1 SINGLE OR MULTIPLE POROSITY CUT-OFF? LEMIGAS SCIENTIFIC CONTRIBUTIONS BAMBANG WIDARSONO VOL. 33. NO. 1, MAY 2010: 1 - 8 SINGLE OR MULTIPLE POROSITY CUT-OFF? A NEW RELEVANCE PROVIDED BY APPLICATION OF A NEW APPROACH By: Bambang Widarsono Researcher at “LEMIGAS” R & D Centre for Oil and Gas Technology Jl. Ciledug Raya Kav. 109, Cipulir, Kebayoran Lama, Jakarta Selatan 12230, Indonesia Tromol Pos: 6022/KBYB-Jakarta 12120, Telephone: 62-21-7394422, Faxsimile: 62-21-7246150 First Registered on 7 April 2010; Received after Corection on 29 April 2010; Publication Approval on : 31 May 2010 ABSTRACT Porosity cut-off is one of the most important parameter normally used to differentiate between reservoir and non-reservoir rocks. Quantity of the parameter certainly dictates reservoir volumes, hence directly influences economics and commerciality of a given oil/ gas field. An ever critical issue in relation to this parameter is whether the use of porosity cut-off has to be established through a single or multiple values especially for heteroge- neous formation rocks. This certainly may lead to different reservoir sizes along with the technical and economical consequences. The study presented in this article is meant to fur- ther investigate this question. The thrust of this study lies on the application of a newly proposed method for determining porosity cut-off. It is put that with this new and reliable method – compared to the traditionally used method – a more conclusive answer can be achieved. For the purpose, a heterogeneous limestone reservoir in West Java – Indonesia is used. Evaluation, analysis, and application of the new method on data from the field’s eight wells have shown that multiple porosity cut-off values are needed for better defini- tions of reservoirs. Application of a single value for these reservoirs can still be regarded as unrepresentative. The fact underlines that – despite the new approach’s reliability – heterogeneity governs more over the use of either single or multiple cut-off values rather than limitations of method. The study also proves that the new method for determining porosity cut-off works well for highly heterogeneous reservoir rocks. Key words: Porosity cut-off, heterogeneous reservoir, multiple porosity cut-off values, new method for porosity cut-off determination. I. INTRODUCTION As universally acknowledged cut-off parameter is used to define a limit of acceptance over a certain parameter or measured entity. Dependent upon the nature of parameter of interest cut-off values are set to distinguish parts of the parameter that are to be ‘included’ or ‘excluded’ from consideration. In case of rock porosity, cut-off value is normally used to distinguish parts of the rock formation that can be regarded as ‘reservoir’ from the rest of formation rocks. It is also well acknowledged that application of porosity cut-off values is much dependent on the level of heterogeneity of reservoir rocks under concern. Relatively homogeneous reservoir rocks may need only a single cut-off value whereas the reverse is true for reservoirs with high level of rock heteroge- neity. However, as clearly put by Worthington (2005), there are no fully satisfactory methods for determin- ing reservoir cut-off parameters and the case is also true for porosity cut-off value. In his comprehensive review over the variously known methods he truth- fully underlined that porosity cut-off value is a diffi- cult parameter to establish due to its indirect relation to permeability cut-off, the parameter that directly separate reservoirs from non-reservoir rocks. Combination of rock heterogeneity and lack of satisfactory method for determining parameter cut-