SPE 165436

Fire-flooding Technologies in Post-Steam-Injected Heavy Oil Reservior:A Successful Example of CNPC Guan Wenlong,Xi Changfeng,Huangjihong,Tang Junshi, China National Petroleum Corporation(CNPC)

Copyright 2013, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Heavy Oil Conference Canada held in Calgary, Alberta, Canada, 11–13 June 2013. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract Steam injection is an effective technology for heavy oil development. With the increasing of the recovery percent, the technical effectiveness and economic efficiency of continuous steam injection will substantially decline. Therefore the fire-flooding can be considered as a follow up EOR technology in these steam-injected reservoir. Because of the complicated secondary water and steam channels distribution, fire flooding in post steam injected reservoir is far different from that in original reservoir. In this paper, the mechanism and problems associated with development engineering of fire flooding in post steam injected heavy oil reservoir was studied systematically by using 1D&3D physical simulation systems and reservoir numerical simulator. The temperature of combustion zone decreased and high temperature zone enlarged because there existed secondary water formed during steam injection which could absorb and carry heat towards producers out of combustion front during fire flooding, but high saturation of water in layer caused by secondary water had less influence on the quantity of fuel deposit and air consumption.

There is a living example of fire-flooding in post-steam-injected heavy oil reservoir in Xinjiang oil field of CNPC, and it is the first major pilot of fire-flooding in this kind of reservoir, namely H1 Block Fire-flooding Pilot. H1 Block had been steam-injected for more than 10 years and oil recovery factor had nearly reached 30%. Before fire-flooding test, the reservoir had been abandoned for many years. In this paper, the fire-flooding model, well pattern, well spacing, and air injection rate was optimized based on the reservoir property and the existed well pattern in H1 Block, and the key techniques of ignition, lifting, and anticorrosion was also selected in the same time. The H1 Block Fire-flooding Pilot was carried out in the late of 2009. Up to now, more than 30 wells produced oil, 16 of them produced stably. The daily oil production increased from nearly 0 to 48t/d , water cup decreased from nearly 100% to 60%, and AOR decreased from more than 5000Sm3/m3 in the beginning to 2200 Sm3/m3 by far. The forecasted ultimate recovery factor will reach 65%.

Introduction

Fire-flooding is usually applied in the reservoir which has not been flooded by water or steam. As fire-flooding has strong adaptability to develop the reservoir[1], it can be considered as a follow-up EOR technology of the low economic profit and high oil recovery reservoirs flooded by water or steam. CSS is now the key technique for the heavy oil reservoirs in China, and most of the reservoirs have been operated in many cycles which results in problems such as high water cut, high SOR and low-efficiency. Some of such reservoirs are suitable for steam flooding and SAGD after CSS. Some are suitable for fire-flooding.

Fire-flooding in post-steam-injected reservoir is different from that in original reservoir, because of the complicated secondary water and steam channels. Under this condition, it is important to find out the influence on fireflooding by secondary water, steam channels and oil residual oil distribution, and the different mechanisms. Since CSS has got high oil recovery, it is suggested to make use of exist well patterns as far as possible during fire-flooding design in order to reduce cost. Besides, the ignition method, lifting technique and corrosion prevention are different from that of original reservoir as the existing of secondary water. Problems mentioned above were studied and the results were applied in pilot using Xinjiang Oilfield Block H1 as prototype.

Jurassic Badaowan (J1b), the target layer of Block H1 is braided river deposition, and the lithology is mainly glutenite. Average effective thickness of the reservoir is 8.2m with average porosity of 25.4% and average permeability of 720×10－3

μm2. The burial depth of the reservoir is 550m, original pressure 6.1 MPa, original temperature 23oC. The viscosity of

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degassed oil in reservoir temperature is 9 000～12 000 mPa·s. The layer is monoclinal structure, and the angle of bedding is 5°. Before fire-flooding, CSS and steamflooding were taken. At the steam injection stage, oil recovery was 28.4%. The well pattern was square, well space was 100m (Fig 1). Because of the high water cut at the later stage of steam injection, the reservoir was abandoned, and all the producers were used to develop other reservoir. According to the history matching of numerical simulation, the oil saturation decreased from 71% to 51%, and the oil saturation in 30m near the well was 20%～40% (Fig 2).

