Abstract
An effective porosity is defined as the ratio of volume of interconnected pore space to total volume of a porous sample. It controls the magnitude of fluid flow and is a key parameter in the assessment of recoverable resources. However, its accurate measurement in tight formations is challenging, due to their complex pore structure and lithofacies heterogeneity. In this study, porosities of sixteen lacustrine shale samples from the second Member of the Kongdian Formation (Ek2) in the Cangdong Sag, Bohai Bay Basin were measured and compared using multiple methods and sample sizes to compare and contrast the effective porosity results. The methods included helium pycnometry (HP; cubes of 1 cm3 and grains at 500-841 μm), water immersion porosimetry (WIP; cubes), mercury intrusion porosimetry (MIP; cubes), and nuclear magnetic resonance (NMR; cubes). Finally, samples were completely sealed using paraffin for bulk density measurements to evaluate the extent of potential clay swelling in shale samples involving probing fluids. Results from the HP, WIP, and MIP methods for skeletal density, bulk density, and effective porosity with cubic samples were compared. While very similar skeletal densities were found for all three methods, a lower bulk density, and therefore lower porosity, from the MIP approach can be attributed to the experimental conditions (e.g., vacuum efficiency, applied pressure, wettability of water/helium vs. mercury) and the probable presence of pores with diameters larger than 50 μm not measurable by MIP. Furthermore, the HP porosity of granular samples with 500-841 μm grain sizes can be regarded as approaching the total porosity. The complicated relationship between WIP and NMR porosities may result from the heat-induced volatilization of moisture in pores during NMR tests, and countercurrent imbibition of water replacing the residual hydrocarbons during the saturation process for sample preparation in both tests. The swelling behavior of the lacustrine Ek2 shale with water is not significant because of the low content of expansive clay minerals. In summary, the WIP and HP methods are recommended for effective porosity measurement, whereas the NMR and MIP methods are invaluable for the measurement of pore-size distribution, with additional information on the effective porosity.
Highlights
The commercial exploration and production of marinesourced shale gas and oil in the United States [1,2,3,4] and marine shale gas in China [5, 6] has led to significant attention being paid to oil resources in lacustrine shale reservoirs in China
Porosity measurements of shale can be generally divided into the following categories: (1) gas expansion porosimetry, such as helium pycnometry (HP) for skeletal density with additional information on bulk density for calculating the porosity [14], as well as low-pressure gas physisorption with N2 or CO2 [15]; (2) three dimensional (3D) imaging techniques, such as microcomputed tomography, nano-CT, and focused ion beam-scanning electron microscopy (FIB-SEM) [16, 17]; (3) fluid immersion porosimetry (FIP), such as FIP with water (WIP) or kerosene (KIP) and dual liquid porosimetry (DLP), as well as mercury intrusion porosimetry (MIP) [18,19,20,21,22]; and (4) radiation detection methods, such as nuclear magnetic resonance (NMR) and small angle neutron scattering (SANS) [23,24,25]
The porosities measured by water immersion porosimetry (WIP) and NMR methods are noticeably different, which may potentially result from moisture loss in pores during NMR measurement and countercurrent imbibition during the saturation process
Summary
The commercial exploration and production of marinesourced shale gas and oil in the United States [1,2,3,4] and marine shale gas in China [5, 6] has led to significant attention being paid to oil resources in lacustrine shale reservoirs in China. Several notable characteristics of lacustrine shales in China, such as lower thermal maturity, generation of fluids with higher viscosities and high wax contents, and limited geographical distributions, pose greater challenges to a cost-effective development compared to marine petroleum systems [11]. In addition to a direct extraction of tight oil, in situ conversion technology plays a significant role in the development of lacustrine shale oil in China [12]. Research Institute of Petroleum Exploration and Development of PetroChina suggested that the recoverable resources of lacustrine shale oil reservoirs in China, with a thermal maturity (Ro) less than 1.0%, in these areas are as large as700‐900 × 108 t using in situ conversion technology [13]. Note that the approaches involving a probing fluid (e.g., He, N2, CO2, H2O, and Hg) measure the effective (or connected portion of pore space linked to sample surface) porosity, while CT, SEM, and SANS detect the total (both connected and nonconnected) porosity; in addition, gas physisorption, MIP, and NMR quantify the pore-size (throat) distribution
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