An Electrical Analog Facility for Hydrocarbon Reservoirs

Abstract

A user-friendly type chart has been constructed as an aid to the petrophysical evaluation of water saturation.It provides a basis for the inter-reservoir comparison of electrical character in terms of adherence to, or departures from, Archie conditions in the presence of significant shaliness or low formation-water salinity.The chart describes a continuum of electrical behavior for both water and hydrocarbon zones.In this respect, it is a generalization of earlier approaches that were necessarily restricted to the water zone.This extension is achieved by adopting a generalized geometric factor, the ratio of formation resistivity to water resistivity, regardless of the degree of hydrocarbon saturation.The type chart relates normalized geometric factor to formation-water conductivity, "shale" conductivity and (irreducible) water saturation.The approach has been benchmarked using data from comprehensively studied reservoirs.The application of the method is described with particular reference to rock types, scale effects, core sampling levels and net reservoir criteria.The use of the type charts is illustrated for reservoir units that show different levels of non-Archie effects.The deliverables include reservoir classification, for interpretation strategy and optimum core-data acquisition, and reservoir characterization in terms of analog porosity and saturation exponents.The principal benefit is a reduction in the uncertainty associated with the well-log analysis of water saturation at an early stage in the appraisal/development process when adequate characterizing data are not available. Introduction One of the ever-present problems in petrophysics is how to carry out a meaningful evaluation of well logs in situations where characterizing information from core analysis is either unavailable or is insufficient to groundtruth the log interpretation satisfactorily.This problem is especially pertinent at an early stage in the life of a field, when reservoir data are relatively sparse.Data shortfalls could be mitigated if there was a means of identifying petrophysical analogs of reservoir character, so that the broader experience of the hydrocarbon industry could be utilized in constructing reservoir models and thence be brought to bear on current appraisal and development decisions.Here, a principal requirement is for type charts of petrophysical character, on which data from different reservoirs can be plotted and compared, as a basis for aligning approaches to data acquisition and interpretation.This need manifests itself strongly in the petrophysical evaluation of water saturation, a process that uses the electrical properties of a reservoir rock to deliver key building blocks for an integrated reservoir model.This problem calls for an electrical analog facility through which the electrical character of a subject reservoir can be compared with others that have been more comprehensively studied. In an earlier paper,[1] type curves were presented for the electrical behavior of sedimentary rocks that were fully saturated with an aqueous electrolyte and were therefore representative of a water zone.The curves covered a very wide range of electrolyte resistivity and excess conductivity, a measure of the electrical manifestation of shaliness.The type curves described a continuum of electrical behavior for these petrophysical properties.They were validated1 using a wide range of electrical data that had been measured on diverse core plugs by different investigators.[2]For purposes of petrophysical evaluation, the type curves allowed a reservoir to be classified as an Archie reservoir if it satisfied the requirement that measured formation factor was effectively independent of electrolyte salinity (Appendix I).[3]Otherwise it was classified as an effectively shaly or non-Archie reservoir (Appendix II).[4]The type curves also allowed data from different reservoirs worldwide to be plotted and compared.They could therefore be used as an analog facility for conditions of full electrolyte saturation.