Enhanced Oil Recovery By In Situ Combustion Environmental Sciences Essay

Enhanced oil recovery is oil recovery by the injection of materials not normally present in the reservoir. In situ Combustion (ISC) is the process of an enhanced oil recovery process to improve the recovery of heavy crude oil. As it is the oldest thermal recovery technique, it has been used for over nine decades with many economically successful projects. Nevertheless, it is regarded as a high-risk process by many, primarily because of many failures of early field tests. Most of those failures came from application of a good process (ISC) to the wrong reservoirs or to the poorest prospects. This paper contains a description of ISC, a discussion of laboratory screening techniques, an illustration of how to apply laboratory results to field design, a discussion of operational practices and problems, and an analysis of field results. For complete review, the case study is done on Balol and Santhal fields in Mehsana.

In-situ combustion has been known since 1888. Mendeleev was the first scientist to suggest the in-situ conversion of coal into combustible gases. Based on the earlier laboratory results, Sheinman and Dubrovai in 1934 proposed the processed the process of oil displacement by means of a moving underground fire-front. A number of field tests, were performed in various regions in the late 1940’s and early 1950’s. The results from these tests indicated that the heat losses were large, therefore the injected hot gases reached the formation zone with zero thermal energy. These studies however were followed by laboratory research field tests and development of mathematical models to simulate in-situ combustion as a result of which this process has been recognized and can be used as a promising method of recovering heavy oil from petroleum reservoirs.

The principle of in-situ combustion is to achieve combustion within the pores of hydrocarbon-bearing reservoir, burning part of the oil in place in order to improve the flow of the unburned part. Combustion is supported by the injection of air into the reservoir at one or more wells. The heat generated during combustion is sufficient to raise the rock to a high enough temperature to enable the combustion front to self propagate after initial ignition by increasing mobility of the fluid.

Methodology

The in-situ combustion process was applied to petroleum reservoirs depending on wide range of characteristics like Nature of formation, depth, temperature, reservoir thickness, permeability, porosity and oil saturation in order to recover oil. Pressure is also a factor but not much critical.

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The process was applied in reservoirs with average permeability ranging from 40 to 8000mD, whereas the oil saturation varied from 25 to 95%. In addition fuel content is one of the most important factors influencing the success of a fireflood process. The fuel content of the reservoir is the amount of coke available for combustion that is deposited on reservoir rock as a result of distillation and thermal cracking. If the fuel content is too low, the combustion process in the reservoir cannot be self sustained. Moreover, a high fuel content requires a large amount of air and high power cost which means low oil production. Gates and Ramey (1980) compared the estimated fuel content by various methods including laboratory results with that of field project data. It has been shown that fuel content determined experimentally in the laboratory by tube -run method can provide a reasonably good estimation of the fuel content obtained in the field.

In situ combustion is basically injection of an oxidizing gas (air or oxygen-enriched air) to generate heat by burning a portion of the resident oil. Most of the oil is driven towards the producers by a combination of gas drive (from the combustion gases), steam and water drive. This process is also called fire flooding to describe the movement of a burning front inside the reservoir. Based on the respective directions of front propagation and air flow, the process can be forward, when the combustion front advances in the same direction as the air flow, or reverse, when the front moves against the air flow.

Reverse Combustion

This process has been studied extensively in laboratories and has been field tested. In brief, it has not been successful economically for two major reasons. First, combustion started at the producer results in hot produced fluids that often contain unreacted oxygen. These conditions require special, high-cost tubular to protect against high temperatures and corrosion. More oxygen is required to propagate the front compared to forward combustion, thus increasing the major cost of operating an in situ combustion project. Second, unreacted, coke-like heavy ends will remain in the burned portion of the reservoir. At some time in the process the coke will start to burn and the process will revert to forward combustion with considerable heat generation but little oil production. This has occurred even in carefully controlled laboratory experiments. In summary reverse combustion has been found difficult to apply and economically unattractive.

Forward Combustion

Forward combustion can be further characterized as “dry” when only air or enriched air are injected or “wet” when air and water are co-injected.

Dry Forward Combustion

The first step in dry forward ISC is to ignite the oil. In some cases auto-ignition occurs when air injection begins if the reservoir temperature is fairly high and the oil reasonably reactive. Artificial Ignition has been induced using down hole gas burners, electrical heaters, and/or injection of pyrophoric agents or steam injection.

