A new technique is presented which detects electrical anisotropy from conventional resistivity logs in multiple wells, allowing the user to identify laminated pay intervals and create horizontal and vertical resistivity curves (Rh and Rv).
The benefits of tri-axial resistivity data in laminated formations are well known and lead to interpretation results with very low uncertainties. However, many laminated reservoirs were logged before these tools become available and interpretations using conventional measurements are generally pessimistic and prone to very large uncertainty.
The electrical anisotropy of a formation is determined from changes in responses of conventional resistivity measurements in multiple wells with different angles of relative dip. Each analysis is limited to formations which exhibit similar characteristics on other logs and which are above the transition zone. This often means dividing the field into smaller areas where the formation is seen to be consistent. Rv and Rh curves are computed from conventional resistivities and modelled anisotropy, while Rv_sh and Rh_sh values are determined from resistivities in thick shale sequences in multiple wells. These measurements are then used in a Laminated Shaly Sand Analysis.
Case study results are presented from a low-resistivity laminated shaly sand pay interval, along with a shaly sand interval which is not laminated. This illustrates how the technique can be used to differentiate laminated formations from those which may be bioturbated or disturbed in some way. Laminated formations show distinct anisotropic effects in high angle wells while bioturbated formations do not.
In practice, conventional interpretations are run over the entire formation with the analysis from laminated formations overriding the conventional interpretation over specific intervals. Uncertainty in the results is also considered, both in terms of the improvements over conventional interpretations and also improvements based on the number of wells analysed.