(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:9)(cid:9)(cid:8)(cid:10)(cid:3)(cid:11)(cid:5)(cid:12)(cid:8)(cid:9)(cid:11)(cid:5)(cid:3)(cid:2)(cid:13)(cid:5)(cid:14)(cid:5)(cid:15)(cid:16)(cid:17)(cid:18)(cid:5)(cid:19)(cid:20)(cid:5)(cid:14)(cid:5)(cid:10)(cid:16)(cid:18)(cid:5)(cid:21)(cid:5)(cid:14)(cid:5)(cid:21)(cid:22)(cid:23)(cid:24) http://dx.doi.org/10.7494/drill.2017.34.2.335 Marcin Kremieniewski*, Stanisław Stryczek**, Rafał Wiśniowski**, Marcin Rzepka*, Andrzej Gonet** INFLUENCE OF BENTONITE ADDITION ON PARAMETERS OF FRESH AND HARDENED CEMENT SLURRY*** 1. INTRODUCTION Sealing slurries and the resulting hardened slurries used for sealing the casing should meet respective standards. The parameters of fresh and resulting hardened cement sheath are modified with a variety of additives and admixtures. Last years have seen a growing interest in the realization of directional and horizontal wells. The parameters of slurries for this type of wellbores should be selected more strictly than in the case of vertical wells [1, 2, 18]. It should be stressed that the properties of each cement slurry mainly depends on the geologic-technical conditions in a well, where the slurry is bonding and the hardened cement sheath is formed. For this reason the influence of these additives and admixtures should be determined, not only on selected (modified) properties of the cement slurry, but also a complex analysis of all parameters in view of the applied additive. The analysis of particular technological parameters of cement slurry is very important because a tan early stage one can detect possible disturbances in the parameters of the designed composition of cement slurry [17]. It is very important because the possible complications are connected * Oil and Gas Institute – National Research Institute ** AGH University of Science and Technology *** Paper realized on the basis of the research work Analysis of possibility of improving the sedi- mentation stability of cement slurries – work of OGI – NRI on the order of the Ministry of Science and Higher Education, archival No.: DK-4100/36/16, order No.: 36/KW/16. Work realized at the Department of Drilling and Geoengineering, Faculty of Drilling, Oil and Gas AGH UST within the Statutory Research Program No. 11.11.190.555 WP2 335 with huge difficulties in the liquidation of, e.g. exhalations or outflows of reservoir medium from the space between and beyond pipes. What is more, the cost of cementing up the wellbore is very high. For this reason the detailed analysis of impact of a single additive should not be omitted in elaborations and scientific publications. 2. ROLE OF BENTONITE IN CEMENT SLURRIES Owing to the varying geological-technical conditions in the wellbore, appropriate additives have to be involved to produce a slurry which would have properties adjusted to the casing cementing project. One of such additives is bentonite, commonly known as montmorillonite [14]. This additive is presently applied in the cement slurry technology because of its ability to regulate numerous parameters. It is fine grained thanks to which can penetrate the filtration cake between the cement particles and eventually lower the filtration of the slurry. Clays and bentonites are composites of made of components build- ing one or more clayey materials. The assume the form of hydrated aluminum silicate of filitosilicate group [6, 11]. The most popular filler of this group of materials is bentonite, which contains at least 85(cid:2) of mineral clays commonly called montmorillonite. Bentonite may expand its original volume several times, when mixed with water. The elastic vis- cosity, yield point and structural strength responsible for maintaining the solid phase, considerably increase. For this reason the use of bentonite is very favorable from the point of view of sedimentation stability of the slurry [3, 4, 5, 7]. Because of the ability to absorb large quantities of water this additive causes homogenization of cement slurry and so prevents delamination of fractions of various weight. This is very important for designing slurries used for sealing of directional and horizontal wells, where the sedimentation stability is particularly important. Owing to the very dense structure of working water, bentonite is also used for designing lower density slurries. The presence of lower