The Role of Filtercake in Wellbore Strengthening

Abstract

Abstract Recent experiments and associated modelling on wellbore strengthening have revealed the importance of the filtercake in inhibiting the growth of fractures that would otherwise cause lost circulation. As a result, this increases the pressure that the wellbore can sustain, during both drilling and cementing. In preventive wellbore strengthening treatments, the rock formation is assumed to have no open fractures. Fractures are induced during the drilling process and the treatment inhibits their growth. This technique is most often used in drilling through depleted formations and has proven to be very effective. The basis of design is that particles of loss prevention material (LPM) in the drilling fluid are sized to enter the fractures, form a bridge within them, and then restrict fluid flow to the tip. A second mechanism has also been identified. Several hundred block tests on sandstone samples have been performed using various fluids, from drilling muds to cements, under various treatment conditions. These tests revealed the importance of filtercake in preventive wellbore strengthening. Particles sized to block a specific fracture were not needed. Provided that the filtercake itself can bridge the very narrow fractures that form at early times, it can prevent the fluid flow into the fracture that then allows the fracture to extend and widen. The properties, in particular the strength, of filtercake are important in determining the effectiveness of this kind of strengthening. Tests were performed to characterize filtercake and LPM effects on filtercake for both water and oil-base muds. Results showed that incorporation of particulate LPM to water-base mud filtercakes has little effect on the filtercake strength, but has appreciable effect on oil-base mud filtercake strength. Filtercake tensile strength is much more difficult to measure, particularly in relationship to the critical fracture width before the filtercake ruptures.