Article Cite This: Energy Fuels 2018, 32, 2962−2972
pubs.acs.org/EF
New Insight into the Characteristics of Tight Carbonate based on Nuclear Magnetic Resonance Wei Duan,†,‡ Gao Zhiqian,*,†,‡ Fan Tailiang,†,‡ Meng Miaomiao,†,‡ Chen Yue,§ Li Yangbing,∥ and Zhang Chenjia†,‡ †
School of Energy Resource, China University of Geosciences (Beijing), Beijing 100083, China Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Enrichment Mechanism, Ministry of Education, China University of Geosciences (Beijing), Beijing 100083, China § Wuxi Research Institute of Petroleum Geology, Exploration and Production Research Institute, SINOPEC, Wuxi, Jiangsu 214151, China ∥ Unconventional Experimental Center, Unconventional Research Institute, CNOOC EnerTech-Drilling & Production Co., Tianjin 300452, China ‡
ABSTRACT: In order to further our understanding of the physical properties of tight carbonate and to explore the use of NMR to identify different rock types, in this study we use thin-section observations, scanning electronic microscopy (SEM), helium porosity, helium pulse decay permeability, mercury injection capillary pressure (MICP), and nuclear magnetic resonance (NMR) to conduct petrographic and petrophysical characteristics studies on 12 carbonate samples. Our results show that nano/ micropores are widely distributed between the micrite and/or dolomite crystals. The correlation between the permeability and porosity of the tight carbonates is poor, while the rapex, which is the apex of the hyperbola in Pittman (1992), is well correlated with the threshold entry pressure, the maximum pore-throat radius, and the average pore-throat radius. On the basis of new cutoff values, we have identified three types of pores: nanopores, which mainly correspond to the intercrystalline pores; micropores, which may be related to the bioerosion or mechanical erosion process of the aragonitic bioclasts; and mesopores, which mainly consist of well-preserved intraparticle porosity related to the diagenetic shielding effect, dissolved intragranular pores, and a few intercrystalline pores. The dissolved bioclastic packstone and dolostone exhibit similar unimodal behavior with a broader wave, while each of the other four lithofacies has a unique NMR signature. The microstructures and diagenesis processes result in different NMR responses in the different rock types. Micrite envelope, neomorphism, and moderate recrystallization of the micrite matrices result in a higher T2 spectrum value and a longer relaxation time, while the high clay content and stylolite have the opposite effect. The dissolved bioclastic packstone has a shorter relaxation time than dolostone, with a similar pore throat distribution. Geological knowledge is needed for the NMR-based core-facies classification and for evaluation of the physical properties of the tight carbonate. petrophysical properties.11,12 Due to interference factors, such as pore coupling and the complex diagenesis of the carbonate, interpreting the NMR signals can be difficult. However, the richness of the NMR signal and the use of NMR logging make this study meaningful. This study aims to characterize the petrophysical properties of tight carbonates and to use the NMR data for identifying different rock types. In this article, (i) petrographical and petrophysical characteristics of the tight carbonate are studied based on NMR, SEM, optical microscopy, and MICP; (ii) the reasons for the unique NMR signatures are analyzed; (iii) the correspondence between the reservoir space types and the relaxation time ranges is built; and (iv) an NMR-based method to further our understanding of the microstructures of tight carbonates is established.
1. INTRODUCTION As a major part of hydrocarbon exploitation, geothermal energy, and storage, carbonate reservoirs have always been of great interest. However, some carbonate rocks are characterized by low porosity and permeability (in this article, tight carbonates are defined as that with a porosity of less than 5% and permeability of less than 0.3 mD), and these have not been studied in detail. In recent years, with the continuous development of reservoir stimulation techniques, tight carbonates have received widespread attention as they might represent underexplored reservoirs.1−4 Studies found that the developed intercrystalline micropores of the tight carbonate have a major influence on their petrophysical properties.5,6 However, due to the complex diagenetic changes they have undergone and their strong heterogeneity, the petrophysical properties of the microporous limestones remain poorly understood.7,8 Nuclear magnetic resonance (NMR) relaxation has been widely used in the energy industry during the past decade.9,10 The rich information provided by the NMR signal can be used to determine the pore-size distribution and to estimate the © 2018 American Chemical Society
Received: November 8, 2017 Revised: January 24, 2018 Published: February 1, 2018 2962
DOI: 10.1021/acs.energyfuels.7b03460 Energy Fuels 2018, 32, 2962−2972
Article
Energy & Fuels
Figure 1. Geological map of the Tarim Basin. The sampled outcrop and well are marked on the map, and the lower panel is a photograph of the NYG outcrop, with sample locations indicated by the colored dots. Porosity was measured using a helium gas expansion porosimeter. The permeability of the cored plug samples was measured using a helium pulse-decay permeameter (NDP605) under an effective stress of 800 psi. In order to measure the distribution of the pore throat radii of the samples, MICP analyses were performed on the 12 core plug samples with an AutoPore IV 9500 device at the Unconventional Research Institute of CNOOC EnerTech-Drilling & Production Co. Mercury is a nonwetting liquid, so the mercury volume is equal to the pore volume connect by each characteristic pore radius.13 It is accepted that the NMR relaxationc time distribution represents the pore size distribution of the rock sample when the pore space is fully water saturated.15 Coates et al. (1999) provide a detailed explanation of the theoretical basis for the use of NMR.16 The relaxation time (T2) is a characteristic time that represents the magnetic decay of the protons within the fluid after polarization.16 T2 can be calculated from eq 1.