Figure : schematic illustration of the in-situ combustion process (Source)

After ignition the combustion front is propagated by a continuous flow of air. As the front progresses into the reservoir, several zones exist between injector and producer as a result of heat and mass transport and the chemical reactions. The above figure is an idealized representation of the various zones and the resulting temperature and fluid saturation distributions. In the field there are transitions between zones.

A. The burned zone is the volume already burned. This zone is filled with air and may contain small amounts of residual unburned organic solids. As it has been subjected to high temperatures, mineral alterations are possible. Because of the continuous airflow from the injector, the burned zone temperature increases from injected air temperature at the injector to combustion front temperature at the combustion front.

B. The combustion front is the highest temperature zone. It is very thin, often no more than several inches thick. It is in this region that oxygen combines with the fuel and high temperature oxidation occurs. The products of the burning reactions are water and carbon oxides. The fuel is often misnamed coke. In fact it is not pure carbon but a hydrocarbon with H/C atomic ratios ranging from about 0.6 to 2.0. This fuel is formed in the thermal cracking zone just ahead of the front and is the product of cracking and pyrolisis which is deposited on the rock matrix. The amount of fuel burned is an important parameter because it determines how much air must be injected to burn a certain volume of reservoir.

C/D. The cracking/vaporization zone is downstream of the front. The crude is modified in this zone by the high temperature of the combustion process. The light ends vaporize and are transported downstream where they condense and mix with the original crude. The heavy ends pyrolize, resulting in CO2 , CO, hydrocarbon gases and solid organic fuel deposited on the rock.

E. The steam plateau. This is the zone where some of the hydrocarbon vapors condense. Most of those condense further downstream as the steam condenses. The steam plateau temperature depends on the partial pressure of the water in the gas phase. Depending on the temperature the original oil may undergo a mild thermal cracking, often named visbreaking that usually reduces oil viscosity.

F. A water bank exists at the leading edge of the steam plateau where the temperature is less than steam saturation temperature. This water bank decreases in temperature and saturation downstream, with a resulting increase in oil saturation.

G. The oil bank. This zone contains most of the displaced oil including most of the light ends that result from thermal cracking.

H. Beyond these affected areas is the undisturbed original reservoir. Gas saturation will increase only slightly in this area because of the high mobility of combustion gases.

Wet Forward Combustion

A large amount of heat is stored in the burned zone during dry forward in situ combustion, because the low heat capacity of air cannot transfer that heat efficiently. Water injected with the air can capture and advance more heat stored in the burned zone. During wet combustion injected water absorbs the heat from the burned zone, vaporizes, moves through the burning front and condenses, expanding the steam plateau. This results in faster heat movement and oil displacement. Depending on the water/air ratio, wet combustion is classified as: (1) incomplete when the water is converted into superheated steam and recovers only part of the heat from the burned zone, (2) normal when all the heat from the burned zone is recovered, and (3) quenched or super wet when the front temperature declines as a result of the injected water.

ISC requires particular attention to air compression, ignition, well design, completion, and production practices. Air compression causes high temperatures because of the high c p / cv ratio of air. Compressor design must consider these high temperatures to ensure continuous, sustained operations free from the corrosive effects of air and the explosion hazards of some lubricating fluids. Mineral oils are not recommended. Synthetic lubricants withstand the higher temperatures and offer lower volatility and flammability than conventional lubricants.

In order to achieve the combustion in the petroleum reservoir, mainly Spontaneous ignition and Artificial ignition are the two methods that are used for heavy oil recovery. Ignition can occur spontaneously if the oil is reactive, the reservoir temperature high enough, and the reservoir is reasonably thick. Down hole gas-fired burners allow good control of the temperature of injected gases and may be operated at a greater depth than other methods. The disadvantages include the need to run multiple tubing strings in the injection wells. Catalytic heaters run at lower temperatures but are expensive. Electrical heaters can be lowered with a single cable, and can provide excellent temperature control. They can be reused repeatedly. There is, however, a depth limitation because of electrical power losses in the cable. Chemically enhanced ignition may require handling and storage of dangerous materials. Steam may be used to locally increase reservoir temperature and facilitate auto ignition. It suffers from depth limitation because of wellbore heat losses, but when the conditions are right it can be a very simple and effective method for ignition. Combustion process was also employed as primary and tertiary recovery processes.

Applications

In situ combustion can be applied to many different reservoirs. Some suggested screening guidelines are:

Nature of the Formation : The rock type is not important provided that the matrix/oil system is reactive enough to sustain combustion. As in any drive process, high permeability streaks are detrimental. Swelling clays may be a problem in the steam plateau area.

