132 Int. J. Oil, Gas and Coal Technology, Vol. 7, No. 2, 2014

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1 132 Int. J. Oil, Gas and Coal Technology, Vol. 7, No. 2, 2014 Production performance of water alternate gas injection techniques for enhanced oil recovery: effect of WAG ratio, number of WAG cycles and the type of injection gas Jigar Bhatia School of Petroleum Technology, Gandhinagar, , India J.P. Srivastava Institute of Reservoir Studies, Oil and Natural Gas Corporation, Ahmedabad, , India Abhay Sharma School of Petroleum Technology, Gandhinagar, , India and Department of Mechanical Engineering, Indian Institute of Technology, Hyderabad, Medak, , India Jitendra S. Sangwai* School of Petroleum Technology, Gandhinagar, , India and Department of Ocean Engineering, Indian Institute of Technology (IIT), Madras, Chennai, , India Fax: *Corresponding author Abstract: Production performance of a water alternate gas injection (WAG) method has been reported for the effect of several operating parameters, such as, WAG injection cycles, viz., single cycle WAG and five-cycle WAG and the tapered WAG at the reservoir conditions of 120 C and 230 kg/cm 2 for hydrocarbon gas and CO 2 gas. It is observed that the number of cycles in the WAG injection process affects the recovery of oil from the core sample. It is Copyright 2014 Inderscience Enterprises Ltd.

2 Production performance of water alternate gas injection techniques 133 observed that the tapering in the WAG injection process improves the recovery of oil initially in place. The observations on the effect of gases revealed that the CO 2 gas with five-cycle WAG process gives higher incremental recovery than the five cycle WAG process using hydrocarbon gas. It is observed that the saturation profile of CO 2 WAG injection shows the better gas saturation in the core as against the hydrocarbon gas in the WAG process. [Received: April 30, 2013; Accepted: October 7, 2013] Keywords: enhanced oil recovery; EOR; water alternate gas; WAG; gas trapping; incremental oil recovery; hydrocarbon pore volume; HCPV. Reference to this paper should be made as follows: Bhatia, J., Srivastava, J.P., Sharma, A. and Sangwai, J.S. (2014) Production performance of water alternate gas injection techniques for enhanced oil recovery: effect of WAG ratio, number of WAG cycles and the type of injection gas, Int. J. Oil, Gas and Coal Technology, Vol. 7, No. 2, pp Biographical notes: Jigar Bhatia is currently working as an Instrumentation Engineer at ICAM Technologies Pvt. Ltd. Surat. It is basically the recognised system integrator for Rockwell Automation. He completed his BTech in Instrumentation & Control Engineering from Nirma University, Ahmedabad in 2008 and MTech. in Petroleum Engineering from PanditDeendayal Petroleum University, Gandhinagar, India in J.P. Srivastava is currently working as Reservoir Engineer in National Oil Company, ONGC at Mumbai, India. He worked in Institute of Reservoir Studies, Ahmedabad from dealing with laboratory investigation and selection of gas-based EOR process to enhance recovery from mature fields of ONGC. He has published over six papers in conferences of international reputes. His research interest lies mainly in the field of gas-based EOR techniques and reservoir characterisation through pressure transient analysis. Abhay Sharma is currently working as Assistant Professor in the Department of Mechanical and Aerospace Engineering at Indian Institute of Technology Hyderabad, Hyderabad, India. He obtained his MTech and PhD in Mechanical Engineering from IIT Roorkee in 2001 and 2008, respectively. His research interest lies mainly in modelling and optimisation manufacturing processes. Jitendra S. Sangwai is currently working as Assistant Professor in the Petroleum Engineering Program, Department of Ocean Engineering at Indian Institute of Technology Madras, Chennai, India. He obtained MTech (2001) and PhD (2007) in Chemical Engineering from IIT Kharagpur and IIT Kanpur, respectively. He worked with Schlumberger dealing with flow assurances issues and on several commercial projects. He has published over 55 papers in international journals and conferences of international repute. He holds seven patents. His research interest lies mainly in the field of gas hydrates, enhanced oil recovery and flow assurance. This paper is a revised and expanded version of a paper entitled Investigations on gas trapping phenomena for different EOR-water alternate gas injection methodologies presented at International Petroleum Technology Conference 2012, IPTC 2012, Bangkok, Thailand, 7 9 February 2012.

