Published by the DTI Oil & Gas Directorate for the reservoir engineering and IOR community in the UK.
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Gas Condensate Well Productivity

Robert Mott (robert.mott@ecl-winfrith.com) a consultant principal reservoir engineer with ECL Technology Ltd describes the results of a recently completed joint industry project investigating and evaluating the various phenomena that influence productivity in gas condensate reservoirs (co-author Andrew Cable).

Introduction
Well productivity is a key issue in the development of many gas-condensate reservoirs.  Accurate predictions of well productivity are needed to select the best development plan, to optimise the number of wells and to set gas sales contracts.

AEA Technology (now ECL Technology Ltd) has recently completed a joint industry project aimed at an improved understanding of some of the key issues affecting gas-condensate well productivity.  The work included core flood experiments on gas-condensate fluids at reservoir conditions, and the development of methods for calculating gas-condensate well performance, both in field-scale reservoir simulation models and simpler reservoir engineering calculations.

When the pressure in a condensate well falls below the dew point, a region of high liquid saturation builds up around the well, impairing the flow of gas and reducing productivity.  It is essential to take account of this ‘condensate blockage’ effect when calculating well productivity.  It is also important to understand the two high-velocity phenomena which can have a significant impact on condensate well productivity - the increase in mobility at high capillary number (sometimes referred to as ‘positive coupling’ or ‘velocity stripping’), and inertial or non-Darcy flow.

Gas Condensate Relative Permeability Measurements
The most important parameter in determining the impact of condensate blockage is the effective gas permeability in the region close to the well.  It is essential to understand the gas-oil relative permeability and how it changes at the high velocities which can occur close to the well.  High-velocity effects are usually modelled through correlations in terms of the capillary number, a dimensionless number that measures the relative importance of viscous and capillary forces.

A key objective of this project was to develop a procedure for measuring all of the relative permeability data needed for gas-condensate reservoir management on a routine basis and at lower cost.  We have developed a new experimental technique, the pseudo-steady-state technique [1], which can provide a more cost-effective way of acquiring gas-condensate relative permeability data.  We have been providing service studies based on this method since 2001.

During the joint industry project we have measured near-well gas condensate relative permeabilities on three cores; a sandstone outcrop core, a sandstone reservoir core, and a carbonate reservoir core.  The work has concentrated on low permeability rocks with permeabilities in the range 3 to 9 mD, as well productivity is more likely to be important in low permeability reservoirs.

All of the cores showed a significant increase in relative permeability at high capillary number, and we were able to match the results using empirical correlations, as illustrated in Figure 1.

Figure 1. Data for gas relative permeability versus capillary number on 9 mD sandstone outcrop core, compared with correlation of Saevereid et al [2]  

Engineering Calculations of Gas Condensate Well Productivity
The most accurate way of calculating gas condensate well productivity is by fine-grid numerical simulation, either in single-well models with a fine grid near to the well, or in full field models using local grid refinement.  However, while numerical simulation is suitable for detailed forecasting of reservoir behaviour, there are many applications where this level of modelling is not justified, and simpler engineering calculations are more appropriate.

Simpler calculations are particularly useful to provide rapid forecasts of well deliverability, for sensitivity studies to assess the impact of parameters such as relative permeability or PVT properties, or to estimate the benefits of fractured or horizontal wells.  They may also be more appropriate where accurate data on reservoir, fluid or rock properties are not available, or as part of an integrated study involving issues such as pipelines, surface facilities, drilling schedules and gas sales contracts. 

We have developed an Excel spreadsheet model for simple forecasts of gas condensate reservoir performance, combining a material balance model of the reservoir with a 2-phase pseudopressure model for well inflow-performance.  The pseudopressure model can take account of condensate blockage and high velocity effects.  This work is described in detail in a paper at the 2002 SPE Annual Conference [3].

The spreadsheet model has been tested by comparison with results of fine-grid numerical simulation for a number of different cases, including fractured and horizontal wells. Figure 2 shows results for a vertical well in a rich gas condensate reservoir with 10 mD permeability.  Two different relative permeability models are used – one which ignores high velocity effects and one which allows for an increase in relative permeability with capillary number.  The results show the significant improvement in productivity due to the high-velocity effect, and the good agreement between the spreadsheet and simulation results in both cases.

Figure 2. Gas production profiles for vertical unfractured well, including the change in relative permeability with capillary number.

Acknowledgements
Financial support for this work was provided by BP, Petroleum Development Oman, Texaco North Sea UK Ltd and the UK Department of Trade and Industry.

References

1. Mott, R., Cable, A. and Spearing, M: ‘Measurements of Relative Permeabilities for Calculating Gas-Condensate Well Deliverability’, SPEREE, December 2000.
2. Seavareid, A., Whitson, C.H. and Fevang, O.: “An Engineering Approach to Measuring and Modelling Gas Condensate Relative Permeabilities,” paper SCA-9930, presented at the Society of Core Analysts conference, Golden, Colorado, August 2-4 1999.
3. Mott, R.: ”Engineering Calculations of Gas Condensate Well Productivity”, paper SPE 77551, to be presented at the SPE Annual Technical Conference Exhibition, San Antonio, Texas, 29 September–2 October 2002.

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