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Title: Well test analysis of low permeability medium-rich to rich gas condensate homogeneous and layered reservoirs
Author: Kgogo, Thabo C.
ISNI:       0000 0004 2699 1295
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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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.
Supervisor: Gringarten, Alain Sponsor: PetroSA
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral