CEMENTING GLASSES FOR OIL-WELLS APPLICATIONS

Transcription

1 CEMENTING GLASSES FOR OIL-WELLS APPLICATIONS Perera Yibran*, Aiskely Blanco Jiménez Wuilmen, George Quercia PDVSA Intevep. Well Construction and Maintenance Department, Urb. Santa Rosa, sector el Tambor, Los Teques, Edo. Miranda, P.O. Box 76343, Caracas 1070-A, Venezuela Abstract Venezuela has one of the biggest reserves of heavy crude oil on the world and the steam-injection is one of the techniques employed to extract these types of oil. Unfortunately, the conventional cementing materials used for the heavy crude oil wells do not show a good performance after two/three steam-injection cycles and the wells need be repaired several times, thus increasing the operation costs. Lately, the blast furnace slags are used as cementing material for these oil wells due to their thermal/chemical stability. Unfortunately, Venezuela does not produce these slags, so they are imported with their disadvantage associated, such as: the heterogeneous chemical composition, the amount of remaining crystalline phases and the elevated cost due to the blast furnace slag milling process. This research concerns the employment of a conventional glass production process to synthesize new amorphous materials with novel chemical compositions and high purity controls, which will be applied as new cementing materials for heavy crude oil wells. The characterization was made using Scanning Electron Microscopy (SEM), Energy Disperse Spectroscopy (EDS), Thermal Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and X-Ray Diffraction (XRD). Finally, a slurry formulation was made from the synthetic glasses and was compared with a commercial slag slurry in order to evaluate their thermal/mechanical behaviors. Keywords: cementing glasses, oil wells, cement, slags, heavy crude oil. * Pereray@pdvsa.com; Tlf. +58 (0) ; fax: +58 (0)

2 Introduction The zonal isolation in the annulus of a producing well is the driving force behind the research for new cementing materials and cementing technologies [1]. In Venezuela specifically, steam injection is the most popular method carried out today to stimulate the oil production for heavy crude oil-wells. Unluckily, the casing and the conventional oil-well cements are not available to support thermal cycles between short periods of steam stimulation followed by production, because during a steam injection process the temperature can exceeds 500ºF and the CSH gel from the cement becomes alpha dicalcium silicate hydrate (α-c SH), this phenomenon is known as strength retrogression, that decreases the mechanical properties of the cement [1,,3]. Finally, the cementing systems do not show a good performance after two/three steam injection cycles and the wells need to be repaired many times, increasing the operation cost. The cementing material used in this type of well must be able to withstand elevated temperature exposure and thermal cycling associated with steamflood wells and these materials must show some specific characteristics like a strong cement with low permeability [1,]. To solve the thermal degradation of conventional Portland cement used for heavy crude oil-wells, the Mud-to-Cement (MTC) technology has been applied in diverse fields where steam injection process is required. The MTC is a unique cementing technique where the water based drilling fluid is mixed with a steel manufacturing byproduct know as Blast Furnace Slag (BFS) and some chemical activators to be transformed into cement slurry. Besides it is important to emphasize that the BFS is more compatible with mud than the conventional Portland cement, normally used in well cementing operations [1,,3]. Venezuelan steel manufacture does not use the blast furnace system and the national slags do not have the physical and chemical specification required for the MTC technology, so the BFS is imported with its disadvantages associated, such as: heterogeneous chemical composition, the amount of remaining crystalline phases and the elevated cost due to the BFS milling process. In order to continue with the MTC technology applications in Venezuela due to the potential advantages over conventional slurries and according to their thermal/chemical stability during steam injection procedure, the synthesis of new amorphous materials with novel chemical compositions and high purity control from the cofusion of national minerals and then cooling the resulting melt at sufficient rate to avoid crystallization is proposed. The ceramic glasses synthesized by quenching a melt with chemical composition close to the BFS could be used as a cementing material for the MTC technology. The cementing glasses are produced from different minerals like Kaolin, Dolomite, Calcium Carbonate, Silica, etc. with a final chemical composition in the quaternary phase field of the CaO-MgO-Al O 3 -SiO (MCAS) system.