Depth: Depth should be large enough to ensure containment of the injected air in the reservoir. There is no depth limit, except that this may affect the injection pressure.

Pressure: Pressure will affect the economics of the process, but does not affect the technical aspects of combustion.

Temperature: Temperature will affect auto ignition but is otherwise not critical.

Reservoir Thickness: Thickness should be greater than about 4m (15 ft) 2,3 to avoid excessive heat losses to surrounding formations. Very thick formations may present sweep efficiency problems because of gravity override.

Permeability: This has to be sufficient to allow injection of air at the designed air flux. The air injectivity is especially important for heavy oil reservoirs. Conditions are favorable when kh /μ is greater than about 5md m/cp.3

Porosity and Oil Saturation: These have to be large enough to allow economic oil recovery. The product, φ So , needs to be greater than 0.08 for combustion to be economically successful.

Oil Gravity: This parameter is not critical. Insitu viscosity has to be low enough to allow air injection and resulting oil production at the design rate.

Oil Nature: In heavy oil projects the oil should be readily oxidizable at reservoir and rock matrix conditions. The laboratory experiments can also determine the amount of air needed to burn a given reservoir volume. This is key to the profitability of the process.

Current Status of In-Situ Combustion

The in-situ combustion process is attractive economically, provided it is applied to petroleum reservoirs containing approximately 50% oil saturation. The fuel content is one of the important parameters for combustion support at a relatively low air/oil ratio. Although laboratory experiments can provide some basic understanding of the process, the primary evaluation factor is a field application before the process is employed on a large scale.

The present status of oil production by in-situ combustion in the United States is nearly 11,000 bbl/day. The commercial dry ISC project at Romania is the largest project of its kind and it has been in operation for more than 34 years. The Balol and Santhal projects in India have been in operation for more than seven years and have been applied in a wet mode. Currently, combined all these three projects produce approximately 2300m3 /day. It is likely that very little laboratory research can be performed to improve the displacement efficiency of this process. With continued improvement of the in-situ combustion technology, it is almost certain that some form of this process, such as dry, wet, and partially quenched combustion, will find greater application in the coming years.

Currently, commercial In situ combustion projects are

Economic Evaluation

It is recognized that the success or failure of an enhanced oil recovery process depends on the economic evaluation. An economic study completed by Wilson and Root (1966), which is based on a modified form of two-dimensional model presented by Chu, compares the cost of heating a reservoir. The cost comparison was studied for a reservoir either in the presence of steam injection or forward combustion without oil production. The main consideration was to determine heating cost of the same dimensions of a reservoir by either steam injection or by forward combustion. The following conclusions were drawn from this study;

(1) Combustion is favored over steam injection as the sand thickness decreases the pressure increase.

(2) As the coke deposition increases, steam injection is favored over the combustion process.

(3) As the heated distance in the reservoir increases, reservoir heating by combustion is more favorable as compared to steam injection.

(4) Decreased injection rated favors the cost of steam injection relative to air.

(5) Increased wellbore losses with increasing depth favor combustion.

Conclusions

It has been shown that in-situ combustion process is suitable to displace oils of gravities greater than 10 degree API. The average oil recovery by employing in-situ combustion is 50%. The major amount of oil is recovered before breakthrough of the combustion zone. For heavy oils, about 50% crude oil recovery occurs after breakthrough, whereas low-viscosity oil production declines very rapidly following breakthrough. The breakthrough of combustion zone can be recognized by an increase in gas production and its oxygen content. This is followed by a sharp increase ranging from 100 degree to 200 degree Fahrenheit in bottom hole temperature. In addition, the increase in water cut of the produced oil also indicates the breakthrough of the combustion zone. At the same time, pH of the produced water decreases, which is usually due to increase in the content of ions such as iron and sulphate.

CASE STUDY

IN-SITU COMBUSTION AT MEHSANA, GUJARAT.