3 134 J. Bhatia et al. 1 Introduction Enhanced oil recovery (EOR) methods, also referred to as tertiary oil recovery methods, are employed when primary and secondary recovery methods do not improve the production from brownfields. It is a well-known fact that the world average of oil recovery factor is estimated to be 35% (Tayfun, 2007) thus almost more than 60% of the oil initially in place (OIIP) remains in the reservoir after the primary and the secondary recovery. There is, therefore, an enormous incentive for development of a field through EOR methods aimed at recovering some portion of the remaining oil keeping in view of increasing oil prices and the energy demand worldwide. There have been several kinds of EOR methods that can be used and are shown in Figure 1, such, as, polymer-flooding, alkaline-surfactant-polymer flooding, gas-injection, thermal techniques, such as, in-situ combustion, steam injection, etc. The applicability of several of these techniques to a given field depends on various factors. Out of these, gas-injection-based EOR methods are one of the most preferred methods for low to medium API oil brownfields due to their simplicity and economic advantages. One of the derivative methods of gas-injection techniques is the water alternate gas (WAG) injection methods, wherein water and gas are injected intermittently. Oil recovery by the WAG injection has been attributed to contact of upswept zones, especially recovery of attic or cellar oil by exploiting segregation of gas to the top or accumulation of water toward the bottom. The WAG injection techniques has the potential for increased microscopic displacement efficiency because the residual oil after gas flooding is normally lower than the residual oil after water flooding, and three-phase zones thus obtained lowers the remaining oil saturation. Thus, the WAG injection can lead to improved oil recovery by combining better mobility control and contacting upswept zones, and by leading to improved microscopic displacement. Figure 1 Different methods of EOR EOR METHODS CHEMICAL INJECTION THERMAL BIOLOGICAL Miscible gas injection Microbial enhance oil Immiscible gas injection recovery Polymer flooding Alkaline flooding Surfactant flooding Stream flooding In-situ combustion Hot water Source: Green and Willhite (2003) 1.1 Gas-injection method Gas-injection-based EOR methods are one of the most frequently used methods for EOR (Kulkarni and Rao, 2005). In this method hydrocarbon or inert gas is injected in to the reservoir containing residual oil. The components of the gas get dissolved with the lighter components of the oil which helps to reduce the viscosity and increase the sweep efficiency in the presence of a chasing fluid such as water. The component exchange processes between the injected gas and reservoir oil causes heavy and light compositions in the reservoir which separately moves towards the production side. Different gases are used in the gas-injection methods, such as, nitrogen, hydrocarbon gas (HC), flue gas and

4 Production performance of water alternate gas injection techniques 135 CO 2 gas. Some of the injectants such as, CO 2, help to increase oil production by means of oil viscosity reduction, oil swelling and solution gas drive (Green and Willhite, 2003). The use of specific gas depends on the availability of gas at the field. Previously liquefied petroleum gas (LPG) and hydrocarbon gas were used for injection. But gradually as price of natural gas increases, their priority got reduced. Gas-injection method can broadly be classified as immiscible and miscible gas-injection, depending upon their miscibility with the oil at reservoir condition. In immiscible gas-injection process the gas is injected at lower pressure into the reservoir. It is further classified as dispersed gas-injection and crestal gas-injection according to the injection region. In dispersed gas-injection, gas is directly injected in to the oil bearing zone of the reservoir. This method is used in the thin production zone. In crestal gas-injection method, gas is injected in to the gas cap above the oil bearing zone. For this process, vertical permeability of the reservoir should be high in order to push the oil towards the production end. Miscible gas-injection method can be broadly classified as high pressure dry gas miscible displacement, enriched gas miscible displacement and miscible slug flooding. A large change in the mobility of gas and oil is observed in case of the gas-injection methods due to difference in the viscosity of gas to the oil and water at the reservoir conditions. This results in early breakthrough of the gas to the production side due to its high sweep velocity. In order to control the sweep velocity of the gas, water and gas are injected intermittently. This method is called as WAG injection method. Oil recovery by WAG injection is due to the segregation of gas to the top and accumulation water at the bottom resulting in the recovery of attic or cellar oil. As the residual oil after gas flooding is typically lower than that of the water flooding, in addition to the formation of three-phase zones, which may result in lowering the remaining oil saturation, therefore, WAG injection shows the potential for increased microscopic displacement efficiency. Thus, WAG injection can lead to improved oil recovery by combining better mobility control and contacting upswept zones, and by leading to improved microscopic displacement. Some factors such as wettability, interfacial tension, connate/fossil water saturation and gravity segregation increases the complexity to the design of a successful WAG flood. The WAG injection methods can be classified as miscible WAG, immiscible WAG, hybrid WAG and simultaneous water alternate gas (SWAG) methods (Christensen et al., 2001). Several screening criterion are to be considered before the application of WAG technique for any particular field operation. These are mainly, reservoir pay thickness, vertical permeability of the reservoir, availability of the gas, type of formation, mobility ratio, etc. The aim of the current work is to evaluate the performance of the different gas-injection methodologies for a given brownfield in India. It includes comparative studies on different WAG injection methods and to verify their effects on the production enhancement from the given field. Core flooding experiments are performed at close to the reservoir conditions of the pressure and temperature to identify: a b c the effect of WAG injection method for various WAG cycles the recovery efficiency for different methods using different gases like hydrocarbon gas and CO 2 gas at reservoir condition the effect of tapering on the WAG performance.