2. MATERIALS AND METHODS Ten samples were collected from the NYG (Nanyigou) outcrop in the northwestern margin of the Tarim Basin. Due to the similar strata, sedimentary characteristics, and structural evolution, the Yingshan Formation and the Yijianfang Formation exposed at the NYG outcrop have been considered to be analogues of the subsurface reservoirs in the Tahe area.13 The Lower Ordovician Yingshan Formation is divided into two parts by the unconformity (T76). The lower part is composed of medium-thin bedded dolomite, algal limestone, and grainstone, and the upper part is made up of medium-thin bedded algal limestone and micritic limestone interbedded with medium bedded fine crystalline algal dolomite. Yingshan Formation mainly developed a carbonate ramp facies depositional system in the NYG outcrop. Yijianfang Formation mainly includes bioclastic packstones and grainstones in its lower part and thick-bedded bioclastic wackestones and mudstones in its upper part, indicating a rise in sea level. The Samples were numbered consecutively from P1 to P10. P1−P4 were collected from the Yijianfang Formation, while P5−P10 were collected from the Yingshan Formation. In addition, the two additional samples were collected from the core plugs of S115 in the Tahe area. The location of the sampled outcrop is shown in Figure 1. Plugs of 5 cm long and 2.5 cm diameter were cored from the samples for petrophysical analysis. Thin sections with a fluorescence epoxy resin were prepared. The rock texture/petrography and the diagenesis were carefully studied under transmitted light using a polarizing microscope. In order to obtain a full overview of the micrite microtexture and to further study the pore systems, SEM analyses were carried out.
1/T2 = ρ2 (S /V )
(1)
where ρ2 is the surface relaxivity; S is the surface area; and V is the volume. The NMR experiments were carried out at the Unconventional Research Institute of CNOOC EnerTech-Drilling & Production Co. The RecCore 2500 instrument was used. The number of echoes was set as 10 000, and the echo spacing was set at 300 μs during the test. The operations were as follows: first, the 12 samples were dried at 60 °C for 24 h. Then, they were vacuumed for 48 h until no weight change was observed at room temperature. Finally, the plug samples were saturated with distilled water under vacuum. The samples were assumed to be 100% water saturated after the above-described 2963
DOI: 10.1021/acs.energyfuels.7b03460 Energy Fuels 2018, 32, 2962−2972
Article
Energy & Fuels process.17 The relaxation time (T2) was measured for each sample. Optical microscopy, SEM, and MICP allow for the in-depth understanding of the NMR signals of the samples.
commonly indicate shallow turbulent water in a high energy platform shoal environment. 3.1.3. Grainstone. The grainstone consists of abundant clasts (about 80%) and shallow-water bioclasts (about 20%) with sparry cement. The clasts have a variety of shapes and particle sizes. Pores were rarely observed in the thin section due to cementation by crystalline calcite. Sparry cements and locally circumgranular cement rims were present in the detrital particles. Thus, these samples were deposited in an open and photic water environment. 3.1.4. Bioclastic Packstone. The lithofacies contain abundant echinoderm bioclasts with micrite calcite infilling and crystalline calcite locally. The micrite infilling indicates a moderate wave energy environment, while the widely distributed biological detritus and subsorted bioclasts suggest that they were formed near the shelf-margin reefs. Therefore, the lithofacies were deposited behind the reef and sand shoal barrier. Dissolved pores are easily observable. 3.1.5. Dolostone. The dolostone consists of euhedral to subeuhedral crystalline dolomite. The crystal size varies from 50 to 200 μm. Dissolution also occurred around the dolomite crystals. 3.1.6. Mudstone. The mudstone lacks grains and mainly consists of micrite with rare clay minerals. The mudstone experienced compression, and stylolites filled by bitumen can be seen in the thin section. The mudstone facies formed in a restricted environment with low hydrodynamics.20 3.2. Pore-Network Characteristics. The pore sizes were divided into different groups by different scholars. Casteleyn et al. (2010) and Rashid et al. (2017) defined three pore sizes in tight carbonate reservoirs based on mean pore size, including nanointercrystalline pores (pore diameter (dp) < 1 μm), microintercrystalline pores (1 < dp < 10 μm), mesointragranular pores (dp > 10 μm), and moldic pores.7,21 Hassal et al. (2004), Maliva et al. (2009), and Periere et al. (2011) proposed cutoff values for micropore, mesopores, and macropores of 0.5 μm and 5 μm.6,22,23 In this article, the pore sizes were divided into three classes: nano-, micro-, and mesopores. Pore diameters below 1 μm were defined as nanopores, and values >1 μm and
