- Email: [email protected]
Experimental study of air foam flow in sand pack core for enhanced oil recovery
Author’s Accepted Manuscript Experimental study of air foam flow in sand pack core for enhanced oil recovery Shuai Hua, Yifei Liu, Qinfeng Di, Yichong Chen, Feng Ye www.elsevier.com/locate/petrol
PII: DOI: Reference:
S0920-4105(15)30095-4 http://dx.doi.org/10.1016/j.petrol.2015.08.021 PETROL3162
To appear in: Journal of Petroleum Science and Engineering Received date: 8 October 2013 Revised date: 8 July 2015 Accepted date: 31 August 2015 Cite this article as: Shuai Hua, Yifei Liu, Qinfeng Di, Yichong Chen and Feng Ye, Experimental study of air foam flow in sand pack core for enhanced oil r e c o v e r y , Journal of Petroleum Science and Engineering, http://dx.doi.org/10.1016/j.petrol.2015.08.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental Study of Air Foam Flow in Sand Pack Core for Enhanced Oil Recovery Shuai Huaa , Yifei Liub , Qinfeng Dia,Yichong Chena , Feng Yea a Shanghai Institute of Applied Mathematics and Mechanics, Shanghai University, China
b Xi’an Shiyou University, China
Abstract As an innovative way of Enhancing Oil Recovery (EOR), the air foam flooding technique comprises the merits of air flooding and foam flooding, which not only has the double effects on profile controlling and oil displacement, but also avoids the gas channeling weakness. In this paper, a series of flow experiments of the simultaneous injection of air and air foam through artificial sand pack core was conducted to investigate changes in gas and oil composition and oil displacement efficiency. The effects of foaming agent concentration and slug were investigated. The results show that in low temperature oxidation process, oxygen content decreases, and carbon dioxide content increases. Aromatic hydrocarbon content decreases, while resins and asphaltene content increases. Oil displacement efficiency of air foam flooding is much higher than air flooding. The Foam blocking ability and the function of profile controlling of the foam, significantly prolongs gas breakthrough. The displacement efficiency was dropped due to the decrease of foaming agent concentration or the decrease of the foam slug.
Keywords air foam flooding; experimental study; low temperature oxidation; enhanced oil recovery
Corresponding author. E-mail address: [email protected] (Qinfeng DI).
1. Introduction
In recent years, much attention has been paid to gas injection recovery, which is one of the important methods for enhanced oil recovery (EOR). The EOR production attributable to gas injection has increased quite rapidly (Yan et al., 2008). Compared to water flooding, gas is easier to inject because of its general favorable mobility. Moreover, gas injection may result in viscosity reduction, volume expansion, proliferation and the reduction of interfacial tension. In recent years, a large number of laboratory tests and field applications have proved that the injection of carbon dioxide, nitrogen, natural gas and flue-gas technology is a highly efficient EOR technology. But due to the shortcoming of inadequate gas supply and high cost, there are a lot of restrictions on the application of these technologies (Moore et al., 1999). Differing from other gas injection methods, air injection technology uses air as the injection medium. Air has no resource limits or cost limits, and is abundant and inexpensive in price (Moore et al., 2002). Therefore, air injection has recently attracted considerable attentions. The oil recovery technology of injecting air with high pressure has its merit, and it is a new promising technology of EOR. Air injection not only has the general effect of gas injection, but also has the effect of crude oil oxidation. When air is injected into a reservoir, the mixture of oxygen and oil produces a low-temperature oxidation reaction, and consumes oxygen generating carbon oxides. The heat of the reaction makes reservoir temperature increase, resulting in increased mobility of oil. For the heterogeneous reservoirs, however, gas channeling and viscous fingering are prone to happen. Pure air injection not only limits the oil displacement efficiency, but also results in some safety hazards, such as the oxygen in the air may channel to the producing well. To solve the problems of gas channeling and viscous fingering in the process of high pressure air injection, a foam system is added when injecting air. The special physical and chemical properties of foam improve the effect of high
pressure air injection (HPAI) (Gauglitz et al., 2002). EOR by the use of foam has been employed during field steam-foam pilots in the late 1980’s and several non-thermal applications of foam in the middle 1990’s (Patzek et al., 1990). Micro-models have proved to be very useful in visualizing foam phenomena in porous media. Researchers have used micro-models to investigate the mechanisms by which foam is generated, destroyed and interact with oil (Nguyen et al., 2000). Haugen et al. performed a series of water, gas, and surfactant core-flood experiments in fractured oil-wet limestone rock. Their results showed that oil recovery by such process is low, i. e. less than 10% of original oil in place (OOIP). When foam was injected, the oil recovery was significantly improved with up to 78% of OOIP (Haugen et al., 2012). Foam as a mobility-control agent has been successful in many field applications, such as in steam foam flood (Patzek et al., 1996), foam-assisted water-alternating-gas injection in the Norwegian Snorre field (Blaker et al., 2002), aquifer remediation (Hirasaki et al., 1997), and alkaline / surfactant / polymer (ASP) flooding (Wang et al., 2001). As an innovative way of Enhancing Oil Recovery (EOR), the air foam flooding technique comprises the merits of air flooding and foam flooding, which not only has the double effects on profile controlling and oil displacement, but also avoids the gas channeling weakness. It’s a creative way of EOR. This cost-effective and safe technology applies to more oil reservoirs in variety, depth and scope. It is ideal for the oil reservoirs with high water content, heterogeneous, and fissures or big pore throat. Another distinctive feature of air foam is low density and high elasticity. As the permeability increases, the apparent viscosity rises accordingly and can effectively adjust the mobility differences between layers with high and
low permeability, which lead to simultaneous movement and achieve layer mobility displacement, enlarge the sweep volume and postpone gas breakthrough (Zhao et al., 2008). Therefore, air foam flooding effectively overcomes the weakness of air channeling and small sweep volume. It is a new way of EOR which has great potential. In the experiments reported in this paper, a series of flow experiments of the simultaneous injection of air and air foam through artificial sand pack core was conducted to investigate changes in gas and oil composition and oil displacement efficiency. The effects of foaming agent concentration and slug were investigated.
