Abstract
Whether hydraulic fractures could connect multiple gas zones in the vertical plane is the key to fracturing treatment to jointly exploit coalbed methane and tight sandstone gas through integrative hydraulic fracturing in tight sandstone–coal interbedded formations. Laboratory true triaxial hydraulic fracturing experiments were conducted on layered specimens with different combination types of natural sandstone and coal to simulate the propagation behavior of hydraulic fractures. The effects of the fracture initiation position, fracturing fluid viscosity and injection rate were discussed. The results showed that different fracture morphologies could be found. When initiating from coal seams, three patterns of fracture initiation and propagation were obtained: (1) The main hydraulic fracture initiated and propagated along the natural fractures and then diverged due to the effects of in situ stress and formed secondary fractures. (2) The hydraulic fracture initiated and propagated in the direction of the maximum horizontal stress. (3) Multiple fractures initiated and propagated at the same time. With the same fracturing fluid viscosity and injection rate, the hydraulic fractures initiating in sandstones had greater chances than those in coal seams to penetrate interfaces and enter neighboring layers. Excessively small or large fracturing fluid viscosity and injection rate would do harm to the vertical extension height of the induced fracture and improvement of the stimulated reservoir volume. Compared with operation parameters (fracturing fluid viscosity and injection rate), the natural weak planes in coals were considered to be the key factor that affected the fracture propagation path. The experimental results would make some contributions to the development of tight sandstone–coal interbedded reservoirs.
Highlights
Coal-bearing strata consist of interbedded coal seams and tight sandstones deposited and jointly preserved in the vertical direction
When the fracture initiation position was located in the coal seam, the initiation and propagation of hydraulic fractures exhibited the following three patterns
The results showed that when the viscosity of the fracturing fluid was low (3 mPa s), the fracturing fluid was easy to seep via the bedding plane and shear slip occurred, which led to an excessive opening of weak planes in coal, increased the complexity of hydraulic fractures in coal and limited the vertical propagation
Summary
Coal-bearing strata consist of interbedded coal seams and tight sandstones deposited and jointly preserved in the vertical direction. To realize co-exploitation of many types of gas successfully, it is necessary to make hydraulic fractures effectively connect different production layers in the vertical direction. With regard to the vertical propagation of hydraulic fractures in layered media, many studies have been carried out including laboratory experiments and numerical simulations. Based on laboratory experiments and field tests, Warpinski et al (1982), Teufel and Warpinski (1983) and Teufel and Clark (1984) revealed that mechanical property differences between layers were not sufficient to prevent the propagation of hydraulic fractures at the interface, whereas the minimum in situ stress difference was critical for the fracture propagation path. Liu et al (2016) studied the influences of different deviation angles, borehole azimuths, perforation parameters and in situ stress on multifracture propagation from layered formations in inclined well hydraulic fracturing. The results showed that a pennyshaped fracture plane would be formed when a layered interface was well cemented. Liu et al (2016) studied the influences of different deviation angles, borehole azimuths, perforation parameters and in situ stress on multifracture propagation from layered formations in inclined well hydraulic fracturing. AlTammar and Sharma (2017) used digital image correlation to resolve full-field displacement and strain when fracturing layered formations
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