Figure 1-Well pattern and productions of single well during CSS and steam-drive

Figure 2- Planar distribution of remaining oil saturation

EOR mechanism of fire-flooding in post-steam-injected reservoir

Long combustion tubing experiments with secondary water

Long combustion tubing model [7] is 2000mm in length and 170mm in diameter. Temperature compensation system was set outside the tube in order to reduce heat loss as much as possible. Data acquisition system recorded temperature, pressure, component of production gas and production rate. Air was injected in the inlet of the model, and was heated by igniter to ignite oil. Gas and fluids were producted in the outlet. Fundamental parameters such as the moving speed of combustion zone, air consumption in unit volume, fuel deposition (consumption) and displacement efficiency were obtained.

In order to study the influence of the fire-flooding effects by secondary water, 2 experiments were designed using the oil and cores from Block H1. In test 1 the tube was packed by sections: the first 1/3 was high water storage section with water saturation of 55%, which represents the reservoir with secondary water; the left 2/3 was original reservoir section with water saturation of 20%. Test 2 was fully packed original reservoir with water saturation of 20%. The ignition temperature was 500oC in both tests, experiment results were in Table 1. Results indicated: ① In both experiments, the air consumption in unit

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volume and fuel deposition (consumption) are nearly the same, which shows that secondary water do not have influence on that two parameters. If the oil could continually combust and it only depended on the oil and reservoir properties but not on the secondary water. ② However, the displacement efficiency was much less in Test 1 than that in Test 2 because of low oil saturation.

Table 1 Long tubing fire-flooding experiment results

TEST ignition

temperature T/℃

air consumption

/(m3·m－3)

fuel deposition /(kg·m－3)

combustion volume/ %

displacement efficiency /%

1 500 322 23.7 82 67

2 500 317 23.1 85 90

We have done long combustion tubing fire-flooding experiment before in the condition of high water cut of Shengli Oilfield Zhen 408. The water cut was constant 47.8% along the length. The phenomenon of thermal front beyond the combustion front was found and the wetting combustion happened under the condition of air injection only.

Test 1 showed that although water storage section was near injection point, the water was firstly driven out by flue-gas as the viscosity difference between oil and water. At the beginning, the water cut was high and the oil production increased slowly. The peak oil production rate occurred when the oil bank with high oil saturation were formed. More details about the oil bank were in references [9-10].

3-D fire-flooding physical experiments

3-D fire-flooding physical experiment facility consists of inject system, physical model, data acquisition and product system[11]. Injection system is made up of air compressor, pump, middle vessel, gas bottles, pipes and valves. Data acquisition is made up of hardware and software, and can collect temperature, pressure and flow rate data. Product system can separate and measures fluid rate produced from model. This is 3-D sand packed model. The inner size of model is 500 mm×500 mm×100 mm, and a well pattern of 1/4 inverted nine spot is set inside, consisting 4 vertical wells (including 1 air injection/ignition well), 2 edge wells and 1 corner well. The well space is 500 mm×707 mm. The thermocouples are uniform fixed in three layers (upper, middle and bottom) of the model. The system can give temperature profiles which can be used to study the distribution of the combustion front in both horizontal and vertical directions. The maximum temperature is 900 ℃ and the maximum pressure is 5 MPa.

