Ravva Produced Water Reinjection


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Ravva - Cairn’s first development success story has been the bedrock of innovation, and the foundation of our success story in the country and the region. Ravva, which in Sanskrit and Telegu means “diamond” showcases the journey of growth that Cairn has been able to achieve in its business. Incidentally, Ravva is the only field in India to get such a unique name indicating the belief of the nation in it.

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Ravva Produced Water Reinjection

  1. 1.               RAVVA Celebrating 16 Years of Technical Excellence Operation | Produced Water Reinjection
  2. 2. Ravva | ProductionOperationProduced Water ReinjectionIncreasing water production is a major environmental, as well as an economic issue for the oil and gasproducers. It’s also an issue for the regulators who are responsible for ensuring produced water dischargerequirements around the globe. Most oil and gas reservoirs have a natural water layer called formationwater beneath the hydrocarbon layer.Also to achieve maximum oil recovery,additional water is usually injected into thereservoir, which may be associated withhydrocarbons in the production. In thecase of some gas production, producedwater can be condensed water. Thus, theliquid that comes out of the reservoir isnot just hydrocarbons, but is frequentlyaccompanied by water. The liquidproduction is in the form of a mixture offree water, an oil/water emulsion and oil.Furthermore, as an oil field matures, theamount of produced water increases. Thisis because after some time, the formationwaters out due to the water injectionprocess. The water-oil ratio varies from reservoir to reservoir. It also varies with time for a particularreservoir. Worldwide 75% of the production is water, but in some places this percentage may increase toas high as 98%.Ravva field started production in March 1993, and in year 1997 water injection in the field was started tomaintain the reservoir pressure above saturation pressure to maximise the recovery.Cairn introduced PWRI at Ravva, which was a first in the country. The Ravva fields are in a mature phaseof operation, resulting in increased water cut or produced water. This coupled with other factors likeeffluent discharged from plant limit as prescribed by the Andhra Pradesh Pollution Control Board(APPCB), also presented challenges in meeting effluent specifications to be disposed in the sea. ThePWRI also has other environmental significance like helping reduce oil sludge through higher oil recoveryfrom produced water and helping reduce consumption of ground water with the treated produced water forwater injection purpose. Usually, the fluids that are removed from the reservoir by the producing well arebrought to the surface and separated into an oil stream, a gas stream and a water stream. The maincomponents of the water stream that is separated are:  Water  Suspended oil  Dissolved oil  Suspended solids (scale, corrosion products, sand, etc.)  Dissolved solids  Dissolved gases (CO2, H2S, O2)  Bacteriological matter  Added materials (treating chemicals, kill fluids, acids, etc.)
  3. 3. Ravva | ProductionOperationBefore setting up the project, a detailed study was conducted to understand the feasibility of the PWRIapplication. In this process various tests were conducted to understand the compatibility of the existingsource of water at various ratios and stages of the cycle. Also it was ensured that the design be feasiblefor the treatment facility to be compatible with the reservoir requirements for the complete lifecycle of thefield.Based on the available data and test samples, a high level study led to the design of a model, which hadthe following objectives:  to include reservoir behaviour, facilities modelling for scaling and to understand the effects on permeability due to oil presence  to study the effects of change in temperature  to determine specification for produced water for re-injection  to identify and mitigate associated risks like injectivity decline, scaling, souring, corrosion issues, etc.  to carry out basic engineering and provide specifications and make qualified cost estimates  to examine various processes which are economically viable for treating produced water in consideration of the lifecycle of the fieldThe produced water to be reinjected originates from the FWKOD, HP and MP crude oil three phaseseparators. The water from all these sources is degassed in the Produced Water Degasser vessel andthen routed to the Produced Water Surge tank for initial oil-water separation. Separated water from theCrude Oil Wash Tank is pumped directly to the Surge Tank. The pump speed is varied by a variablefrequency drive (VFD) in response to an interface level control signal from the crude oil wash tank.The Produced Water Surge Tank is provided with new internals allowing gas and slugs of oil to separate.Sufficient residence time is provided to allow oil droplets to coalesce to form a separate oil layer, whichcan be drained to Recovered Oil Sump manually. The water flows out from the Surge Tank via a hydraulicloop, which would maintain a minimum level in the tank. From the Surge Tank the produced water thenflows into two parallel Induced Gas Flotation (IGF) units, each handling up to 50% of the design flowEach IGF is designed for 100% of design flow and thus can be used as a standalone unit. In case of anyproblem in PWRI facilities, the produced water in excess to ETP capacity is stored in one of the storagetanks by pumping water from the API separator. Once PWRI is online, water is routed to ETP fortreatment and disposal. This situation is envisaged at high water cut condition. The water from the IGFunits is pumped into the mixing tank using low shear positive cavity screw pumps. The level in the IGFs ismaintained at its normal operating level by a level control loop modulating the transfer pump speed usinga variable frequency drive (VFD).At present, the bore well water is collected in the supply header. The number of bore well pumps onlinedepends on the makeup demand of injection water to oil wells. The water is filtered in the existing auto-back-flushing coarse filters, and then routed to the Mixing Tank. The number of bore wells online dependson the injection water quantity required for the reservoir.The produced water is commingled with the bore well water in the Mixing Tank and then fed to theProduced Water Dual Media Fine Filter package. The existing water injection feed pumps are providedsufficient head to overcome the pressure drop through the produced water dual media fine filter and alsoto ensure the required net positive suction head (NPSH) at the Water Injection Pumps locateddownstream of the fine filters.Each new filter is provided with a dedicated flow control loop located downstream of the filter vessel Theinjection pump discharge pressure is controlled by a split range pressure controller deriving signals from
  4. 4. Ravva | ProductionOperationtwo pressure transmitters – one on each side of the pressure control valve in the injection line. Allatmospheric tanks are fuel gas blanketed to prevent air from precipitating the dissolved iron.