Summary
Hydraulic fracturing is a common way to improve productivity of
gas-condensate wells. Previous simulation studies have predicted much larger
increases in well productivity than have been actually observed in the field.
This paper shows the large impact of non-Darcy flow and condensate accumulation
on the productivity of a hydraulically fractured gas-condensate well. Two-level
local-grid refinement was used so that very small gridblocks corresponding to
actual fracture width could be simulated. The actual fracture width must be
used to accurately model non-Darcy flow. An unrealistically large fracture
width in the simulations underestimates the effect of non-Darcy flow in
hydraulic fractures. Various other factors governing the productivity
improvement such as fracture length, fracture conductivity, well flow rates,
and reservoir parameters have been analyzed. Productivity improvements were
found to be overestimated by a factor as high as three, if non-Darcy flow was
neglected. Results are presented that show the impact of condensate buildup on
long-term productivity of wells in both rich and lean gas-condensate
reservoirs.
Introduction
A significant decline in productivity of gas-condensate wells has been
observed, resulting from a phenomenon called condensate blocking. Pressure
gradients caused by fluid flow in the reservoir are greatest near the
production well. As the pressure drops below the dewpoint pressure, liquid
drops out and condensate accumulates near the well. This buildup of condensate
is referred to as a condensate bank. The condensate continues to accumulate
until a steady-state two-phase flow of condensate and gas is achieved. This
condensate buildup decreases the relative permeability to gas, thereby causing
a decline in the well productivity. Afidick et al. (1994) studied the Arun
field in Indonesia, which is one of the largest gas-condensate reservoirs in
the world. They concluded that a significant loss in productivity of the
reservoir after 10 years of production was caused by condensate blockage. They
found that condensate accumulation caused well productivity to decline by
approximately 50%, even for this very lean gas. Boom et al. (1996) showed that
even for a lean gas (e.g., less than 1% liquid dropout) a relatively high
liquid saturation can build up in the near-wellbore region. Liquid saturations
near the well can reach 50 to 60% under pseudosteady-state flow of gas and
condensate (Cable et al. 2000; Henderson et al. 1998).
Hydraulic fracturing of wells is a common practice to improve productivity
of gas-condensate reservoirs. Modeling of gas-condensate wells with a hydraulic
fracture requires taking into account non-Darcy flow. Gas velocity inside the
fracture is three to four orders of magnitude higher than that in the matrix.
Use of Darcy's law to model this flow can overestimate the productivity
improvement. Therefore, it is necessary to use Forchheimer's equation to model
this flow with an appropriate non-Darcy coefficient that takes into account the
gas-relative permeability and water saturation.
© 2009. Society of Petroleum Engineers
View full textPDF
(
995 KB
)
History
- Original manuscript received:
7 July 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
2 April 2009
- Manuscript approved:
20 April 2009
- Published online:
31 July 2009
- Version of record:
9 September 2009
View Cart
