COMPRESSOR SCHEME WILL REDUCE GAS FLARING IN INDIA'S OIL FIELDS
N. Banerjee
Oil India Ltd.
Duliajan, India
To reduce flaring of associated gas, Oil India Ltd. will be increasing compression capacity over a 5 year period.
Although the ideal condition of zero flaring is seldom possible, as long as flaring is limited to 10% of the total gas produced, the operation may be called satisfactory.
It is difficult to control flaring within the satisfactory limits in an aging oil field or where the gas/oil ratio increases rapidly.
The volume of low-pressure gas production also influences the rate of flaring. It may not be economical to compress very low-pressure gas for downstream use. Such gas has to be flared for all practical purposes.
However, if adequate compression facilities are available so that production of low-pressure gas at wellheads can be reduced through gas injection for formation pressure maintenance, then the volume of low-pressure gas can be reduced, thereby reducing flaring.
OIL INDIA'S FIELDS
Oil India's principal oil producing fields are Nahorkatiya, Jorajan, and a number of satellite fields, including a nearly depleted field called Digboi. All these fields are located in the state of Assam in northeast India. An adjacent state, Arunachal Pradesh, has two fields at Kharsang and Kumchai.
Wells that are on regular production are scattered over an area of about 3,200 sq km.
The aggregate production of the company is about 56,000 bo/d out of which about 44,000 bo/d come from the Nahorkatiya and Jorajan fields.
Digboi field is over 100 years old and produces about 400 bo/d.
All of the fields contain multiple reservoirs producing mainly from sandstone formations of the Pliocene, Miocene, and Eocene ages. Well depths vary between 2,000 and 3,500 m.
The average gas-to-oil ratio (GOR) at the wellheads is around 500, although a few wells have GORs as high as 650-700.
COMPRESSION PLANS
The demand for oil necessitates more production from these wells. On account of aging, declining reservoir pressure, and inadequate compression facilities, the volume of low-pressure gas production has been increasing over the years.
The rate of flaring has ranged between 30% and 33% on a daily average. This has been causing tremendous loss of revenue in addition to wasting an important and depleting source of energy. A number of factors are responsible for such high flaring. These are:
- Increasing production of low-pressure gas due to decreasing reservoir pressure.
- Unavailability of gas at a pressure suitable for compressing to the higher pressures needed for gas lift and gas-injection operations.
- Wide market fluctuations in committed gas quantities.
- Inadequate number of compressors to handle the increasing volume of gas.
- Frequent breakdown of compressors because of age and difficulty in getting spare parts.
By analyzing the above, it can be seen that Reasons 1, 2, 4, and 5 relate to compressors directly or indirectly. Reason 3 can be attributed to external factors.
A study on the availability of installed compressor capacity in the fields showed that capacity ranged between 60 and 65% compared to the 85-90% required for efficient operations.
To remedy the situation, plans are to install a number of new compressors in phases over the next few years. This will enhance the installed capacities and at various locations increase the number of standby units to help regular and major maintenance of old compressors.
To remedy the situation arising from a sudden decrease in market demand, two steps have been taken. These are:
- Installation of a supervisory control and data acquisition (scada) system in the major fields to control the production and distribution of the gas through a master control station located at the field headquarters
- Injection of the surplus gas into a few selected gas-storage wells during periods of lean demand with flow back during peak demand.
The schemes were initiated in 1988-89 which was the start of the first phase.
OLD SCHEME
Associated gas is a major portion of the natural gas produced. Owing to natural decline, the volume of low-pressure gas is increasing. The bulk of the gas from the wellhead moves along with crude oil to the various oil collecting stations (OCS) where, after two phase/three phase separation, the gas proceeds to gas compressor stations (GCS).
The low-pressure gas produced at the OCS (below 25 psi) is flared, and the quantity above this pressure is compressed to higher pressures for various uses.
Fig. la shows a typical integrated scheme for utilization of the OCS gas. The mass balance varies from one OCS to another and therefore is not shown in the diagram.
It can be seen that the flaring has to be at a higher rate due to low market demand and the absence of gas storage facilities. The system operates through four types of compressors:
- Low-pressure boosters (LPB) to receive the gas from OCS at 25-30 psi and raise the pressure to 250 psi.
- Gas-lift compressors (GL) to raise the pressure from 250 psi to 1,500 psi.
- Booster compressors (BC) to raise the pressure from 250 psi to 1,500 psi.
- Gas-injection compressors (GI) to raise the pressure from 1,500 psi to 5,000 psi.
COMPRESSION SHORTFALL
In Table 1, the total gas production, available installed capacities of the compressors, and the resulting shortfall are shown. The figures for 1990-91 onwards are estimated based on crude production targets and anticipated changes in GOR computed in various geological studies of the fields.
The installed capacities relate to the capacities available through compressor systems in 1988-89. For the subsequent years, these have been derated because of some phaseout due to old age.
NEW SCHEME
Having identified the anticipated shortfall in the compression capacity, plans are to augment, in phases, the capacities through new additions. These plans include installation of gas storage compressors (GS) to raise the pressure from 150 to 4,500 psi.
Because other methods of oil recovery, such as rod pumps, formation fracturing, nitrogen injection, polymer flooding, etc., are being implemented, the shortfall partly will be taken up by the gas lift and gas-injection compressor capacities.
But in the case of low-pressure booster capacities, the shortfall is planned to be eliminated totally by providing for increased LPG production, storage, and market/internal demands.
Fig. 1b shows the proposed integrated new compressor scheme.
To reduce flaring, additional compressors at the oil collecting station for recovering tank vapors and stabilizing gas recovery will use the gas below 25 psi, until 10 psi.
Thus, by implementing the new compressor scheme, only very low-pressure gas at the oil collecting station will be flared. This is expected to stay well within 10% of the total gas produced.
The need for flaring because of fluctuating market offtake will be totally eliminated through scada and gas storage schemes.
Table 2 shows the various phases of the proposed new compressor scheme.
Table 3 exhibits the aggregate compressor capacities that will be available in the coming years based on present and the proposed compressors under procurement.
PROJECT IMPLEMENTATION
The installation of these new compressors will require the development of associated infrastructural facilities such as piping, sheds with overhead cranes, foundations, operators' cabin, office, roads, lights, power supply, security fencing, fire protection, etc.
In some locations these will be extensions to existing facilities, and in other locations new facilities will be built.
The compressors will be horizontal-balance opposed, reciprocating type, air cooled, natural gas-engine driven, and designed for continuous operation. The delivery times of the machines vary between 18 months and 42 months for the various phases.
The setting up of the infrastructural facilities is determined from the phased delivery program of each type of machine. Therefore, installation and commissioning can be done as soon as the compressor is available at the site.
With the commissioning of the first phase, the rate of gas flaring has come down to around 25%.
When the scada and gas-storage system becomes operational, the flaring is estimated to be reduced to around 15%.
By the terminal year 199394, when the second phase and third phase will also be commissioned, the flaring rate will be decreased to below 10%.
It is anticipated that the total investment in the project will be paid back within the terminal year, 1993-94, thereby establishing the financial viability of such a scheme.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.