This study investigates near-wellbore effects during well testing in low
permeability, single- and multi-layered, medium-rich to rich, gas condensate
reservoirs. Theoretical results obtained from compositional simulations are
validated with actual well test data.
We first study well test behaviours for a range of gas condensate fluids with
increasing condensate to gas ratios (CGR), from lean to medium-rich to rich.
We verify that, during a drawdown below the dew point pressure, a
condensate bank forms around the wellbore for all fluids studied. We show
that, in the case of a medium-rich gas, as pressure increases above the dew
point pressure in a subsequent build up, part of the condensate bank closer to
the well dissolves into the gas, with the fluid returning to being a single-phase
gas. This is different from what happens with rich gas, where the bank
disappears completely; and with lean gas, where condensate saturation at the
end of a drawdown and in the subsequent build up are very similar. Lean and
medium-rich gas condensate fluids yield three-region radial composite
derivative behaviours corresponding to dry gas away from the well,
condensate bank, and capillary number effects in the immediate vicinity of the
well. Only two-region radial composite behaviours are created in the case of
rich gas fluids, as rates required to see capillary number effects are not
reached in practice.
We then study layered systems and show that composite behaviour due to
condensate bank and a multi-layer behaviour are superimposed, with the
condensate bank appearing on top of multi-layer effects. In addition, the
production rate ratio of the most permeable layer rate to the total rate tends to
one as the least permeable layer is choked by its condensate bank.
We also investigated gravity effects and conclude that gravity has little impact
on pressure response once the condensate bank develops near the wellbore
and in particular does not create a partial penetration behaviour.
Lastly, we show that drilling horizontal wells and hydraulically-fracturing
vertical wells improve well productivity when pressure is below the dew point
pressure. Condensate drop-out effects are minimized when wells are
fractured prior to being produced.