Engineering preparation for a hydrocarbon miscible flood is primarily concerned with selection of a solvent composition having the best chance of attaining the oil recovery objective. Emphasis is placed on PVT experiments and simulation, slim tube tests and core floods.
Once the solvent is selected, emphasis shifts to identifying the NGL and gas volumes required from each hydrocarbon supply source. The results are then used for economic evaluations, equipment design, application to government bodies and supply negotiations.
The current "Industry Standard" procedure assumes the solvent to be a rich gas at reservoir conditions. Accepted physical property correlations are then used to subdivide the total solvent volume into mixing gas and NGL-equivalent gas, followed by conversion to NGL liquid using average gas equivalence factors.
The impact of these calculations on solvent injection costs can be important. For example, if a project requires one million cubic metres of NGL, each 1 percentile error in NGL requirement changes the solvent cost 0.2 - 1.0 million dollars. This emphasizes the need to avoid uncertainties associated with assumptions.
The advent of densitometers adapted to high pressure hydrocarbon streams has made assumptions and approximations unnecessary. A new method is proposed based on the measured solvent density at reservoir conditions and measured NGL density at de1ivery conditions.
An important step in the design of a hydrocarbon miscible flood project is the development of a solvent design. The decision whether to make the solvent first- or multi-contact miscible usually derived from the expected working reservoir pressure and the properties of the reservoir oil. If the reservoir pressure exceeds 10–20 MPa and the oil is highly volatile, the two-phase region resulting from mixing with butane and lighter hydrocarbons may be quite small. Theremay not be much difference in solvent enrichment between the two options. If the reservoir oil ismuch denser and/or undersaturated, first contact miscible fluid may have to be so rich that economics prevent its use.
Once the basic process decision has been made, the required minimum miscibility concentration at reservoir working pressure and temperature is usually defined by a combination of PVT experiments and simulations and/or slim tube tests. Althoughat first glance it might appear that the options for making up miscible solvents are limitless, this isnot usually the case. There is often only one or two sources for natural gas liquids available within reasonable transportation distance of the project. There is often only a single source of mixing gas available. Solvent mixtures are, therefore, limited to combinations of a small number of sources.
The combination having minimum ethane plus concentration and yet still generating over 90% oil recovery from a slim tube is designated the minimum miscibility concentration, provided the slim tube test demonstrates no immiscible discontinuities in density and the final effluent is a single-phase olvent-oil mixture. The solvent enrichment is then increased 5 - 10% to give a safety margin to cover reservoir uncertainties.
The calculation of the solvent volume required is made independently, based on reservoir rock diffusion and dispersion parameters.