. ë After curing time condition Increase is caused by salt ë Second mold, a cylindrical form with inside diameter water in the formation, as the electrolyte charges. and height of 1 inch and 2 inches respectively. It is completed with caps both at the top and the bottom. In cementing operations, the second point above is an This mold is used to test shear bond strength between important condition in increasing cement shear bond cement and casing. strength. ë Third mold, a cylindrical core with the hole diameter DANJUSHEVSKY’s research, which was done 1 inch and height 1.5 inches. This mold consists of six by adding CaO and MgO where those materials are burned holes to place the cement slurry. The specimens from before use as expanding additive at certain temperature, this mold are used to test compressive strength, and shows that the expansion is more than 1%, even up to 15% cement permeability without casing . in particular conditions. 5 RUDI RUBIANDINI , in his research concerning These molds were designed to be placed into the simulator shear bond strength phenomenon, stated that by adding simultaneously before testing. CaO and MgO as expanding additives, with the burning 0 temperatures up to 1600 C and with certain fineness 3.3. Specimens Test which was tested up to 1500C, caused increased shear bond strength that varies from 250% to 1000%. He also This test was done by using API Class G Portland stated that the silica flour concentration less than 30% cement - HSR Type, with the following composition : BWOC did not improve compressive strength, because the C/S ratio was still more than 1.0. ò Water Solids Ratio (WSR) = 44% ò Silica Flour (SiO2) = 35% BWOC ò Neat Magnesium oxide (MgO) varies from 0 to 20% III. LABORATORY TEST BWOC

3.1. Simulator Preparation The medium used in the curing chamber was fresh water. The duration of the tests varied from 24 hours (1 day) to The apparatus for this research was a simulator 30 days (1 month). with a modified pressure curing chamber which can be operated at temperature up to 3500C with pressure up to 3.3.1. Calculation of Compressive Strength 3000 psi. The main parts of the simulator for conditioning cement slurry are : The specimens of cement are removed from the molds, then placed in a hydraulic press where increasing ë Cylindrical simulator, completed with heater and uniaxial pressure is exerted on each until failure. The thermocouple in the inside. compressive strength is then calculated from the pressure ë Maximator pump, used to increase pressure from the at which failure occurred referenced to the cross-sectional compressor up to 6500 psig. area of the specimen. The calculation of compressive ë Safety valve or rupture disk strength can be calculated by using the equation below : ë Reservoir liquid injection reserve  A1 š ë Temperature regulator CS = kP• (1) ë Flowmeter for injection gas • A2  ë Cooling drum where ; ë Manometer and valves to regulate flow distribution of CS = compressive strength, psi. liquid and gas. P = maximum load, psi. A1 = block bearing cross section area at the 3.2. Curing Molds hydraulics mortar, sqin. A2 = specimen cross section area, sqin. For testing both compressive strength and shear k = correction constant, is a function of the bond strength, three types of curing molds were used to height/diameter (t/d) ratio. place cement slurry. These are : 3.3.2. Calculation of Shear Bond Strength

