Abstract
The fracture pattern of rock mass in shale gas reservoirs is one of the main factors affecting the efficiency of hydraulic fracturing. In this paper, physical experiments and numerical modelling were conducted to systematically investigate the effect of the in-situ stress and perforation angle on the hydraulic fracture initiation pressure and location, fracture propagation, and fracture pattern in a horizontal well drilled by Sinopec Corp. in Luojia area of Shengli Oilfield. A total of six different in-situ stress combinations and eight different perforation angles were considered for the stratified rock mass during the hydraulic fracturing. A summary of the fracture initiations and propagation, and the final fracture patterns induced by the hydraulic fracturing in the stratified rock masses reveals that, for the stratified rock masses with the same perforation angle, the larger the in-situ stress ratio (i.e. lower maximum horizontal principal stress when the vertical stress remains constant) is, the lower hydraulic pressure is required for hydraulic fracturing initiation and propagation. Moreover, it is found that, for the stratified rock mass under the same stress ratio, the hydraulic fracturing pressure in the case with a perforation angle of 30° is higher than that in all other cases. Furthermore, it is noted that the effect of the stratification on the hydraulic fracturing becomes weaker with the in-situ stress ratio increasing. It is finally concluded that the results from this study can provide important theoretical guidance for improving the hydraulic fracturing design in order to ensure the effective shale gas reservoir stimulations.
Highlights
Shale gas refers to the natural gas accumulated in organic-rich, dark-coloured or high-carbon shale mainly in the form of adsorbed or free gas [1]
Five cases with the perforation angles of 90°, 60°, 45°, 30° and 0° are simulated to investigate the effect of the perforation angle on the hydraulic fracturing in the stratified rock mass with the in-situ stresses of σV = 61 MPa and σH = 55 MPa
1) For the rock mass medium with the same perforation angle, the larger the stress ratio, the lower the hydraulic pressure required for its complete hydraulic fracturing, i.e., the easier the hydraulic fracture formed
Summary
Shale gas refers to the natural gas accumulated in organic-rich, dark-coloured or high-carbon shale mainly in the form of adsorbed or free gas [1]. Hydraulic fracturing is the most effective shale gas reservoir stimulation method and, correspondingly, the hydraulic fracturing theory and practice have been further advanced. Hydraulic fracturing has become an important technical means to improve the well output and recovery efficiency in low-permeability reservoirs during the exploitation of oil and gas fields [6]. In recent years, driven by the horizontal well staged hydraulic fracturing, great success has been achieved for unconventional natural gas exploitation in extremely low-permeability shale reservoirs in the Granada area of the U. In Luojia area of Shengli Oilfield as an example to conduct a systematic study on the effect of the deviatoric stress and perforation angle on the hydraulic fracture initiation pressure, fracture initiation location, fracture propagation and fracture pattern during hydraulic fracturing [25]. This study can provide important theoretical guidance for improving the reliability of hydraulic fracturing design in stimulating shale gas reservoirs
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