2. Experimental Approach
2.1Fluids Materials
Stock tank oil from Liao He oil field was used for core preparation. The oil viscosity was 2.58 mPa.s, with a density of 0.844 g/cm3, a freezing point of 298K, and a wax content of 7.1%. Formation water of NaHCO3 type with 4199.45 mg/L was used. The air was mainly composed of nitrogen (78.04%), oxygen (21.9%) and Carbon dioxide (0.03%). Under 293K, the air viscosity was 0.0179 mPa˖s, and its density was 1.293 g/L. The foaming agent chosen was HS-403 which is an alkyl benzene sulfonate surfactant. This foaming agent is a white viscous liquid, with density of 1.2 g/cm3 and a PH value of 8.
2.2Core Sample Preparation
The physical simulation of artificial sand pack core modeling is an important scientific
research tool in oilfield development, and plays a quiet important role in the study of secondary and tertiary oil recovery. The steps for making sand pack core models are as follows: (1) Making sand-filled pipe. First of all, the vibrating screen is used to separate the reservoir sand into seven particle sizes. After that, the different particle sizes of sand were mixed based on the reservoir size distribution, and then were compacted with hydraulic sand filling machine. (2) Measuring the absolute permeability of the core. (3) Measuring the effective core porosity. (4) Establishing initial core oil saturation. (5) Calculating physical parameters of the sand pack core assembly. Based on the previous steps, the sand pack core model, which matches the air foam flooding experimental condition, can be obtained. In the experiment, the core model is 32mm in diameter and 5m in length, and the average porosity, permeability and oil saturation are about 20%, 10×10-3 um2 and 60% separately. The model has sufficient length to produce the oxidized zone, the miscible zone and the oil zone after crude oil and oxygen undergo multistage contact. The inclusion of all three zones effectively simulate the LTO process.
2.3Experimental System and Devices
The air foam flooding system is composed of 8 parts: mainly of [1] a high pressure air injection system, [2] a foaming agent injection system, [3] a formation water injection system, [4] an experimental core model, [5] a temperature-pressure control and test system, [6] a gas sampling and detecting system, [7] a back pressure control system, and [8] a measurement monitoring system. A schematic of air foam flooding experimental system is shown in Fig.1. A high pressure air injection system is used to inject air into the core model under constant temperature and constant pressure, mainly through a use of a compressor, pumping
units, and a high-pressure air accumulator and pressure relief regulators (PRR). A foaming agent and a formation water injection system are used to inject the foaming agent and formation water into the core model through a constant flux pump and nitrogen cylinder. The temperature-pressure control and text system consist mainly of a constant temperature oven, a pressure temperature sensor and a M400 data acquisition software. The data of temperature and from a pressure sensor inside a constant temperature oven can be recorded by computer software. Generating a report from data can be achieved at the desired time and format. The gas-sampling detection system is composed mainly of a sampler and a HB0-2B type injection oxygen-detector. The oxygen content at various time checkpoints can be detected in the model through the use of this system. The back pressure control system consists mainly of pressure reducing regulators and back-pressure regulators (BPR). By the control of pressure of the PRR and BPR, the pressure difference at both ends of the model is controlled in order to simulate the actual production pressure difference. The main components of the measurement monitoring system are an oil-gas-water separator, drainage gas recovery device, and a measuring device. By adopting the measuring and detecting system, oil-gas-water separation can be achieved and the volume of oil, gas, water can be measured during the displacement process. The experimental steps are as follows: (1) The experimental instruments are connected based on Fig. 1. (2) The 0.1 PV foaming agent is driven into the model with 0.1 ml/min flow rate. (3) Inject high pressure air under the condition of 30 MPa injection pressure, and 0.2 MPa displacement differential pressures. (4) The model pressure and temperature are obtained by computer. (5) The content of oxygen in the model is tested per 24 hours. (6) The experiment
was stopped when the oxygen content is higher than 14% at the exit.
3 Results and Discussions
3.1 Changes in Gas Composition
After 168 hours, low temperature oil oxidation experiments were conducted at 30MPa and 90℃, using gas sampling exports from the sand pack core model. The experimental results are shown in Tab.1. From Tab.1, it can be seen that in a low temperature oxidation process, oxygen content decreases while carbon dioxide and carbon monoxide content increases. This is mainly due to an oxidative addition reaction which first occurs in hydrocarbons and oxygen, and which produces carboxylic acids, aldehydes, ketones, alcohols, peroxides and other intermediate products. Therefore, the intermediate products will continue to be oxidized to carbon oxide and water. After the reaction, the gas content of C1 ~ C6 increases, which is due to the light component of the oil volatilized into the air, and also led to the increase of the average molecular weight and the relative density of the gas. Nitrogen is an inert gas that does not react with oil. Oxygen content reduction from the oxidation reactions led to the increase of nitrogen-to-oxygen ratio.
3.2 Changes in Oil Composition
After 168 hours, low temperature oil oxidation experiments were conducted at 30MPa and 90℃, using gas sampling exports from the sand pack core model.
3.2.1 Elemental Analysis
Elemental changes in the oil are detected before and after the reaction, and the results are shown in Tab.2. From Tab. 2, it can be seen that oxygen content increases, as oil at the low temperature oxidation process generates the oxygen-containing compounds. Carbon content decreases after the reaction, because the generation of oxygen-containing compounds and crude oil lighter components evaporate into the air. After the reaction, hydrogen content decreases, because this produces a certain amount of water in the reaction process.