Mehsana asset, located in the northern part of Gujarat state in India is the highest oil producing onshore asset of ONGC with annual crude oil production of 2.35 MMT. Its having oil fields producing both heaviest crude and the lightest crude in India with API gravity ranging from 13Ëš – 42Ëš. Balol and Santhal fields form a part of this heavy oil belt with a API gravity 15Ëš-18Ëš. Balol and Santhal field encompass 22.17 MMT and 53.56 MMT of oil in place respectively. The crude is asphaltic in nature containing 6-8% asphaltene and the oil viscosity ranges from 50-450 cps at reservoir pressure of 100 kg/cm² and 70Ëš C temperature. Reservoirs have the permeability of the order of 3-8 darcies and are operating under active water drive. Subsequent Artificial lift methods resulted into high water production than oil. In many wells it became 95-100% and some wells had to be closed due to high water cut. The poor primary and secondary necessitated for In-Situ combustion technique in these fields.

Exploitation of heavy oil from these heavy oil fields was a challenge for Mehsana asset. Based on results of laboratory studies, the In-situ combustion process was identified as the most suitable technique for enhancing the recovery from these fields.

PILOT SCHEME

A pilot test was designed and initiated in 5.5 acre area of southern part of Balol field in 1990-91. The first well CP#10 and thereafter Balol#171 were ignited with the help of foreign experts. The sustained combustion and production gain from nearby producers lead to conceptualization of the commercialization schemes in entire Balol field.

In another attempt, a pilot scheme was also designed for Lanwa oil field and an inverted five slot pattern with four producer wells had been ignited in 1992. At present the commercialization of the scheme is in progress to enhance the production from the field. A pilot scheme is also running since 2002 in Bechraji field with four EOR injectors.

COMMERCIAL SCHEMES

Based on the techno-economic success of Balol Pilot project, commercial schemes were designed for entire Balol field for exploitation of heavy oil. Considering the similarities between the Balol and Santhal oil fields, this EOR technique has been implemented on a commercial scale in 1997 both at Balol and Santhal fields. Presently four commercial schemes viz. Balol Ph-1, Santhal Ph-1, Balol Main and Santhal Main are running successfully. Till date total 61 wells have been ignited in Balol and Santhal under these commercial schemes. More wells are in line for conversion into EOR injectors.

For commercial exploitation of Balol and Santhal fields using In-situ combustion technique, four major air compressor plants, two, each in Balol and Santhal fields were set up. These plants supply compressed air to injector wells at reservoir conditions. Compressors except emergency air compressors at all the plants run on electricity. Combined installed capacity of these four plants is of compressing 4.9 NMm3/day air at maximum pressure of 123 Kg/cm2. Since water is required to be injected subsequently during wet phase, facilities for water treatment and injection are also installed in the respective plants. All these four plants are connected to each other with an integrated air grid network for better utilization of resources. A mobile unit called Ignition trailer is being used to initiate ignition process. Gas burners are used for artificial ignition in Mehsana.

RESULTS

After implementation of the technique, decline in production from Balol and Santhal fields was arrested. A number of wells have started flowing on self which were in artificial mode prior to in-situ combustion process. Production testing data of affected wells show the gradual increase in liquid production and decrease in water cut resulting increase in net oil production. Presently EOR gain from both the fields in the tune of 1200 TPD and air injection is in tune of 1.4MM Nm3/d. Production performance of these fields shows the gradual increase in oil production and decrease in W/C% with increasing number of injectors/air injection rate. It has not only given a new lease of life to Balol and Santhal fields but has also increased the oil recovery factor by 2-3 folds from 6-13% to 39-45%.

OTHER HIGHLIGHTS OF THE PROJECT

ONGC is one of the few organizations in the world, which has taken up In-situ combustion process on such a large scale.

Total 68 wells have been converted in EOR injectors at Mehsana Asset so far.

Most of the EOR injectors are old producer wells. They have been converted to injector wells after proper washing and cleaning of wells.

Ignition is being done in the reservoir at an average depth of 990 meters, having 100 Kg/cm2 pressure and 70 degree Celsius temperature.

Present Air-Oil ratio in these fields is about 1160 Nm3/m3 and Air-Oil ratio on cumulative basis it stands at 985 Nm3/m3, which indicates quite good efficiency of ISC process.

Figure : Production profiles of Santhal and Balol fields (Source)

MAJOR ISSUES

Occurrence of Auto-Ignition:

In Mehsana Gas burner is being used for artificial ignition. In this method air is injected through the annulus and natural gas through tubing. An aluminum plug fitted at the tip of burner prevents air and gas to mix. The plug pops out when gas injection pressure is more than air injection pressure and forms gas-air mixture at the bottom. A pyrophoric chemical is being used to initiate the flame. At well no. Balol # A on 1998 the burner caught fire without lowering pyrophoric liquid. Burner temperature shot up to 910 degree Celsius and was soon controlled by ignition tem members. There was no damage to thermocouple and down-hole assembly in this well. After this incidence auto ignition occurred successively in another three wells. In last two wells Santhal #B and Balol # C, thermocouple got damaged. Ignition experts were unable to establish the reason and remedy for auto ignition. Due to this failure, ONGC had completely suspended all the ignition operations fearing further auto ignition and damage to thermocouple.

