Optimization of reservoir cut-off parameters; a case study in SW Iran

Although reservoir quality cut-off criteria have been used for more than 50 years as a guide for economic decisions, there is still no rational procedure for identifying and applying them in Iranian oil and gas fields. In other words, there are different "rules of thumb" in different sections of the National Iranian Oil Companies for determination of cut-off values. For instance, in one section, values of 10%, 50% and 50% are used for porosity, water saturation and shale content cut-offs, respectively; in another section, cut-off criteria are not used at all, simply an estimate of the time when 20% of oil-in-place could be produced. This paper addresses the optimization of cut-off value estimation from raw and processed petrophysical data based on extracting the most appropriate relationship for permeability as a function of porosity, water saturation and shale content - k = (f)(phi , S (sub w) , V (sub sh) ). The procedure starts by looking at permeability as the key parameter in choosing a cut-off value because sometimes the minimum value (the permeability cut-off) is directly related to economic circumstances and is defined by the client. Regression analysis coefficients of 0.936 and 0.870 were achieved for relationships of the form k = (f) (phi , S (sub w) , V (sub sh) ) in the two petrofacies intervals studied. This leads to specification of minimum k values of permeability and determination of optimum cut-off values for phi , S (sub w) and V (sub sh) . This method is then used to determine optimum cut-off values for the Burgan Member (sandstone) in the Kazhdumi Formation in an offshore oil field in the Persian Gulf. The calculated cut-off values for this case for k = 1.0 mD are phi = 12.5%, S (sub w) = 60% and V (sub sh) = 27%, as opposed to the "standard" corporate values of phi = 10%, S (sub w) = 50% and V (sub sh) = 50%.