3.2.2 Group Composition Analysis
Oil group composition refers to the use of different organic solvents on the different types of components and structures of oil for selective separation, and obtains physical and chemical properties similar mixtures. It is generally divided into saturated hydrocarbon, aromatic hydrocarbon, resin and asphaltene. Group composition changes are detected in the Oil before and after the reaction. The results are shown in Tab.3, from which it can be seen that aromatic hydrocarbon content decreases, while resins and asphaltene content increases. This is mainly because aromatic hydrocarbon is oxidized to resin and asphaltene. The increase of resin and asphaltene will deteriorate the oil’s physical property, and increase oil flow resistance, which is going against EOR.
3.2.3 Chromatographic Analysis
The different numbers of carbon atoms n-alkane distribution in oil, both before and after
the reaction can be detected by the use of chromatographic analysis. The results are shown in Fig. 2. From Fig.2, it can be seen that oil before and after the reaction were mainly composed of C2-C38 n-alkanes with C15 as a main peak. N-alkanes in oil weight ratio (ΣnC21-/ΣnC22+) increases, because parts of the long carbon chain break into short carbon chain in the process of oxidation, resulting in a decrease of heavy component and an increase of light components. The increase of light components improves the physical property of oil, which is beneficial to EOR.
3.3 Air and Air Foam Flooding Displacement Efficiency
Fig. 3 shows the relation between accumulative injection pore volume (PV) and displacement efficiency (recovery factor) through the two experiments. From Fig.3, it can be seen that in air flooding, the displacement efficiency increases gradually with the increase of injection PV. At the point of 0.23PV (the volume under high pressure and high temperature in the model), air breaks through and the oil displacement efficiency is 35.8%. Afterwards, it begins to slow down, and finally the oil displacement efficiency reaches to 38.45%. All this shows that the increase of displacement efficiency in air flooding mainly happens before the air breakthrough. After the air breakthrough, the amount of displaced oil is very limited and the displacement efficiency did not change significantly. In air foam flooding, with the increases of air injection PV, the displacement efficiency rises accordingly. At the point of 0.47PV, air breaks through with the displacement efficiency rises to 73.35%, and the final figure is 74.61%. The way displacement efficiency changes over time is basically the same as that in air flooding.
From the comparison of the two groups of experiments, we are able to see an increase of 36.1% of displacement efficiency in air foam flooding over air flooding. That is because the increase of gas phase viscosity and decrease of gas phase permeability make the gas phase mobility with foam injection far smaller than the gas phase mobility without foam injection. As a result, the amount of gas decreases and that of oil and formation water rises. Also with the foaming agent’s oil-washing effect, the displacement efficiency of air foam flooding remarkably exceeds that of air flooding.
3.4 Air and Air Foam Flooding Oxygen Consumption
At present, a key problem of air flooding technology encountered is the rapid increase of oxygen level in the producing well after air is injected. When the oxygen in the gas mixture in the producing well reaches to the limit of explosion, the wellhead will probably catch on fire or explode (Hua et al., 2010). To study the changes of oxygen level in air flooding and air foam flooding, the gas sampling opening in the experiment model was set up and, through which, the oxygen level in different sampling openings can be measured with oxygen recorder at anytime. In this experiment, four gas sampling openings are set up, No.1 at the entrance of gas (0 m from the entrance), No. 2 at place 1 m from the entrance, No.3 at 3 m from the entrance and No.4 at the exit (5m from the entrance). The monitoring result of oxygen level at different times in air flooding is shown in Fig. 4. As time goes by, the oxygen level at location No. 1 is unchanged with time (21.9%) because it is the air injection inlet. A gradual increase of oxygen level can be seen at No.2, No.3 and No.4 checkpoints. 69 hours later after the air injection, air breaks through and the oxygen level is 16.8% at the exit. 85 hours later after the air injection, the oxygen level at the exit reaches to
20.4%. The monitoring result of oxygen levels at different times in air foam flooding is shown in Fig.5. As the time goes on, the oxygen level at No.1 checkpoint (entrance) is not changed. A gradual increase of oxygen level can also be seen at No.2, No.3 and No.4 checkpoints. 194 hours later after the air injection, air breaks through and the oxygen level is 1.6% at the exit. 216 hours later after the air injection, the oxygen level at the exit reaches to 16.8%. The experimental results showed that the time of gas breakthrough was shortened, and the gas breakthrough oxygen content was apparently decreased. The gas breakthrough oxygen content in air flooding was at 16.8%, and is only 1.6% in air foam flooding. That is due to the the Jamin effect, the function of profile controlling of the foam, and thus the gas fingering is slowed down resulting in effectively prolonging the gas breakthrough time. The reaction time of oxygen and oil increases and the oxygen content decreases gradually when the gas breakthrough. The changes of crude oil composition are similar to the crude oil low temperature oxidation experiments. Carbon content and hydrogen content decreases while oxygen content increases in the crude oil after the reaction. Aromatic hydrocarbon content decreases, while resins and asphalting content increases.
3.5 Effects of Concentration of Foaming Agent on Displacement Efficiency
To study the effects of concentration of the foaming agent on displacement efficiency, two experiments of foam flooding with foaming agent concentration of 1% and 0.5% were conducted. From Fig.6, one can see that a decrease of 7.8% of displacement efficiency lead to a decrease of 0.5% of foaming agent concentration. This is mainly because the decrease of
foaming agent concentration leads to a shortening of foam life and a decrease in the foam volume that can reduce the foam plugging effect and cause a decrease of oil displacement efficiency.
3.6 Effects of Slug of Foaming Agent on Displacement Efficiency
To study the effects of slug volume on displacement efficiency, two experiments of foam flooding were conducted using a foaming agent slug of 0.1PV and 0.05PV. From Fig.7, one can see that a decrease of 16.52% of displacement efficiency leads to a decrease of 0.05PV in the foaming agent slug. This is mainly due to the decrease of foam slug, resulting in excessive loss of the foaming agent, gas through the forward moving slug and finally breaking quickly which leads to the decrease of the oil displacement efficiency.
