Lost Circulation and Wellbore Strengthening by Yongcun Feng & K. E. Gray

Author:Yongcun Feng & K. E. Gray
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

Fig. 3.13Fracture mouth aperture versus time during the wellbore ballooning event

Fig. 3.14Fluid loss/gain rate versus time during the wellbore ballooning event

Figure 3.12 indicates that the downhole pressure decreases after the initiation of the natural fracture. At the early time of fracture propagation, downhole pressure fluctuations are observed. This downhole pressure behavior is a part of the solution of the simulation, which is quite different from existing models that assume a constant downhole pressure (as an input) during mud loss. The fracture extends to the end of the natural fracture at about 20 s. After that, the downhole pressure, fracture width, and mud loss rate reach relatively constant values with continuing mud circulation. At this period, the mud loss rate is dominated by fluid leak off into formation, rather than by fracture growth.

With pumps stopped, the downhole pressure drops almost immediately to the hydrostatic pressure of 12 MPa. As a result, the fracture aperture decreases to zero, but at a rate slower than the decrease of downhole pressure. With fracture closing, fluid flows out of the wellbore with a gradually decreased rate as shown in Fig. 3.14.

Figure 3.15 illustrates the cumulative fluid volume lost into the formation with time during the circulation and pump-off periods, which is the time integral of the flow rate in Fig. 3.14. The total amount of mud lost into the formation during drilling period is about 0.025 m3, while the final fluid loss at the end of pump-off period is 0.015 m3, indicating a fluid gain of 0.01 m3 after the stop of drilling. The fluid volume that does not return during pump-off period is the part of fluid filtration into the formation. The results demonstrate that the proposed model can simulate the entire process of a borehole ballooning event.

Fig. 3.15Cumulative fluid loss versus time during the wellbore ballooning event

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