Greenhouse gas sequestration in aquifers saturated by natural gases

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1 GOSPODARKA SUROWCAMI MINERALNYMI Tom Zeszyt 3/1 JAN LUBAŒ*, MARCIN WARCHO *, PIOTR KRÊPULEC**, TADEUSZ WOLNOWSKI** Greenhouse gas sequestration in aquifers saturated by natural gases 1. The Poznañ Trough megastructure and CO 2 storage potential project Permian structure of Poznañ Trough megaaquifer represents a great potential for long-term underground CO 2 storage on 5000 km 2 area. The aquifer is naturally saturated by native hydrocarbon gases and its tightness is confirmed by the presence of many local gas accumulations in top areas of the structure. The Poznañ Trough is divided into two areas. The firs one, west part riches from Lwówek-Nowy Tomyœl to Poznañ. It is about km width and 35 km long. The Poznañ Trough is limited from the north site by Rokietnica-Siekierki elevation. There have been discovered a lot of hydrocarbon gas reservoirs in this part of structure. The second area, east one, extends from Poznañ to Pleszew. It is about 80 km long. This dismemberment is caused by differences in inclination of the formation top of the Rotliegend. The average depth of Rotliegend top formation is m. The fragment (Ujazd-Grodzisk) of seismic profile the structure is presented in Figure 2. It was affirmed that in about 50 wellbores located there the reservoir water is saturated with hydrocarbon gases. Some studies determined that the gas in water solution is 2.4 Sm 3 of gas in 1 Sm 3 of formation brine. So it is calculated that gas deposits (dissolved in aquifer) in the Poznañ Trough mega structures can rich even Sm 3 on 5000 km 2 area [2]. ** Oil & Gas Institute. ** Polish Oil & Gas Company.

2 300 Fig. 1. Poznañ Trough location on the map of Rotliegend area in Poland [1] Rys. 1. Lokalizacja koryta poznañskiego na mapie Rotliegend w Polsce [1] Fig. 2. Fragment of seismic profile of the Poznañ Trough structure Rys. 2. Fragment profilu sejsmicznego struktury koryta poznañskiego

3 There was assumed to calculation that: average porosity = 10%, formation area = 5000 km 2, average gas in water solution = 2.4 Sm 3 /Sm 3, average reservoir temperature = 80 C, average reservoir pressure = 30 MPa. Gas saturation can decrease together with depth so realistic amount of hydrocarbon gas deposits can be equal about Sm 3. The Poznañ Trough megastructure can be compared with huge siphon bottle nearly full filled with water saturated by gas. Such a deep saline aquifer has a great potential of CO 2 storage. Exploration of Poznañ Trough permian megastructure with geological confirmed tightness, located close to large CO 2 emitters such a Poznañ heat and power generating plant is very important. Poznañ is a big city located in the neighbourhood of many oil and gas reservoirs (Fig. 3). A lot of these fields are partially or completely exploited, that makes additional capabilities of CO 2 storage there. These reservoirs present culminations of extensive rotliegend structures. The structure capacity which is outside of the reservoirs remains as a formation saturated with water and gas in solution. Studies performed in Polish Oil and Gas Institute have confirmed that it s possible to inject the acid gases directly to bottom water zone. We performed at PVT Laboratory research into phase properties and reinjection model simulations using PVT equipment for 301 Fig. 3. Power plant and gas reservoirs around Poznañ area Rys. 3. Elektrownia i zbiorniki gazu wokó³ Poznania

4 302 Borzêcin case. On the ground of experiences with process of reinjection of acid gases on Borzecin field, it is expected that it will be proceeds process of displacement the hydrocarbon (in-situ) gases (dissolved in bottom water) by CO 2 directly injected into bottom water of Poznañ Trough megastructure. This phenomenon should take place due to the significant differences in the solubility the natural hydrocarbon gases and CO 2 in the formation brine. This property is very interesting because in practice, it allows minimizing greenhouse effect and at the same time to replenish the gas resources in gas cap with displaced gases originally dissolved in bottom water. Also the routes of existing gas pipelines make real simplification in designing the pipelines transporting CO 2 into area of exploited gas reservoirs located on Poznañ Trough megastructure. It is proper to mention that very close to Poznañ heat and power generating plant has been identified at 2000 m depth a structure, which can be also (after geological research) considered as a potential CO 2 storage formation for the future projects. 2. Injection of acid gases into the Borzêcin gas-water zone Displacement of the native gas which originally saturates the underlying water with acid gases injected into reservoir may increase the recoverable gas reserves. Such a displacement process enables replenishing the gas cap by volume equivalent to the methane gas dissolved in the underlying waters. Such a project was implemented in industry in The project reported here presents reinjection of acid gases containing 60% of CO 2 and 15% of H 2 S into an aquifer directly underlying the Borzêcin gas reservoir (Fig. 4). Fig. 4. Diagram showing acid gas reinjection into Borzecin gas reservoir Rys. 4. Diagram ukazuj¹cy reiniekcjê gazu kwaœnego do zbiornika gazu Borzêcin