3. Conclusions
Preliminary conclusions of the air foam flooding experimental study are as follows: (1) In a low temperature oxidation process, oxygen content decreases, while carbon dioxide and carbon monoxide content increases. Aromatic hydrocarbon content decreases, while resins and asphaltene content increases. N-alkanes in oil weight ratio (ΣnC21-/ΣnC22 +) increases. (2) The oil displacement efficiency of air foam flooding is much higher than air flooding. Before the air breakthrough point in air flooding and air foam flooding, air or air foam injection will observably enhance crude oil displacement efficiency. After the air breakthrough point, the displacement production of crude oil is quite small, and the increase of displacement
efficiency is within a very narrow range. (3) The foam blocking ability and the function of profile controlling of the foam can significantly prolong gas breakthrough. The gas breakthrough oxygen content fell sharply, which effectively decreased the potential safety hazard in the process of air injection. (4) A decrease of 7.8% of displacement efficiency in the concentration of the foaming agent dropped from 0.1% to 0.05%. A decrease of 16.52% of displacement efficiency in the slug of the foaming agent dropped from 0.1PV to 0.05PV.
Acknowledgments
This research is supported partly by the National Science Funding of China (50874071), the Chinese National Programs for High Technology Research and Development (SS2013AA061104 ), Shanghai Program for Innovative Research Team in Universities, Shanghai Leading Academic Discipline Project (S30106)，The Excellent Academic Leading Person Program of Science and Technology Commission of Shanghai Municipality（12XD1402500）, Shanghai Leading Talents Project. Part of the ability of local colleges and universities in Shanghai construction projects (12160500200).
References
Fengpeng Yan, Lixing Yang et al. Effect Analysis of Enhancing Ultra-low Permeability Oilfield Recovery by Air-foam Flooding. Journal of Yanan University (Natural Science Edition),2008;27(4):58-60 Moore, R.G., Laureshen, C.J., Ursenbach, M.G. et al. 1999. A Canadian Perspective on In Situ Combustion. J Can Pet Technol 38 (13): 1-8.
Moore R G， Mehta S A. A guide to high pressure air injection (HPAI) based oil recovery[C]. SPE75207, 2002. Gauglitz, P.A., Friedmann, F., Kam, S.I. et al. 2002. Foam Generation in Porous Media. Presented at the SPE/DOE Improved Oil Recovery Symposium, Tulsa, 13-17 April. SPE-75177-MS. Patzek T.W. and Koinis M.T. 1990. Kern River Steam-Foam Pilots. Journal of Petroleum Technology 42(4):496-503. Nguyen, Q.P., Alexandrov, et al. 2000. Experimental and Modeling Studies on Foam in Porous Media A Review. SPE International Symposium on Formation Damage Control. USA. Haugen, Å., Fernø, M.A., Graue, A., and Bertin, H.J. 2012. Experimental Study of Foam Flow in Fractured Oil-Wet Limestone for Enhanced Oil Recovery. SPE Res Eval & Eng 15 (2): 218-228. SPE-129763-PA. Patzek, T.W. 1996. Field Applications of Steam Foam for Mobility Improvement and Profile Control. SPE Res Eng 11 (2): 79-86. SPE-29612-PA. Blaker, T., Aarra, M.G., Skauge, A. et al. 2002. Foam for Gas Mobility Control in the Snorre Field: The FAWAG Project. SPE Res Eval & Eng 5 (4): 317-323. SPE-78824-PA. Hirasaki, G.J., Miller, C.A., Szafranski, R. et al. 1997. Field Demonstration of the Surfactant/Foam Process for Aquifer Remediation. Paper. SPE 39292 Wang, D., Cheng, J., Yang, Z. et al. 2001. Successful Field Test of the First Ultra-Low Interfacial Tension Foamflood. SPE 72147 Jinsheng Zhao.Study on Nitrogen Foam Flooding in Uniform Mobility for EOR after Polymer Flooding.China University of Petroleum(East China),Doctoral Dissertation,2008.4：5-6 Shuai
Hua,Yifei
Liu
et
al.The
Problem
of
Reservoir
Air
Countermeasure.Oil-Gasfield Surface Engineering,2010,29(11);47-48
Injection
Technology
and
Its
Tab.1 Air Component Changes Before and After Reaction Detection results Air before reaction Air after reaction
O2
N2
CO2
CO
C1-C6
N2/O2
Relative density
Average molecular weight
21.9%
78.04%
0.03%
0
0
3.57
0.9662
27.98
16.86%
81.71%
0.62%
0.17%
0.63%
4.85
0.9995
28.93
Note: figures in the table1, 2, and 3 are for mass percentage.
Tab.2 Oil elemental Changes before and after Reaction Elemental
O（%）
C（%）
H（%）
N（%）
Oil before reaction
5.52
81.16
12.54
0.06
Oil after reaction
6.69
79.77
11.20
0.08
Tab.3 Oil Group Composition Changes Before and After Reaction Group composition
Saturated hydrocarbon
Aromatic hydrocarbon
Resin
Asphaltene
Oil before reaction
51.0%
31.2%
11.4%
6.4%
Oil after reaction
50.0%
25.6%
17.0%
7.4%
Fig. 1 Schematic of Air Foam Flooding Experimental System
Fig.2 Oil Before and After the Reaction N-alkane Distribution Changes
Fig. 3 Relations between Injection Pore Volume and Displacement Efficiency
Fig. 4 Changes of Oxygen Content at Different Monitoring Point with Time in Air Flooding
Fig. 5 Changes of Oxygen Content at Different Monitoring Point with Time in Air Foam Flooding
Fig.6 Effects of Concentration of Foaming Agent on Displacement Efficiency
Fig.7 Effects of Slug of Foaming Agent on Displacement Efficiency
Highlights •A new experimental model of air foam flooding process is designed. •Quantitative research is conducted about the changes of oil and air composition. •Oil displacement efficiency of air foam flooding is much higher than air flooding. •The changes of oxygen level in air flooding and air foam flooding are studied.
