Gas Injection to Fractured Carbonate - Repressurisation (original) (raw)
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The purpose of the three experiments described in this paper was to investigate gas injection in fractured carbonate reservoirs, focusing on the effect of capillary continuity, re-pressurization by equilibrium gas, and injection of non-equilibrium gases. A composite core made from carbonate core material from a Middle East reservoir was placed vertically in a core holder with free space in the annulus between core and cell body. The core was saturated with oil phase and surrounded by gas. Synthetic fluid systems consisting of C1, C3, and nC10 were used. Pressure and temperature were selected to give proper IFT vs. capillary pressure and block height. Series of gas injections were performed on the composite core as a single block, with all plugs in capillary contact, and with two types of barriers in the middle of the composite core, establishing two blocks. Experimental data from the study include oil and gas production data, and gamma attenuation in-situ saturation profiles. Capillary pressure and relative permeabilities were measured on plugs similar to those used in the composite core. The experiments showed a significant effect of increasing the pressure from 200 to 240 bars, especially for the two block configuration. The oil recoveries from equilibrium gas injection with a barrier in the core were significantly lower than in the experiment with all plugs in contact. An experiment with a thin, perforated barrier gave similar poor oil recovery as with a thick barrier, indicating poor capillary contact in both cases. Injection of a rich non-equilibrium gas recovered almost 100 % of the oil, whilst injection of lean non-equilibrium gas also improved the oil recovery. A compositional reservoir simulation model was set up to simulate the experiments. The simulation model gave a good match of the experimental results after history matching.
SPE reservoir evaluation & engineering, 2023
Gas injection is the most widely applied recovery method in light, condensate, and volatile oil carbonate reservoirs. Gas has high displacement efficiency and usually results in a low residual oil saturation in the part of the reservoir that is contacted with gas. The displacement efficiency increases when the injected gas is near-miscible or miscible with the oil. In addition to nitrogen and hydrocarbon gas projects, carbon dioxide (CO 2) enhanced oil recovery (EOR) has been the dominant gas EOR process. Gas-based EOR has been implemented in both mature and waterflooded carbonate reservoirs. In this paper, we present the results of a detailed experimental study aimed at understanding the performance and efficiency of gas injection in carbonate reservoirs. A series of immiscible and miscible gas injection coreflood experiments were performed using limestone reservoir cores under different injection strategies. To minimize laboratory artifacts, long cores were used in the experiments, and to observe the effect of gravity, both 2 in. diameter and 4 in. diameter (whole core) were used. The experiments were performed under reservoir conditions using live crude oil. The core wettability was restored by aging the core in crude oil for several weeks under reservoir temperature. Hydrocarbon gas (methane) was used as the immiscible injectant, and both CO 2 and a mixture of 50% C 1 and 50% CO 2 were used as miscible injectant. All gas injection experiments were performed using vertically oriented cores, and the gas was injected from the top unless it is stated otherwise. The main parameters investigated in this study are as follows: 1. The effect of miscibility on oil recovery for both continuous gas injection and water alternating gas (WAG). 2. The effect of gravity on gas sweep efficiency compared to waterflooding. 3. The effect of gas-oil interfacial tension (IFT) on oil recovery when using the same oil. 4. The effect of oil type on oil recovery using the same injected gas at miscible and immiscible conditions. 5. The effect of immiscible gas injection on subsequent miscible gas injection performance. 6. Impact of CO 2 cycle length on ultimate oil recovery. 7. The impact of the order of fluid injection where multiple WAG injection cycles were performed in separate experiments after water or gas injection. The main conclusions of this study are as follows: 1. As expected, miscibility has a significant impact on displacement efficiency and oil recovery where miscible gas recovered more than 20% extra oil compared to immiscible gas. 2. A significant variation in oil recovery is observed for miscible gas injection (i.e., more than 10 saturation units difference) depending on the minimum miscibility pressure (MMP) between the injected gas and crude oil, even when both experiments are performed at miscible conditions using the same injected gas. 3. The performance of tertiary CO 2 flood was adversely affected by the slug of immiscible gas injected. Therefore, it is not recommended to have immiscible gas injection before miscible gas injection. 4. Regardless of injected gas type, gas injection with similar IFTs achieved similar oil recovery. 5. During WAG experiments, starting the injection cycles with water or gas did not have any impact on the ultimate oil recovery for both miscible and immiscible cases for one of the reservoirs, while WAG_G (WAG starting with gas injection) recovered more oil for another reservoir. 