Cementation Exponents in Middle Eastern Carbonate Reservoirs

Abstract

Summary. The cementation exponent, m, is a major factor of uncertainty in the calculation of hydrocarbon/water saturation in heterogeneous carbonate reservoirs. Hydrocarbon saturations as high as 70 or 80%, calculated with the conventional value m=2, may disappear completely with m values of 3 and 4 (Fig. 1), which are quite common in carbonate rocks. Laboratory data obtained in heterogeneous carbonate reservoirs often show a wide scatter (Fig. 2). Constant (average) m values are usually used in spite of such scatter, and data points with m values much higher than the average value are often rejected to obtain straight-line trends. This paper presents the results of a study of the relationship between variable m values measured on core plugs and detailed carbonate rock types. The scatter in the data is analyzed in terms of rock type. We conclude that the high m values are often representative for specific rock types and that these values should not be rejected but applied selectively in log analysis over those intervals where these rock types occur. Introduction The cementation exponent (lithological exponent), m, plays an important role in the calculation of hydrocarbon/ plays an important role in the calculation of hydrocarbon/ water saturations from electrical wireline logs, with the Archie equation, FR=Rt/Rw =1/ m. As calculated from laboratory measurements on core plugs, m varies considerably in complex lithologies. In carbonates, the variation in m is very large. As a result, the use of average values, even if based on many laboratory measurements, and particularly the use of the literature value m=2, may lead to large inaccuracies in the calculated saturations. Geologic analysis of several hundreds of samples on which formation resistivity experiments have been performed showed that more accurate values for m can be performed showed that more accurate values for m can be given if a distinction is made between detailed rock types. Rock types with mainly interparticle or intercrystalline porosity (e.g., grainstones and sucrosic dolomites) show porosity (e.g., grainstones and sucrosic dolomites) show m values close to 2. Rock types with more tortuous and/or poorly interconnected porosity (e.g., moldic lime poorly interconnected porosity (e.g., moldic lime grainstones), however, show well-defined trends of increasing m with increasing porosity. In several cases the m value ranged from about 2 at 5% porosity to 5.4 at 35% porosity. Similar trends, but with narrower ranges, were porosity. Similar trends, but with narrower ranges, were found for more complex and mixed porosity types. Scatter can be reduced further by distinguishing between permeability classes. The physical mechanism of electrical permeability classes. The physical mechanism of electrical conductance in moldic rocks is still not well understood and requires further field and laboratory research. The electromagnetic propagation (EPT) log has been a promising tool (in conjunction with standard electrical and porosity logs) for deriving m in hydrocarbon-bearing zones, a normally difficult task with standard techniques (e.g., resistivity logs, Pickett plots, and sonic logs), and good confirmation has been obtained from core data in some cases. Many assumptions, however, need to be made, and it is considered too early to generalize these results. Middle Eastern carbonate reservoirs are very heterogeneous in terms of rock types. Application of the defined average values and trends therefore requires the reservoir to be split into layers on the basis of the dominant rock type (porosity type) within each layer and the selective application of different petrophysical parameters. Layering can be defined on the basis of cores and/or logs or a detailed geologic field model that allows layers and rock types to be identified by log correlation calibrated with cores. Such field models require an integrated and coordinated effort by geologists, petrophysicists, and reservoir engineers and should lead to reconciliation between data sets that, in heterogeneous reservoirs, often appear to be conflicting (e.g., saturations from logs compared to capillary pressure curves, and/or well testing results). The m value is only one of many parameters that affect such comparisons. The Archie Relation Although Archie described the relationship between the formation resistivity factor, FR, and porosity, as being exponential, FR = 1/ m,...................................(1) based on empirical observations mainly on sandstones, he was aware of the occurrence of variations and scatter and that these-are related to rock variations in heterogeneous reservoirs. He recognized that m was not constant and that the term "cementation factor" implied some relationship between m and the degree of cementation in sandstones. SPEFE P. 155