2327 ARIS BUNTORO, RUDI RUBIANDINI R.S.

In this test, it is assumed that the casing is real the curve in Figure-2. However, the shear bond strength clean. The shear bond strength test is done in the same from 1000C to 1500C increases slightly, and also from manner as the compressive strength test, but with a 1500C to 2000C and from 2000C to 2500C. driving stick and cylindrical holding. The pressure at The effect of temperature on compressive strength which initial shear occurred is the maximum shear stress. for 24 hours curing time is illustrated in Figure-4. The Shear bond strength can be calculated by using the compressive strength significantly decreases at 1500C, equation below : increases sharply at 2000C, but sharply decreases from 2000C to 2500C. However, for 3 - 5% BWOC  A1  concentration neat magnesium oxide, the strength SBS = P• (2) •  increases significantly. The behavior of the curves in pdh Figure-5 (3 days curing time) is inverse to that in Figure- where ; 4, with the compressive strength significantly increasing at SBS = shear bond strength, psi. 1500C to 2000C, but decreasing at 2500C. However, for 3 - P = maximum shear stress, psi. 5% concentration neat magnesium oxide, values sharply A = block bearing area at the hydraulic increase at 1500C, but sharply decrease at 2000C, and mortar, sqin. slightly increase at 2500C. Figure-6 (7 days curing time) d = inside diameter of casing, in. shows a trend that is similar to Figure-4, but at 1500C the h = the height of specimen in cylindrical compressive strength decreases, and slightly increases at mold, in. 200 to 2500C. However, for 0% BWOC concentration MgO (by adding silica flour only), the compressive strength increases for temperatures of 100 to 2000C, but IV. RESULTS OF THE LABORATORY TESTS decreases at 2500C. AND DISCUSSION The trends in Figures-1, 2 , 3, 4, 5 and 6 indicate that the values of shear bond strength and compressive 4.1. Results of The Laboratory Tests strength significantly change at 1500C for all concentrations with 24 hours, 3 days, and 7 days curing The results of the laboratory tests can be seen in time. Thus, the temperature of 1500C is called a data plots for each specimen test. Thus, the concentration dangerous transition temperature. It agrees with the of neat magnesium oxide (MgO) which provides the best results of RUDI RUBIANDINI.5 shear bond strength can be selected. The trends in the curves of Figures-1, 2, 3, 4, 5 Figures-1, 2 and 3 depict the effect on shear bond and 6 can be evaluated to determine the best concentration strength of neat magnesium oxide (MgO) as an expanding 0 0 0 of neat magnesium oxide as an expanding additive in well additive at temperatures of 100 C, 150 C, 200 C and 0 0 cementing, especially at high temperature up to 250 C. 250 C; at constant pressure (1500 psi); with durations of Figure-1 illustrates that after 24 hours curing curing of 24 hours (1 day), 3 days and 7 days. Figures-4, 5 time the 3 - 5% BWOC concentration of neat magnesium and 6 depict the effect on compressive strength of oxide as an expanding additive provides the highest value magnesium oxide (MgO) as an expanding additive at 0 0 0 0 0 of shear bond strength, even though at 150 C the shear temperatures of 100 C, 150 C, 200 C and 250 C; at bond strength decreases sharply, but the minimum shear constant pressure (1500 psi); with durations of curing of bond value is more than 100 psi.4 In this case, the 24 hours (1 day), 3 days and 7 days. minimum shear bond strength value at 1500C is 628 psi. From Figure-2 it is seen that after 3 days curing time the 3 4.2. Effect of Temperature - 5% BWOC concentration of neat magnesium oxide also provides the highest shear bond strength. Figure-3 (after 7 Figure-1 (24 hours curing time) shows that the 0 days curing time) also shows the same trend. However, at shear bond strength significantly decreases to 150 C, 0 0 0 the transition temperature (150 C) the shear bond strength increases sharply at 200 C, then at 250 C increases values are less than those for the 10 - 20% BWOC MgO slightly. However, for the 20% BWOC concentration of concentration. the neat magnesium oxide, its bonding decreases from 0 0 The addition of 3 - 5% BWOC concentration of 200 C to 250 C. Figure-2 (3 days curing time) indicates neat magnesium oxide provides the highest value of the inverse trend to 24 hours curing time, with the shear 0 0 compressive strength at 250 C after 24 hours curing time, bond strength increasing sharply at 150 C, then increases 0 0 0 although at 150 C its value is slightly lower, but this value slightly at temperature of 200 C and 250 C. However, for is more than 1000 psi, i.e. 1153 psi, as seen in Figure-4. 3 and 20% BWOC concentration of the neat magnesium 0 Figure-5 also indicates that the 3 - 5% BWOC oxide, bonding decreases at temperatures of 150 C and concentration of neat magnesium oxide provides the best 2000C, then at 2500C increases slightly. Figure-3 (7 days 0 value of compressive strength. From Figure-6 it is seen curing time) shows that the curve at 150 C is similar to that after 7 days curing time the 3 - 5 % BWOC neat

