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Investigation of the Influence of Diffusion on the Closure Stress on Particles for Wellbore Strengthening Applications

The loss of circulation is among the most important geomechanical issues in drilling engineering, as drilling fluids escape from the wellbore towards the rock formation. This event is considered a problem because it causes, non-productive time, high costs from drilling fluids losses, stuck pipe, induced kicks or in some cases loss of the entire well. Events of loss of circulation take place when the hydraulic pressure in the well exceeds (F.I.P) fracture-initiation pressure and (F.P.P) fracture-propagation pressure of the formation, a situation very common in wells with a narrow mud window. The wellbore strengthening methods are employed to increase F.I.P or F.P.P or both so as to prevent the loss circulation (Feng & Gray, 2016a,b,c; Morita & Fuh, 2012). The fracture resistance method is employed for wellbore strengthening, in which particles that normally resemble proppants are introduced in the drilling fluid and are directed in the unwanted hydraulic fracture to act as a plug. These particles are deposited near the wellbore or at some distance inside the fracture. Of the plentiful literature it is recognised that a lack of understanding exists of the precise role of the various factors influencing the fracture propagation resistance. One of these factors is the stress change at the tip location after plugging which has proofed to be highly effective. During plugging, fluid pressure drops as the fracture is sealed thus creating a pressure difference between the fluid in the wellbore Pw and the fluid inside the fracture Pf. Poroelasticity introduces an extra pressure drop between Pf and the formation pore pressure Pp. These drops create stress change at the tip thereby causing I Ic k k ≤ thus providing an effective fracture resistance (Feng & Gray, 2016a,b,c; Morita & Fuh, 2012). An open question that exists is issue of the stresses at the plug location when the closure stresses will act on the particles. In other words, the stress concentration at the plug location is ignored. In this work, we investigate whether a new fracture will be created at the plug location during wellbore strengthening operations in elastic and poroelastic formations by creating and letting a fluid driven fracture to propagate up to 5m and then plugging it for a distance of 1m. During the process, the in-situ stresses are recovered and a scaled Griffith criterion is applied to investigate the stress instabilities over the fracture path.