6. Gravity has a significant impact on oil recovery for both miscible and immiscible gas injections. A significant difference is observed in oil recovery when comparing CO 2 injection on 2-in.-and 4-in.-diameter core samples or when comparing horizontal vs. vertical immiscible gas injection and WAG experiment. 7. The longer the CO 2 slug size, the higher the oil recovery observed in gas injection experiments. The results of this study provide a rich and rarely available set of experimental data that can help improve and optimize gas and WAG injection in oil-wet carbonates. Introduction Carbonate reservoirs contain more than 50% of the world's hydrocarbon reserves and, on average, have recovery factors between 30 and 40%. Carbonate reservoirs are, in general, geologically more complex than clastic reservoirs as they have more complex pore systems, are highly heterogeneous, and often have dual porosity systems (in addition to fractures). Most carbonate reservoirs also show mixed-wet
Secondary and tertiary gas injection in fractured carbonate rock: Experimental study
Journal of Petroleum Science and Engineering, 2008
The use of CO 2 has received considerable interest as a method of EOR but a major drawback is its availability and increasing cost. Therefore, as the number of CO 2 injection projects increase, an alternative must be considered to meet the economic considerations. For this reason attention has been directed to nitrogen injection which may be a good substitute for CO 2 . The purpose of the experiments described in this paper was to investigate the efficiency of oil recovery by CO 2 and N 2 in fractured carbonate rock. The combined effects of gravity drainage and component exchange between gas in fracture and oil in matrix on oil recovery in fractured reservoirs subjected to CO 2 or nitrogen gas injection are experimentally studied. Laboratory experiments have been carried out on a low permeable outcrop chalk, as an analogue to a North Sea reservoir rock. This was surrounded by a fracture, established with a novel experimental set-up. The experiments aimed to investigate the potential of oil recovery by secondary and tertiary CO 2 and nitrogen gas injection at high pressure high temperature condition. The matrix block was saturated using recombined binary mixture live oil (C 1 -C 7 ), while the fracture was filled with a sealing material to obtain a homogeneous saturation. The sealing material was then removed by increasing the temperature which in turn creates the fracture surrounding the core. Gas was injected into the fracture at pressures above the bubble point of the oil. Oil recovery as a function of time was monitored during the experiments. Results from secondary gas injection experiments indicate that CO 2 injection at elevated pressure and temperature is more efficient than N 2 injection. Results from tertiary gas injection experiments also show that injection of CO 2 could significantly recover the oil, even after waterflooding, compared to N 2 injection.
CO2 and C1 Gas Injection for Enhanced Oil Recovery in Fractured Reservoirs
SPE International Conference on CO2 Capture, Storage, and Utilization, 2010
Gas injection serves as a main enhanced oil recovery (EOR) method in fractured-vuggy carbonate reservoir, but its effect differs among single wells and multi-well groups because of the diverse fractured-vuggy configuration. Many researchers conducted experiments for the observation of fluid flow and the evaluation of production performance, while most of their physical models were fabricated based on the probability distribution of fractures and caves in the reservoir. In this study, a two-dimensional physical model of the karst fault system was designed and fabricated based on the geological model of TK748 well group in the seventh block of the Tahe Oilfield. The fluid flow and production performance of primary gas flooding were discussed. Gas-assisted gravity flooding was firstly introduced to take full use of gas-oil gravity difference, and its feasibility in the karst fault system was examined. Experimental results showed that primary gas flooding created more flow paths and achieved a remarkable increment of oil recovery compared to water flooding. Gas injection at a lower location was recommended to delay gas breakthrough. Gas-assisted gravity flooding achieved more stable gas-displacing-oil because oil production was at a lower location, and thus, the oil recovery was further enhanced.
Investigation of Oil Recovery in Fractured Carbonate Rock by
Energy and Environment Research, 2016
The purpose of the three experiments described in this paper is to investigate the efficiency of secondary andtertiary gas injection in fractured carbonate reservoirs, focusing on the effect of equilibrium gas,re-pressurization and non-equilibrium gas. A weakly water-wet sample from Asmari limestone which is the mainoil producing formation in Iran, was placed vertically in a specially designed core holder surrounded withfracture. The unique feature of the apparatus used in the experiment, is the capability of initializing the samplewith live oil to obtain a homogeneous saturation and create the fracture around it by using a special alloy whichis easily meltable. After initializing the sample, the alloy can be drained from the bottom of the modified coreholder and create the fracture which is filled with live oil and surrounded the sample. Pressure and temperaturewere selected in the experiments to give proper interfacial tensions which have been measured experimentally.Series of sec...