2328 THE EFFECT OF NEAT MAGNESIUM OXIDE (MgO) AS EXPANDING ADDITIVE ON CEMENT SHEAR BOND STRENGTH AT HIGH TEMPERATURE UP TO 2500C magnesium oxide concentration gives fair compressive able to increase both shear bond strength and strength, although its value is less than for the 0 - 1.5% compressive strength after 24 hours curing time. BWOC MgO concentration. 2. The concentration of neat magnesium oxide as From these evaluations, it can be recommended expanding (MgO) additive from 3 to 5% BWOC will that the 3 - 5% BWOC concentration of neat magnesium provide excellent shear bond strength and fair oxide as expanding additive which is burnt up to 14000C compressive strength in geothermal and oil well provides the best shear bond strength and fair cements at high temperature of up to 2500C. compressive strength for geothermal and oil well 3. For a curing time of up to 3 days there is a significant cementing at high temperature of up to 2500C. effect on both shear bond strength and compressive strength, but after 7 days curing time, the increase in 4.3. Effect of Time both shear bond strength and compressive strength is less. Figures-7 and 8 depict the effect of time on shear 4. Addition of neat magnesium oxide (MgO) to cement bond strength and compressive strength respectively. slurries will decrease compressive strength and Figure-7 and 8 show that for curing time up to 3 days, increase shear bond strength. there is a significant effect of time on shear bond strength and compressive strength. After 7 days curing time there is a slightly lower rate of increase in shear bond strength REFERENCES and compressive strength . Two days of curing will yield high shear bond 1. ADAMS, N.J. : “Drilling Engineering, A complete strength and compressive strength, thus 2 days waiting on Well Planning Approach”, PennWell Publishing cement will improve safety on drilling forces for the next Company, Tulsa, Oklahoma, 1985. trajectory. 2. CARTER, et al : “Expanding Cement for Primary Cementing”, Petroleum Technology, 1966. 4.4. Effect of Neat Magnesium Oxide as Expanding 3. EILER, L.H., NELSON, E.B. AND MORAN, L.K. : Additive “High Temperature Cement Compositions- Pectolite, Scawtite, Truscottite, or Xonotlite : Addition of neat magnesium oxide as an Which Do You Want ?”, J. Petroleum Engineering, expanding additive will increase the CaO/SiO2 (C/S) ratio Vol. 35, pp. 1373-1377, July 1983. more than 1.0. The C/S ratio value significantly effects 4. NELSON, E.B., “Well Cementing”, Schlumberger the compressive strength at high temperature. Educational Services, Houston, Texas, 1990. Figures-4 to 6 indicate that there is a decrease of 5. RUDI RUBIANDINI R.S. : “Perubahan Sifat Fisik compressive strength value with the addition of neat Semen Pada Temperatur dan Tekanan Tinggi, magnesium oxide as an expanding additive into cement Serta Kandungan CO2 dan H2S Tinggi”, Jurnal slurry. In these figures it is seen that cement slurry without Teknologi Mineral, No. 1, Vol.II/1995. neat magnesium oxide (by adding silica flour only, 0% 6. RUDI RUBIANDINI R.S., R.-Ing. Ir. : “Teknik BWOC MgO) provides the highest compressive strength Pemboran II”, Jurusan Teknik Perminyakan, UPN values. However, Figure-1 to 3 illustrate the increase in “Veteran” Yogyakarta, 1993. shear bond strength with the addition of MgO as an 7. SWAYZE, M.A. : “Effect of High Pressure and expanding additive. Temperature on Strength of Oil-Well Cement”, International Petroleum Association, Oklahoma, April 1954. V. CONCLUSIONS 8...... : “Dowell Schlumberger Cementing Technology”, Published by Nova Communications We have reached the following conclusions from Ltd. London, England for Dowell Schlumberger, this laboratory study on the effect of neat magnesium 1984. oxide (MgO) as an expanding additive on cement shear 9...... : “Studi Penyemenan Sumur Panasbumi”, 0 bond strength at high temperature of up to 250 C : Laporan Akhir, Proyek Kerjasama PERTAMINA Div. Panasbumi dengan Lembaga Penelitian-ITB, 0 1. The temperature of 150 C is a dangerous transition Bandung, 1994. temperature, because there is no specific composition

2329 ARIS BUNTORO, RUDI RUBIANDINI R.S.