5 136 J. Bhatia et al. WAG processes which have been studied and discussed in this work (on the basis of WAG cycles) are, 1 single cycle WAG using HC gas 2 five cycles WAG using HC gas 3 tapered WAG (with increasing and decreasing WAG ratio) using HC gas 4 five cycles WAG using CO 2 gas. The following Section 2 provides the experimental details of the present investigation followed by the outcomes of the experimental work and discussion thereon. 2 Experimental details The experiments were performed using the in-situ core sample obtained from the reservoir and fitted in the core pack, which was then kept horizontally during all the experiments. The gas and oil samples were collected from the separator and recombined in the laboratory with given gas-oil ratio (GOR) so as to become representative of the in-situ reservoir fluid. The recombination process is discussed in detail elsewhere (Bhatia, 2010). The experiments were performed using the recombined separator fluid as a reservoir fluid in the core sample and the hydrocarbon or CO 2 gas with water as a mean for injection in the core sample during WAG process. The water was injected at 20 cc/hr and gas was injected at 10 cc/hr, which remained same for all the experiments as mentioned above. The basis to choose these injection rates for water and gas are purely based on our experience of several laboratory studies done in-house to mimic the scaled-up water and gas injection rate that are possible in real field applications. The water and gas ratio remained same except for the experiments where the effect of tapering was studied. The details on the ratio of water and gas used are described later in experimental procedure Section Properties of the experimental fluids and the reservoir The composition of the hydrocarbon gas used for injection is given in Table 1, which was obtained by using gas chromatographic technique. The major component of the injection gas was methane (about 90%) of the total concentration. The gas contains around 2% CO 2. The gas gravity was observed to be gm/cc. Another gas used for the WAG process was pure CO 2. The basic reservoir data and rock properties are given in Table 2. The given reservoir is a sandstone reservoir and is under depletion. API gravity of the oil was about 42 which indicates the light oil reservoir.

6 Production performance of water alternate gas injection techniques 137 Table 1 Composition of the injected gas in mole fraction obtained by gas chromatography Component Mole fraction N CO C C C i-c n-c i-c n-c C C C C C Total Table 2 Basic data for the reservoir and the core sample experiments Details on the reservoir and the core sample Sr. no. Parameters 1 Reservoir rock type Sandstone 2 Initial reservoir pressure (kg/cm 2 ) Current reservoir pressure (kg/cm 2 ) Bubble point pressure (kg/cm 2 ) Reservoir temperature ( C) Density of oil (gm/cc) at 128 C Stock tank oil density at 15.5 C API gravity of oil Oil FVF (v/v) Specific gravity of gas Solution GOR (v/v) Core length (cm) Core diameter (cm) Avg. permeability (md) Experimental set-up High pressure apparatus was selected for the core flooding experiments. All the flooding experiments were performed at the reservoir pressure of 230 Kg/cm 2 and temperature of

