Summary
A fine-grid simulation is needed to capture the buildup of a condensate bank
near wells operating below the dewpoint pressure. However, full-field
simulations with a sufficiently fine grid will often not be feasible or will
require very long computational times. A semianalytical method has been
developed that can be used to predict the gas- and condensate-production rates
from such wells accurately and that has some advantages over the pseudopressure
approach. The semianalytical method includes the effects of capillary number
(high velocity) and non-Darcy flow. The new method has been implemented in a
compositional-reservoir simulator and verified with fine-grid compositional
simulation results for both lean and rich gas-condensate fluids. Pressures,
saturations, relative permeabilities, viscosities, and densities calculated
with the semianalytical method are in excellent agreement with the results of
fine-grid compositional simulations. Coarse-grid simulations with gridblock
sizes on the order of 200 ft, coupled with the semianalytical method in
gridblocks with wells, yielded production rates as accurate as fine-grid
simulations with gridblock sizes on the order of 2 ft. The method was tested
for single-layer, multilayer, and multiwell gas-condensate reservoirs and was
found to give accurate results.
Introduction
The prediction of well deliverability in gas-condensate reservoirs is a
complex problem. Once the pressure falls below the dewpoint, a condensate bank
builds up near the wellbore, which reduces the relative permeability to gas.
This may cause a significant decline in the well productivity and dominate the
pressure- and production-rate behavior. The difficulty arises in capturing this
near-wellbore phenomenon accurately because it is a two-phase flow problem with
large changes in relative permeability, and hence the equations are highly
nonlinear and do not lend themselves to analytical solutions.
The high velocity of the gas near the wellbore results in a high capillary
number, which may increase the relative permeability of the gas significantly,
and add to the complexity and nonlinearity of the problem. Also, near
very-high-rate gas-production wells, non-Darcy flow further increases the
complexity and nonlinearity. The combined effects of relative permeability,
capillary number, and non-Darcy flow need to be accurately modeled to estimate
the gas-condensate-well deliverability accurately. The changes in the gas and
condensate viscosity and density near the well may also be important in some
cases. Coarse-grid simulations do not capture these effects accurately near the
wells where they matter the most and dominate the production rates.
Fine-grid compositional simulations or simulations using local grid
refinement (LGR) near the wells can be used to obtain an accurate estimate of
the well deliverability. However, these methods have the disadvantage of large
run times, especially for full-field problems with many producing zones and
other complexities. There are numerical errors associated with LGR, and these
are difficult to assess in general. No comparisons to LGR were made in this
work.
Several investigators have used pseudopressure functions to estimate well
deliverability. These methods are simple and have been shown to yield useful
predictions.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
7 June 2004
- Meeting paper published:
26 September 2004
- Revised manuscript received:
28 August 2007
- Manuscript approved:
18 September 2007
- Version of record:
25 February 2008
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