Article pubs.acs.org/EF
Reservoir Characterization of Tight Sandstones Using Nuclear Magnetic Resonance and Incremental Pressure Mercury Injection Experiments: Implication for Tight Sand Gas Reservoir Quality Xinhe Shao,†,‡ Xiongqi Pang,*,†,‡ Fujie Jiang,†,‡ Longlong Li,†,‡ Yuying Huyan,†,‡ and Dingye Zheng†,‡ †
Basin and Reservoir Research Center and ‡State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China ABSTRACT: A series of experiments including porosity and permeability measurements, thin section and scanning electron microscopy (SEM) observations, incremental pressure mercury injection (IPMI), and nuclear magnetic resonance (NMR) were conducted to systematically characterize the pore structure of tight sandstone from the Lower Shihezi Formation of Permian (P2x) in the northeastern Ordos Basin, China. The influences of pore types, pore size distribution, and fractal characteristics on reservoir quality of tight sandstones are also investigated. Results show that the studied tight sandstones generally possess poor quality and complex pore structure. The porosity and permeability range from 4.08% to 17.56% (average 9.22%) and from 0.05 to 16.66 mD (average 2.49 mD), respectively. Five pore types were observed in thin section and SEM images: primary intergranular pores, intergranular dissolution pores, intragranular dissolution pores, micropores within clay aggregates, and microfractures. The pore throats are mainly hairy/fibrous, inhibiting the connectivity between pores. Three types of pore structures were identified in the mercury-injection curves and pore size distribution curves from the IPMI experiment and in the T2 relaxation time spectrum obtained by NMR. Both experiments yielded consistent classifications, and their combination was necessary to analyze the pore structure effectively. In general, permeability and porosity are positively related and depend on pore types. Large numbers of small pores confer high storage capacity, whereas small numbers of larger pores improve the flow capability. In the high porosity−permeability zone, larger pores also determine the storage capacity. The P2x tight sandstone is fractal, and macropores are more heterogeneous while micropores are more homogeneous. The fractal dimensions of macropores are good indicators of the reservoir quality of the P2x tight sandstone as larger fractal dimension values of macropores reflect poor reservoir quality.
1. INTRODUCTION Tight sandstone is defined as a reservoir with porosity less than 12% and permeability less than 1 mD.1,2 Such reservoirs have become globally recognized for their huge resource potential in hydrocarbon exploration.3−5 The prospecting potential of petroleum exploration largely depends on the quality of the reservoir.6 Unlike conventional reservoirs, the quality of tight sandstone reservoirs cannot be assessed from their porosity and permeability alone, because their physical properties can be poor and their pore-throat geometry very complex.7,8 The quality of such reservoirs is mainly determined by the pore structure, which relates to pore type and size distribution.9,10 Therefore, to proficiently assess the exploitation potential of hydrocarbon reserves in tight sandstone reservoirs, we must elucidate the petrophysical properties and pore structure of the reservoirs.11 The pore structures of reservoirs have been investigated by various techniques, including thin section analysis,6,12 scanning electron microscopy (SEM),12 X-ray computer tomography (CT), 1 3 incremental p ressure mercury injection (IPMI), 4 , 9 , 1 4 − 1 8 and nuclear magnetic resonance (NMR).7,19−23 However, each technique has its advantages and disadvantages in pore structure characterization. For example, thin section analysis and SEM can directly clarify the two-dimensional topography and the sizes of the pores and throats, but cannot provide the three-dimensional distribution or quantitative data of the pore-throat size.12 X-ray CT provides © XXXX American Chemical Society
information on the pore structure size and the geometry and topology of the pore network; however, its use is restricted by the high cost.13 The data from IPMI experiments provide the characterization parameters relating to the pore−throat connections and are useful for studying the pore body characteristics;14 however, their application distorts the skeletal porous structure, particularly in reservoirs with low permeability.15−18 Moreover, this technique requires drying of the samples, which alters the porosity, permeability, and fabric of samples with high clay content.15 NMR has been widely applied in reservoir characterization,19−22 and it reveals the porosity, permeability, and pore-size distribution parameters.23 In many recent studies of sandstones, the pore sizes have been derived in laboratory-based NMR studies and compared with those derived from images.24,25 However, the complex pore structure of tight sandstone limits the interpretation of NMR measurements of these materials in practice.26,27 In summary, as the pore structures considerably differ between tight sandstone and conventional reservoirs, the former are difficult to characterize by any single technique or traditional research methods.7 Instead, the pore structures of tight sandstone must be systematically studied by different techniques and methods. Received: April 25, 2017 Revised: August 5, 2017 Published: August 27, 2017 A
