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Estimation of reserves potential for near-critical systems


Key technical results and breakthroughs were achieved in the following fields:
Description of thermodynamics and interfacial properties of near-critical fluids (treated within Task 1):
- Measurement of the bulk thermodynamic properties (phase envelops, liquid dropout curves, densities and compositions) for multi-component mixtures and real gas condensate fluids
- A simple calculation procedure for the thermodynamic behaviour of near-gas condensates easy to implement in reservoir simulators
- A global crossover EOS incorporating the singular behaviour near the critical point and classical behaviour far away from it for pure fluids. The asymptotic behaviour satisfies universal scaling laws with universal critical exponents and universal amplitude ratios
- Acquisition of gas/oil interfacial tensions for real and synthetic gas condensates as a function of pressure and composition (pendant drop or interface laser-light scattering spectroscopy). Values as low as 0.0058 mN/m have been determined with an overall accuracy better than 2%
- A mean field gvdW theory to calculate liquid vapour coexistence curves and IFT for multi-component mixtures of molecular fluids. The method has been tested on alkane mixtures pressurised with methane, nitrogen or carbon dioxide, ternary and four component systems
- A thorough understanding of the wetting behaviour of simple oils on water. It has been demonstrated that the equilibrium wetting behaviour of oils on water in the presence of gas does not obey the conventional picture. In particular, the location of the different wetting states can be predicted as a function of oil composition (alkane chain length), brine salinity, presence of surface-active agents, temperature and pressure. This understanding arises primarily from ellipsometry measurements of equilibrium oil film thicknesses.

Modelling of fluid flow in low permeability reservoirs related to the porous medium structure and properties (far field and near-wellbore region)(treated within Task 2).
* Dependence of the relative perme abilities depend on the capillary number even for capillary numbers as low as 10-7.
- Severe trapping of the condensate phase outside the critical point vicinity, accompanied by important hysteresis on the fluids mobility.
- A fractal model to calculate gas condensate relative permeability and critical condensate saturation as functions of the capillary number and the Bond number, the dimensionless parameters expressing viscous to capillary and gravity to capillary forces ratios. A very satisfactory agreement between model prediction and experimental results.
Examination of different approaches to model high flow rate effects. The most promising approach was to model inertial flow effects separately through the non-Darcy flow factor FND, in which case the relative permeability could be modelled as a function of capillary number.
Near-critical hydrocarbon systems are becoming increasingly important as more high-pressure/high temperature reservoirs are being exploited. When due to depletion the pressure drops below the dew point a new phase appears (condensate) whose properties do not differ much from the other phase (gas).
Improving the description of the behaviour of reservoir fluids in the oil/gas near-critical region and the determination of fluid transport parameters in porous media under near-critical conditions has been the basis of the first RESPONS project. The obtained knowledge may not be directly applicable for very low permeability reservoirs or near-wellbore regions where different displacement mechanisms are expected to determine flow characteristics. In the North Sea many of the newly discovered fields fall into this category. The fluids properties affecting the displacement mechanisms (interfacial tensions, densities and wetting characteristics) vary significantly (by up to three orders of magnitude) in near-critical systems when the pressure varies as a result of reservoir depletion or approach to the wellbore. These variations are poorly captured by the existing conventional reservoir simulators.
The proposed extension will focus on the following questions:
a. What is the behaviour of gas condensate in real porous medium with extremely low permeability ( +-10-5 Darcy)?
Does it present any similitude with flow in ordinary porous media?
b. How the condensate behaviour under near-wellbore conditions can be modelled and predicted? These questions are the subject of strong industrial interest demonstrated by the fact that some of the partners of the project will get financial support from oil and gas companies.

The objectives of the project will be:

- Improving description of near-critical fluids within the porous medium and incorporating the new models in existing simulators.

- Improving understanding of the processes occurring in the near-wellbore region of a gas condensate reservoir.

- Improving understanding of the physical phenomena dominating fluid flow in very low permeability reservoirs and relate them to the porous medium structure and properties.

- Improving modelling of the above phenomena which are not included in conventional simulators.

The work programme will be divided into three main tasks:

Task 0 will consist in choosing representative fluid/rock systems and defining the operating conditions for the high rate (near-wellbore) condensate experiments.

Task 1 will consist in studying, both from experimental and modelling points of view, the fluid bulk and interfacial properties that are needed to estimate the reserves in near-critical hydrocarbon fields and to describe the transport of high rate condensate flow in tight porous media . Results obtained in this Task will be incorporated in existing thermodynamic codes and reservoir simulators.

Task 2 is devoted to the study of the behaviour of near-critical systems in a real porous medium from an experimental and a theoretical point of view. The relative importance of the various phenomena which affect behaviour in tight reservoirs and near-well region will be investigated. Based on the experimental results models will be developed and included in reservoir simulators. Finally the impact of these new models on reservoir performance prediction will be assessed.

The achievement of these objectives should result in improvement of: reserves estimates, gas condensate reservoirs management, and accuracy and confidence in predicting well productivity behaviour by taking into account a physical description of the fluid system and the porous rock.
Fulfillment of the objectives of the project will lead to development of powerfull thermodynamic codes and reservoir simulators for describing production and exploitation of near-critical reservoir systems.

Funding Scheme

CSC - Cost-sharing contracts


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Participants (5)

AEA Technology Plc.
United Kingdom

DT2 8DH Dorchester
Rue Lhomond 24
75231 Paris
S.p. Andersens Vei 15 B
United Kingdom
Crown Street
L69 7ZD Liverpool
Technische Universiteit Delft
134,Julianalaan 134
2628 BL Delft