Wellbore strengthening (WBS) has been applied as an effective method for preventing lost circulation in the field. Recent research has indicated the important role of filtercake in wellbore strengthening. Filtercakes can potentially prevent fracture initiation and cover pre-existing narrow fractures and drilling-induced fractures at the initiation stage. The ability of the filtercake to effectively strengthen the wellbore expectedly depends on its capability to maintain its integrity and provide the barrier to isolate pressure and fluid transmission between the wellbore and the fracture. Currently, there is no well-defined method to evaluate the filtercake's ability to resist rupture over a fracture aperture.

In this research, a modified permeability apparatus was applied to evaluate the quality of drilling fluid filtercake for the first time. A new criterion defined as filtercake rupture resistance is proposed to characterize the filtercake quality regarding its ability to sustain pressure over a fracture aperture. Experimental tests were conducted to investigate how filtercake thickness and filtercake yield strength affect the rupture resistance. The mechanism of filtercake in sealing the narrow fractures is explored and it was observed that solid's plugging/bridging plays the vital role in this mechanism. A thicker and stronger filtercake also contributes to accelerating the process of complete fracture sealing. The results of this research can be utilized as a reference that guides the optimization of drilling fluid for continuously strengthening the wellbore.

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