PII: DOI: Reference:
S0920-4105(15)30095-4 http://dx.doi.org/10.1016/j.petrol.2015.08.021 PETROL3162
To appear in: Journal of Petroleum Science and Engineering Received date: 8 October 2013 Revised date: 8 July 2015 Accepted date: 31 August 2015 Cite this article as: Shuai Hua, Yifei Liu, Qinfeng Di, Yichong Chen and Feng Ye, Experimental study of air foam flow in sand pack core for enhanced oil r e c o v e r y , Journal of Petroleum Science and Engineering, http://dx.doi.org/10.1016/j.petrol.2015.08.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental Study of Air Foam Flow in Sand Pack Core for Enhanced Oil Recovery Shuai Huaa , Yifei Liub , Qinfeng Dia,Yichong Chena , Feng Yea a Shanghai Institute of Applied Mathematics and Mechanics, Shanghai University, China
b Xi’an Shiyou University, China
Abstract As an innovative way of Enhancing Oil Recovery (EOR), the air foam flooding technique comprises the merits of air flooding and foam flooding, which not only has the double effects on profile controlling and oil displacement, but also avoids the gas channeling weakness. In this paper, a series of flow experiments of the simultaneous injection of air and air foam through artificial sand pack core was conducted to investigate changes in gas and oil composition and oil displacement efficiency. The effects of foaming agent concentration and slug were investigated. The results show that in low temperature oxidation process, oxygen content decreases, and carbon dioxide content increases. Aromatic hydrocarbon content decreases, while resins and asphaltene content increases. Oil displacement efficiency of air foam flooding is much higher than air flooding. The Foam blocking ability and the function of profile controlling of the foam, significantly prolongs gas breakthrough. The displacement efficiency was dropped due to the decrease of foaming agent concentration or the decrease of the foam slug.
Keywords air foam flooding; experimental study; low temperature oxidation; enhanced oil recovery
Corresponding author. E-mail address: [email protected] (Qinfeng DI).
1. Introduction
In recent years, much attention has been paid to gas injection recovery, which is one of the important methods for enhanced oil recovery (EOR). The EOR production attributable to gas injection has increased quite rapidly (Yan et al., 2008). Compared to water flooding, gas is easier to inject because of its general favorable mobility. Moreover, gas injection may result in viscosity reduction, volume expansion, proliferation and the reduction of interfacial tension. In recent years, a large number of laboratory tests and field applications have proved that the injection of carbon dioxide, nitrogen, natural gas and flue-gas technology is a highly efficient EOR technology. But due to the shortcoming of inadequate gas supply and high cost, there are a lot of restrictions on the application of these technologies (Moore et al., 1999). Differing from other gas injection methods, air injection technology uses air as the injection medium. Air has no resource limits or cost limits, and is abundant and inexpensive in price (Moore et al., 2002). Therefore, air injection has recently attracted considerable attentions. The oil recovery technology of injecting air with high pressure has its merit, and it is a new promising technology of EOR. Air injection not only has the general effect of gas injection, but also has the effect of crude oil oxidation. When air is injected into a reservoir, the mixture of oxygen and oil produces a low-temperature oxidation reaction, and consumes oxygen generating carbon oxides. The heat of the reaction makes reservoir temperature increase, resulting in increased mobility of oil. For the heterogeneous reservoirs, however, gas channeling and viscous fingering are prone to happen. Pure air injection not only limits the oil displacement efficiency, but also results in some safety hazards, such as the oxygen in the air may channel to the producing well. To solve the problems of gas channeling and viscous fingering in the process of high pressure air injection, a foam system is added when injecting air. The special physical and chemical properties of foam improve the effect of high
pressure air injection (HPAI) (Gauglitz et al., 2002). EOR by the use of foam has been employed during field steam-foam pilots in the late 1980’s and several non-thermal applications of foam in the middle 1990’s (Patzek et al., 1990). Micro-models have proved to be very useful in visualizing foam phenomena in porous media. Researchers have used micro-models to investigate the mechanisms by which foam is generated, destroyed and interact with oil (Nguyen et al., 2000). Haugen et al. performed a series of water, gas, and surfactant core-flood experiments in fractured oil-wet limestone rock. Their results showed that oil recovery by such process is low, i. e. less than 10% of original oil in place (OOIP). When foam was injected, the oil recovery was significantly improved with up to 78% of OOIP (Haugen et al., 2012). Foam as a mobility-control agent has been successful in many field applications, such as in steam foam flood (Patzek et al., 1996), foam-assisted water-alternating-gas injection in the Norwegian Snorre field (Blaker et al., 2002), aquifer remediation (Hirasaki et al., 1997), and alkaline / surfactant / polymer (ASP) flooding (Wang et al., 2001). As an innovative way of Enhancing Oil Recovery (EOR), the air foam flooding technique comprises the merits of air flooding and foam flooding, which not only has the double effects on profile controlling and oil displacement, but also avoids the gas channeling weakness. It’s a creative way of EOR. This cost-effective and safe technology applies to more oil reservoirs in variety, depth and scope. It is ideal for the oil reservoirs with high water content, heterogeneous, and fissures or big pore throat. Another distinctive feature of air foam is low density and high elasticity. As the permeability increases, the apparent viscosity rises accordingly and can effectively adjust the mobility differences between layers with high and
low permeability, which lead to simultaneous movement and achieve layer mobility displacement, enlarge the sweep volume and postpone gas breakthrough (Zhao et al., 2008). Therefore, air foam flooding effectively overcomes the weakness of air channeling and small sweep volume. It is a new way of EOR which has great potential. In the experiments reported in this paper, a series of flow experiments of the simultaneous injection of air and air foam through artificial sand pack core was conducted to investigate changes in gas and oil composition and oil displacement efficiency. The effects of foaming agent concentration and slug were investigated.