DOI: 10.1021/acs.energyfuels.7b01184 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
continental facies.32,33 According to drilling data and field outcrop studies, the sequence of the late Paleozoic strata (from top to bottom) is as follows: the Shiqianfeng formation (P3s) of the upper Permian, the upper Shihezi formation (P2s) and lower Shihezi formation (P2x) of the middle Permian, the Shanxi formation (P1s) and Taiyuan formation (P1t) of the lower Permian, the Benxi formation (C3b) of the upper Carboniferous, and the Majiagou formation (O2m) of the middle Ordovician (Figure 2). This study focuses on the lower Shihezi formation (P2x), consisting of braided river delta sediment, and on a tight sandstone reservoir that is mainly distributed in an underwater distributary channel with relatively good physical properties.28 P2x is subdivided into four parts from the fifth member (P2x5) to the eighth member (P2x8). P2x contains a thick gas layer, and some production wells have yielded a high productivity gas flow. The direct cap rock of the tight gas reservoir is that of mudstone interbedded in P2x, and the regional cap rock is the overlying mudrock and silty mudstone at the top of P2s and the bottom of P3s, P1s, P1t, and C3b coal measure source rocks are the main source rocks for the P2x reservoir. Gas generation occurred at the end of the Triassic, and the hydrocarbon generation and expulsion peaked from the Late Jurassic to Early Cretaceous (the main reservoirforming stage). The source rock evolution was halted by the uplift of the formation between the Late Cretaceous and the present. In addition, the tight gas reservoir was formed before 50 Ma and was later adjusted and altered.28
The Lower Shihezi Formation (P2x) in the northeastern Ordos Basin, China, is a typical tight sandstone reservoir with a large gas production potential28 However, a limited number of studies have been conducted on the reservoir’s characteristics and most researches focused on the control of sedimentation and diagenesis within the reservoir.28 Therefore, our study has three objectives: (1) to analyze the physical properties and pore types of the Lower Shihezi Formation reservoir in the northeastern Ordos Basin through casting thin section analysis and SEM; (2) to characterize the pore structure from capillary pressure curves and the T2 spectrum obtained from IPMI and NMR, respectively; and (3) to discuss the complexity of pore network of P2x tight sandstone with fractal dimensions from IPMI experiment. The results of this study should assist in comprehensive evaluations of the reservoir and provide a theoretical basis for exploring and exploiting the gas field in the northeastern Ordos Basin, China.
2. GEOLOGICAL SETTING The Ordos Basin is the second largest sedimentary basin in China. Located in north-central China, this nonmarine basin contains Paleozoic strata covering an area exceeding 37 × 104 km2,.29−31 Several giant gas fields have been discovered in this basin, one of which (the Sulige gas field) is the largest in China.30 The sample area is located on the western margin of the Jinxi fault-fold belt in the Ordos basin (Figure 1). The area
3. SAMPLES AND MEASURING TECHNIQUES In accordance with the study objectives and data constraints, samples were selected from the P2x reservoir drill cores of eight wells in the northeastern Ordos Basin. Routine petrophysical measurement was performed on 30 samples to obtain the information on porosity and permeability of the P2x tight sandstone. The porosity was measured using a PoroTm300 porosimeter, and the permeability was measured using a Low perm-meter with air flowing through samples. Thin section observation was performed using a Leica DMLP polarizing microscope. Samples were impregnated with bluedye resin to highlight the pores. SEM observations were performed on freshly broken rock fragments coated with a thin layer of gold and a JSM-5500LV scanning electron microscope equipped with a QUANTAX400 energy dispersive X-ray spectrophotometer was used. The observation was performed at magnifications ranging from 100 to 15 000 with an acceleration voltage of 20 kV. The pore-throat geometry in the reservoir rocks was studied by IPMI. In the early period of mercury intrusion there was a relatively horizontal stage; a gradual slope and longer stage represents the centralized distribution of pore sizes and wellsorted granulars. A horizontal stage lying close to the X-axis reflects a large pore radius. In addition, the pore volume, maximum and average pore-throat radii, maximum mercury intrusion saturation, residual mercury pressure, and other reservoir characterization parameters were obtained from the intrusion and extrusion curves. Thus, through IPMI, pore structure was characterized based on patterns of the mercury intrusion and extrusion curve. Permeability contribution distribution were obtained from fluid flow capacity in pores with different radii uing eq 1, while detailed descriptions of the method can be referred to the study by Wall:34
Figure 1. Location of the study area and map of the Ordos Basin.
