Article pubs.acs.org/EF
Fractal Analysis of Pore Network in Tight Gas Sandstones Using NMR Method: A Case Study from the Ordos Basin, China Xinhe Shao,†,‡ Xiongqi Pang,*,†,‡ Hui Li,†,‡ and Xue Zhang†,‡ †
Basin and Reservoir Research Center, China University of Petroleum, Beijing 102249, China State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China
‡
ABSTRACT: To characterize the pore structure and quantify fractal dimensions of tight gas sandstones, a case study is performed on the Lower-Middle Permian tight sandstones in the Ordos Basin in China by conducting a series of experiments including X-ray diffraction (XRD) analysis, routine petrophysical measurements, thin section and scanning electronic microscope (SEM) observations, and nuclear magnetic resonance (NMR) experiment. The studied tight sandstones mainly consist of quartz and clay minerals, and pore types include primary intergranular pores, inter- and intragranular dissolution pores, as well as micropores associated with clay aggregates; T2 spectra reflect three types of pore size distributions in the studied samples, indicating a rather irregular pore distribution pattern in tight sandstones; NMR can estimate porosity of tight sandstones accurately, and movable-fluid porosity from NMR can better reflect the permeability of tight sandstones than total porosity. Two fractal dimensions, Dbnd (with respect to bound-fluid pores) and Dmov (with respect to movable-fluid pores), are calculated to be 1.1135−1.8116 (average 1.4750) and 2.6816−2.9932 (average 2.8921), respectively. Dbnd increases with the decrease of detrital quartz content and the increase of clay mineral content, whereas Dmov increases with the increase in authigenic quartz content and the decrease of detrital quartz content; fractal dimensions can reflect the physical properties of tight sandstones, as large Dbnd and Dmov values typically result in low movable-fluid porosity and permeability; the pore network of tight sandstones can be considered as a dual-scale pore system based on fractal theory, whereas Dbnd and Dmov can reveal the roughness of bound-fluid pore surface and the distribution of movable-fluid pores, respectively. This study shows that NMR fractal dimension can be employed as an effective indicator to characterize the pore network of tight sandstones.
1. INTRODUCTION In recent years, tight sandstones have received a lot of interest due to the great hydrocarbon productivity after suitable technologies such as hydraulic fracture treatment, horizontal wells are developed.1,2 Unlike conventional reservoirs, it is the pore structure, rather than total porosity, that controls flow capacity, water saturation and producible pore volumes of tight sandstone reservoirs.3 Therefore, precise description and characterization of pore system are essential steps to understanding the producibility of tight gas sandstones.1 Pore system of tight sandstone is a global research focus, as it is difficult to characterize because of the great complexity, which is attributed to various pore sizes, poor connectivity, and strong heterogeneity.2,4 Many techniques have been applied to investigate the pore characteristics of tight sandstones in previous studies, such as environmental scanning microscopy,5 X-ray fluorescence,6 X-ray computer tomography scanning,7,8 nuclear magnetic resonance (NMR),9 and mercury porosimetry.10 Among these, NMR has become a widely used analytic tool to estimate the physical properties and study the pore structure of rocks, as it can provide reliable information on pore size distribution and saturation of pore fluid.11−14 Some researchers suggested that NMR is an effective method for nondestructively quantifying the full-range pore throat structures of rocks, compared to other technologies such as mercury intrusion porosimetry and N2 adsorption.15,16 Furthermore, NMR experiments were performed on core plugs in laboratory to provide guidance for the interpretation of NMR loggings and the construction of petrophysical models, showing © XXXX American Chemical Society
that the application of NMR experiment in hydrocarbon exploration is helpful.17−19 Fractal theory, which uses a fractal dimension D to analyze the regularity of pore network quantitatively, has been considered as an effective method to characterize the complex pore structure of porous materials.20−23 Previous studies have proven that the pore structures of hard rocks own fractal characteristics.24−27 Many researchers have investigated fractal characteristics of shales and coals using low-pressure N2 adsorption and NMR experiment.26,28−31 Some researchers studied pore throat structures of tight sandstones using high-pressure mercury injection method based on fractal theory.32−34 These studies all suggested that unconventional reservoirs have rather irregular and heterogeneous pore networks, and fractal theory is an effective method to study the pore structure. For instance, Lai et al.32,33 studied the fractal characteristics of small and large pores in tight sandstones respectively, and suggested that several factors during mercury injection may result in the calculation of abnormally high fractal dimension values of large pores; Li et al.34 performed fractal analysis on Chang 7 member tight sandstones in the Ordos Basin, China, and defined the fractal dimensions for micropores, transitional pores and mesopores, respectively. However, there are only a few studies related to fractal characteristics of tight sandstones using NMR experiment. A series of problems remain unsolved: What are the fractal characteristics of Received: April 10, 2017 Revised: August 29, 2017 Published: August 31, 2017 A