3 Experimental Raw materials, melting and sample preparation The process consists of mixing in an exact ratio the different raw materials previously characterized by MEB and EDS (Table 1), to obtain two () homogeneous and controlled final chemical composition glasses, followed by the melting in a platinum crucible at 1500ºC for 3 h. The liquid was quenched in water at 0ºC to suppress crystallization. The synthetic glasses were milling to get controlled particles size observed by SEM and chemically analyzed by EDS. The averages shown in Table, were calculated from ten measurements for each sample. Table 1. Chemical composition of the raw materials (%w/w) used to obtain the cementing glasses: Compounds Dolomite % Ca carbonate % Kaolin % Silica % CaCO MgCO SiO Al O Fe O Na O K O TiO Table. Chemical composition of the cementing glasses and the commercial slag (%w/w): Oxides Teorical range (%) Comp. 1 (%) Comp. (%) Commercial Slag (%) CaO MgO SiO Al O Fe O Na O+K O < TiO < SO MnO Differential scanning calorimetry and thermal gravimetric analysis Simultaneous differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) measurements were performed using a 0ºC/min heating rate. Both raw minerals mixed to obtain the final synthetic glasses weighing, were placed in a platinum pan, and an empty platinum pan was used as standard. Data were recorded using a computerdriven data acquisition system. Due to the BFS is a commercial product this material were not evaluated with these techniques.

4 X-ray diffraction To determinate the final amorphous degree from the synthetic glasses and to investigate the remnant crystalline phases composition from the commercial slag, all the samples were exposed to X-ray radiation, using the copper Kα (λ = 15,418 Å) and a graphite monocromator as filter. The cementing glasses for MTC technology A MTC technology procedure was used to design a slurry from the synthetic glasses and the water based drilling fluid that could be used for steam injection wells [1,]. The glasses and also the commercial slag employed in this research were activated by reaction with a high ph solution where the bonds in the network structure of the synthetic glasses are easily broken and hydration of these amorphous materials were promoted. For the slurry formulation were used soda caustic and soda ash as activators, a polymeric based drilling fluid and other compounds like Silica Fluor and antifoam were used to formulate a slurry with 13.5 lpg (1.6 g/cc) of density for all the cases. Laboratory simulation of the steam injection process The mechanical properties produced in some cementing materials by the thermal effects when the temperature increases due to steam injection in the well, were simulated using a Ultra Sonic Analyzer (UCA) curing chamber. The samples were subjected to constant pressure of 000 psi and 10ºF for 4 h. Then, the chamber temperature was increased to 350ºF using a heating rate controlled by the chamber simulating the heating that the cementing material experiences during the steam injection process for 4 h, followed by a temperature decrease to cooling the system to room temperature, this step is called first thermal shock. Then, this procedure was repeated in order to simulated a total of two injection cycles. This equipment reported the compressive strength and was related to the temperature changes during the test, the data were recorded using a computerdriven data acquisition system. Raw minerals Results and discussion When the minerals are mixed in a controlled ratio to get a final specific chemical glass compositions, the capacity to be more reactive during the melting process is achieved for their surface area. In general, increased fineness results in more homogeneous chemical reaction at high temperatures [5]. The same way is observed for the synthetic glasses, when the particle size decreases the reactivity into the MTC slurries increase and the final cement material has a better strength development. Unluckily, in practice, fineness is limited by economic and performance considerations and factors such as setting time and shrinkage [5,6]. Fig. 1 shows the SEM and EDS analysis indicating the final morphology glasses after the milling process with a regular structure of cleavage observed in broken glasses. The particles size correspond with the ASTM C used to adequate the commercial slag materials for Concrete and Mortars applications.

5 Commercial Slag Comp. Comp. 1 Figure 1. MEB and EDS analysis from the different cementing glasses and the commercial slag. DSC and TGA analysis The minerals mixed to produce the two synthetic glasses were studied by DSC/TGA simultaneous analysis to know the decomposition and transformation temperatures for each specific final chemical glass compositions. Fig. shows the curves from the first mineral mix powders of kaolin, dolomite and calcium carbonate to produce the cementing glass of comp. 1. The TGA curve shows a very strong loss mass at temperature between ºC that correspond to the calcium carbonate (CaCO 3 ), and dolomite MgCa(CO 3 ) decomposition. After 900ºC is observed a very low slope change related to a final step of CO remnant gas-separation during the melting process ( ºC) and this process continues to the final temperature analyzed (1600ºC). These reactions can be represented as [7]: CaCO 3 CaO + CO begin at 40 C MgCa ( CO MgO + CaCO + CO begin at 80 C 3 ) 3 ) MgO + CaO 3 + MgCa ( CO CO begin at 960 C

6 HeatFlow /µv/ 4 EXO TG / mg TGA 900 C 1050 C 140 C 180 C 1305 C 0 DSC C 160 C 1470 C C C 1400 C Temperature [ C] Figure. TGA/DSC from the minerals mixed to formulate the glass comp. 1. The DSC curve from the minerals mixed (comp. 1) shows different events, the big first one occurs at the same temperature between ºC where endothermic peaks are reported and related to the solid state reactions between the calcium and magnesium oxides generated during the CaCO 3 and MgCa(CO 3 ) thermochemical decomposition to form magnesium and calcium silicates with a final exothermic peak at about 1050 C as follows [7]: MgO + SiO MgSiO 3 between C CaO + SiO CaSiO 3 between C additional, can occur the following reaction [7]: CaSiO 3 MgSiO 3 CaMg (SiO3 ) + between C The second big event is related to the melting step that shows a group of exothermic peaks indicating a sequential melting process of diverse phases, this step begin at 160ºC and the last one is related to the temperature at about 1450ºC. The multiple reactions are associated to the dissolution of silicates and the reaction with other aluminum phases obtained from the kaolin decomposition. The high melting point of this composition can be attributed to the high aluminum phases related to incorporation of kaolin in the composition that have a contribution of about 1.0% in the final glass. Fig. 3 shows the minerals mixed to produce the glass comp. (kaolin, dolomite, calcium carbonate and silica), in this system the chemical precursor mineral balance included the silica as a new mix component. The TGA curve is more exact and the final temperature of carbonates decomposition is at 870ºC related to the first solid reaction with the production of CaO and MgO reactive oxides associated to the CO gasseparation from the system. The slop at temperature up to 900ºC is more constant and

7 do not show several changes. The DSC curve shows a first endothermic peak at 450ºC related to the dehydroxylation of kaolin and formation of metakaolinite according to the reaction [8]: Al O3 SiO H O Al O3 SiO + H O between C HeatFlow /µv/ 4 EXO DSC TG / mg TGA 1300 C C C 1380 C Temperature [ C] Figure 3. TGA/DSC from the minerals mixed for the glass comp.. An event associated to the loss mass at 870ºC is observed as a big endothermic peak and between C is observed a exothermic success related to the formation of calcium and magnesium silicates as showed for the first glass composition. Finally, a reduction of the melting temperature compared with the glass comp. 1 is obtained with a specific endothermic peak at 1380ºC, temperature at which the mass shows a total transformation from solid to liquid. Additionally, this glass comp. shows the lower density obtained from this synthesis process (Table 3). This effect can be attributed to the decrease in the amount of aluminum phases and the increase of silica in the composition that is one of the most common glass-former component. The melting point reduction of this second glass composition is an improvement for the synthesis of these glass materials related to the energy consumption reduction that influence directly the economic production considerations. Table 3. Some properties from the cementing glasses and the commercial slag evaluated: Properties Glass comp. 1 Glass comp. Commercial Slag Density (g/cc) C/S ratio Hydraulicity* *Hydraulic Index = (CaO+MgO+Al O 3 )/SiO 1.0 [5] After the production of the cementing glasses and their chemical characterization some properties were measured from them and compared with the commercial slag. In order to predict the hydraulic activity (latent) and to understand the strength retrogression

8 phenomenon in these systems, the Hydraulic Index (HI) and de C/S ratio are calculated and are shows in Table 3 also the density of both cementing glasses and the commercial slag evaluated are reported. Glass content The amorphous degree of the synthetic glasses is considered to be the most significant variable and certainly the most critical for hydraulic activity (latent). Several factors influence the vitrification degree achieved to the quenching, but the most important variable influencing the nature of the glasses is the temperature at which the furnace is tapped. The rate of quenching, which influences the glass formation, is thus the predominant factor affecting the strength of cementing glasses. CaCO 3 remanent a) Intensity [a.u.] b) c) Amorphous halo Θ [ ] Figure 4. DRX. (a) commercial slag. (b) Glass comp. 1. (c) Glass comp.. The high amorphous degree from both synthetic glasses and the remnant crystalline phases of the commercial slag were identified using XRD diffraction patterns. Fig. 4 (a) and (b) shows that the glasses (comps. 1 and, respectively) are in total amorphous phases without any remnant crystalline phases, the fast quenching rate cooled the glassy structure that is essential to reactivity. Fig. 4 (c) shown the remnant crystalline phase of Calcium Carbonate (CaCO 3 ) into the commercial slag but this material shows a glass content > 95 %. It has been heuristically reported that generally, the glass content of the slag should be in excess of 90% to show satisfactory properties [5]. Thermal/mechanical properties The compressive strength is the most important property of cementing materials quality. In Fig. 5, the compressive strength values for the two different glasses based systems and the commercial slurries are given. Data are present from measurements taken initially after curing and then after each subsequent thermal shock cycles. An important step during the slag and cementing glasses hydration is the development of mechanical properties at the initial curing. The hydration of these systems lead to

