TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIn the literature it has been shown that a bilinear flow regime, characterized by a quarter slope line on the logarithmic derivative diagnostic plot, can arise for horizontal wells draining fractured or layered reservoirs with transient dualporosity behavior during the intermediate linear flow period. In the case of layered reservoirs the bilinear flow is a result of the linear flow in the more permeable layer toward the well and of the linear flow from the less permeable layer by which the more permeable layer is fed. In the case of fractured reservoirs the bilinear flow occurs as a result of the linear flow in the fracture planes and of the linear flow from the matrix toward the fractures. Therefore, bilinear flow is expected to end in relatively short times. However, bilinear flow was also observed for a long period in the case of two horizontal wells draining a layered, heterogeneous limestone reservoir. The reservoir is characterized by matrix porosity only but highly permeable subvertical zones have been detected due to consistent mud losses occurred during well drilling, and by well logging. The two wells are drilled in the more permeable layer of the depositional sequence encountered at each well location. A 3D numerical model of the reservoir was set up accounting for reservoir heterogeneity in an attempt to reproduce the pressure trend observed during well tests, which consistently showed a quarter slope pressure derivative. Results showed that the vertical heterogeneity would not lead to a bilinear flow regime although the permeability contrast between layers could locally exceed two orders of magnitude, probably because such contrast would not keep constant throughout the reservoir. The quarter slope pressure derivative could only be reproduced when the horizontal heterogeneity was accounted for, assuming that the more permeable subvertical zones would intersect the whole oil-bearing formation.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIn the literature it has been shown that a bilinear flow regime, characterized by a quarter slope line on the logarithmic derivative diagnostic plot, can arise for horizontal wells draining fractured or layered reservoirs with transient dualporosity behavior during the intermediate linear flow period. In the case of layered reservoirs the bilinear flow is a result of the linear flow in the more permeable layer toward the well and of the linear flow from the less permeable layer by which the more permeable layer is fed. In the case of fractured reservoirs the bilinear flow occurs as a result of the linear flow in the fracture planes and of the linear flow from the matrix toward the fractures. Therefore, bilinear flow is expected to end in relatively short times. However, bilinear flow was also observed for a long period in the case of two horizontal wells draining a layered, heterogeneous limestone reservoir. The reservoir is characterized by matrix porosity only but highly permeable subvertical zones have been detected due to consistent mud losses occurred during well drilling, and by well logging. The two wells are drilled in the more permeable layer of the depositional sequence encountered at each well location. A 3D numerical model of the reservoir was set up accounting for reservoir heterogeneity in an attempt to reproduce the pressure trend observed during well tests, which consistently showed a quarter slope pressure derivative. Results showed that the vertical heterogeneity would not lead to a bilinear flow regime although the permeability contrast between layers could locally exceed two orders of magnitude, probably because such contrast would not keep constant throughout the reservoir. The quarter slope pressure derivative could only be reproduced when the horizontal heterogeneity was accounted for, assuming that the more permeable subvertical zones would intersect the whole oil-bearing formation.