2. Experimental Approach
2.1Fluids Materials
Stock tank oil from Liao He oil field was used for core preparation. The oil viscosity was 2.58 mPa.s, with a density of 0.844 g/cm3, a freezing point of 298K, and a wax content of 7.1%. Formation water of NaHCO3 type with 4199.45 mg/L was used. The air was mainly composed of nitrogen (78.04%), oxygen (21.9%) and Carbon dioxide (0.03%). Under 293K, the air viscosity was 0.0179 mPa˖s, and its density was 1.293 g/L. The foaming agent chosen was HS-403 which is an alkyl benzene sulfonate surfactant. This foaming agent is a white viscous liquid, with density of 1.2 g/cm3 and a PH value of 8.
2.2Core Sample Preparation
The physical simulation of artificial sand pack core modeling is an important scientific
research tool in oilfield development, and plays a quiet important role in the study of secondary and tertiary oil recovery. The steps for making sand pack core models are as follows: (1) Making sand-filled pipe. First of all, the vibrating screen is used to separate the reservoir sand into seven particle sizes. After that, the different particle sizes of sand were mixed based on the reservoir size distribution, and then were compacted with hydraulic sand filling machine. (2) Measuring the absolute permeability of the core. (3) Measuring the effective core porosity. (4) Establishing initial core oil saturation. (5) Calculating physical parameters of the sand pack core assembly. Based on the previous steps, the sand pack core model, which matches the air foam flooding experimental condition, can be obtained. In the experiment, the core model is 32mm in diameter and 5m in length, and the average porosity, permeability and oil saturation are about 20%, 10×10-3 um2 and 60% separately. The model has sufficient length to produce the oxidized zone, the miscible zone and the oil zone after crude oil and oxygen undergo multistage contact. The inclusion of all three zones effectively simulate the LTO process.
2.3Experimental System and Devices
The air foam flooding system is composed of 8 parts: mainly of [1] a high pressure air injection system, [2] a foaming agent injection system, [3] a formation water injection system, [4] an experimental core model, [5] a temperature-pressure control and test system, [6] a gas sampling and detecting system, [7] a back pressure control system, and [8] a measurement monitoring system. A schematic of air foam flooding experimental system is shown in Fig.1. A high pressure air injection system is used to inject air into the core model under constant temperature and constant pressure, mainly through a use of a compressor, pumping
units, and a high-pressure air accumulator and pressure relief regulators (PRR). A foaming agent and a formation water injection system are used to inject the foaming agent and formation water into the core model through a constant flux pump and nitrogen cylinder. The temperature-pressure control and text system consist mainly of a constant temperature oven, a pressure temperature sensor and a M400 data acquisition software. The data of temperature and from a pressure sensor inside a constant temperature oven can be recorded by computer software. Generating a report from data can be achieved at the desired time and format. The gas-sampling detection system is composed mainly of a sampler and a HB0-2B type injection oxygen-detector. The oxygen content at various time checkpoints can be detected in the model through the use of this system. The back pressure control system consists mainly of pressure reducing regulators and back-pressure regulators (BPR). By the control of pressure of the PRR and BPR, the pressure difference at both ends of the model is controlled in order to simulate the actual production pressure difference. The main components of the measurement monitoring system are an oil-gas-water separator, drainage gas recovery device, and a measuring device. By adopting the measuring and detecting system, oil-gas-water separation can be achieved and the volume of oil, gas, water can be measured during the displacement process. The experimental steps are as follows: (1) The experimental instruments are connected based on Fig. 1. (2) The 0.1 PV foaming agent is driven into the model with 0.1 ml/min flow rate. (3) Inject high pressure air under the condition of 30 MPa injection pressure, and 0.2 MPa displacement differential pressures. (4) The model pressure and temperature are obtained by computer. (5) The content of oxygen in the model is tested per 24 hours. (6) The experiment
was stopped when the oxygen content is higher than 14% at the exit.
3 Results and Discussions
3.1 Changes in Gas Composition
After 168 hours, low temperature oil oxidation experiments were conducted at 30MPa and 90℃, using gas sampling exports from the sand pack core model. The experimental results are shown in Tab.1. From Tab.1, it can be seen that in a low temperature oxidation process, oxygen content decreases while carbon dioxide and carbon monoxide content increases. This is mainly due to an oxidative addition reaction which first occurs in hydrocarbons and oxygen, and which produces carboxylic acids, aldehydes, ketones, alcohols, peroxides and other intermediate products. Therefore, the intermediate products will continue to be oxidized to carbon oxide and water. After the reaction, the gas content of C1 ~ C6 increases, which is due to the light component of the oil volatilized into the air, and also led to the increase of the average molecular weight and the relative density of the gas. Nitrogen is an inert gas that does not react with oil. Oxygen content reduction from the oxidation reactions led to the increase of nitrogen-to-oxygen ratio.
3.2 Changes in Oil Composition
After 168 hours, low temperature oil oxidation experiments were conducted at 30MPa and 90℃, using gas sampling exports from the sand pack core model.
3.2.1 Elemental Analysis
Elemental changes in the oil are detected before and after the reaction, and the results are shown in Tab.2. From Tab. 2, it can be seen that oxygen content increases, as oil at the low temperature oxidation process generates the oxygen-containing compounds. Carbon content decreases after the reaction, because the generation of oxygen-containing compounds and crude oil lighter components evaporate into the air. After the reaction, hydrogen content decreases, because this produces a certain amount of water in the reaction process.
