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 = ƒ(ϕ, S w, V 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 = ƒ (ϕ, S w, V 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 ϕ, S w and V 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 ϕ = 12.5%, S w = 60% and V sh = 27%, as opposed to the ‘standard’ corporate values of ϕ = 10%, S w = 50% and V sh = 50%.