The Petrophysics of Problematic Reservoirs

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Introduction Petrophysics provides the building blocks for integrated reservoir models. It encompasses the analysis of well logs run on wireline and drillstring, conventional and special core analysis, mud logging, and formation testing and fluid sampling. The subject has open- and cased-hole cultures. For deviated and horizontal wells, and/or in the presence of dipping beds, petrophysical analysis is 3D and it should account for formation anisotropy, particularly transverse/ longitudinal differences in rock properties where reference directions are relative to bedding. To manage the current task, the discussion notionally refers to water-wet reservoirs sensed by openhole well logs, although there are exceptions where the reservoir also is the source. Beyond this, the subject matter has been selected on the basis of topicality. Petrophysical Evaluation Most commonly, petrophysics is concerned with the technical evaluation of laboratory data and downhole measurements for reservoir properties such as shale-volume fraction Vsh, porosity f, permeability k, net/gross reservoir, water saturation Sw, and net/gross pay. The subject has a philosophy of indirectness, in that, often, it is not possible to measure a required reservoir property directly. Therefore, it is necessary to measure some other property that is related to the required property. For this reason, petrophysics is built around a framework of interpretive algorithms that relate measurable parameters to reservoir parameters. Usually, these algorithms are empirical with some conceptual reference. This means that quantitative petrophysical interpretation is, mostly, data driven and that the interpretive algorithms change from reservoir to reservoir. This, in turn, requires that each reservoir be investigated separately and thoroughly. For general formation evaluation, see Warner and Woodhouse (2007); for net-pay evaluation, see Worthington (2010a). Archie Reservoirs. Petrophysical interpretive procedures usually are described in terms of an idealized clastic reservoir, which is the textbook reference and is sometimes termed an ’Archie?? reservoir because it broadly matches the requirements for the application of the fundamental Archie equations that provide the quantitative basis for welllog analysis (Archie 1942). Attributes of an Archie reservoir are listed in Table 1. Although these conditions were not itemized explicitly by Archie (1942), they are implicit in the use of the Archie equations on the basis of many years of application.