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Characterization of Volcanic Reservoirs Requires Comprehensive NMR Tests

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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 T2 cutoff determination. The paper includes a step-by-step description of the experimental procedure.

Experimental Results and Discussion

The paper includes detailed discussion of the results, including numerous tables and plots. Specific areas of discussion include NMR porosity and a comparison of conventional porosity and NMR porosity between samples from conventional reservoirs and those from volcanic reservoirs with different lithologies, analysis of NMR porosity error, centrifugal force suitable for calibration of T2 cutoff, variation characteristics of T2 cutoff of volcanic reservoirs, and applications.

Conclusions

General conclusions include the following.

  • Unlike sandstone and carbonate plugs, NMR porosity of volcanic plugs at a fully brine-saturated state is dependent on rock lithology.
  • NMR porosities of trachyte, trachytic volcanic, and granite porphyry are significantly less than those of conventional porosities measured by the Archimedes method, which means that accurate identification of reservoir-interval lithology is a primary prerequisite before correct interpretation of NMR logging.
  • Paramagnetic minerals—mainly iron (Fe) and manganese (Mn) elements—contained in volcanic reservoirs are the fundamental cause of this abnormal phenomenon.
  • The critical values of Fe and Mn elements are approximately 2% and 0.06% by weight, respectively, above which point the NMR porosity will be considerably less than the conventional porosity, suggested by inductively coupled plasma-atomic emission-spectrometry tests on 14 representative plugs.
  • A new NMR porosity-corrected formula was developed to improve interpreted quality of NMR logging. Suitable capillary pressure for determination of T2 cutoff of volcanic reservoirs is 400 psi—three times greater than the commonly recommended standard (100 psi) for sandstones.
  • Calculated T2 cutoff ranges from 3 to 120 ms; average value is 50 ms.
  • T2 cutoff between different volcanic reservoirs and lithologies exhibits significant differences.
  • The laboratory NMR results were used to interpret NMR logging of the Xushen reservoir of the Daqing field and aided in detailed reservoir evaluation. The outcome of interval selection and high-productivity well completion performed on the basis of NMR logging interpretation is very encouraging.

More-detailed conclusions of the paper include the following.

  • Paramagnetic mineral content of rhyolite, tuff, rhyolitic agglomerate lava, rhyolitic breccia lava, and basalt in the volcanic reservoir is lower and their NMR porosity is more accurate, while Fe and Mn content of trachyte, trachytic volcanic breccia, granite porphyry, and andesite are higher, and their NMR porosity is much lower than the conventional porosity.
  • When Fe and Mn content are lower than 1% and 0.02%, respectively, NMR porosity is in better accordance with the conventional porosity. When Fe and Mn content are higher than 2.5% and 0.06%, respectively, NMR porosity is much lower than the conventional porosity.
  • It is determined that the centrifugal force suitable for the calibration of reservoir T2 cutoff of low-permeability volcanic gas reservoirs is 2.76 MPa, which is also the lower limit of centrifugal force corresponding to movable reservoir fluid, and the lower limit of throat radium for fluid flowing is approximately 0.05 μm (Fig. 1).
  • Average T2 cutoff of 102 plugs of low-permeability volcanic rocks from Daqing, Jilin, and Xinjiang fields is 49.31 ms, which is higher than that of sandstone reservoirs but lower than that of carbonate reservoirs. Value and distribution ranges of T2 cutoff of volcanic rocks vary greatly in different areas and for different lithologies, but after lithological subdivision, the distribution range of T2 cutoff is decreased slightly. T2 cutoff of volcanic rocks is ranked as Daqing, Jilin, and Xinjiang from highest to lowest. The average T2 cutoff of Daqing rhyolite is the highest, and its distribution range is the largest. T2 cutoff of Xinjiang granite porphyry, basalt, and andesite is the lowest.
Fig. 1—Water saturation of six rock plugs under different centrifugal forces.
 
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.

Characterization of Volcanic Reservoirs Requires Comprehensive NMR Tests

01 August 2019

Volume: 71 | Issue: 8

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