DOI: 10.1021/acs.energyfuels.7b01007 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Map of the Ordos Basin and sampling location. Thin section observation was performed on Sample 1 to 8 using a Leica DMLP polarizing microscope. Samples were grinded to 0.03 mm thickness, and were impregnated with blue epoxy resin to highlight the pores. In addition, thin sections were stained with an alizarine-S and potassium ferrocyanide solution for the identification of carbonate minerals such as calcite, ferrocalcite, dolomite, and ankerite. Pore types were identified through the morphology, location, and paragenetic relation. SEM analysis was conducted on Sample 1 to 8 to further describe the mineral composition and pore structure characteristics. Prior to the experiment, samples were broken into small rock fragments, and the fresh broken surface of each sample was coated with a thin layer of gold. The experiment was performed using an EVO/MA15 scanning electron microscope equipped with an energy dispersive X-ray spectrometer. The observation was at magnifications ranging from 100 to 1500 with an acceleration voltage of 20 kV. NMR measurement was performed using a Recore 3100 NMR analysis meter. Two sets of NMR measurements were conducted: one at 100% water saturated condition (Sw) and another at irreducible water condition (Sir). All samples were first water-saturated and conducted for NMR experiment. After the initial measurements, they were centrifuged to obtain an irreducible water condition at a centrifuge capillary pressure of 1.4 MPa by a PC-1 petroleum core centrifuge. During the measurement, the number of echoes was 1024 and the echo spacing was 0.2 ms; the scanning and stacking number was 128; the waiting time was 5s; the salinity was 45 000 ppm. In this study, T2 distribution curves and parameters such as T2 cutoff time (T2c), NMR porosity under water saturated and irreducible water conditions were derived.
different types of pores in tight sandstones? Are there any differences between fractal characteristics of tight sandstones and shales, coals or other lithologies? Therefore, developing fractal dimensions by NMR method can provide a brand new cognition of pore network of tight sandstones. In this paper, a case study of the Lower-Middle Permian tight sandstones in the Ordos Basin is carried out to investigate the fractal characteristics of tight sandstones. First, we analyzed the mineral compositions, physical properties, and pore network morphology; then, we used NMR experiment to obtain the information on pore structure characteristics and fractal dimensions of tight sandstones. Additionally, the relationships between mineral compositions, physical properties, and fractal dimensions were discussed. The effectiveness of using fractal theory to characterize the pore network of tight sandstones is thus recognized.
2. SAMPLE AND METHODS 2.1. Samples. Fifteen tight sandstone samples were cut from drilling cores from Permian strata in the northeastern Ordos Basin, and were given serial numbers from 1 to 15. The sample wells are among the most important exploration wells drilling to the Lower-Middle Permian tight reservoirs in the studied area, and the drilling cores are not contaminated by drilling mud. The Ordos Basin is the second largest basin in China covering an area of 37 × 104 km2,35,36 and is located in the central China (Figure 1). It belongs to craton marginal depression basin, and is tectonically stable. The basement of the basin consists of the Archean and Proterozoic metamorphic crystalline rock, and the overlying sediments include Paleozoic, Mesozoic, and Cenozoic strata.37 Samples in this study are tight sandstones from the Lower Shihezi Formation in Middle Permian, the Shanxi and Taiyuan Formation in lower Permian. These formations are all important exploration targets in the Ordos Basin and possess abundant tight gas resources.36 The sedimentary environment of these formations is fluvial-deltaic environment, and sandbodies tend to develop in the distributary channels interbeded with mudstones.35,37 2.2. Experiments. To systematically analyze the fractal characteristics of the tight sandstones, we performed a series of experiments in this study, including X-ray diffraction (XRD) analysis, routine petrophysical experiments, thin section and scanning electron microscope (SEM) observations and NMR measurement. XRD analysis was conducted to determine the mineral compositions of sandstones. The experiment was performed using a D/max-2500 X-ray diffractometer, and samples were crushed to 100 mesh size prior to the experiment. The routine petrophysical measurements were conducted following the Chinese Oil and Gas Industry Standard GB/T 29172−2012. The porosity was measured using a PoroTm300 porosimeter, and the permeability was measured by Low perm-meter with flowing air through samples.
