For long horizontal wells, the pressure drop along the well can exceed the drawndown between the wellbore and the reservoir. Therefore, it is very important to calculate the wellbore pressure profile with accuracy when determining the productivities of horizontal wells.

A comprehensive wellbore model, based on a transient, two-fluid approach, has been implemented in a general purpose reservoir simulator. The model can handle black oil, miscible, thermal, and compositional cases. The wellbore is represented by one dimensional finite-difference grid-blocks which are simultaneously solved with reservoir grid-blocks. A mechanistic model of multiple-phase fluid flow is employed to calculate flow regime identification, interfacial drag, and friction losses between the tubing wall and fluids. Phase changes and heat transfer inside the wellbore are also rigorously modeled.

Several horizontal well examples including the seventh SPE comparative project are studied. It is found that the production profile along the well can be very different between the cases with and without wellbore hydraulics. However, for most cases, the wellbore hydraulics impact on the ultimate oil recovery is minor unless the reservoir is poorly communicated within itself. In addition, we use the transient wellbore model to study the performance of a horizontal steam injector and the transient wellbore behavior of a shut-in well.

The use of the comprehensive wellbore model can be very time-consuming from the CPU standpoint, especially for extremely transient cases. For CPU time efficiency, we also have implemented a simplified approach to model the wellbore hydraulics, based on a steady state correlation. The phase changes and heat transfer inside the wellbore are neglected in the simplified approach. The results from this simplified approach are compared with those from the more rigorous comprehensive wellbore model.

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