Tan, Leichuan (China University of Petroleum) | Li, Ningjing (China Petroleum Pipeline Engineering Corporation) | Gao, Deli (China University of Petroleum) | Ren, Shaoran (China University of Petroleum) | Adeeb, Samer (University of Alberta) | Wang, Zhengxu (China University of Petroleum) | Gu, Yue (China University of Petroleum) | Li, Wenlong (China University of Petroleum) | Chen, Xuyue (China University of Petroleum)
ABSTRACT: Hydraulic fracturing practices in shaly unconsolidated sandstone reservoirs readily result in complex fractures due to high shale content, strong plasticity, and fracture toughness. This paper introduces a new method for manufacturing shaly unconsolidated sandstone that is supported by experimental results. In addition to the laboratory experiment, a Particle Flow Code (PFC) numerical model was established based on the relevant physical properties, mechanical parameters, and fluid-solid coupling theory. The fracture propagation law of shaly unconsolidated sandstone was comprehensively assessed. Shale content was found to significantly influence fracture propagation. Straight fractures tend to transform into circuitous pinnate fractures accompanied by seepage fracture zones as shale content increases. There is clear stress chain directivity accompanied by uneven distribution of stress after stress field loading, which produces shear stress as the fractures expand. The shear force created by shale can exceed easily the shear strength, leading to shear failure, uneven stress distribution and uneven compaction. The results of this study may provide a workable basis for optimizing the hydraulic fracturing process in shaly unconsolidated sandstone reservoirs.
Shaly unconsolidated sandstone reservoirs can be considered as the transition zone from unconsolidated sandstone to mud shale with typical characteristics of relatively poor physical properties, resulting in low oil and gas production rate and bad recovery. Due to the weak cementing properties, the shaly unconsolidated sandstone often indicates distinctly different mechanical properties from general sandstone, such as the significant elastoplastic features in hydraulic fracturing process. In view of such kind of low permeability reservoirs, whether the fracturing technology can create effective artificial fractures is the primary criterion to determine the success or failure of the fracturing measure.
Hydraulic fracturing technology has been widely used in the oil and gas industry for many decades, which can be regarded as a definitely impactful measure to rise the yield. Warpinski et al., 1979 pointed out that when fracturing is carried out in unconsolidated sandstone, there will be forming a very complex fracture. It was argued by Settar et al., 1989 that there will be fluid filtration phenomenon when unconsolidated sandstone is fractured, and the emerged fractured zone could be the cardinal reason for this. After recent years development, the physical simulation experiments of general sandstone are very developed both in theory and practice nowadays. Researchers (Chen et al., 2000; Khodaverdian et al., 2000; Gao et al., 2016; Tan et al., 2018; Feng and Gray, 2018) used sandstone specimens with the size of 300 mm×300mm×300 mm to simulate and analyze the effects of stress field, rock fracture toughness and internal branches on the fracture propagation. Bohloli et al., 2006 considered the influence of various types of fracturing fluid on the hydraulic fracturing technology, arguing that this technology is very suitable for the unconsolidated sandstone layer, and the fractures produced under the strong stress condition are relatively short. Jia et al., 2007 conducted the true three axis simulation experiment using fine sand and cement mixed with the proportion of 1:3, finding there is great influence of inclination angle, borehole azimuth and perforation on the fracture extension law. Jasarevic et al., 2010 and Feng et al., 2016 observed that micro fracture existed in the hydraulic fracturing of the unconsolidated sandstone. Meanwhile, the filtration zone can be easily formed as well.