Determining Residual Oil With The Nuclear Magnetism Log

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Abstract
Given a reservoir with an immobile oil saturation, a mobile water phase, and proper open-hole conditions, the nuclear magnetism logging tool can be used to determine the amount of residual oil and the vertical distribution of the residual oil near the wellbore. Unique to the method is that the interpreter can control the precision of the results obtained. Introduction Methods of determining oil in place using underground surveys are indirect; that is, secondary properties are measured and the desired primary characteristics are derived from empirical relations with these secondary properties. Most of these methods must account properties. Most of these methods must account for contributions that both rock and fluid make to the measured signal. The nuclear magnetism log (NML) is unique because its signal is from the pore fluid alone. In this report we present a technique that takes advantage of this response characteristic of the NML and enables us to estimate immobile oil after water displacement.The NML tool detects protons in molecules of whatever liquids are contained in a formation immediately adjacent to the wellbore. In Ref. 1 it has been shown that if both water and oil are present the total response is equal to a contribution present the total response is equal to a contribution from the oil phase (which is relatively uninfluenced by confinement in rock pores), plus a contribution from the water phase (which is highly influenced by the pore size and matrix properties of the rock).The tool that was used to obtain the field results presented here caused magnetic fields to be applied presented here caused magnetic fields to be applied to the formation as shown in Fig. 1a. During time Tp the induced magnetization grows toward an equilibrium value proportional to the field strength Hp according to(1) Upon removal of the high field, the induced magnetization decays toward a lower equilibrium value proportional to the low-field strength, HL, according to(2) The growth and decay of the induced magnetization described by Eqs. 1 and 2 are illustrated in Fig. 1b.After the low field is removed the protons respond to the presence of the earth's magnetic field and proton free-precession takes place in the process of proton free-precession takes place in the process of achieving equilibrium. An exponentially decaying voltage with decay constant T2 and amplitude proportional to the magnetization is induced in the proportional to the magnetization is induced in the coil of the tool and is expressible as(3) Fig. 1b illustrates the induced voltage as it decays during the free-precession period. The second term of Eqs. 1 through 3 is a sum because the T1 of the response from the water will depend upon the size of the pore in which it is contained. JPT P. 226

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