7 138 J. Bhatia et al. 128 C. The schematic of the core flooding experiment is shown in Figure 2. The heart of the set-up is the core pack which holds the actual core sample at reservoir conditions. The core pack is placed in the oven which is maintained at reservoir temperature. Pressure gauges are used to indicate the pressures at inlet and outlet of the core pack. The pressure in the core pack is maintained at reservoir conditions by using positive displacement pump (Ruska ) which injects the fluid (gas, oil, water) at different flow rates in the core pack. The inlet pressure is regulated by same positive displacement pump through which kerosene has been used as a displacing fluid to displace any gas or liquid from the gas cell/buffer cell/rocking cell into the core pack. The gas cell contains gas (HC or CO 2 ) to be injected during the WAG process. The buffer cell contains water (2% KCl) which is used as a buffer to displace oil or water. The rocking cell is used to prepare the live oil (recombined fluid) from the oil and gas samples collected from the separator. A backpressure regulator regulates the flow from the core outlet by maintaining constant pressure difference at the input and the output side. The produced fluid (water, gas and oil) collected in the separator flask at the outlet of the core pack indicates the quantity of the produced fluid and one end of the flask connected to the gas meter indicates the quantity of the produced gas during the WAG process. Steel pipe of 1/8" diameter is used for fluid transportation within the experimental set-up. The experimental setup described here was same for all the experiments carried in this work. As the current experiment set-up consist of a horizontal core flood reactor having a core diameter of about 3.8 cm, which is sufficiently small, we assume that the flow of the fluid in the core sample is predominantly unidirectional. In the current set-up, the control over vertical sweep may not possible due to the small core diameter. This may need better set-up which can quantify and control the vertical sweep of the injected fluid (Hadia et al., 2007). Before carrying our actual experiments on the WAG process, initial preparation is done on the recombination of reservoir fluids using the separator sample of the oil and gas and the determination of GOR and formation volume factor (FVF) of the recombined reservoir fluid. The GOR and FVF of recombined fluid are then matched within the accepted limit with the reservoir GOR and FVF in order to check the reliability of the recombination process. Figure 2 Schematic diagram of the experimental set-up used for displacement studies (see online version for colours) Pump-I Pump-II High pressure experiments Hot Air Oven Porous medium Back Pressure Regulator Gas volume measurement Gasometer Water Cell Gas Cell Liquid (oil and brine) volume measurement Buffer Cell Rocking Cell

8 Production performance of water alternate gas injection techniques Experimental procedure The experimental procedure for all the WAG cases studied mainly consists of the preparation of the core pack, cleaning and drying of the core pack, evacuation of the core pack, determination of the pore volume (PV) with saturation of the brine solution, determination of the hydrocarbon pore volume (HCPV) by displacing the brine solution with heavy and light paraffin oil. The obtained value of the PV ( 60 cc) and HCPV gives the connate/fossil water saturation inside the core. The core pack is then cleaned using kerosene for studies with recombined fluid. Subsequently, the cleaned core pack is then saturated with the recombined oil prepared in the laboratory. This is followed by the secondary water flood until oil saturation in the core reaches to the residual oil saturation. After completion of the water flooding, to produce the residual oil from the core, WAG injection is started. The overall process resembles to the actual recovery process a reservoir may undergo during its production phase. The details on other experimental procedures related to core pack preparation and cleaning, absolute permeability determination, PV and HCPV determination, oil saturation and water flooding procedure can be found elsewhere (Bhatia, 2010) and not discussed here. In the subsequent Section 2.3.1, a brief discussion on the process of tertiary gas-injection of WAG method is presented Tertiary gas-injection The tertiary gas-injection is carried out mainly by WAG process using hydrocarbon gas and CO 2 gas. Different WAG methods applied for EOR were single cycle WAG, five cycle WAG (with HC gas and CO 2 separately), and tapered WAG (with increasing and decreasing WAG ratio). For single cycle WAG and five cycle WAG total 1 PV (1 PV = 60 cc with ± 0.5 cc) of gas and water was injected intermittently with the WAG ratio of 1:1 at the end of water flooding experiment. For tapered WAG injection method a total of 1.5 PV gas and water was injected intermittently at the end of water flooding experiment. In tapered WAG (with increasing and decreasing WAG ratio) WAG ratios, as given in Table 3,were selected and used for the experimental study. Table 3 Cycles Injection WAG ratio for different cycle of tapered WAG methods Increasing WAG ratio WAG ratio for tapered WAG (water :gas) Decreasing WAG ratio 1 3:5 3:1 2 3:4 3:2 3 1:1 1:1 4 3:2 3:4 5 3:1 3:5 Total of five experiments, namely, single cycle WAG (with hydrocarbon gas), five cycle WAG (with hydrocarbon gas), five cycle WAG (with CO 2 gas), and tapered WAG using HC gas (with decreasing and increasing gas tapering) were investigated. In core-flooding experiments, total PV (about 60 cc) of the core was divided according to the number of cycles. In a single cycle WAG process, PV (about 60 cc) was divided as 0.5 PV (about 30 cc) gas and 0.5 PV (about 30 cc) water and were injected accordingly. Similarly, for