5 The reinjected gases are by-products of amine gas sweetening process (Fig. 5). Such a method of acid gas disposal where the injection zone is in hydrodynamical contact with a gas-bearing reservoir has not been referenced to in the literature. In this method the injected gas dissolves in the underlying water which has a hydrodynamic contact with the gas horizon and thus may influence the composition of the produced gas. The acid gas reinjection into the Borzêcin gas horizon has been in operation from the moment when 67% of gas ( Sm 3 ) was produced. The original gas reserves of the Borzêcin gas field were Sm 3 of gas. Before designing injection facility, the PVT experiments were carried out. They indicated that the upward movement of H 2 SandCO 2 to the gas cap would be very slow owing to the high solubility of these gases in the reservoir waters, which was much higher than that of the native gas. The laboratory experiments indicated that: Solubility of native gas which contained 65% of hydrocarbons, 35% of nitrogen and small volumes of H 2 SandCO 2 was 1.55 Sm 3 of gas per 1 Sm 3 of reservoir water at 58 C and 97 bars. Solubility of acid gas which contained 60% of CO 2, 15% of H 2 S, 20% of hydrocarbons and 5% of nitrogen was 13 Sm 3 of gas per 1 Sm 3 of reservoir water at the same temperature and pressure as specified above; this means that it was 8.4 times grater than solubility of native gas (Fig. 6). 303 Fig. 5. The picture of Borzecin acid gas reinjection installation Rys. 5. Widok instalacji do reiniekcji gazu kwaœnego w Borzêcinie

6 304 Phase diagram, presented in Figure 7 (constructed using the computer simulation of PVT experiments) indicates that the acid gas remaines in a gaseous phase during whole injection process. Acid gas dissolves in reservoir water preferentially displaces the originally dissolved natural gas. Fig. 6. Solubility of gases in water of Borzecin reservoir in reservoir temperature 58 C Rys. 6. Rozpuszczalnoœæ gazów w wodzie w zbiorniku Borzêcin w temperaturze zbiornika 58 C Fig. 7. Phase transmission diagram for acid gas compression and injection stage (case studies for Borzecin reservoir) Rys. 7. Diagram przejœcia faz dla etapów sprê ania gazu kwaœnego i iniekcji (przypadek zbiornika Borzêcin)

7 A considerable drop of injection pressure from 10.4 MPa to 6.6 MPa was recorded after of Sm 3 of acid gas was injected into reservoir. This drop of injection pressure was probably caused by an increased permeability due to a chemical interaction between carbonate reservoir rocks and injected acid gas with high CO 2 concentration (60%), vide Figure 7. The decrease of injection pressure and related decrease of power consumption improved the economical effectiveness of the whole project. The PVT test results indicated that volume of methane gas displaced from reservoir water is an increasing function of volume of CO 2 injected into reservoir (Fig. 8). 305 Fig. 8. The changes of reservoir parameters. The drop of injection pressure from 10.4 MPa to 6.6 MPa was recorded after injection of Sm 3 of acid gases Rys. 8. Zmiany parametrów zbiornika. Spadek ciœnienia iniekcji z 10,4 MPa do 6,6 MPa zarejestrowano po wtrysku Sm 3 gazów kwaœnych Fig. 9. Volume of methane displaced from reservoir water versus volume of injected CO 2 (based on PVT tests) Rys. 9. Iloœæ metanu wypychanego ze zbiornika wody a iloœæ wtryskiwanego CO 2 (na podstawie prób PVT)

8 306 In cooperation with AGH-University of Science and Technology [3], computer models simulating the acid gas injection into reservoir were used for prediction of the acid gas distribution pattern and for evaluation of possible changes in chemical composition of produced gas. The predicted CO 2 and H 2 S concentrations in produced gas indicates that an increase of CO 2 content appears much earlier than an increase of H 2 S concentration. This is caused by ahighco 2 content in the injected gases which is four times as large as H 2 S concentration. The predicted concentration of CO 2 in production wells is shown in Figure 10. Fig. 10. Predicted CO 2 concentration in gas produced from various wells Rys. 10. Przewidywane stê enie CO 2 w gazie z ró nych odwiertów Fig. 11. Measured CO 2 concentration in gases produced from various wells of Borzecin Rys. 11. Mierzone stê enie CO 2 w gazach produkowanych z ró nych odwiertów w Borzêcinie