3.2.2 Group Composition Analysis
Oil group composition refers to the use of different organic solvents on the different types of components and structures of oil for selective separation, and obtains physical and chemical properties similar mixtures. It is generally divided into saturated hydrocarbon, aromatic hydrocarbon, resin and asphaltene. Group composition changes are detected in the Oil before and after the reaction. The results are shown in Tab.3, from which it can be seen that aromatic hydrocarbon content decreases, while resins and asphaltene content increases. This is mainly because aromatic hydrocarbon is oxidized to resin and asphaltene. The increase of resin and asphaltene will deteriorate the oil’s physical property, and increase oil flow resistance, which is going against EOR.
3.2.3 Chromatographic Analysis
The different numbers of carbon atoms n-alkane distribution in oil, both before and after
the reaction can be detected by the use of chromatographic analysis. The results are shown in Fig. 2. From Fig.2, it can be seen that oil before and after the reaction were mainly composed of C2-C38 n-alkanes with C15 as a main peak. N-alkanes in oil weight ratio (ΣnC21-/ΣnC22+) increases, because parts of the long carbon chain break into short carbon chain in the process of oxidation, resulting in a decrease of heavy component and an increase of light components. The increase of light components improves the physical property of oil, which is beneficial to EOR.
3.3 Air and Air Foam Flooding Displacement Efficiency
Fig. 3 shows the relation between accumulative injection pore volume (PV) and displacement efficiency (recovery factor) through the two experiments. From Fig.3, it can be seen that in air flooding, the displacement efficiency increases gradually with the increase of injection PV. At the point of 0.23PV (the volume under high pressure and high temperature in the model), air breaks through and the oil displacement efficiency is 35.8%. Afterwards, it begins to slow down, and finally the oil displacement efficiency reaches to 38.45%. All this shows that the increase of displacement efficiency in air flooding mainly happens before the air breakthrough. After the air breakthrough, the amount of displaced oil is very limited and the displacement efficiency did not change significantly. In air foam flooding, with the increases of air injection PV, the displacement efficiency rises accordingly. At the point of 0.47PV, air breaks through with the displacement efficiency rises to 73.35%, and the final figure is 74.61%. The way displacement efficiency changes over time is basically the same as that in air flooding.
From the comparison of the two groups of experiments, we are able to see an increase of 36.1% of displacement efficiency in air foam flooding over air flooding. That is because the increase of gas phase viscosity and decrease of gas phase permeability make the gas phase mobility with foam injection far smaller than the gas phase mobility without foam injection. As a result, the amount of gas decreases and that of oil and formation water rises. Also with the foaming agent’s oil-washing effect, the displacement efficiency of air foam flooding remarkably exceeds that of air flooding.
3.4 Air and Air Foam Flooding Oxygen Consumption
At present, a key problem of air flooding technology encountered is the rapid increase of oxygen level in the producing well after air is injected. When the oxygen in the gas mixture in the producing well reaches to the limit of explosion, the wellhead will probably catch on fire or explode (Hua et al., 2010). To study the changes of oxygen level in air flooding and air foam flooding, the gas sampling opening in the experiment model was set up and, through which, the oxygen level in different sampling openings can be measured with oxygen recorder at anytime. In this experiment, four gas sampling openings are set up, No.1 at the entrance of gas (0 m from the entrance), No. 2 at place 1 m from the entrance, No.3 at 3 m from the entrance and No.4 at the exit (5m from the entrance). The monitoring result of oxygen level at different times in air flooding is shown in Fig. 4. As time goes by, the oxygen level at location No. 1 is unchanged with time (21.9%) because it is the air injection inlet. A gradual increase of oxygen level can be seen at No.2, No.3 and No.4 checkpoints. 69 hours later after the air injection, air breaks through and the oxygen level is 16.8% at the exit. 85 hours later after the air injection, the oxygen level at the exit reaches to
20.4%. The monitoring result of oxygen levels at different times in air foam flooding is shown in Fig.5. As the time goes on, the oxygen level at No.1 checkpoint (entrance) is not changed. A gradual increase of oxygen level can also be seen at No.2, No.3 and No.4 checkpoints. 194 hours later after the air injection, air breaks through and the oxygen level is 1.6% at the exit. 216 hours later after the air injection, the oxygen level at the exit reaches to 16.8%. The experimental results showed that the time of gas breakthrough was shortened, and the gas breakthrough oxygen content was apparently decreased. The gas breakthrough oxygen content in air flooding was at 16.8%, and is only 1.6% in air foam flooding. That is due to the the Jamin effect, the function of profile controlling of the foam, and thus the gas fingering is slowed down resulting in effectively prolonging the gas breakthrough time. The reaction time of oxygen and oil increases and the oxygen content decreases gradually when the gas breakthrough. The changes of crude oil composition are similar to the crude oil low temperature oxidation experiments. Carbon content and hydrogen content decreases while oxygen content increases in the crude oil after the reaction. Aromatic hydrocarbon content decreases, while resins and asphalting content increases.
3.5 Effects of Concentration of Foaming Agent on Displacement Efficiency
To study the effects of concentration of the foaming agent on displacement efficiency, two experiments of foam flooding with foaming agent concentration of 1% and 0.5% were conducted. From Fig.6, one can see that a decrease of 7.8% of displacement efficiency lead to a decrease of 0.5% of foaming agent concentration. This is mainly because the decrease of
foaming agent concentration leads to a shortening of foam life and a decrease in the foam volume that can reduce the foam plugging effect and cause a decrease of oil displacement efficiency.
3.6 Effects of Slug of Foaming Agent on Displacement Efficiency
To study the effects of slug volume on displacement efficiency, two experiments of foam flooding were conducted using a foaming agent slug of 0.1PV and 0.05PV. From Fig.7, one can see that a decrease of 16.52% of displacement efficiency leads to a decrease of 0.05PV in the foaming agent slug. This is mainly due to the decrease of foam slug, resulting in excessive loss of the foaming agent, gas through the forward moving slug and finally breaking quickly which leads to the decrease of the oil displacement efficiency.