The 3-D fire-flooding experiment contains the following processes: ① Experiment preparation: According to the property of Block H1 and the fire-flooding similarity criteria, the porosity, permeability and saturation of 3-D model were designed, and the work on fluids preparation, core, fluids physical property test, transducer calibration and igniter tests was done based on this. ② Model packing: This process mainly includes model well/igniter installation, transducer installation, leak test, core packing, water saturation and oil saturation. In order to simulate the high water saturation in the post-steam-injected, crude oil, formation water and the sand of reservoir H1 were pre-mixed and packed [12]. The packed model was homogeneous, the porosity was 38%, the permeability was 73 μm2, the oil saturation was 58% and the water saturation was 37% (initial gas saturation was 15% ). ③ Air test: Before ignition, N2 was injected to test the connection between injector and producers, and the initial temperature field was built to simulate the reservoir condition. ④ Fire-flooding experiment: this process includes ignition, increase injection rate, stable fire-flooding and stop injection. During experiment, the 3-D combustion front distribution was monitored by calculating key temperature, pressure and flow rate data. The producer should be closed when the production fluid temperature was over 300 oC or the oxygen concentration in produced gas was over 10%. Experiment ended when all producers were closed.

Figure 3 was the model opened at 300min after the ignition/injection. The ignition/injection well was at the upper left corner where the white zone was burned zone in which the oil was burned up and white sands left. Uncomplete combustion coke existed below the combusted zone and the coke was thicker near the producer. The outside edge of the combusted zone was the black coke zone (different from the bottom, this coke was the fuel for the follow-up combustion). The residual oil was existed between the black coke zone and the production zone. According to the calculation, the combusted zone took up 37.9% of model volume, the coke (below the combusted zone and front of combustion zone) took up 22.5% of model volume, the residual oil took up 39.6%. According to the calculation of production oil, the oil recovery was 65.6%, the accumulate air/oil ratio was 2 157 m3/m3.

In the 3-D fire-flooding experiment, unswept zone by combustion zone(residual oil zone) took up nearly 40%, but the oil recovery achieved 65%, which shows that: � most oil of the residual oil zone was produced; � the oil saturation of the coke zone was quite low. In order to confirm this, the oil saturations of different zones were tested by thermogravimetry analysis method[12], and the final oil saturation of residual oil zone was 35%～42% which was obviously lower than original oil saturation. The oil saturation of coke in front of combustion zone was 10.2%～12.3%. The average oil saturation in the

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bottom coke zone was 18.6% which was a little higher than the coke in front of the combustion zone, which indicated that although the combustion was uncompleted in the bottom, most of the crude oil was driven out by the high temperature fluid (flue gas, steam and hot water).

Figure 3- Distribution of coke zone in the model after 300 min of ignition/injection

Pilot testing project

Fire-flooding pilot deployment and developing performance prediction

According to geological research, residual oil distribution evaluation and numerical simulation optimization, the pilot test area was selected, shown as Fig.4. The main points of the pilot project includes: ① New wells were drilled based on old well pattern, the well space was 70m; ②Ignition/injection was operated in the new wells, the ignition temperature was controlled at 450～500oC; ③ 3 well groups parallel to the structure top contour were selected to process area fire-flooding. After the combustion zone communicated among the 3 well groups, line fire-flooding was started and ensure the combustion front moving from up-structure to down-structure; ④ Dry air injection combustion was used; ⑤ In area well pattern stage, the single well injection rate increased to 40 000 m3/d step by step, and in line fire-flooding, the single well injection rate was 20 000 m3/d; ⑥ Injection well was completed with corrosion resisting treatment, and the pump with gas anchor was used for lifting.

There were 3 development stages in fire-flooding pilot: At stage 1, it was ignited in 3 wells, and area fire-flooding formed in 5 plot well pattern with 70m well space; In stage 2, When the fire front moved to the old wells (shown in Figure 5a), an angular 7 plot well pattern formed with 100m and 140m well space made up by the center injector and 6 new drilled well, and it was still area fire-flooding (shown in Figure5b); In stage 3, it was regulated and controlled to make the 3 nearby combustion zones communication, then turned the other 4 new wells of the injection well array into injection wells, forming

Figure 4- The well pattern in the fire-flooding test area

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a 7 wells (together with original 3 injection wells) injection array, and then the line fire-flooding began (shown in Figure 5c). The numerical simulation predicted the area fire-flooding oil recovery was 15% in 4 a, and section line fire-flooding oil recovery was 18% in 6 a. Accumulated enhance oil recovery of the fire-flooding after post-steam-injected was 33%. Influenced by the aeolotropism and the steam injection, although the combustion zones connect together in line fire-flooding, the fire front was not even. The fire front moved fast in the high porosity and high permeability zone. Some oil may be trapped and could not be produced s (see the red ring in Figure 5c). In order to avoid the situation mentioned above, controlling operations to production wells in the high porosity and high permeability zone should be taken in fire-flooding process.

