When a well is hydraulically fractured, the propagation of the fracture away from the wellbore is dictated by the far field stresses in the reservoir. However, the fracture initiation from the wellbore depends strongly on the near wellbore stress state created by drilling the well. Misaligned fracture initiation and propagation planes can reduce the wellbore-to-reservoir connectivity causing operation failure and high post fracturing skin.
Currently creating multiple fractures along a horizontal openhole requires mechanical isolation means such as openhole packers or sand plugs. They can be costly and time consuming. In addition, there is no control of fracture initiation within one isolated section. Undesirable competing fractures within the zone can occur to impact the fracture length. Significant improvement can be made if the factors controlling multiple fracture initiation without mechanical isolation can be understood.
Experimental work in multiple fracture initiation has been rare, controlled multiple fracture initiation is non-existent. Therefore a series of laboratory experiments was performed in a true tri-axial stress frame to investigate how multiple fractures can be initiated in a controllable fashion. In the tests, notches at specific locations along the openhole wellbore were created. The impact of the notch depth on the orientation of the hydraulically induced fractures was studied.
In addition to the experiments, continuum fracture mechanics modeling using finite element was also conducted to rationalize the experimental observations of fracturing initiation process in the rock.
The results of block tests provided new insight in multiple fracture initiation. By monitoring the real time acoustic emission events, the sequence of fracture creation as wellbore pressure increased was visualized. The finite element modeling gives simple criteria to explain the observed orientation of initiated fracture as a function of notch depth.