density additives, e.g. microsphere may cause delamination of slurry as a result of which light microsphere fractions will float on the surface of the prepared slurry. Bentonite addition to work water increases the rheological parameters of the cement slurry thanks to which the liquid slurry becomes homogeneous and the cement sheath obtains a uniform structure after the slurry hardens [9, 12, 13]. Bentonite is also used as a filler thanks to which smaller quantities of cement can be applied. Recipes of this type can be used for obtaining a very light slurry of high water/cement ratio [8, 11]. Another design condition for this type of slurries is the cost factor. In this case the physicochemical conditions are ignored and they get poor because of the increased water/cement ratio. When designing a slurry (depending 336 on the required properties) up to 20(cid:2) (bwoc1) of bentonite is added. The analysis of the literature [11, 15, 16] reveals that the concentrations above 6(cid:2) may necessitate using of a plastifier to lower the viscosity and structural strength. According to API2 standard, the working water should be increased in the slurry by 5.3(cid:2) per each 1(cid:2) (bwoc) of bento- nite for all cement classes. The increase of bentonite addition in the cement slurry results in the lowering of slurry density with the simultaneous increase of rheological parame- ters and weakening of mechanical parameters [11, 13]. The influence of bentonite on the compression of hardened cement slurry is presented in Figure 1. According to the obtained data [10, 11] the use of bentonite in cement slurry may result in lower permeability which is not favorable in a situation of contact with sulfate and corrosive waters. Fig. 1. Influence of bentonite addition on compressive strength of exemplary slurry Literature abounds in partial results of bentonite impact on the parameters of slurry; besides this is a (simple) slurry containing only cement and water. Therefore an analysis was performed to see the impact of bentonite on parameters of slurries present- ly used in industry and which contain a few to ten or so various modifiers, as presented further in this paper. 1 Bwoc – by weight of cement 2 API – American Petroleum Institute 337 3. INVESTIGATIONS Analyses aimed at determining the influence of bentonite on the parameters of fresh cement slurry were performed at the Laboratory of Sealing Slurries OGI-NRI in line with standards: PN-85/G-02320 Cements and sealing slurries for cementing in wellbores; PN-EN 10426-2 Oil and gas industry. Cements and materials for cementing wells. Part 2: Analyses of drilling cements and API SPEC 10 Specification for materials and testing for well cements. The presented investigation consisted of analyses of impact of bentonite on parame- ters of cement slurry. For determining the effect of bentonite on parameters of cement slurry and the result- ing cement sheath in the wellbore, eight slurry compositions were analyzed. For reference recipes were performed for comparison’s sake. Four modified slurries contained bento- nite. The influence of bentonite on the parameters of slurry in a possibly broad range of wellbore conditions was analyzed for temperatures 30°C, 50°C, 70°C and 90°C, and the relating pressures 5 MPa, 18 MPa, 25 MPa and 35 MPa, respectively. Natural sodium bentonite from Zębiec was used in the tests. Drilling cement G HSR (used in industry) was used as a binder. Portland cement CEM I 32,5R was a(cid:3)plied in shallow wells of low downhole temperatures and pressures. During realization of the cycle of tests all reci- pes contained cement G HSR to eliminate discrepancies among results stemming from the use of various types of binder. For the sake of sealing up the matrix of the cement sheath, a 20(cid:2) microcement admixture was applied in all recipes. A constant water/cement ratio of 0.50 was assumed; only in the recipe designed for wellbore temperature of 70°C and pressure 25 MPa the ratio equaled to 0.57. The assumed content of antifiltration coefficient and swelling agent totaled to 0.2(cid:2) bwoc and 0.3(cid:2) bwoc (Tab. 1). Constant quantity of latex (10(cid:2) bwoc) and latex stabilizer (1(cid:2) bwoc) were admixed. The slurries were based on 3(cid:2) salt content in the working water (bwow3). The defoamer, liquefier, accelerating agent and retarder were added in