9 140 J. Bhatia et al. five cycles WAG process, 0.1 PV of gas and 0.1 PV of water were injected in five cycles sequentially so as to make total injection equal to 1 PV (about 60 cc). Attanucci et al. (1993) observed that in case of tapered WAG process an injection of total 1.5 PV gives better results. In the present study, the experiments for tapering WAG were carried using 1.5 PV of gas and water as total injections. In case of tapered WAG (with decreasing WAG ratio) more amount of gas was injected in the first cycle and was gradually decreased in the subsequent cycles. Amount of water to be injected during each cycle remained constant. In case of tapered WAG (with increasing WAG ratio) similar procedure was followed but in reverse direction. The details on the quantity of gas and water used for each of the above processes are given in the Table 4. Table 4 Type of process/ number of cycles Details on the amount of water and HC gas used for tapered WAG process Tapered WAG (increasing WAG ratio) Tapered WAG (decreasing WAG ratio) Amount Fraction WAG ratio Amount Fraction of of (cc) of total PV for each of (cc) total PV Water Gas per cycle cycle Gas Water per cycle : : : : : : : : : :5 Total WAG ratio for each cycle The hydrocarbon gas collected from the adjacent field and pure CO 2 obtained from other sources were used as injection gas. The pressure and temperature conditions of the core pack were kept at reservoir condition and the injection rate for water was maintained at 20 cc/hr and for gas was maintained at 10 cc/hr to avoid the early breakthrough of the gas. The brine, oil and the gas volumes produced at the end of the experiment were measured from the separator (flask) and gas meter readings and tabulated as a function of time. Material balance procedure was used to calculate the saturations of oil, gas and water components Chasing water post WAG process The chasing water was injected to get the additional recovery of HCPV after the WAG injection process. Chase water helps to push the trapped gas and water in the core pack, with that combined oil also gets produced at the production side. In this experimental study maximum of 0.5 PV (around 30 cc) chasing water was injected after the completion of the WAG injection process. The results tabulated during process are discussed in the following Section 3. 3 Results and discussion The core-flooding experiments are carried out to verify the effect of different parameters of the WAG injection methods. The main objectives of this work are to study the

10 Production performance of water alternate gas injection techniques 141 efficiency of different WAG processes and the parameters affecting the production enhancement, viz., tapering of gas, WAG cycles, and type of the injected gas. The results are discussed in the subsequent section with respect to the incremental oil recovery over water flooding. 3.1 Oil recovery The oil recovery from the above experimental data can be expressed as the displacement efficiency or recovery in percentage of the total HCPV and can be calculated as, ( ) HCPV Q0 FVF V L Displacement efficiency (%HCPV) = 100 (1) HCPV where Q 0 is the flow rate of oil and V L is the total line volume. V L is actually a kind of dead volume of oil remained in the production tubing of the core flood apparatus and which need not to be accounted as oil recovered. The results on the displacement efficiency (in % of total HCPV) with respect to the total PV of fluid (water/gas-water) injected for different WAG processes are shown in Figures 3 to 7. It is to be noted that for all the experiments, a water flooding is carried out prior to each WAG process to represent the secondary oil recovery using water flooding. The water flooding process required a total of about PV of the water to be injected in the core sample (refer to Table 5). Percentage of recovery obtained using the water flooding prior to WAG process is also given in Table 5 and observed to be in the range of 47 to 57%. Actual WAG process starts after the end of the water flooding. The WAG processes consume the fluids (gas and water) in different ranges of PV and have been shown in Table 4 and are cumulative on the x-axis of Figures 3 to 7 after the pre-water flooding section. Recoveries in percentage of HCPV have been shown for each of these processes in Figures 3 to 7. At the conclusion of each WAG process, chasing water is flown through the core pack to see any incremental recovery. The recovery obtained for different phases of each process visible in Figures 3 to 7 are tabulated in Table 5. The maximum recovery is noticed in CO 2 five cycle injections (about 97.86% of HCPV), and next maximum recovery is in tapered WAG injection (decreasing WAG ratio) (about 72.48% of HCPV). The maximum incremental recovery over the water flood is seen with CO 2 gas with five cycle WAG injection (about 40.2% of HCPV), and the next is noticed with tapered WAG HC gas-injection with increasing WAG ratio (about 23.92% of HCPV). The maximum recovery with CO 2 is obtained probably due to its better miscibility with the crude oil (in the core pack) at reservoir conditions as compared with the HC gas used in WAG processes. Better recovery is obtained in case of tapered WAG injection (decreasing WAG ratio) as against all WAG processes using HC gas is due to an increased sweep efficiency governed by an initial dissolution of maximum amount of gas with the crude oil in the first cycle, thus helping better mobility in the pore of the core sample. This results in an increased relative permeability of oil in the core sample which is enhanced by the subsequent water cycle in the WAG process. The recovery is affected by different parameters like WAG cycles, type of the injected gas, tapering, etc. The effects of these parameters are discussed in the following Section 3.2.