Low IFT gas–oil gravity drainage in fractured carbonate porous media
Journal of Petroleum Science and Engineering, 2010
This paper addresses a study of gas-oil gravity drainage in fractured carbonate rock subjected to gas injection in low interfacial tension. The purpose of the experiments described in the paper was to investigate gas injection in fractured carbonate reservoirs in both secondary and tertiary cases (after water injection), focusing on gravity drainage using equilibrium gas followed by re-pressurization. Gas injection experiments were performed on 20 cm long and low permeable outcrop chalk core surrounded with a fracture established with a novel experimental setup in reservoir conditions. The core was saturated with binary mixture live oil consisting of C 1 and C 7 of a known composition, while the fracture was filled with sealing material to obtain a homogeneous saturation. After core initialization, the sealing material was removed by increasing the temperature to higher than its melting point and displaced by live oil. Gas was then injected into the fracture and gravity drainage experiments were performed in low interfacial tension (< 0.5 mN/m) where the IFT between the phases were measured experimentally by selecting the proper pressure and temperature. Experiments were performed at different pressures and reversibility of the effect of the interfacial tension was checked by re-pressurization process. The oil recovered from the bottom side of the block was measured versus time. Based on the results of this study, the recovery of oil showed a significant increase by re-pressurization in gravity drainage process. It was also clear that low IFT gravity drainage is capable to recover a significant amount of oil in fractured reservoirs even after water injection.
Enhanced oil recovery by CO2injection in carbonate reservoirs
Energy and Sustainability V, 2014
The majority of carbonate reservoirs have low porosity and permeability in general because of having a high amount of matrixes that make a heterogeneous reservoir, however high permeable layers are fractured. This study shows the effect of carbon dioxide injection on the oil recovery factor using an ECLIPSE 300 compositional reservoir simulator for 3D modelling and the change of carbonate components reaction during CO 2 injection in experimental work. In addition, a high recovery factor has been recorded during miscible CO 2 injection compared to immiscible injection. Water alternative gas (WAG) has been used as an enhanced oil recovery (EOR) method to overcome an unfavourable mobility ratio of CO 2 flooding. Miscible CO 2 injection with the aid of WAG has also had a great impact on the dissolution of carbonate components in dissolving calcite and dolomite components. Consequently, CO 2 flooding has a relatively low recovery factor without any EOR techniques such as gravity stable displacement, WAG or mobility control. CO 2 injection below minimum miscibility pressure (MMP) reduces CO 2 emission, while it takes too long time to maintain reservoir pressure. On the other hand, CO 2 flooding above MMP improves pressure maintenance; causes oil swelling, and increases the oil density.
THAI ("toe-to-heel" air injection) is a novel thermal enhanced oil recovery method that has been studied in the case of conventional sandstone systems. It involves vertical injection and horizontal producer wells in a line drive configuration and is regarded as a promising tool for some reservoirs. However, its applicability on fractured carbonate rocks as an in-situ combustion-based method remains questionable. The aim of the present work was to study the feasibility of the THAI process in the presence of parallelepiped networked fractures to study how the presence of networked fractures affects on the process outcome. Both conventional and fractured THAI models and the effect of different operational and reservoir parameters on the process performance have been considered. Networked fractures reduced the performance of the THAI process. Parameters such as ultimate oil recovery factor, °API quality of the produced oil, front temperature and sweep efficiency were all reduced in non-conventional model as compared to the conventional one, but the process has high performance even after. In the case of networked fractures, the air injection rate should be optimized since the ultimate oil recovery is reduced after a limit, and there is also higher possibility of carbonate rock decomposition, oxygen breakthrough and water emulsions production for higher injection rates. Heterogeneity ratio reduction from 1000 to 100 has a significant effect on process performance since the combustion fronts in matrix and fractures systems have higher opportunity to become coupled together. THAI process start up should be optimally chosen for a system, considering the fact that the higher initial oil saturation at the start, leads to the higher ultimate oil recovery.
2016
The purpose of the three experiments described in this paper is to investigate the efficiency of secondary and tertiary gas injection in fractured carbonate reservoirs, focusing on the effect of equilibrium gas, re-pressurization and non-equilibrium gas. A weakly water-wet sample from Asmari limestone which is the main oil producing formation in Iran, was placed vertically in a specially designed core holder surrounded with fracture. The unique feature of the apparatus used in the experiment, is the capability of initializing the sample with live oil to obtain a homogeneous saturation and create the fracture around it by using a special alloy which is easily meltable. After initializing the sample, the alloy can be drained from the bottom of the modified core holder and create the fracture which is filled with live oil and surrounded the sample. Pressure and temperature were selected in the experiments to give proper interfacial tensions which have been measured experimentally. Series of secondary and tertiary gas injection were carried out using equilibrium and non-equilibrium gas. Experiments have been performed at different pressures and effect of reduction of interfacial tension were checked by re-pressurization process. The experiments showed little oil recovery due to water injection while significant amount of oil has been produced due to equilibrium gas injection and re-pressurization. Results also reveal that CO 2 injection is a very efficient recovery method while injection of C 1 can also improve the oil recovery.