quantities presented in Table 1. A constant quantity of bentonite was used, i.e. 0.5(cid:2) (bwow) and 0.3(cid:2) (bwow) of bentonite for a slurry to be used in a wellbore of a temperature 90°C and pressure 35 MPa. The measured quan- tity of bentonite was mixed with working water prior to the slurry components. These criteria and the mixing proportion were used for working out four reference recipes and four modified recipes, on the basis of which the influence of bentonite on parameters of slurry could be determined for particular temperatures. Detailed quantities of compo- nents are given in Table 1. The analysis of the obtained results shows to the lowering of density of slurries after adding bentonite as compared to the basic slurries (Fig. 2 and Tab. 2). In the slurry designed for a wellbore with temperature 70°C and pressure 35 MPa, where water/cement ratio was higher (w/c = 0.57) the density of slurry was not observed to drop after adding bentonite. 3 Bwow – by weight of water 338 Table 1 Recipes of cement slurries for wellbore conditions Binder: cement G HSR = 100%, microcement 20.0(cid:2) No. CFoacmtNoproo (s.w i ti/ocn) °emperature [C] Pressure [MPa] Defoamer Antifiltration agent Liquefier Latex Latex stabilizer KCl (bwow) Bentonite Swelling agent CaCl 2 Retarder T No. 1 1. w/c = 0.50 C Pa 0.3 0.20 0.20 10.0 1.0 3.0 – 0.3 4.0 – ° M 2. No. 2 305 0.3 0.20 0.20 10.0 1.0 3.0 0.50 0.3 2.0 – w/c = 0.50 No. 3 3. w/c = 0.50 C Pa 0.3 0.20 0.10 10.0 1.0 3.0 – 0.3 – 0.10 ° M No. 4 508 4. 1 0.3 0.20 0.10 10.0 1.0 3.0 0.50 0.3 – 0.10 w/c = 0.50 No. 5 5. w/c = 0.57 C Pa 0.4 0.20 0.20 10.0 1.0 3.0 0 0.3 – 0.05 ° M No. 6 705 6. 2 0.4 0.20 0.20 10.0 1.0 3.0 0.50 0.3 – 0.05 w/c = 0.57 No. 7 7. w/c = 0.50 C Pa 0.5 0.20 0.25 10.0 1.0 3.0 – 0.3 – 0.25 ° M No. 8 905 8. 3 0.5 0.20 0.25 10.0 1.0 3.0 0.30 0.3 – 0.25 w/c = 0.50 Fig. 2. Results of analyses of spillability density and settling of water for fresh cement slurries 339 3 10 28 14 19 2 3 4 4 s u arat 6 14 30 16 20 3 4 6 7 p m Fann apat r.p.m.] 100 45 65 55 62 27 31 49 50 uts fro** [j.F. 200 78 97 89 96 49 55 85 87 o ead- 300 107 128 119 128 70 78 120 123 R 0 0 7 7 7 1 0 6 1 0 9 1 0 1 3 4 1 2 6 1 2 2 2 1 1 2 2 Bc 0 2 9 3 0 2 2 2 ]nim : srh[ 00 4:5 3:1 3:3 3:1 4:5 6:0 6:5 5:5 emit gninekcihT 1 rries * Bc 20 35 17 45 10 27 02 23 slu 30 4: 2: 3: 2: 4: 5: 6: 5: ment noitartliF 3/ m min] 6.0 6.0 6.0 6.0 0.0 2.0 2.0 9.0 ce [c30 4 3 5 6 7 9 8 9 2 d s able alyze retaw fo gniltteS (cid:4)(cid:2)(cid:5)[%] 0.2 0.1 0.2 0.4 0.3 0.4 0.0 0.2 n unit T n n Parameters of a ) tnn oiyt o ai ]]]tty]p ia3aay3ltm PPitmigtnbliMM/in sea/gbgnimlk[[arlke uit[ iDp[se esdySarre(tuueSisssmssneeerrDPP Top Center Bottom [mm] 1 885 1 880 1 885 1 890 245 aPM 51 830 1 830 1 830 1 830 220 1 840 1 830 1 840 1 850 215 aP 811 830 1 830 1 830 1 830 205 1 780 1 765 1 780 1 795 290 aP 521 780 1 780 1 780 1 780 205 1 840 1 835 1 840 1 845 265 aP 531 830 1 830 1 830 1 830 255 e: ** j.F. – Fa= 40 min= 60 min r ]C ° []C e°r[u tpamreepTmeT C°03 C°05 C°07 C°09 atu°C °C per7090 m tnemec fo epyT RSH G e t g n hinn .oN . oyNrrulS 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 Denotations* Time of reac°30C = 10 mi50°C = 25 mi 340 The sedimentation stability of bentonite slurry considerably improved as compared to the slurries without bentonite admixture. Density values at particular measuring points during the tests on the sedimentation column are presented in Table 2 and Figure 4. A density difference in Figure 5 was presented to visualize the improvement of sedimenta- tion stability. The density difference between the top and central, and between central and bottom measuring point of reference slurries range from 5 kg/m3 to 15 kg/m3. After using bentonite the slurry revealed a homogeneous structure, with uniform density at all measuring points (top, central and bottom part of the measuring column). Fig. 3. Stand for determining sedimentation stability of cement slurry and settling of water (left: sedimentation column, right: settling cylinders) Fig. 4. Density of slurry in particular measuring points of sedimentation column 341 Fig. 5. Comparison of density differences at particular measuring points of sedimentation column (difference between top and central part, and central and bottom part of the column) The use of bentonite also resulted in lower spillability; the highest reduction of this parameter