Characterization of Volcanic Reservoirs Requires Comprehensive NMR Tests

Abstract

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper IPTC 19378, “Comprehensive Evaluation of NMR Characteristics of Complex Volcanic Reservoirs With Different Types of Rock Lithology,” by Junchang Sun, Shijie Zhang, Jieming Wang, Hekun Guo, Chun Li, Hongcheng Xu, Sinan Zhu, and Kai Zhao, PetroChina, prepared for the 2019 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2019 International Petroleum Technology Conference. Reproduced by permission. This paper discusses a study undertaken to gain better understanding of nuclear magnetic resonance (NMR) characteristics of volcanic reservoirs with different lithologies. Results of the study confirmed that accurate identification of reservoir-interval lithology, achieved through comprehensive laboratory NMR investigation of the volcanic rock, is a primary prerequisite of correct interpretation of NMR logging of volcanic reservoirs. The research results play an important role in guiding in-depth understanding of NMR characteristics of volcanic reservoirs and the implementation of NMR reservoir evaluation in corresponding areas. Introduction The introduction of NMR to the petroleum industry in the 1990s led to a breakthrough in logging. NMR can distinguish between movable and irreducible fluid, and is currently the only logging technology that can determine reservoir permeability. When used to interpret and evaluate complex reservoirs such as volcanic rocks and low-resistivity oil and gas layers, NMR provides logging analysts with new ideas and techniques to deal with various difficulties. By analyzing the NMR relaxation-time (T2) spectrum of rock plugs at a fully brine-saturated state, the distribution of reservoir pore size can be evaluated, and important reservoir evaluation parameters, including effective porosity, NMR permeability, and movable fluid saturation, can be obtained. Numerous studies have shown that, compared with sandstones and carbonates, volcanic reservoirs are much more complex and heterogeneous because of the special eruption diagenesis mechanism, many types of rock lithology, various mineral compositions, and a broad range of pore sizes. The models currently used for NMR logging interpretation are suitable only for sedimentary reservoirs. Consequently, accurate characterization of volcanic reservoirs using advanced NMR logging requires a comprehensive laboratory NMR investigation of volcanic rock. Materials and Methods For the study described in the complete paper, NMR sample analysis technology was adopted to perform NMR measurements and other related tests on a total of 108 low-permeability rock plugs from the Xushen reservoir of the Daqing field, the Changling reservoir of the Jilin field, and the Dixi reservoir of the Xinjiang volcanic gas field. The rock plugs comprised nine types of lithology representing the main producing formation lithology from the three reservoirs. Tests included CT scans, thin-section petrography, mercury injection, and mineral-composition analysis. Centrifuge tests were conducted with maximum centrifugal forces up to 500 psi to explore the suitable capillary pressure for T2cutoff determination. The paper includes a step-by-step description of the experimental procedure.