It is well known that in higher-permeability natural gas reservoirs turbulence, at times referred to as non-Darcy effects, may become the dominant influence on production. For vertical wells turbulence has been relatively well understood, deliverability equations have been adjusted to account for the phenomenon and, correlations have been presented a long time ago. Furthermore, field-testing techniques have been established to obtain the non-Darcy coefficient.

Surprisingly, similar work has not been done for horizontal wells. One of the reasons is that horizontal wells have been the completion of choice in North America primarily for very low permeability reservoirs where turbulence effects are of no consequence.

However, as the international petroleum industry starts looking at high-permeability gas reservoirs, turbulence effects in horizontal wells can no longer be ignored. Past work on the subject has been sketchy and in the one or two literature sources where it was addressed, the calculations are demonstrably wrong, not taking into account the crucial near-well effects. Also, quite important is that the few turbulence correlations used in the past, for some unexplainable reason have ignored the porosity.

We present here appropriate correlations to account for turbulence effects in horizontal wells and show that while horizontal wells, even with turbulence, provide considerably higher performance than vertical wells, the reduction from ideal expectations can be substantial. For example, at a permeability of 100 md, while for vertical wells the reduction between real and ideal production may approach 40%, for horizontal wells, it may approach 30%. The influence of porosity is considerable and can account for another 30% deviation among reasonable porosity values

We finally offer a large range of parametric studies that involve reservoir thickness, permeability anisotropy and horizontal well length. Turbulence effects have a tendency to reduce the expected beneficial ideal effects from longer wells or permeability isotropy because of the implicit "suicide" effects deriving from higher production rates.

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