Critical pressure of closure fracture reopening and propagation: Modeling and applications

- With fracture dip direction, dip angle and the stress state on fracture surface considered, a new model of the critical pressure for reopening and propagation of closure fracture is established.
- The reopening and propagation mechanism for closed fracture surface is analyzed. Moreover, the critical mechanics condition for closed fracture mouth reopening and tip propagation is given out.
- A corresponding solution algorithm is put forward to calculate the critical pressure.
- The critical pressure for closure fracture reopening and propagation in the fractured formation of HF oilfield in Middle East is obtained by the model. And the mud loss problem was successfully avoided.

The reopening and propagation of natural closure fracture is one of the key causes of lost circulation in the fractured formation, which is directly related to safe and efficient drilling. In this paper, considering dip angle and dip direction of the fracture, the stress state on closed fracture surface near borehole is analyzed. Meanwhile, combining the critical mechanics condition of fracture reopening and propagation, a new model of the critical pressure for reopening and propagation of natural closure fracture is established based on rock mechanics theory and Linear Elastic Fracture Mechanics (LEFM), and the corresponding solution algorithm is proposed. Furthermore, the new model is utilized in investigating reopening and propagation of natural closure fracture, which intersects with the horizontal well along direction of the minimum horizontal principal stress σh, for R1 formation of HF oilfield in the Middle East. And the stress distribution on the fracture surface and critical pressure contour is presented. The result indicates that the normal stress and shear stress on the fracture surface near the wellbore are of central symmetry. Moreover, critical pressure distribution of reopening and propagation of closed fracture is symmetrical. The critical reopening pressure of natural closure fracture of R1 formation is consistent with the actual mud loss pressure, therefore the new model is reliable. This work provides a scientific foundation for drilling fluid density design for fractured formation. It can effectively avoid drilling and completion leakage, which eventually prevents drilling problems and dwindles drilling cost, as well as offer a theoretical support for fast, safe and efficient drilling.