Wireline T1 Logging

Abstract

With the recent introduction of NMR Logging While Drilling tools and down-hole NMR Fluid Analyzers, the more fundamental T1 measurement has made its appearance in the oil patch. In both these applications, the preference of T1 over T2 has been its insensitivity to motion, since T1 measurements effectively eliminate the detrimental effects arising from tool motion or fluid flow. Experience has shown that the acquisition and interpretation of T1 logs recorded in the less hostile wireline environment is quite straightforward, mainly due to the absence of tool motion, while yielding several benefits over T2 logs. T1 is a critical fluid NMR parameter and can play a prominent role in fluid typing applications. Also, T1 data sets are more compact and the acquisition is more efficient. Finally, T1 logging offers the potential of substantially increased logging speeds. This paper outlines the current T1 logging practice, and substantiates the advantages of T1 logging using real-life data. Examples included demonstrate the value added by wireline T1 measurements into the overall petrophysical understanding of in-situ reservoir fluids. Introduction The spin-lattice (longitudinal) relaxation time T1 contains important surface and bulk relaxation information used in the determination of critical petrophysical answers such as irreducible water saturation, and hydrocarbon properties. In contrast to more popular T2, T1 is not affected by fluid diffusivity, internal gradients or instrumentation artifacts. Although much of the earlier petrophysical work was based on T1 measurements1,2,3, due to technical limitations4, T2 logging became the method of choice in the early nineties. The more fundamental T1 measurement has made its appearance in the industry with the recent introduction of NMR Logging While Drilling tools and down-hole NMR Fluid Analyzers5,6. With both these devices, the preference of T1 over T2 has been its insensitivity to motion, since T1 measurements effectively eliminate the detrimental effects arising from tool motion or fluid flow. Experience has shown that the acquisition and interpretation of T1 logs recorded in the less hostile wireline environment, where (lateral) tool motion is not an issue, is straightforward and even has some benefits over T2 logs. T1 logs provide the same critical NMR petrophysical information available from T2 logs, in terms of porosity, irreducible water saturation, and permeability. Furthermore, determination of the hydrocarbon type, or calculation of the flushed zone hydrocarbon saturation using T1 logs can be much simpler in certain applications. T1 logging also offers the potential of increased logging speeds, which could address a major stumbling block in the general acceptance of NMR logging as a routine application. These statements should not be interpreted to downplay the role of T2 logs; there are many synergies to be exploited between T1 and T2 logs, as demonstrated in the case study presented in the paper. The main objective of this paper is to document the feasibility of wireline T1 logging. Following brief discussions on the physics of the measurement and signal processing, a case study is used to demonstrate the value added by T1 logs. A second objective, once again using real data, is to prove that the quality of the answers, or the level of uncertainty in T1 logs, is at least as good as that of T2 logs. The T1 measurement The first and most fundamental step in any NMR measurement is to align the magnetic nuclei with a magnetic field. This alignment process, or polarization, is not instantaneous and takes some time. The time constant associated with it is called T1. In reservoir rocks, the value of the apparent (or measured) T1 depends strongly on the characteristics of the fluids and the confining pore space. Two distinct relaxation mechanisms acting in parallel determine the longitudinal relaxation time:Equation 1 Where the subscripts B and S correspond to bulk and surface relaxation, respectively.

Challenges for Everyday NMR: An Operator's Perspective

“…Note that in the pay zone (marked by the red strip in the depth axis of both logs), the T 2 distribution contains a single peak, while the T 1 log has two (excluding the weak signals present at early times due to bound water). The stronger peak in the T 1 distribution, centered around 2 seconds is due to gas, while the peak at around 200 ms is due to mud filtrate 19 . Because of diffusion, the water and gas signals are superimposed and direct detection of gas is not possible.…”