Investigation the relationship between nuclear magnetic resonance and acoustic velocity for improving the evaluation of tight gas reservoirs

ABSTRACT

Nuclear magnetic resonance (NMR) and acoustic logging play different roles in formation evaluation. Establishing a relationship model between the two is helpful for joint inversion or evaluation of the physical properties of the formation content. The rock pore structure is used as a link to analyze the theoretical relationship between the transverse relaxation time (T₂) spectrum of NMR and acoustic velocity. In this paper, the relationship between NMR T₂ and shear-wave velocity is determined by using NMR-acoustic velocity joint experiment. Considering that both NMR T₂ spectrum and Shear-wave velocity (Vs) are highly related to porosity and pore structure, a formula based on the relationship between Vs and T_2gm (NMR T₂ geometric mean) was established. It is found that the T_2gm is a power function of the shear-wave velocity. The respective coefficients in this formula for different lithologies were derived through petrophysical measurements of core samples (both NMR and ultrasonic experiments). The relationship model displays the same power-law relations as fitting expression from the experimental data.After that, there is a relation-model application for field data further validated effectiveness and reliability by contrasting the Shear-wave velocity determined by NMR logging with the direct measurements. This model establishment also helps to predict the mutual prediction of NMR and acoustic information of rocks. Based on the sensitivity of the vertical and transverse wave ratios of the formation to the gas-bearing properties of the reservoir, a NMR-acoustic velocity joint gas-bearing identification map was established. An extended case study demonstrated that the cross-plot between NMR logging and acoustic logging is also applicable to natural gas-bearing reservoir identification. As an important supplement and perfection of the existing methods, the relation model proposed in this paper offers a new thought for the petrophysical and in-situ field studies.

KEYWORDS:

• Nuclear magnetic resonance
• shear-wave velocity
• petrophysical experiment
• power function
• gas-bearing identification map

Additional information

Funding

This work is supported by the National Natural Science Foundation of China [41574118, 41174099, and 41404091], Shandong Province Natural Science Foundation [ZR2014DQ004], Self-Determined and Innovative Research Funds [17CX05008], Applied Basic Research Projects of Qingdao [15-9-1-63-jch], and National Science and Technology Major Project [2017ZX05009001].

Notes on contributors

Haitao Li

Haitao Li,   PhD candidate, study in petrophysical analysis and logging data processing.

Feng Xu

Feng Xu, senior engineer, research in complex reservoir well logging evaluation.

Xuquan He

Xuquan He, senior engineer, mainly studies logging data processing of carbonate reservoir.

Li Bai

Li Bai, junior engineer, research in carbonate reservoir logging data analysis.