Determination method for T2 cut-off value of nuclear magnetic resonance logging

Abstract

The invention relates to a determination method for a T2 cut-off value in a T2 spectrum of nuclear magnetic resonance logging, in particular to a method for constructing a T2 cut-off value according to the pore structure characteristics reflected by T2 spectrum intensity at each time point, of a T2 spectrum of nuclear magnetic resonance logging. The determination method disclosed by the invention is capable of saving the time and cost of demarcating the T2 cut-off value in the original lots of experimental analysis for a rock core, does not need to establish an empirical formula on the basis of T2 spectrum classification to evaluate the T2 cut-off value, and realizes dynamic and automatic evaluation for the T2 cut-off value. The method realizes scientific, simple and rapid evaluation for 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] (1) [0004] In the formula: is the echo amplitude varying with time t measured by NMR logging; [0005] T 2i is the transverse relaxation time of the i-th component, i=1,...,N; [0006] is corresponding to T 2i NMR porosity of components, i=1,…,N; [0007] Multi-exponential fitting can be performed by formul...

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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

[0045] 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, there are certain reliability problems, and it is difficult to popularize and apply

[0048] (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

[0049] (3) Directly based on NMR logging T 2 (saturated T 2 spectrum) to determine the T of each point 2 The method of the 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 data prediction effect in many regions is not ideal

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