11 142 J. Bhatia et al. Table 5 Summary of results for WAG injection methods WAG injection pattern Type of injection gas Amount of Water injected pre-wag process (PV) Chasing water injected after WAG process (PV) Total PV injected including for WAG process Recovery with water flooding Recovery (%HCPV) Incremental recovery over water flood Total recovery Incremental recovery during chase water Residual oil saturation (%HCPV) Single cycle WAG HC gas Five cycle WAG HC gas Tapered WAG (increasing WAG ratio) HC gas Tapered WAG (decreasing WAG ratio) HC gas Five cycle WAG CO 2 gas Effects of various operating parameters Number of WAG cycle Zhang et al. (2010) observed that by increasing the number of the WAG cycles in gas-injection methods helps to get more recovery of the oil from the reservoir. The effects of WAG cycle are also studied in this work to see the applicability for the given reservoir. The results obtained are shown in Figures 3 and 4 for single cycle WAG and five cycle WAG process using HC gas. The single cycle WAG process using HC gas shows 12.74% incremental recovery (recovery obtained after the initial water flooding) and five cycle WAG process using HC gas (no tapering) shows about 17.16% of HCPV incremental recovery over the water flooding. This indicates that the number of cycle affects the recovery of HCPV. Increment in the number of WAG cycle improves the recovery for the same amount of gas utilisation. However, in some of the studies it is observed that the recovery does not improve significantly even increasing the number of WAG cycles, probably due to increased water saturation and reduced discontinuity of the oil phase (Dong et al., 2005) Effect of tapering The increase or decrease in the water to gas ratio during the WAG cycles is known as tapering phenomena in the WAG process. It is also known as the hybrid WAG method (Christensen et al., 2001). In the tapered WAG process, gas-injection after the initial

12 Production performance of water alternate gas injection techniques 143 water flood pushes the oil to the production well in case the oil saturation is high, and if it is low then it will displace the oil to the higher water saturated channels and some of the gas stays in the small channels and resist the water mobility (Dong et al., 2005). Increasing the tapering form a WAG ratio (water: gas) of 1:1 to 1:2 to 1:3 followed by chase water increases the efficiency of the oil recovery (Attanucci et al., 1993). Results due to the present experimental study on tapering with both increasing and decreasing WAG are shown in Figures 5 and 6. It is evident that increasing WAG ratio shows more recovery than decreasing WAG ratio during each cycle. The large quantity of gas injected during the first cycle (refer to Table 5) helps to get connected with more residual oil and makes it move towards the high permeable area. Subsequently, the water cycle flowing through this highly permeable area (channels) helps to push the oil-gas system towards the production well. The experimental results for tapered WAG with increasing WAG ratio show that there is no further oil production after three cycles of WAG injection, which indicate that tapered WAG injection method can be efficiently used for lower WAG cycles for the equivalent oil recovery. It is also observed that the chase water also plays an important role during tapering of gas. The oil recovery obtained in case of tapered WAG with decreasing WAG ratio is about 68.25% of the HCPV before the chase water flooding. An additional recovery of 4.2% is achieved after the chase water flooding at the end of WAG process resulting in a total 72.48% recovery from the given experiment. The chasing water is a concluding process and acts similar to water cycles in a WAG process only except that sufficient quantity of water is injected in the core in order to get the maximum possible recovery from the reservoir. The current study may not be sufficient enough to deduce on the optimum WAG ratio required for field application. This may depend upon the reservoir fluid and rock properties, in addition to the type of gas injection being used. In addition, economics may play a vital role in deciding an optimum value for the WAG ratio. WAG process may help to reduce the viscous fingering effect associated with sample gas injection techniques, thanks to the better mobility ratio provided by the alternate water injection. However, these phenomena may depend on the reservoir fluid properties, such as, viscosity and density, which may have significant impact on the optimum WAG ratio Effect of injecting gas The results for five cycle WAG using HC gas and CO 2 gas are shown in Figures 4 and 7, respectively, and tabulated in Table 5. The results show that CO 2 injection in five cycles WAG gives recovery of about 97.86% of HCPV, which is very high compared to the recovery of five cycle injection of hydrocarbon gas (about 71.3% of HCPV). This is due to the fact that compared to the hydrocarbon gas CO 2 gas is having better miscibility with the crude oil at the reservoir condition that helps in increasing the solution GOR of the oil and also helps in reducing viscosity of the oil. This probably results in the solution gas driven production and increasing the relative permeability of the oil phase. The reservoir condition of pressure and temperature of 230 kg/cm 2 and 120 C shows that the CO 2 gas may be at near supercritical state at the reservoir condition resulting in better miscibility with the reservoir fluid.