3. Conclusions
Preliminary conclusions of the air foam flooding experimental study are as follows: (1) In a low temperature oxidation process, oxygen content decreases, while carbon dioxide and carbon monoxide content increases. Aromatic hydrocarbon content decreases, while resins and asphaltene content increases. N-alkanes in oil weight ratio (ΣnC21-/ΣnC22 +) increases. (2) The oil displacement efficiency of air foam flooding is much higher than air flooding. Before the air breakthrough point in air flooding and air foam flooding, air or air foam injection will observably enhance crude oil displacement efficiency. After the air breakthrough point, the displacement production of crude oil is quite small, and the increase of displacement
efficiency is within a very narrow range. (3) The foam blocking ability and the function of profile controlling of the foam can significantly prolong gas breakthrough. The gas breakthrough oxygen content fell sharply, which effectively decreased the potential safety hazard in the process of air injection. (4) A decrease of 7.8% of displacement efficiency in the concentration of the foaming agent dropped from 0.1% to 0.05%. A decrease of 16.52% of displacement efficiency in the slug of the foaming agent dropped from 0.1PV to 0.05PV.
Acknowledgments
This research is supported partly by the National Science Funding of China (50874071), the Chinese National Programs for High Technology Research and Development (SS2013AA061104 ), Shanghai Program for Innovative Research Team in Universities, Shanghai Leading Academic Discipline Project (S30106)，The Excellent Academic Leading Person Program of Science and Technology Commission of Shanghai Municipality（12XD1402500）, Shanghai Leading Talents Project. Part of the ability of local colleges and universities in Shanghai construction projects (12160500200).
References
Fengpeng Yan, Lixing Yang et al. Effect Analysis of Enhancing Ultra-low Permeability Oilfield Recovery by Air-foam Flooding. Journal of Yanan University (Natural Science Edition),2008;27(4):58-60 Moore, R.G., Laureshen, C.J., Ursenbach, M.G. et al. 1999. A Canadian Perspective on In Situ Combustion. J Can Pet Technol 38 (13): 1-8.
Moore R G， Mehta S A. A guide to high pressure air injection (HPAI) based oil recovery[C]. SPE75207, 2002. Gauglitz, P.A., Friedmann, F., Kam, S.I. et al. 2002. Foam Generation in Porous Media. Presented at the SPE/DOE Improved Oil Recovery Symposium, Tulsa, 13-17 April. SPE-75177-MS. Patzek T.W. and Koinis M.T. 1990. Kern River Steam-Foam Pilots. Journal of Petroleum Technology 42(4):496-503. Nguyen, Q.P., Alexandrov, et al. 2000. Experimental and Modeling Studies on Foam in Porous Media A Review. SPE International Symposium on Formation Damage Control. USA. Haugen, Å., Fernø, M.A., Graue, A., and Bertin, H.J. 2012. Experimental Study of Foam Flow in Fractured Oil-Wet Limestone for Enhanced Oil Recovery. SPE Res Eval & Eng 15 (2): 218-228. SPE-129763-PA. Patzek, T.W. 1996. Field Applications of Steam Foam for Mobility Improvement and Profile Control. SPE Res Eng 11 (2): 79-86. SPE-29612-PA. Blaker, T., Aarra, M.G., Skauge, A. et al. 2002. Foam for Gas Mobility Control in the Snorre Field: The FAWAG Project. SPE Res Eval & Eng 5 (4): 317-323. SPE-78824-PA. Hirasaki, G.J., Miller, C.A., Szafranski, R. et al. 1997. Field Demonstration of the Surfactant/Foam Process for Aquifer Remediation. Paper. SPE 39292 Wang, D., Cheng, J., Yang, Z. et al. 2001. Successful Field Test of the First Ultra-Low Interfacial Tension Foamflood. SPE 72147 Jinsheng Zhao.Study on Nitrogen Foam Flooding in Uniform Mobility for EOR after Polymer Flooding.China University of Petroleum(East China),Doctoral Dissertation,2008.4：5-6 Shuai
Hua,Yifei
Liu
et
al.The
Problem
of
Reservoir
Air
Countermeasure.Oil-Gasfield Surface Engineering,2010,29(11);47-48
Injection
Technology
and
Its
Tab.1 Air Component Changes Before and After Reaction Detection results Air before reaction Air after reaction
O2
N2
CO2
CO
C1-C6
N2/O2
Relative density
Average molecular weight
21.9%
78.04%
0.03%
0
0
3.57
0.9662
27.98
16.86%
81.71%
0.62%
0.17%
0.63%
4.85
0.9995
28.93
Note: figures in the table1, 2, and 3 are for mass percentage.
Tab.2 Oil elemental Changes before and after Reaction Elemental
O（%）
C（%）
H（%）
N（%）
Oil before reaction
5.52
81.16
12.54
0.06
Oil after reaction
6.69
79.77
11.20
0.08
Tab.3 Oil Group Composition Changes Before and After Reaction Group composition
Saturated hydrocarbon
Aromatic hydrocarbon
Resin
Asphaltene
Oil before reaction
51.0%
31.2%
11.4%
6.4%
Oil after reaction
50.0%
25.6%
17.0%
7.4%
Fig. 1 Schematic of Air Foam Flooding Experimental System
Fig.2 Oil Before and After the Reaction N-alkane Distribution Changes
Fig. 3 Relations between Injection Pore Volume and Displacement Efficiency
Fig. 4 Changes of Oxygen Content at Different Monitoring Point with Time in Air Flooding
Fig. 5 Changes of Oxygen Content at Different Monitoring Point with Time in Air Foam Flooding
Fig.6 Effects of Concentration of Foaming Agent on Displacement Efficiency
Fig.7 Effects of Slug of Foaming Agent on Displacement Efficiency
Highlights •A new experimental model of air foam flooding process is designed. •Quantitative research is conducted about the changes of oil and air composition. •Oil displacement efficiency of air foam flooding is much higher than air flooding. •The changes of oxygen level in air flooding and air foam flooding are studied.