Method and apparatus for measuring oil content of tight reservoir based on nuclear magnetic resonance

Patent metadata

Field	Value
Publication number	US-10884155-B2
Application number	US-201816157480-A
Country	US
Kind code	B2
Filing date	Oct 11, 2018
Priority date	Oct 17, 2017
Publication date	Jan 5, 2021
Grant date	Jan 5, 2021

Abstract

A method and an apparatus for measuring oil content of a tight reservoir based on nuclear magnetic resonance includes applying a pulse sequence to a tight reservoir rock, and after applying a first pulse and a last pulse in the pulse sequence, applying a gradient magnetic field to the tight reservoir rock, respectively, directions of the two applied gradient magnetic fields being opposite to each other, wherein the pulse sequence is composed of three 90° pulses; acquiring a nuclear magnetic resonance signal of the tight reservoir rock; and determining oil content of the tight reservoir rock according to an intensity of the nuclear magnetic resonance signal. The method can accurately distinguish an oil phase nuclear magnetic resonance signal and a water phase nuclear magnetic resonance signal in nanopores of tight reservoir rock, thereby effectively improving the accuracy of the detection result of the oil content of the tight reservoir rock.

First claim

What is claimed is:

1. A method for measuring oil content of a tight reservoir based on nuclear magnetic resonance, comprising:
a pulse sequence and magnetic field application step: applying a pulse sequence to a tight reservoir rock, and after applying a first pulse and a last pulse in the pulse sequence, applying a gradient magnetic field to the tight reservoir rock, respectively, directions of the gradient magnetic field applied twice being opposite to each other, wherein the pulse sequence is composed of three 90° pulses;
acquiring a nuclear magnetic resonance signal of the tight reservoir rock;
determining oil content of the tight reservoir rock according to an intensity of the nuclear magnetic resonance signal,
wherein before determining oil content of the tight reservoir rock according to an intensity of the nuclear magnetic resonance signal, the method includes
judging whether a type of the nuclear magnetic resonance signal is an echo signal; and
if yes, determining that the nuclear magnetic resonance signal is an oil phase nuclear magnetic resonance signal.

2. The method for measuring the oil content of the tight reservoir according to claim 1 , wherein the pulse sequence and magnetic field application step comprises:
applying a first 90° pulse to the tight reservoir rock, and applying a first gradient magnetic field to the tight reservoir rock within a preset time period after the first 90° pulse is applied;
applying a second 90° pulse to the tight reservoir rock, when an oil phase magnetization vector corresponding to an oil phase substance and a water phase magnetization vector corresponding to a water phase substance in the tight reservoir rock are in positive and negative directions of a second axis, respectively, wherein the second axis is perpendicular to a first axis that is parallel to a direction of an initial magnetic field;
applying a third 90° pulse when the oil phase magnetization vector is in the direction of the initial magnetic field and the water phase magnetization vector is turned from an opposite direction of the initial magnetic field of the first axis to the second axis, and applying a second gradient magnetic field to the tight reservoir rock within a preset time period after the third 90° pulse is applied, wherein the first and second gradient magnetic fields have a same intensity and opposite directions.

3. The method for measuring the oil content of the tight reservoir according to claim 2 , wherein applying a first 90° pulse to the tight reservoir rock, and applying a first gradient magnetic field to the tight reservoir rock within a preset time period after the first 90° pulse is applied comprises:
applying a first 90° pulse to the tight reservoir rock, so that the oil phase magnetization vector and the water phase magnetization vector are both turned from the opposite direction of the initial magnetic field of the first axis to a same direction of the second axis; and
applying a first gradient magnetic field to the tight reservoir rock for a first time duration, within a preset time period after the first 90° pulse is applied, so that the oil phase magnetization vector and the water phase magnetization vector are rotated to positive and negative directions of the second axis, respectively.