A close study of all four cases of auto ignition revealed that gas injection was used to be done at full discharge rate of gas compressor. Due to this sudden release of huge amount of gas, a very rich mixture of air and gas forms making situation vulnerable for auto ignition. To overcome this problem, ignition team came up with an idea to put a cushion of an inert gas in the tubing before starting gas injection. At the time of plug pop up, now this inert gas release first afterwards natural gas comes in contact with air. This cushion provide ample time between plug pop up and release of natural gas which facilitate in regulating the gas injection rate to prevent formation of unwanted combustible mixture. The whole idea was put up before the management which was promptly agreed and broke the dead lock of suspended ignitions. After adoption of this technique till date no case of auto ignition encountered.

EFFECTIVE UTILIZATION OF AIR COMPRESSOR

Compression of air at high pressure is a costly affair because of huge consumption of electricity. To minimize this wastage of energy and for optimize the utilization of air compressors, it was thought to connect all the four plants with a common air grid. Subsequently the air grid was constructed using 6″ and 4″ dia pipelines as required. Now compressors are being run as per the total air requirement. By using this grid, on an average INR 2.0 Crores per month (USD 5.3 million per annum) are being saved as electricity charges.

FAILURE OF AFTER COOLER OF AIR COMPRESSOR

Running of large air compressor is difficult in India especially during summer due to high temperature. It may lead to explosion at compressed air piping due to accumulation of carryover lubricants and high discharge temperature. Two incidents of bursting of 3rd stage (Final stage) after coolers of HP compressor had taken place at a compressor plant of Santhal field. As a remedy synthetic lubricant has been introduced. Further regular chemical cleaning of the lines is being carried-out and monitoring of operational parameters has been intensified.

OOZING OF AIR/FLUE GASES

In Mehsana, mostly old wells were used for injection as well as for production. In some cases failure of casing or cementation have observed and has caused pressure built-up in outer casing and even in some cases oozing of gases/air from well site has also been observed. The remedies are

1) New additives for cementation (like thermal cements and calcium aluminates) have been introduced which help to withstand higher temperatures.

2) It is recommended to cement the casing to the full depth in case of new injector wells to prevent the risk of coming out of gas into overlying permeable layers.

3) It is suggested by IEOT (ONGC Institute) to have casing of API 5CT L-80 13 Cr steel in new injector wells and tubing in all wells.

4) New injector wells are being drilled to suit specially for in-situ combustion.

5) Regular monitoring of injection pressure, annulus pressure and outer casing pressure.

Research Work

Figure : showing the working model made in the laboratory

The working model for the In situ combustion was made in laboratory. In this model Injection well and the production well is present on the left and right side respectively, gas injection at high pressure, igniter is taken as the kitchen lighter, test tube is made as an artificial reservoir and ignition zone near the artificial reservoir and also the temperature showing device at the bottom of the production well. This model can be compared to the real conditions with the help of the following diagram.

Figure : In situ combustion process (source)

There were many challenges during the modeling. These challenges were faced according to the need, economy and the factors available. For example reservoir simulation was not perfect, combustion zone was not able to be built exactly in the pores due to lack of oxygen supply. Hence I discover that this process is very economical as compared to other EOR processes but it is very risky as injection of gas should be done at correct place and ignition should be controlled then this process acts as magic recover the oil to 65%. I was successful in recovering the oil but the simulation problem was a main constraint of this working model as that requires a whole laboratory for its working. Hence according to my research heat loss should be minimum, combustion should be in controlled manner are the major challenges that should be overcome.

And these can be overcome by calculating the area in which injection is to be done and what should be the ignition system use for ignition (whether a chemical can be used, artificial igniter at the combustion can be used or if the temperature of the bottom of the hole is very high that can give spontaneous ignition) should be preplanned according to the condition.

The latest and important factor is the chemical injection to ignite the heavy crude oil, let us suppose the oil present there is very heavy oil that cannot be directly ignited; for that situation a chemical can be injected inside which will burn first and then increases the temperature of the respective zone to such an extent that the oil present there will ignite and the further process should start.