For single phase flow, it is commonly accepted that wall friction along the completed wellbore may reduce inflow performance. For two-phase flow, interface friction and resulting liquid loading are usually more important than wall friction and may govern inflow performance. The paper provides quantitative understanding of how liquid loading along the wellbore affects inflow performance. It shows that liquid loading can be reduced and inflow performance improved by proper completion design.
Pressure drop due along the completed section of the wellbore may have significant impact on the overall inflow performance. Methods to predict and counteract detrimental effects of such pressure drop have been proposed by different authors. They all assume single phase flow.
In multi-phase flow, heavier and more viscous fluids tend to flow at lower velocity than lighter fluids. This causes interface friction and relative accumulation, loading, of the heavier fluid, which contributes to pressure drops along the completed wellbore. Where the flow velocity in the wellbore is low, as towards the well bottom, liquid loading may govern the pressure drop. It may therefore be suspected that the production of a second fluid will result in quite different inflow performance than predicted by single phase flow models. Laboratory experiments by Ihara and others did not identify principal differences. However, this may be due to their design of equipment and choice of experimental conditions.
The current paper considers analytic and numerical modelling of two-phase, steady-state inflow performance, accounting for pressure drop along the completed section of the wellbore. The problem can be quantified by combination of concepts already used for modelling of inflow to wellbores and tubular two-phase flow. However, liquid loading in the wellbore leads to unusual boundary conditions, that require special solution methods.
The solution method developed in the paper enables fast and mathematically accurate prediction of two phase inflow performance. This also shows that liquid loading along the completed section may govern inflow performance. However, proper well design reduces liquid loading and improves the effective inflow performance.
For notational convenience, a gas reservoir with associated liquid is considered below. However, the model may with minor modifications be applied to other immiscible fluids such as oil and water, or to fluid with solids.
The inflow to the wellbore is assumed proportional to pressure drawdown. Thus, the gas inflow rate per unit length of completed wellbore is expressed as
(1)
The reservoir pressure, is assumed to be in static equilibrium.
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