Ugljikov dioksid utiskuje se kroz utisne bušotine u iscrpljena naftna i plinska ležišta zbog njegovog trajnog zbrinjavanja u svrhu smanjenja emisije u atmosferu ili zbog produljenja proizvodnog vijeka ležišta. Budući da je ugljikov dioksid korozivan, pri njegovu utiskivanju iznimno je važno pravilno odabrati bušotinsku opremu i zaštitne cijevi.
U okviru doktorskog rada ispitano je sveukupno 109 cementnih kaša različitog sastava na temelju čega je za ispitivanje kvalitete cementnog kamena odabrano njih osam i to: (1) na bazi Portland cement API klase G, (2) na bazi Portland cement API klase G s dodatkom 20 % zeolita na masu cementa, (3) na bazi Portland cement API klase G s dodatkom 30 % zeolita na masu cementa, (4) na bazi Portland cement API klase G s dodatkom 40 % zeolita na masu cementa, (5) na bazi geopolimera – mješavina zgure i vapna, (6) na bazi geopolimera – mješavina letećeg pepela i vapna, (7) na bazi kalcij-aluminatnog cementa s dodatkom staklenih mikrokuglica i (8) na bazi kalcij-aluminatnog cementa s dodatkom lateksa.
Obavljena su ispitivanja fizikalnih svojstava cementnih kaša (gustoća, reološka svojstva, filtracija, izdvajanje slobodne vode, sedimentacija, vrijeme zgušnjavanja), fizikalnih svojstava cementnog kamena (tlačna čvrstoća, poroznost, propusnost), kemijsko-mineraloških svojstava cementnog kamena (promjena mase s porastom temperature, određivanje mineralnih faza), te strukturno-morfološka analiza (izgled uzoraka prije i nakon izlaganja okolini zasićenoj s CO2).
Rezultati ispitivanja pokazali su da cementni kamen nastao iz cementnih kaša pripremljenih od Portland cementa sa zeolitom ima veću tlačnu čvrstoću, ali istovremeno i veću propusnost u odnosu na cementni kamen nastao iz cementne kaše pripremljene od Portland cementa bez dodatka zeolita. Cementni kamen nastao iz cementnih kaša pripremljenih od geopolimera nije pokazao otpornost prema koroziji uzrokovanoj ugljikovim dioksidom. Cementni kamen nastao iz cementnih kaša pripremljenih od kalcij-aluminatnog cementa potvrdio je otpornost prema koroziji uzrokovanoj ugljikovim dioksidom, međutim, ispitivanjima je ustanovljena pojava pukotina u strukturi cementnog kamena koja je značajno povećala njegovu propusnost i uputila na nužnost daljnjeg istraživanja u svrhu boljeg optimiranja sastava cementne kaše.
|Abstract (english)|| |
Injection of carbon dioxide into depleted oil and gas reservoirs is one of the most promising solutions for its long-term capture and storage, and is thereby directly connected to global reduction in emissions of atmospheric greenhouse gases. However, carbon dioxide is a corrosive gas which makes the selection of casing, downhole equipment and cement very challenging in the well design. Casing and downhole equipment should be either made from steel alloys with high chrome content or protected by regular injection of corrosion inhibitors in the case of steel casing pipe. The cement based on Portland cement is susceptible to corrosion in the CO2 laden fluids or in CO2 supercritical state. The corrosion process commences with the carbonisation of free Ca(OH)2 and the C-S-H gel. Upon the consumption of all available quantities of Ca(OH)2, the resulting calcium carbonate is converted into a soluble bicarbonate, which is then released from the hardened cement matrix. Consequently, porosity and permeability of the hardened cement increase concomitant with a decrease in its compressive strength, which could lead to poor binding between casing and the hardened cement on one hand, and the hardened cement and the rock formations on the other hand, which may ultimately result in the migration of the injected CO2 towards the surface. Thus, special types of cement should be used. These could comprise of non-Portland-based cement or cement blends based on Portland cement with the addition of materials which increase hardened cement CO2 corrosion resistance. Cement slurries made of these special cement systems have to be optimised in order to obtain rheological and filtration properties required for their application under well conditions.
In this doctoral thesis alltogether 109 optimizations of cement slurries were performed which resulted in selection of 8 slurries for further tests on hardened cement: (1) comprising API Portland cement class G, (2) comprising API Portland cement with the addition of 20 % zeolite BWOC, (3) comprising API Portland cement with the addition of 30 % zeolite BWOC, (4) comprising API Portland cement with the addition of 40 % zeolite BWOC, (5) comprising geopolymer based on slag-lime blend, (6) comprising geopolymer based on fly ash-lime blend, (7) comprising calcium aluminate cement with the addition of glass microspheres and (8) comprising calcium aluminate cement with the addition of latex.
Application of optimized cement slurries was presented using the computer simulation of re-lined 101,6 mm (4“) production casing cement job at Žutica and Ivanić oil fields (Croatia), where carbon dioxide is injected underground to enhance the oil production (EOR – Enhanced Oil Recovery). Special considerations were given to the flow of cement slurry
through a narrow annular space between the re-lined production casing having the diameter on these oil fields of 101,6 mm (4“) or 88,9 mm (3 1/2“) and the existing 139,7 mm (5 1/2") production casing. Annular space between the two casings ranges from 11,35 mm to 17,7 mm (0,447“ – 0,697“), which differs from the recommended value of 19,1 mm (3/4“) (Adams and Charrier, 1985).
Tests performed in this research included determination of physical properties of cement slurries (density, rheological properties, filtration, free water content, sedimentation, thickening time), determination of physical properties of hardened cement (compressive strength, porosity, permeability), mineralogy analysis (determination of mass loss as a function of increasing temperature, determination of mineral phases) and morphology analysis (hardened cement samples changes in shape and colour before and after exposure to CO2 environment).
Test results have shown that hardened cement containing Portland cement supplemented with zeolite has increased compressive strength concurrent with increased permeability when compared to hardened cement based on Portland cement without zeolite. Further on, mineralogy analysis has confirmed an overall carbonization of all hardened cement samples based on Portland cement with the addition of zeolite, which lends support for the conclusion that hardened cement based on these compositions is not resistant to CO2. However, increased values of compressive strength points to the presence of secondary binding reactions which took place during the hydration due to pozzolanic properties of zeolite, while unexpectedly increased porosity and permeability could have arisen as a consequence of excessive zeolite content in the cement slurry composition that had not been completely consumed due to insufficient quantity of calcium hydroxide. Taken into consideration that zeolite is a porous material, its surplus, which did not react with calcium hydroxide, has increased an overall hardened cement porosity and consequently permeability.
Hardened cement samples based on slag-lime and pozzolan-lime cement blends did not show an adequate CO2 resistance. Compressive strength of all tested samples was significantly lower, while porosity and permeability were higher than that of reference hardened cement samples based on Portland cement. Additionally, cracks were noticed in the cement matrix of these samples.
Hardened cement samples based on calcium aluminate cement confirmed the resistance to CO2 corrosion. However, cracks in the cement matrix were noticed, which significantly increased samples' permeability. These findings warrant further research based on improved cement slurry compositions.