Modeling high-angle wells in laminated pay reservoirs

Reservoir performance predictions require an accurate method for computing well completion indices. Well indices for slanted wells are especially difficult to compute in anisotropic reservoirs. Previous solutions based on line-source approximations and coordinate transforms cannot capture the effects of anisotropy, nonradial geometry, and nonuniform flux for thin-bedded reservoirs. For reservoirs with near-zero vertical permeability, highly deviated wells (well trajectory within 10 degrees of the bedding planes) can be modeled accurately using an infinite-conductivity fracture approximation. In this approximation, the semimajor axis of the elliptical intersection of the wellbore with the bedding plane is substituted for the fracture half-length. The resulting pseudoskin is a function of deviation angle only, and is valid for thin beds or systems with zero vertical permeability. The applicability of the fracture approximation was verified using fine-grid simulations. The deviation pseudoskin must be applied in addition to any mechanical or non-Darcy skin. In general, the predicted performance improvement due to deviation will be moderate, with pseudoskins of not less than approximately -4. The proposed approximation has been applied to field-scale simulation, including a thin-bedded turbidite interval in the Gulf of Mexico. In systems with near-zero vertical permeability, each simulation layer should be coupled to the simulator in only one grid block, regardless of how many grid blocks in that layer lay along the well trajectory. Otherwise, the well-block transmissibility is too high, and the performance prediction is optimistic. The correct well-to-reservoir coupling was verified by simulations on a refined grid.