Experimental study on hydrofracture propagation through perforated wellbore in naturally fractured Guanyinqiao calcareous mudstone under true triaxial stress

Abstract

Hydraulic fracturing technology has become successful in enhancing the permeability of unconventional gas reservoirs. However, due to the geological complexity, considerable challenges remain in understanding hydraulic fracturing of calcareous mudstone. In the main production area of shale gas reservoirs in South China, the tight interlayer of Guanyinqiao calcareous mudstone widely spreads in Wufeng-Longmaxi shale, which could heighten the risk of stimulation treatments and hinder fracturing stimulation efficiency. To investigate the characteristics of hydrofracture propagation in naturally fractured Guanyinqiao calcareous mudstone, we conducted a series of fracturing tests through perforated wellbore under true triaxial stresses. The features of fracturing treatment pressure curves, effect of stress difference and natural fracture on hydrofracture geometry, perforations breakdown modes, and the distinction between calcareous mudstone fracturing and shale fracturing were analyzed. It revealed that the breakdown pressure of perforated fracturing was lower than that of the open hole fracturing, and a smaller perforation phase angle and higher horizontal stress difference could facilitate the hydraulic fracture initiation. The natural fractures mainly developed along the bedding plane, which could cause step-over or diversion of hydrofracture at weakly cemented bedding interfaces. When horizontal stress difference increased from 4 MPa to 8 MPa, the propagation orientation of the dominant hydrofracture would change from oblique mode to transverse mode. The rock brittleness and anisotropy significantly influenced fracture geometry. The hydrofracturing in less brittle and anisotropic calcareous mudstone would produce a single dominant vertical hydrofracture, by contrast, multiple hydrofractures were prone to initiate from the perforations at a lower injection pressure in brittle shale. The understanding of hydrofracture propagation through perforated wellbore in calcareous mudstone may provide some basic data for gas reservoir fracturing engineering and academic application.