9 their partial dissolution in mixing water and to the precipitation of an amorphous hydrate calcium silicate (C-S-H) gel (i.e. poorly crystalline and amorphous calcium silicate hydrate of indefinite composition) and hydrate aluminates and silicoaluminates [8]. Normally, an augmentation of the Al O 3 -content increases the early strength of the cementing system. In Fig. 5, the glass comp. 1 shows the most high Al O 3 -content (table 1) and also the most high hydraulic index (table 3) and compressive strength during the initial curing step. Figure 5. Effect of the thermal shock on compressive strength for all the systems tested. The cementing glasses and BFS hydration are more temperature dependent than Portland cement hydration. Fig. 5 indicates that the cementing glass comp. tested is more thermally stable than the comp. 1 glass, but both cementing glasses shown a higher compressive strength values than the commercial slag system evaluated. It can be attributed to the homogeneous amorphous chemical composition of the synthetic glasses that shows low amount of contaminant according to the chemical characterization shown in the table 1. This factor plus the adequate fineness of the precursor materials alone influence the cementitious/pozzolanic efficiency when are used specifically for the MTC technology [5,6]. On the other hand, according to different references during the hydratation of diverse cementing/pozzolanic materials, the C-S-H gel gives a big portion of mechanical properties and stability to the final structure, this C-S-H gel is very stable below 30ºC. At higher temperatures the C-S-H gel is transformed into a hydrate dicalcium alphasilicate (α-c SH) phase that is highly crystalline, denser than C-S-H gel and produces a compressive strength loss with a permeability increase on set cement materials. This phenomenon is know as strength retrogression [1,,9,10]. To prevent the α-c SH formation Silica Fluor is used to stimulate the formation of other phases as the tobermorite (C 5 S 6 H 5 ) and Xonotlite (C 6 S 6 H) which render to the cement high mechanical properties increasing the chemical bonding and the C/S ratio must be near to 1 [,3,9,10]. In Fig. 5 is possible to observe that the glass comp. 1 (C/S = 1,30) and the slag commercial (C/S = 1,) systems shows a strength retrogression after the first thermal shock, but the glass comp. (C/S = 1,09) do not show this phenomenon may be attributed to the increase of Silica in this glass chemical composition. The C/S ratio for

10 the glass comp. is closer to one (1), and it is required for cementing systems with minimal strength retrogression. Conclusions Cementing glasses with composition in the quaternary phase field of the MgO-CaO- Al O 3 -SiO (MCAS) system were prepared by a classical way of quenching a melt. The new glasses show a higher homogeneous chemical compositions preceding from the high purity natural minerals. These amorphous materials do not show remnant crystalline phases. The correct thermochemical decomposition relationship of different minerals produced the two cementing glasses with chemical compositions very close to the commercial BFS. Both cementing glasses developed pozzolanic properties when are used as raw materials for Mud-to-Cement (MTC) technology. Specifically, the cementing glass comp. shows a thermal/mechanical properties improvement during the curing step compared with the other systems evaluated, and does not showed strength retrogression after the two simulated thermal shocks. Finally, the MTC laboratory slurry formulate with the glass comp. could be adequate for oil-wells that employ steam injection procedure. References [1] A. Blanco; A. Colina; W. Rodríguez and R. Bolivar; Effective Pay Zone Isolation of Steam Injection Wells ; SPE 53689; PDVSA Intevep; [1999]. [] Nelson, E.B; Well Cementing ; Schlumberger Educational Service, Houston, Tx. [1990]. [3] A. Blanco; W. Rodríguez; R. Bolivar; C. Cadenas and J. Soto; Effect of steam injection on microstructural and mechanical properties of cement and Blast Furnace Slag ; 1th ICMA; [1999]. [4] R. Eriksson and S. Seetharamn; Thermal Diffusivity Measurements of some synthetic CaO-Al O 3 -SiO Slags. Metallurgical and Materials Transactions B; 35B; pp. 461; [004]. [5] S.C. Pal; A. Mukherjee and S.R. Pathak; Investigation of hydraulic activity of ground granulated blast furnace slag in concrete ; Cement and Concrete Research 33, pp ; [003]. [6] H. Binici; H. Temiz and M.M. Köse; The effect of fineness on the properties of the blended cements incorporating ground granulated blast furnace slag and ground basaltic pumice ; Construction and Building Materials; [006]. [7] C. Laville; Guia curso de Materiales Vitreos MT-1 ; Dep. de Ing. de Mat. U.S.B.; [1998]. [8] G. Kakali; T. Perraki; S. Tsivilis and E. Badogiannis; Thermal treatment of Kaolin: the effect of mineralogy on the pozzolanic activity ; Applied Clay Science 0; [001] [9] W. Rodriguez; M.I. Specht; A. Blanco; R. Garcia and J. Soto; Morphological changes of aluminates phases using refractory material in cement slurry ; PDVSA Intevep. [10] A. Saasen; B. Salmelid; N. Blomberg; S.P. Young and H. Justnes; The use of Blast Furnace Slag in North Sea Cementing Application ; SPE 881; [1994].