Figure 5- Planar distribution of oil saturation in different stages of the fire-flooding

Key project engineering

Ignition

Spontaneous ignition, chemical ignition, electrical ignition and gas/liquid fuel burner are main ways to ignite the reservoir[15]. Since domestic reservoir original temperature is below 70oC, it will take 1 month to ignite formation spontaneously and the combustion will not be complete. There are two mature ways to ignite: chemical catalytic ignition under the condition of steam preheating; high power electrical ignition. Both Shenli Oilfield and Xinjiang Block H1 pilots are using electrical ignition[16]. According to combustion tubing experiment, the ignition temperature should be controlled above 450 oC.

Air injection system

The operating pressure and delivery capacity of the air compressor should meet the needs of reservoir engineering. Different from waterflooding, polymer flooding and steamflooding, fire-flooding should inject air to the reservoir continuously to ensure the combustion front moving stably. If the injection stopped for a long time, the combustion may extinguish which would lead serious problem to the development because ignition can not happen in original injection well (lack of fuel), another ignition well is needed. So a back up compressor should be prepared while designing the injection system.

Lifting technology

According to the numerical simulation, there are four production stages(figure 6): ① drain of water: In the first 6 months, displace secondary water by flue gas. During this period, the normal piston pump was suitable. ② response and production-rate increase stage: During the stage, liquid production and water cut decreased sharply, the thermal front had not reached the production well and the production liquid temperature did not increase obviously. Viscosity of the production oil were high, assisted drainage is needed. ③ stable producing. The main problem in this stage was to make the pump to suit the high gas/liquid ratio. ④ oxygen control and limited production. In this stage, the production liquid temperature was high (150-200 oC) and oxygen may break through, so the operation of limited production and water-cool-down in the wellbore should be taken to delay the well closing time.

Anti-corrosion of the wellbore and surface facilities

The corrosion is usually caused by produced CO2 in the flue gas. In the pilot, anti-corrosion treatment and tests should be taken in production wellbore, separator on the surface and gas draining pipes. Since the high water cut and high concentration of CO2 , the wellbore and pipes need strong ability for anti-corrosion.

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Another corrosion easily neglected is the rich oxygen corrosion in wellbore of injectors. Under the condition of high pressure injection, the oxygen concentration in the surface of the wellbore is much higher than the atmosphere (usually tens and hundreds of that in atmosphere). In the situation of long time injection, the rate of oxygen corrosion greatly increases, and in the serious situation, lots of produced ferric oxide flakes will block the embrasure [17] and results in the injection pressure rise or injecting failure. So it is necessary to consider safeguard procedures for the wellbore of injectors.

Production gas monitor In order to judge the combustion condition during the fire flooding, all the produced gas components should be monitored. Especially, the contents of CH4、CO2、O2、CO in the production gas should be detected. In the pilot, portable gas analyzer can be used for online monitor, or taking samples for laboratory GC analysis. The GC analysis is relatively complex but relatively accurate, and is more suitable for CO and H2S test.

When the thermal front moved to the bottom of the producer, the production liquid temperature rose obviously, the oxygen may break through soon, so the oxygen concentration should be detected carefully. In common, explosion will not happen when the oxygen concentration is below 10% [18]. However, the producer should be closed when the oxygen concentration was above 5% for safety consideration.

Production performance of the pilot

Performance in initial and current condition

In December 2009, air injection began with hH008 and hH010 well groups, and in June 2010, air injection started with hH012 well group. According to the production gas components analysis, the oxygen concentration decrease quickly after the ignition, and reached 0 in 5 d; the CO2 concentration increased to 16% step by step; the CO concentration was around 0.5%, and had the trend to reduce. The combustion state shows well in the reservoir.