13 144 J. Bhatia et al. Figure 3 Displacement efficiency vs. PV injected for single cycle WAG injection using HC gas as injectant (see online version for colours) FLOODING INJECTION CHASE Displacement Efficiency (%HCPV) WAG CYCLE Pore volume injected (cc) Figure 4 Displacement efficiency vs. PV injected for five cycle WAG injection using HC gas as injectant (see online version for colours) 90 Displacement Efficiency (%HCPV) FLOODING WAG CYCLE CHASE Pore volume injected (cc)

14 Production performance of water alternate gas injection techniques 145 Figure 5 Displacement efficiency vs. PV injected for tapered WAG injection (with increasing WAG ratio) using HC gas as injectant (see online version for colours) Displacement Efficiency (%HCPV) FLOODING WAG CYCLE GA S CHASE Pore volume injected (cc) Figure 6 Displacement Efficiency (% HCPV) Displacement efficiency vs. PV injected for five cycle WAG injection (with decreasing WAG ratio) using HC gas as injectant (see online version for colours) FLOODING WAG Cycle CHASE Pore volume injected (cc)

15 146 J. Bhatia et al. Figure 7 Displacement Efficiency (%HCPV) Displacement efficiency vs. PV injected for five cycle WAG injection using CO 2 gas as injectant (see online version for colours) FLOODING WAG CYCLE Water CHASE Pore volume injected (cc) The recovery of the oil from the reservoir is a strong function of saturation of different phases present in the core (Dong et al., 2005). Trapping is also an important parameter that affects the recovery of oil (Ghomian et al., 2008). Trapped gas refers to the immobile gas saturation remaining after the rock is flooded with oil or water. Land s (1968) model gives a relationship between final and initial gas saturation. It was found that a general relationship between initial gas saturation and trapped gas saturation as give below in equation (2). 1 1 C = (2) S S gt gi where S gt = final gas saturation, S gi = initial gas saturation, and C is the Land s trapping constant of the rock. Value of C reflects the final gas saturation in the reservoir after the WAG injection process. A small value of C shows higher final hydrocarbon gas (from WAG process) saturation in the core. This shows that the hydrocarbon gas has been trapped in the core by efficiently displacing the crude oil from the core, thus, increasing recovery of the oil from the core. Trapped gas creates significant hysteresis effects (during drainage and imbibition) and reduces the relative permeability of the water, especially, in the mixed-wet and oil-wet reservoirs (Rogers and Grigg, 2000). The results of saturation of phases and Land s trapping constant for the experiments have been calculated for all the WAG cases studied. The results on saturation of different phases are shown in Figures 8 and 9 and Table 6. The numerical values on the saturation of fluids in different WAG processes along with their corresponding Land s parameters are shown in Table 6. It is observed from the saturation profiles for each of the WAG processes that

16 Production performance of water alternate gas injection techniques 147 the gas saturation is increased quite significantly in almost all of the WAG processes. This is in accordance with the low value of the Land s parameter. The saturation profile of CO 2 WAG injection shows the better gas saturation in the core as against the use of hydrocarbon gas. The single cycle WAG process is observed to be less efficient as compared to tapered WAG. The tapered WAG with increasing WAG ratio is observed to be efficient in enhancing recovery as compared to that of tapering WAG with decreasing WAG ratio as evident from the Land s parameter. Five cycles WAG with hydrocarbon gas shows the higher gas trapping next to the WAG process with CO 2 (five cycles). Other parameter studied in these experiments is gas utilising factor which indicated as the required quantity of gas required to produce 1 bbl of oil and are given in Table 7. This factor is calculated as in below equation (3), volume of oil produced Gas utilising factor = (3) quantity of gas injected during process The result shows that the CO 2 gas gives the maximum oil recovery with minimum quantity of gas-injection. Miscibility of gas can be considered as one of the factors for this incremental recovery. Figure 8 Saturation of phases during single cycle WAG injection (see online version for colours) Notes: WF: water flooding, G1: gas-injection, W1: water injection (WAG process), S w,g,o = saturation of water, gas and oil. The macroscopic displacement efficiency due to water was observed to be highest in case of tapered WAG and CO 2 WAG injection, which is in the range of 41 to 44% against 21 to 24% due to the gas. A five cycle WAG process was observed to be the lowest displacement efficiency due to water which is about 14%. However, the five cycle WAG has highest displacement efficiency due to gas, which is about 42% followed by the CO 2 WAG injection (38%). In addition, in the present study, all the experiments were carried out with the same experimental set-up and for favourable mobility ratio. The approximate estimate of areal sweep efficiency was found to be in the range of 70 to 80% for most of the experimental conditions. The single cycle WAG was found to have lowest areal seep