generally presents a single-slope configuration and a relatively simple structure, although local differences exist. Magmatism from the Zijinshan structure has uplifted the eastern central part of thestudy area, forming a relatively developed fault system with a ring-radial distribution.28 The sediments in the study area date from the middle-upper Cambrian, lower Ordovician, upper Carboniferous, and lower Permian and comprise marine-continental transitional facies to B
DOI: 10.1021/acs.energyfuels.7b01184 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 2. Generalized Upper Paleozoic Quaternary stratigraphy of the northeastern Ordos Basin, showing the major petroleum system elements.
c=
(2i − 1) ·rn2 n
·100 2
∑i = 1 (2i − 1) ·rn
technology to the attenuation curve.35 In the T2 relaxation time spectra, fluids with long and short T2 relaxation times typically occupy large and small pores, respectively. A large integral area of a T2 spectra implies that there is more fluid in the pores of core samples; a longer T2 relaxation time reflects the existence of pores with larger pore size; and a shorter T2 relaxation time reflects the existence of pores with smaller pore size. Therefore, by constructing a T2 spectrum, we can indicate the ratios of pores with different sizes to the total pore volume and hence characterize the pore size distribution in the reservoir. To this end, we subjected 15 core samples (diameter 2.54 cm; length 4 cm) to NMR analysis using an HD/DH2002 NMR spectrometer. NMR measurements of all samples were performed under the 100% water-saturated condition (Sw) with distilled water and the irreducible water condition (Sir)
(1)
where c is the permeability contribution of pores with the same size; n is the group of pores divided according to their sizes; ri is the pore radius, μm, and r1 > r2>···>ri...>rn. In this experiment, 23 cylindrical samples (of diameter and length 2.5 cm and −4 cm, respectively) were tested using a AutoPore IV 9505 pore analyzer. Maximum intrusion pressure is 101.32 MPa, corresponding to a pore-throat radius of 1.0 nm. NMR reveals the physical properties of a reservoir and the T2 relaxation spectra reflect the movable fluid and oil-bearing saturation parameters. In practical NMR measurements, the fluid content of the pores can be calculated from the ratios of different T2 relaxation times, applying mathematical inversion C
DOI: 10.1021/acs.energyfuels.7b01184 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels after centrifugation at 25 °C and 689 × 103 Pa (100 psi). The NMR parameters are as follows: echo spacing, 0.2 ms; waiting time, 5 s; numbers of scans, 128; echo numbers, 1024; water salinity, 32 000 ppm; experiment temperature, 35 °C; and humidity, 55%.
4. RESULTS AND DISCUSSION 4.1. Physical Properties and Pore Types. The porosity and permeability of the P2x reservoir vary markedly throughout the study area (Table 1). The porosity of the study samples Table 1. Porosity and Permeability Obtained from Routine Petrophysical Experiments of the Studied Samples well
depth (m)
porosity (%)
permeability (mD)
Lin4 Lin4 Lin4 Lin4 Lin4 Lin4 Lin4 Lin4 Lin4 Lin4 Lin4 Lin4 Lin4 Lin4 Lin4 Lin4 Lin4 Lin6 Lin6 Lin6 Lin6 Lin6 Lin6 Lin17 Lin19 Lin21 Lin26 Lin33 Lin33 Lin36
1550.33 1550.51 1550.83 1551.06 1551.69 1552.17 1552.69 1553.05 1593.69 1593.76 1594.34 1595.26 1596.19 1597.95 1599.97 1600.50 1601.34 1646.20 1646.45 1647.00 1648.69 1649.02 1650.00 1677.23 1641.12 1759.84 1811.50 1558.96 1539.10 1656.85
10.29 9.64 11.05 15.22 16.65 13.04 17.56 14.47 7.70 7.90 4.62 5.14 8.52 4.76 9.20 8.34 8.27 9.95 4.08 7.55 10.06 8.93 6.37 6.7 7.3 10.3 7.7 7.1 8 10.2
0.93 2.18 1.98 11.66 1.13 1.05 16.66 15.66 0.11 0.12 0.05 0.06 0.21 0.09 1.96 0.14 0.15 0.15 1.11 0.08 0.72 0.46 0.22 3.7 3.3 4.1 1.7 1.05 0.9 3
Figure 3. Relationship between porosity and permeability of the P2x tight sandstones.
shaped and formed mainly by dissolution of feldspars; these pores are small (pore diameters are typically