For the pilot test project, in initial 6-10 months, the secondary water was displaced by flue gas after ignition, the water cut was nearly 100%(Fig.6). After the 6-10months, the oil bank was formed, then production oil increased gradually. After transferring to line well pattern on June 2012, the production oil increased fast and the water cut was stable below 70%. Nowadays, the oil rate is about 49t/d, the air injection rate is about 100,000m3/d, the air oil ratio is about 2200 Sm3/m3(Fig.7), the good performance is gained for this kind of abandoned post-steam-injected heavy oil reservoir . The forecasted ultimate recovery factor will reach 65%.

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Figure 6 - Daily oil production of the pilot

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Figure 7 - Monthly AOR of the pilot

Figure 8 shows the planar and vertical distribution of oil saturation up to the end of 2012 provided by reservior numerical simulation. Although there are 7 injection wells in line in the pilot nowadays, the 7 burned regions do not yet conneted each other. It is a long way from being a really linear fire-flooding mode.

By the end of 2012, the total oil production of H1 pilot was reach more than 23,000 tons and the recovery percent in fire flooding stage was about 6%. Considering that most of the producers were in their upward production growth stages and the useful producers were very limited in the first year of fire flooding, the real oil recovery rate during fire flooding was between 2.5% to 3%. This oil recovery rate would be higher and higher with promotion of fire flooding process forward.

Figure 8 - Distribution of oil saturation up to the end of 2012

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Figure 9 - Oil composition variation before and after in-situ combustion

Figure10 - Bottom hole temperature of well h2071 in April 2011

Produced oil before ISC

Produced oil of ISC

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Figure11 - Typical crude oil emulsion in the wellhead of producer

Figure12 -Emulsified oil and separated oil

High temperature oxidation mode and crude oil pyrolysis

From production oil monitoring and analysis, oil composition changed after in-situ combustion due to high temperature oxidation reactions had taken place in the formation. Figure 9 gives the compositions of initial crude oil and producted oil of ISC tested by GC. After a period of fireflooding, the light hydrocarbons which carbon number less than 40 were found in the production oil. These light hydrocarbons were normally not in the initial crude oil, which means high temperature cracking had happened in the process of fireflooding.

Figure 10 gives the bottom hole temperature of the production well h2071 at a distance of 70m away from the injection/ignition well hH010. During April 2011, the temperature had been above 400℃ for more than two weeks. From April 13 to April 14, the temperature rosed higher than 700℃ which implied the combustion front had arrived the well h2071 at that time. This is a clear evidence that high temperature combustion reactions and cracking had really happened. The viscosity of production oil was about only one-fifth to one-third of the original crude oil in the same place.

Phenomenon of crude oil emulsion could be found in most of production wells (Figure 11 and Figure 12) since fire flooding became effective. Emulsion fluid from a producer usually corresponds to its stable production phase and water cut keeps relatively stable (from 55% to 75%). From the shading of the color of emulsion fluid one can give a general judgment on its water cut. Figure 12 gives a picture of a sample of emulsion which in 70% water cut and a sample of crude oil which separated from water. Phenomenon of emulsion is often connected with high temperature condition, oxidation reaction process, and multiphase flow through porous media.

Oil emulsion (70%water)

Separated oil

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Summary

1.Dry combustion mode and top to down displacement pattern is adaptable mode for post-steam-injected heavy oil development testified by over 3 years pilot in H1 Block of Xinjiang oilfield of CNPC.

2.The high temperature combustion reaction and thermal cracking had been taken place in the H1 pilot resulted in changes in composition of production oil in contrast with original crude oil.

3. The performance of the pilot gives us enough confidence to take fire flooding as a main follow-up technology for post- steam injected heavy oil reservoir development. The ultimate oil recovery will be more than 65% and will increase more than 30% on the basis of steam injection development predicted by 3D lab experiments and reservoir numerical simulations. At precent, the real oil recovery rate of fire flooding is about 2.5%~3% and will be higher and higher with the fire flooding process forward.

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