A nuclear magnetic resonance logging t 2 How to determine the cut-off value

Abstract

The invention relates to a method for determining the T2 cutoff value in the NMR logging T2 spectrum, in particular to a method for constructing the T2 cutoff value based on the pore structure characteristics reflected by the T2 spectrum intensity at each time point of the NMR logging T2 spectrum. The present invention can save the original time and cost of the experimental analysis of a large number of rock cores to calibrate the T2 cut-off value; at the same time, it does not need to establish the T2 cut-off value based on the empirical formula based on the classification of the T2 spectrum, and the T2 cut-off value can be dynamically and automatically obtained. It realizes the scientific, simple and fast calculation of the T2 cut-off value.

Description

technical field [0001] The invention relates to a nuclear magnetic resonance logging T 2 Spectrum T 2 The method for determining the cut-off value, especially a method based on nuclear magnetic resonance logging T 2 Spectrum at each time point T 2 The pore structure characteristics reflected by the spectral intensity construct T 2 cutoff method. Background technique [0002] In the known technical field, since the introduction of nuclear magnetic resonance technology into oil and gas logging exploration, the understanding of pore fluid and pore structure in oil and gas logging has made a qualitative leap. NMR response parameters: [0003] [0004] In the formula: Echo(t) is the echo amplitude changed with time t obtained by NMR logging; [0005] T 2i is the transverse relaxation time of the i component, i=1,...,N; [0006] φ i is corresponding to T 2i NMR porosity of components, i=1,...,N; [0007] Multi-exponential fitting can be performed by formula (1) to fi...

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Problems solved by technology

[0018] Determination of T by mercury intrusion capillary pressure curve method 2 There are two problems with the cut-off value: (a) The determination of the pressure of mercury injection, the pressure is too small, the free fluid is not completely discharged, the calculated T 2 It will be too large; if the pressure is too large, a part of the bound water will be squeezed out, resulting in the obtained T 2 If the cut-off value is too small, the pore structure of the rock sample will also be destroyed; (b) when the pores contain heavy oil (viscosity greater than 20cp), due to the high viscosity, it is not easy to come out, and the relaxation time is short, it will interact with the irreducible water The signals are superimposed

[0044] The method determines T in the comprehensive physical index 2 The cut-off method is an improvement, considering the influence of the difference in pore structure on the irreducible water saturation, but there is a problem with the NMR logging T 2 In the process of spectrum classification according to shape, manual intervention is required, which is greatly affected by human subjective factors; in addition, the use of normal function fitting lacks a certain theoretical basis, and there are certain reliability problems, so it is difficult to popularize and apply

[0046] (1) Calculation of core test scale to obtain NMR logging T 2 The advantage of the cutoff is that an accurate T 2 However, the limitation in its application is that core samples are required, and core test analysis or other auxiliary logging methods are required, which leads to high cost, long period, and limited core test due to differences in rock structure characteristics. Insufficient data to reveal real patterns

[0047] (2) Calculation of NMR logging T with continuous scale of original formation 2 cut-off value, as determined by the comprehensive physical index T 2 Cut-off method, this method belongs to the general empirical formula, and needs a large amount of core porosity and permeability data as the basis, the application range of this empirical formula is limited, it is difficult to apply to other areas, and the parameters in the formula are not unique , subject to changes in strata

[0048] (3) Directly based on NMR logging T 2 (saturated T 2 spectrum) to determine the T of each point 2 The method of cut-off value, the existing problem is that the NMR logging T 2 The spectrum is classified, and T is obtained according to the empirical formula 2 cut-off value, to get rid of the core experiment and directly use NMR logging to get T 2 Spectrum while quickly determining each depth point T 2 The target of the cutoff value, but this method encounters certain problems in the promotion, and the prediction effect of data in many regions is not ideal

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