17 148 J. Bhatia et al. efficiency of 70%, while those with five cycle WAG was found to have areal sweep efficiency of about 80%. The five cycle CO 2 WAG was observed to have highest areal sweep efficiency of 90%. It is to be noted here that, as the core sample used in this study are relatively small and homogeneous, the vertical sweep may be completely absent. This may result in decrease of overall efficiency of the process at reservoir scale. We expect the reduction in the efficiency may be of the order of 20 to 30% of the measured efficiency at the laboratory scale, which may largely depend upon the local reservoir conditions and may vary from field to field. The general conclusion from this study is that the CO 2 -WAG gives better relative displacement efficiency as compared to other processes studied in this work. Figure 9 Saturation of phases during (a) five cycle WAG injection, (b) tapered WAG injection (increasing WAG ratio), (c) tapered WAG injection (decreasing WAG ratio) and (d) five cycle CO 2 WAG injection (see online version for colours) (a) (b) (c) (d) Notes: WF: water flooding, G: gas-injection, W: water injection (WAG process); S w,g,o = saturation of water, gas and oil.

18 Production performance of water alternate gas injection techniques 149 Table 6 Summary of saturation profile results and Land s trapping constant WAG injection pattern Single cycle WAG Five cycle WAG Five cycle WAG Tapered WAG (increasing WAG ratio) Tapered WAG (decreasing WAG ratio) Injecting gas Saturation (%PV) Initial Final Sw Sg So Sw Sg So Land s gas trapping constant C Hydrocarbon gas Hydrocarbon gas Carbon dioxide gas Hydrocarbon gas Hydrocarbon gas Table 7 Gas utilisation factor for different WAG cycle WAG injection pattern Type of the injection gas Volume of gas injected during WAG process (cc) Gas utilisation factor Single cycle WAG HC gas Five cycle WAG HC gas Five cycle WAG CO Tapered WAG (increasing WAG ratio) HC gas Tapered WAG (decreasing WAG ratio) HC gas Conclusions The implementation of any EOR process should intend experimental verification of process parameters and cost effectiveness of the process. The experimental study, followed by simulation and a pilot project implementation provides the better estimation of the process parameters at the field scale. This study consists of comparative study of different WAG injection process for the core sample collected from the brown field and the live oil prepared in the laboratory, from sample of oil and gas collected from the field separator. The experimental work is done at the reservoir temperature at 120 C and pressure at 230 kg/cm 2. The single cycle WAG (with HC gas), five cycle WAG (with HC gas and CO 2 gas) and tapered WAG using HC gas (with increasing/decreasing WAG ratio) have been investigated experimentally. Based on the experimental results some important conclusions are derived for implementation of WAG process.

19 150 J. Bhatia et al. The main conclusions from this study are given below, The WAG injection process gives the better sweep control, mobility control of water and gas phases, and improves the total recovery of HCPV. Therefore almost all the gas-injection methods now converted to the WAG injection methods. The numbers of cycles in the WAG injection process affect the recovery of oil from the reservoir. The results show that for the same volume of injecting fluid in single cycle and five cycle process the incremental recovery of HCPV has been noticed as 12.74% and 17.16%, respectively. The tapering during the WAG injection process helps to improve the recovery of the HCPV. Increasing WAG ratio during the process gives better incremental recovery than decreasing WAG ratio. The tapered WAG process with decreasing WAG ratio does not show recovery after three cycles of WAG process. This limits use of more cycles of WAG process as they are not economically feasible. The trapping of the gas shows the better effect on the recovery of oil and velocity of the water through core pack. Chasing water injection after WAG process helps to get the incremental recovery of oil. The tapered WAG injection method with increasing WAG ratio gives around 4.2% incremental recovery during chasing water flooding after WAG process. The CO 2 gas at the reservoir temperature and pressure conditions is close to supercritical behaviour and thus helps to improve the recovery of oil during WAG injection process. Gas trapping constant for various variants of WAG is found within the range of 1.3 to 2.8. This shows better gas trapping phenomena in the WAG process. Abbreviations and nomenclature CW Chase water G Gas EOR Enhanced oil recovery FVF Formation volume factor HC Hydrocarbon gas HCPV Hydrocarbon pore volume HCPV Hydrocarbon pore volume LPG Liquefied petroleum gas PV Pore volume Q 0 Flow rate of oil SWAG Simultaneous water alternate gas V L Total line volume W Water WAG Water alternate gas.

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