3. NMR THEORY NMR is the interaction between the hydrogen nucleus and magnetic field. Total NMR relaxation (T2) time is associated with surface relaxation, bulk relaxation of fluid precession, and diffusion relaxation caused by gradient field.38 In a watersaturated sample, the T2 time is proportional to the pore size and the amplitude of the T2 curve directly reflects the porosity of the sandstone.39,40 When the applied electromagnetic field is uniform in the NMR experiment, the diffusion relaxation can be ignored, and T2b is negligible in water saturated pores.11 Therefore, a relationship between pore size and T2 time can be established using a simplified equation11 1 S = ρ2 T2 V
(1)
Where ρ2 is the surface relaxivity, μs/m; S is the surface area to V volume ratio (specific surface area) of the pores, μs−1. The application of NMR allows the determination of boundand movable-fluid pores in sandstones and carbonate rocks, and T2c is a most commonly used parameter to discriminate the B
DOI: 10.1021/acs.energyfuels.7b01007 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels bound and movable fluid in the rock.41,42 To estimate T2c values, the T2 distribution curves are obtained from samples under water-saturated and centrifuged (irreducible water saturation) conditions, respectively. The method to calculate T2c value is shown in Figure 2, and detailed discussions can be referred to previous studies.27,42,43
The porosity of samples from routine petrophysical experiment ranges from 3.9 to 11.6% with an average of 8.0%, and the permeability ranges from 0.18 to 0.98 mD with an average of 0.47 mD (Table 2), suggesting a poor reservoir quality. Table 2. Physical Properties of the Tight Sandstone Samples from Routine Petrophysical Experiment and NMR Experiment routine petrophysical experiment
Figure 2. Schematic diagram showing NMR T2 spectrum and the method to obtain T2c values.
4. RESULTS 4.1. Lithology and Physical Properties. The mineral compositions of the studied samples are listed in Table 1. Quartz is the most abundant mineral in these samples except Sample 13, which contains high content of clay minerals. The high content of clay minerals is because of the great compositional heterogeneity of the tight sandstones or strong alteration of detrital feldspars. Overall, the content of quartz ranges from 27 to 91 wt % with an average of 72.6 wt %, the content of feldspar ranges from 0 to 19 wt % with an average of 4.6%, and the content of clay minerals ranges from 3 to 59 wt % with an average of 15.7 wt %. Carbonate minerals in the studied tight sandstones mainly consists of calcite and ankerite, with the total content ranging widely from 0 to 15 wt % (average 4.1 wt %). Other minerals such as pyrite, siderite, and gypsum are also identified, and their contents are typically lower than 3 wt %.
NMR experiment
sample
porosity (%)
permeability (mD)
bound-fluid porosity (%)
movable-fluid porosity (%)
total porosity (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
9.3 11.6 3.9 9.5 8.5 9.4 8.9 9.3 5.4 7.4 5.9 9.0 10.6 6.4 5.1
0.56 0.98 0.28 0.93 0.51 0.18 0.33 0.46 0.26 0.37 0.32 0.75 0.45 0.28 0.35
5.7 6.2 4.2 3.8 6 6.9 5.8 6.1 4.2 4.2 5.1 4.6 4.7 5.9 4.4
3.4 4.1 0.2 4.9 2.9 1.8 1.4 3 1.4 3 1.2 6 3.4 0.8 0.9
9.1 10.3 4.4 8.7 8.9 8.7 7.2 9.1 5.6 7.2 6.3 10.6 8.1 6.7 5.3
According to the definition by Surdam,44 all samples can be identified as tight sandstones (porosity
