Underbalanced drilling has gained popularity during the last years as it is well-suited to the drilling of low-pressured reservoirs and provides a method to prevent formation damage, especially in horizontal wells. The production rate of formation fluids during drilling carries information on reservoir characteristics. In particular, the permeability profile along the well may be inferred from a combination of well pressure and total rate.

The transient well inflow can be interpreted as a complex well test. The complexity arises from the fact that the boundary conditions (i.e. the well trajectory open to flow) change with time during the test. Moreover, the transient rate is influenced by variation in underbalance (i.e. well pressure) and well penetration rate. However, the signal (transient rate) carries detailed information on the permeability profile along the well. Analysis is preceded by the elimination of the effects of the variable underbalance and drill speed using deconvolution techniques. An accurate approximation of the permeability profile can be obtained by a direct analysis (i.e. without having to resort to history matching) of the corrected well influx. Note that only the total well rate trqnsient is required rather than the full inflow profile along the open well trajectory.

In previous work we considered the analysis of the transient rate during underballanced drilling of a vertical well under a constant underbalance. Both the variable underbalance and the more complex outer boundary conditions of a horizontal well (the transient is very likely to see the cap-and base-rock boundaries during the test) distinguish the current study from the previous one.

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