Abstract
Summary Crucial issues in formation evaluation are the determination of porosity, permeability, hydrocarbon volumes, and net-to-gross ratio. Nuclear magnetic resonance (NMR) logging provides measurements that are directly related to these parameters. The NMR response of fluids contained in pores is governed by their T2- and T1-relaxation times, diffusion coefficient, and whether or not they wet the rock. In the case where fluids possess a sufficiently large contrast in these properties and NMR data have been acquired with suitably chosen acquisition parameters (i.e., wait times and/or inter-echo times) a separation of water, oil, and gas NMR responses can be made. From these separate NMR responses the hydrocarbon volumes, porosity, and permeability estimates are subsequently calculated. Key in these applications is the ability to include all the acquired log NMR data into the processing towards the desired end result. Methods exist to derive hydrocarbon volumes from T2 distributions or from echo decay data. However, these are all methods in which the difference between just two acquisitions that only differ in either wait time or inter-echo time are considered. Over the past years we have developed, tested, and employed an alternative processing technique named multi-acquisition NMR (Mac-NMR). MacNMR takes any number of log acquisitions (wait time and/or inter-echo time variations) and simultaneously inverts them using a rigorous forward model to derive the desired water and hydrocarbon T2 distributions. In this paper, we discuss the concepts of MacNMR and demonstrate its versatility in NMR log processing. An example will illustrate its benefits.
Published Version
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