Abstract
Hydraulic fracturing is widely applied to economic gas production from shale reservoirs, but the effect of the shale mineral composition on the physical-chemical reactions during hydraulic fracturing is still poorly understood. To develop a foundational understanding of chemical interactions occurring on shale with different mineralogical compositions, two different types of mineral composition marine shale (carbonate-poor and carbonate-rich) from the Niutitang Formation were reacted with slick water fracturing fluid in a laboratory reactor at 100°C and 50 MPa for three days. To identify the changes of the pore structure characteristics during hydraulic fracturing, the field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), low-temperature nitrogen adsorption, and porosity measurement were performed on the original and treated shale samples. After the slick water treatment, the dissolution of pyrite in carbonate-poor shale (obtained from Youyang County, labeled as YY) was observed by FE-SEM and XRD analyses, while in carbonate-rich shale (obtained from Guzhang County, labeled as GZ), the carbonate dissolution was observed. Results from the low-temperature N2 adsorption and porosity measurement demonstrated that the variation trend of pore structure characteristics for the YY and GZ shale samples was quite different after reacting with slick water fracturing fluid. For YY shale, the specific surface area, total pore volume, and porosity increased after the reaction, whereas an opposite trend was observed in the GZ shale. Moreover, the fractal dimension analysis illustrated that the pore surface became less rough and the pore structure became more complex in the YY shale, whereas the degree of pore surface roughness and pore structure complexity of the GZ shale was reduced. The results demonstrated that the initial mineralogical composition of shale played an important role in pore structure alteration during hydraulic fracturing.
Highlights
Unconventional shale gas exploitation from lowpermeability shale reservoir rocks has increased worldwide over the last decade owing to the comprehensive development of multistage hydraulic fracturing and horizontal well-drilling technologies [1, 2]
The surface pore structure of the two shale samples before and after treatment with slick water fracturing fluid is shown in the field emission scanning electron microscopy (FE-SEM) images (Figures 3 and 4)
The results showed that the specific surface area of YY shale increased from 1.55 m2/g to 7.73 m2/g, and the total pore volume increased from 0.00466 cm3/g to 0.01407 cm3/g after reaction with the slick water
Summary
Unconventional shale gas exploitation from lowpermeability shale reservoir rocks has increased worldwide over the last decade owing to the comprehensive development of multistage hydraulic fracturing and horizontal well-drilling technologies [1, 2]. China, Canada, and Australia, among other countries, are developing programs to exploit shale gas resources [4]. After nearly ten years of exploration and research, China has constructed four major marine shale gas fields including Changning, Zhaotong, Weiyuan, and Jiaoshiba in the southern Sichuan Basin [5, 6]. By the Geofluids end of 2017, the shale gas annual industrial production was 9:1 × 109 m3, which showed a rapid increase within a short period of China’s shale gas exploration and development [6]. The highpressure fracturing fluids are injected into the shale formation, releasing the adsorbed gas and creating complex fracture networks for gas transport [7]
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