Recognition and Development of Low-Resistivity Pay
- Paul F. Worthington (Gaffney, Cline & Associates)
- Document ID
- Society of Petroleum Engineers
- SPE Asia Pacific Oil and Gas Conference and Exhibition, 14-16 April, Kuala Lumpur, Malaysia
- Publication Date
- Document Type
- Conference Paper
- 1997. Society of Petroleum Engineers
- 5.6.1 Open hole/cased hole log analysis, 5.1 Reservoir Characterisation, 5.2 Reservoir Fluid Dynamics, 4.1.2 Separation and Treating
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In unfractured reservoirs, low-resistivity pay zones can usually be identified with one or more of the following: laminated reservoir/non-reservoir sequences, formations with multi-modal pore-size characteristics, sediments with anomalously high surface area, and reservoir characteristics that extend beyond the range of applicability of interpretative algorithms, e.g. reservoirs containing very fresh formation waters. In all these cases, dry oil has been produced in the presence of high interpreted water saturations, in many different parts of the world. It is therefore important to have a generalised facility for recognising low-resistivity pay as early as possible in the life of a prospect.
A systematic procedure is described for the identification of low-resistivity and low-resistivity-contrast pay zones in primary reservoirs. The approach acknowledges that a reservoir rock is a coupled physicochemical system, the method is generic and robust, it is conceptually simple, and it is structured in a manner that is easy to understand. The scheme is modular and it is arranged hierarchically to reflect maturing data scenarios. Therefore it can be progressively refined during the appraisal and development stages. The essence of the method is the definition and calibration of reliable interpretative procedures through quality-assured reference data from key wells by admitting only validated reservoir characteristics. Examples illustrate how failure to do this can result in much loss of value.
The principal thrust is to facilitate the re-evaluation of other wells within the same reservoir system without the need for additional logging programmes. The proposed interpretation scheme also constitutes a basis for the incorporation of new logging technology when this becomes established. The end-product is a flexible petrophysical interpretation scheme for these complex reservoirs that benefits from cost-effectiveness, portability, a higher degree of exactness and consequently a reduced uncertainty. Introduction
The problem of identifying low-resistivity pay through wireline log analysis has been recognised for over 30 years, with much of the early focus on the Texas and Louisiana Gulf Coast of the United States. During the past three decades an increasing number of examples have come to light elsewhere. Today there are documented records of low-resistivity pay in other parts of the conterminous United States, Alaska, Brazil, Venezuela, Argentina, the North Sea, continental Europe, north Africa, the Middle East, India, southeast Asia, Japan and China. Indeed, the related petrophysical literature has become voluminous during the past ten years, presumably because the market-driven requirement to add maximum value with minimum cash outlay has re-emphasised the need for the most effective and efficient completion programmes (Table 1). The subject area therefore remains highly topical, both technically and commercially.
Low-resistivity pay is a relative term rather than an absolute descriptor. It exists when there is a lack of positive contrast in measured electrical resistivity between zones that contain and produce hydrocarbons in commercial quantities and zones that are water-bearing, within the same reservoir system. Therefore, as used here, the term low-resistivity pay also includes low-resistivity-contrast pay zones and does not distinguish on the basis of absolute resistivity, in which respect this treatment differs from certain others. The rationale is that an intra-reservoir quantitative comparison of what constitutes low-resistivity pay is more meaningful than an inter-reservoir analysis, for it is strictly within the context of the former that completion decisions are made. P. 157^
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