Summary
The recently introduced measurement of total porosity from nuclear magnetic resonance (NMR) tools can help to identify the hydrocarbon type and to improve the determination of formation total porosity (?t) and water saturation (Swt) in combination with other openhole logs. In shaly formations, porosities are difficult to estimate in the presence of hydrocarbons, especially those for gas and light oils. Water saturations are even more difficult to estimate because critical parameters such as clay cation exchange capacities/unit pore volume (QV), the formation factor (F) and formation water resistivity (Rw) might not be known. The latter quantities are essential inputs into the Waxman-Smits and dual-water model saturation equations.
In the typical case of shaly gas-bearing formations, both the total porosity corrected for the gas effect and the gas saturation (Sxgas) in the flushed zone can be derived by combining total NMR porosity (?NMR) and density porosity (?density) Adding resistivity logs such as Rxo) and Rt helps to differentiate between gas and oil. Furthermore, the flushed zone water saturation (Sxot) computed from 1?Sxgas can be used in many ways. One procedure uses Sxot in conjunction with the Rxo saturation equation to determine QV or F. Another technique uses Sxot in conjunction with the saturation point (SP) to estimate QV when Rw is known. Yet, another method estimates QV directly from the NMR short relaxation time part of the T2 distribution and use Sxot in conjunction with SP to estimate Rw. The new interpretation procedure follows the sequential shaly sands approach: first, determine porosity, second, determine shaliness, and, third, determine saturation. The new procedure improves on the classical method by offering new ways to compute QV, F and Rw, The methodology is applied to a number of field examples.
Introduction
Recently, Freedman et al. have shown how to combine ?NMR and ?density to estimate the gas-corrected total porosity ?t and the flushed zone gas saturation in the density magnetic resonance (DMR) method.1 In this paper we build upon their work and integrate NMR logs with other openhole logs in new ways to improve formation evaluation.
For hydrocarbon identification, two simple techniques that combine total NMR porosity, density, shallow and deep resistivity logs are shown in a field example. The techniques are simple enough to give a real-time answer when NMR is logged in combination with the above logs.
For water saturation determination, total porosity corrected for the hydrocarbon effect and QV is essential. Classical shale sand log analysis first estimates porosity from the density neutron, then corrects for the hydrocarbon effect using Sxot from an Rxo tool in an iterative loop.2 The DMR method does not require any iteration since the linear forms of the density and NMR response equations provide an exact analytical solution of the flushed zone total porosity and saturation. Sxot can then be used in conjunction with an Rxo tool to compute other petrophysical parameters such as QV or F. On the other hand, quantitative use of the SP log in shale sand log analysis was demonstrated by Smits3 in 1968. Integrating SP with NMR and other openhole logs allows the estimation Rw or QV in a two-step SP inversion procedure.4 Both the above techniques to determine QV are applied to a field example. In a second field example, NMR and SP logs are used to compute varying Rw in a fresh water example using continuous QV estimated directly from the NMR short T2 time distribution.
The new interpretation methodology is readily extendable to complex lithology, although a multitools solver approach such as the ELAN™ processing method might be preferred.5 (ELAN is a trademark of Schlumberger.)
Quicklook Hydrocarbon Identification
Gas identification with the DMR method is unambiguous when the deficit between density porosity and total NMR porosity is large [e.g., 6 pore units (p.u.) or more]. When the deficit is not large (a few p.u.), one is not sure whether light oil is present or some gas remains in the pore space after flushing. Because the DMR results depend on the input (gas or light oil), of hydrocarbon type whereas the shallow resistivity does not (it only sees the water phase), it is possible to determine the hydrocarbon type by simply comparing the DMR results with the Rxo results.
Rxo?Rt Method
A simple method is to compare the flushed zone water saturation determined by the DMR method with the flushed zone water saturation determined from the Rxo tool. If the two saturations agree (meaning that the DMR gas hypothesis is correct) and the Rt tool indicates hydrocarbon, then the hydrocarbon is gas. If the two saturations disagree (meaning that the DMR gas hypothesis is incorrect) and the Rt tool indicates hydrocarbon, then the hydrocarbon is light oil.
In the zones where Swt<0.7 (a given saturation cutoff), hydrocarbon is present, and if Sxot, DMR-gas? Sxot, Rxo, then gas or light oil is present.
Challenge of Water Saturation in Low Resistivity Cretaceous Reservoirs