was observed in a slurry having the highest water/cement ratio (slurry No. 6 in Tab. 2), where the spillability totaled to 205 mm (in the reference slurry for this recipe the spillability equaled to 290 mm). The analysis of water settling reveals that this parameter lowers only in slurry No. 2 (settlement 0.1(cid:2)) as compared to the reference slurry, i.e. 0.2(cid:2). The successive recipes had an increasing tendency of water settlement (Tab. 2 and Fig. 2). The lowering of water settlement in slurry 2 as compared to other slurries can be explained by the participation of montmorillonite present in bentonite, causing a higher water demand , with addition of strongly hygroscopic calcium chloride used for shortening the time of bonding. The remaining recipes which do not contain CaCl show a comparable increasing tendency 2 of water settlement values. Analogous behavior of slurry as that of water settlement was observed during the anal- ysis of slurry filtration values. Only in slurry No. 2, containing bentonite, the filtration lowered to 36 cm3/30 min as compared to the reference slurry (filtration 46 cm3/30 min). Also in this case we may assume the synergy of montmorillonite and calcium chloride, which is amplified by the increased temperature in the filtration press. In the remaining cases, after adding bentonite the filtration of slurries was higher than in the reference slurries (Tab. 2). The analysis of bentonite as an additive revealed that the time of thickening of cement slurry shortened – both at the beginning of thickening (30 Bc4) and at the end of thickening (100 Bc). The above effect was obtained for all recipes. The obtained results are presented in Table 2 and Figure 6. 4 Bc – unit of time of thickening of cement slurry while taking measurements in a consist- meter (Bearden unit) 342 (cid:13)(cid:22) (cid:4)(cid:3) (cid:15)(cid:21) (cid:18) (cid:10) (cid:19)(cid:6)(cid:20) (cid:18) (cid:18) (cid:17) (cid:15)(cid:16) (cid:7)(cid:8)(cid:6) (cid:14)(cid:6) (cid:13) (cid:13)(cid:3) (cid:5) (cid:12) (cid:11) (cid:10)(cid:3) (cid:7)(cid:8)(cid:6)(cid:9) (cid:5)(cid:6) (cid:4) (cid:2)(cid:3) Fig. 6. Parameters of thickening time of cement slurry The rheological parameters are compared in Figure 7 and Table 3 to better visualize the differences taking place after the additives were used. The yield point described in the Hershel–Bulkley model visibly increases. Plastic viscosity in the Casson model was observed to lower in slurries 2 and 4 in temperatures 30°C and 50°C. The Casson viscosity tends to increase in bentonite-modified slurries Nos. 6 and 8, designed for sealing casing at temperatures 70°C and 90°C. This can be connected with the use of large quantities of defoamer, i.e. 0.4(cid:2) bwoc and 0.5(cid:2) bwoc for slurries in temp. 70°C and 90°C, as compared to 0.3(cid:2) bwow of defoamer in slurries at temp. 30°C and 50°C. Fig. 7. Results of analyses of rheological parameters of cement slurries 343 Table 3 Rheological parameters of cement slurries Rheolo- Rheolo- Slurry No. gical gical 1 2 3 4 5 6 7 8 model parameters Dynamic viscosity 0.0998 0.1168 0.1100 0.1165 0.0673 0.0729 0.1125 0.1152 [Pa⋅s] Newton model Correlation coefficient 0.9861 0.9410 0.9775 0.9660 0.9974 0.9942 0.9939 0.9938 [–] Plastic viscosity 0.0899 0.0947 0.0966 0.0994 0.0647 0.0689 0.1063 0.1086 [Pa⋅s] Yield point Bingham 6.9568 15.6283 9.4213 12.0442 1.8117 2.8498 4.4093 4.6330 [Pa] model Correlation coefficient 0.9985 0.9989 0.9973 0.9968 0.9990 0.9979 0.9975 0.9976 [–] Consistency coefficient 2.0184 6.8483 2.7547 3.9319 0.2593 0.4115 0.5591 0.6102 [Pa⋅sn] Ostwald– Exponent 0.5263 0.3611 0.4952 0.4493 0.7894 0.7306 0.7524 0.7425 de Waele [–] model Correlation coefficient 0.9618 0.9298 0.9643 0.9591 0.9990 0.9934 0.9963 0.9957 [–] Casson viscosity 0.0635 0.0504 0.0657 0.0630 0.0586 0.0602 0.0943 0.0954 [Pa⋅s] Yield point Casson 2.9038 9.5669 4.1373 5.9079 0.2450 0.9044 0.6961 0.7981 [Pa] model Correlation coefficient 0.9993 0.9962 0.9998 0.9997 0.9998 0.9994 0.9992 0.9993 [–] Yield point 4.8629 13.7108 6.1903 8.3565 0.5304 0.7428 0.9597 1.1676 [Pa] Consistency coefficient 0.1914 0.1842 0.2734 0.3099 0.1242 0.1803 0.2929 0.2948 Herschel– [Pa⋅sn] Bulkley Exponent model 0.8916 0.9045 0.8509 0.8371 0.9064 0.8620 0.8547 0.8568 [–] Correlation coefficient 0.9999 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 [–] 344