9 307 The CO 2 content was expected to increase in two wells already in 2004, i.e. after 8 years of continued injection. The CO 2 concentration in the remaining wells will be on a constant level by As shown in Figure 11, a good agreement between predicted and measured data is observed, i.e. increase of CO 2 concentration was initially observed in B4 well, followed by increase of H 2 S content in the same well in High partial pressure of H 2 SandCO 2 components and elevated temperature are the factors which promote the corrosion process. However, the tests indicated that the true corrosion is much lower then its potential value. The wall thicknesses of steel pipes of the acid gas injection facility were checked between Borzêcin acid gas underground storage is currently the unique training industry on-shore project in Europe for investigations and studies of sequestration processes. The cumulative amount of CO 2 injected into the aquifer up to now is about Sm 3. REFERENCES [1] W o l n o w s k i T. Perspektywy poszukiwañ z³ó ropy naftowej i gazu ziemnego na ni u polskim. Wiadomoœci Naftowe i Gazownicze. Materia³ wydrukowany z serwisu [2] Karnkowski P.,1979 Formowaniesiêz³ó gazuziemnegonaobszarzeprzedsudeckim.naftanr8 9. [3] S t o p a J., L u b a œ J., R y c h l i c k i St., 2006 Underground Storage of Acid Gas In Poland Experiences And Forecasts. 23rd World Gas Conference, Amsterdam. SEKWESTRACJA GAZU CIEPLARNIANEGO W FORMACJACH WODONOŒNYCH NASYCONYCH GAZAMI NATURALNYMI S³owa kluczowe Efekt cieplarniany, sekwestracja, emisja CO 2, Borzêcin, Niecka Poznañska Streszczenie Stwierdzono, e bez rzeczywistych wysi³ków maj¹cych na celu zmniejszenie emisji gazu cieplarnianego temperatura globalna bêdzie wzroœnie o kilka stopni, zmieniaj¹c klimat œwiata. Wêgiel jest Ÿród³em 95% produkcji elektrycznoœci w Polsce. Z tego wzglêdu w naszym kraju wymagana jest technologia niskiej emisji wêgla z wychwytywaniem i podziemnym sk³adowaniem CO 2. G³êbokie s³one formacje wodonoœne maj¹ najwiêkszy d³ugofalowy potencja³ sk³adowania CO 2, ale istnieje wiele problemów z ich eksploracj¹ i kwalifikacj¹ wskutek braku potwierdzenia szczelnoœci. Bardzo wa ne jest zmniejszenie kosztu ich eksploracji prowadzonej g³ównie przez kosztowne wiercenia. W istniej¹cych megaformacjach wodonoœnych nasyconych naturalnym gazem szczelnoœæ jest potwierdzana obecnoœci¹ wielu lokalnych nagromadzeñ gazu w górnych strukturach. W niniejszym opracowaniu przedstawiamy megaformacje wodonoœne Niecki Poznañskiej nasycone przez rodzime gazy naturalne. Ta megastruktura stanowi wielki potencja³ dla d³ugofalowego podziemnego sk³adowania CO 2 na powierzchni 5000 km 2. Instytut Ropy Naftowej i Gazu oraz PGNiG zdoby³y znaczne doœwiadczenie w dziedzinie wychwytywania i sk³adowania gazu kwaœnego. Gaz kwaœny zawieraj¹cy 60% CO 2 i 15% H 2 S wprowadzany ponownie do formacji wodonoœnych bezpoœrednio pod borzêciñskim zbiornikiem gazu jest u ytkowany od 1996 r. Oprócz parametrów technologicznych, dokonaliœmy równie analizy procesu przemieszczenia rodzimego gazu ziemnego, który pierwotnie nasyca wodê gazami kwasowymi wtryskiwanymi do zbiornika. Taki

10 308 proces przemieszczenia pozwala na uzupe³nienie iloœci gazu objêtoœci¹ równowa n¹ metanowi rozpuszczonemu w wodzie. Wyniki badania PVT wskazuj¹, e objêtoœæ metanu przemieszczonego z wód zbiornika jest wprost proporcjonalna do objêtoœci CO 2 wtryskiwanego do zbiornika i e stê enie gazu kwaœnego w gazach wêglowodorowych przemieszczanych ze zbiornika stopniowo zwiêksza siê. GREENHOUSE GAS SEQUESTRATION IN AQUIFERS SATURATED BY NATURAL GASES Key words Reenhouse effect, sequestration, CO 2 emission, Borzêcin, Poznañ Trough Abstract It has been estimated that without real efforts to reduce greenhouse gas emission the global temperatures will rise by several degrees altering the world s climate. The coal contributes 95% to electricity generation in Poland. Therefore, low carbon emitting technology with the capture and underground storage of CO 2 is required in our country. Deep saline aquifers have the largest long-term storage potential of CO 2, but there are many problems with their exploration and qualification due to the lack of tightness confirmation. It is very important to reduce the cost of their exploration performed mainly by expensive drilling. In existing mega aquifers saturated by natural gases their tightness is confirmed by the presence of many local gas accumulations in top structures. In this paper we present Poznañ Trough megaaquifer naturally saturated by native natural gases. This megastructure represents a great potential for long-term underground CO 2 storage on 5000 km 2 area. Oil & Gas Institute and Polish Oil & Gas Company has gained a lot of experience in acid gas capture and storage. The acid gas containing 60% of CO 2 and 15% of H 2 S reinjected into an aquifer directly underlying the Borzêcin gas reservoir has been in operation since Apart from technological parameters we also analyzed the process of displacement of native natural gas which originally saturates the underlying water by acid gases injected into reservoir. Such a displacement process allows to replenish the gas cap by volume equivalent to methane gas dissolved in underlying water. The PVT study results indicate that volume of methane gas displaced from reservoir waters is in direct proportion to volume of CO 2 injected into reservoir and that acid gas concentration in hydrocarbon gases being displaced from reservoir is gradually increasing.