A New Method For Deriving Flow-Calibrated Permeability From Production Logs1

Accurate prediction of permeability from well logs in carbonate reservoirs is notoriously difficult. In the Tengiz field, a giant carbonate reservoir in western Kazakhstan, a method has recently been developed to calculate apparent permeability (APERM) based on flow rate from production logs. Incorporation of this flow-calibrated apparent permeability into the static geologic earth model offers an elegant solution to the long-standing problem of how to best incorporate dynamic production log data into a reservoir model. A reservoir model recently built using APERM resulted in a significant improvement over previous methods where only static log-based permeability transforms were used to populate the earth model. Conventional log-based permeability transforms are designed to characterize matrix permeability but not the excess permeability due to fractures and vuggy porosity common in carbonate reservoirs. The APERM method is used for accurately characterizing total permeability (matrix + excess) and for identifying inaccurate permeability predictions in older wells with poor log quality or limited log data. The log-based permeability predictions are more accurate in recent wells with modem logs, but excess permeability identification and quantitative permeability estimation from static well logs is still problematic. The apparent permeability is calculated by solving Darcy's law on an interval basis, using as input the flowing and static pressures, and well, reservoir, and fluid properties. The method makes several simplifying assumptions, but the resulting errors are second order in nature, and the method offers improvements over using conventional static log-based transform permeability. The method is enhanced by the ability to derive coarse-scale zonal layer pressures with multirate production logs, and use these pressures as input into the permeability derivation. The apparent permeability from production logs is then used as a benchmark to adjust the transform permeability derived from static well logs using a variable multiplier. This technique has the advantage of preserving the original fine-scale heterogeneities of the wireline-based permeability logs while calibrating their magnitudes. Future plans are to investigate a correspondence between APERM and rock types as well as statistical transforms with open-hole logs. The log-based transforms can then be used in wells or intervals without PLT data, improving the accuracy of permeability population of the reservoir model.