A New Method for Deriving Flow Calibrated Permeability from Production Logs

摘要

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 (PLT) 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 PLT data into a reservoir model. A reservoir model recently built using APERM resulted in a step change 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 & vuggy porosity common in carbonate reservoirs. The APERM method is used for both 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 modern logs, but hot streak 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 our knowledge of flowing and static pressures, plus 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 our ability to derive coarse scale zonal layer pressures with multi-rate PLTs, and use these pressures as input into the permeability derivation. The apparent permeability from PLT 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 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 accuracy of permeability population into the reservoir model.