4. The method for measuring the oil content of the tight reservoir according to claim 2 , wherein applying a second 90° pulse to the tight reservoir rock comprises:
applying a second 90° pulse to the tight reservoir rock and waiting for a second time duration, turning the oil phase magnetization vector and the water phase magnetization vector from the second axis to the direction of the initial magnetic field of the first axis and the opposite direction thereof, respectively, and then turning the water phase magnetization vector from the opposite direction of the initial magnetic field of the first axis to the second axis.

5. The method for measuring the oil content of the tight reservoir according to claim 2 , wherein applying a third 90° pulse, and applying a second gradient magnetic field to the tight reservoir rock within a preset time period after the third 90° pulse is applied comprises:
applying a third 90° pulse, so that the oil phase magnetization vector is turned from the direction of the initial magnetic field of the first axis to the second axis, and the water phase magnetization vector is turned from the second axis to the direction of the initial magnetic field of the first axis; and
applying a second gradient magnetic field to the tight reservoir rock for a third time duration, within a preset time period after the third 90° pulse is applied, so as to offset the first gradient magnetic field applied to the tight reservoir rock.

6. The method for measuring the oil content of the tight reservoir according to claim 1 , wherein determining that the nuclear magnetic resonance signal is an oil phase nuclear magnetic resonance signal comprises:
if the nuclear magnetic resonance signal is an echo signal, determining that an oil phase magnetization vector corresponding to an oil phase substance in the current tight reservoir rock is turned from a second axis to a direction of an initial magnetic field, and then determining that the current nuclear magnetic resonance signal is an oil phase nuclear magnetic resonance signal,
wherein the second axis is perpendicular to the direction of the initial magnetic field.

7. The method for measuring the oil content of the tight reservoir according to claim 1 , further comprising:
if the nuclear magnetic resonance signal is not an echo signal, determining that the nuclear magnetic resonance signal is composed of a water phase nuclear magnetic resonance signal and an oil phase nuclear magnetic resonance signal;
adjusting at least one of a first time duration, a second time duration, and a third time duration;
wherein, the first time duration is a persistent period of an application of a first gradient magnetic field to the tight reservoir rock, the second time duration is a time interval between a second 90° pulse and a third 90° pulse in the pulse sequence, and the third time duration is a persistent period of an application of a second gradient magnetic field to the tight reservoir rock, the first and second gradient magnetic fields having opposite directions;
and re-executing the pulse sequence and magnetic field application step based on at least one of the first time duration, the second time duration, and the third time duration having been adjusted.

8. The method for measuring the oil content of the tight reservoir according to claim 1 , wherein determining oil content of the tight reservoir rock according to an intensity of the nuclear magnetic resonance signal comprises:
acquiring oil content of the tight reservoir rock according to a relationship curve between a preset intensity of the nuclear magnetic resonance signal and the oil content.

9. The method for measuring the oil content of the tight reservoir based on nuclear magnetic resonance according to claim 8 , wherein the relationship curve is pre-acquired in the steps of:
determining intensities of nuclear magnetic resonance signals and oil contents of a plurality of tight reservoir rocks with known oil contents
linearly fitting the intensities of the nuclear magnetic resonance signals and the oil contents of the tight reservoir rocks, so as to acquire a relationship curve between the intensity of the nuclear magnetic resonance signal and the oil content.

10. The method for measuring the oil content of the tight reservoir based on nuclear magnetic resonance according to claim 7 , wherein the first time duration is 0.01 ms to 6 ms.

11. The metho

Assignees

• Petrochina Co Ltd

Inventors

• Tian Hua
• Zhang Shuichang
• Liu Shaobo
• Zou Caineng
• Liu Keyu
• Lu Xuesong
• Zhao Mengjun
• Jiang Lin
• Ma Xingzhi
• Zhang Bin
• Fan Junjia
• Yang Peiqiang

Classifications

• G01R33/4828

  Resolving the MR signals of different chemical species, e.g. water-fat imaging · CPC title

• G01N24/082

  Measurement of solid, liquid or gas content · CPC title

• G01R33/31

  Temperature control thereof · CPC title

• G01R33/56341

  Diffusion imaging · CPC title

• G01V3/32 Primary

  operating with electron or nuclear magnetic resonance · CPC title