Objectif
Aim of the project is to demonstrate the technical and economical feasibility, by realizing a full scale system, of "the diluent lift" technology, applied to the production of heavy oil from a subsea well.
The project includes the feasibility study, with the definition of the basic transport parameters, which influence the fluid-dynamic behaviour of the system and functional demonstrative tests, basic and detailed engineering, construction and offshore installation of a full scale system in the Vega field, offshore Sicily, in Italy.
The study and the field tests demonstrated the technical feasibility of connecting submarine wells to an existing platform by a sealine, and lifting the crude (heavy crude 16 API) to the platform by diluting it with virgin naphta.
The "diluent lift" method is based on pumping through a dedicated sealine the light diluent to the well bottom, lowering pressure losses of the oil in the production string and in the production sealine.
The laboratory rheological analyses have allowed to select the optimum diluent: virgin naphta with specific gravity 0.7.
The maximum diluent percentage recommended according to the tests is 50-60%; higher quantities could cause asphaltines precipitation.
The optimum percentage of virgin naphta varies in function of well flow rate, PI and Water Cut, as follows :
5 - 15% Q oil = 1000-2000 BOPD WC = 0%
15 - 20% Q oil = 800-1600 BOPD WC = 20%
25 - 30% Q oil = 600-1200 BOPD WC = 40%
45% Q oil = 400- 800 BOPD WC = 60%
For production rates of 400-800BOPD and WC > 50-60%, it is necessary to add a jet pump to the diluent lifting.
This pump is moved by the diluent itself, and allows to obtain an increase in well head pressure (PTHP) up to 200 psi.
The tests on well VEGA 16 have demonstrated that the jet pump, in addition to improve the flow rate of the crude, improves the crude-diluent mixing. This result in turn gives an increase in FTHP of some 20-40 psi.
The injection pressure of the diluent from the platform is between 1500 and 2500 psi, with pump power of 25 HHP (flow rate = 500 BPD).
All fluid dynamic calculations have been carried out by computer programs, carefully set both on the basis of field data, collected during diluent lift tests at wells Vega 16 and Vega 22, and on the basis of laboratory rheological analyses results. This has allowed to obtain reliable programs, with errors on pressure drop lower than 2%.
The first phase of the project is the feasibility of the system by theoretical activities and experimental one, and defines the transport and chemical-physical parameters of the system. In this phase, the technological knowledge on which is based the system is defined from both transport (viscosity, multiphase flow behaviour, fluids compatibility, etc.) and subsea system configuration point of view.
Based on a preliminary diluent selection, laboratory and field tests are foreseen to define the final diluent composition and to demonstrate the system feasibility.
Main activities and objectives are:
- CHEMICAL COMPATIBILITY TESTS :
The mixture of a light hydrocarbon liquid (the "diluent") and a heavy crude oil (such as the Vega oil), depending on the fluid characteristics and their mixing ratios, must be carefully analyzed from the point of view of the asphaltines stability.
- RHEOLOGICAL ANALYSIS :
The diluent injection, besides the gravitational pressure drop reduction in verticallines (mainly in the well), gives also a reduction of the liquid viscosity. This brings to the reduction of the frictional pressure drop in the transportation system.
- TRANSPORTATION SYSTEM DEFINITION :
The whole transportation system is defined in this phase to allow the following fluid-dynamic analysis which evaluates the pressure drop in the system.
That means to define the following:
- well completion configuration
- subsea production line size and thermal insulation
- surface system up to the tie-in point to the existing facilities.
- FLUID DYNAMIC ANALYSIS :
When the pressure in the production line falls below the bubble point value (at the actual temperature) a gas phase begins to exist. The transport analysis must study a multiphase flow (oil+diluent, vapour, water, oil+water emulsion). - FLUID MIXING PROCESS DEFINITION :
The diluent is injected at the bottom of the well through a dedicated well completion which must be specifically developed for the application of "diluent lift" technology.
An important point is to investigate the mixing process between crude oil and diluent, to avoid by-passes and dishomogeneity in the mixture, which can greatly reduce the effectiveness of the lifting process.
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Coordinateur
20124 Milano
Italie
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