Study on Wellbore Stability of Shallow Sediments in Deepwater Drilling
Method: In this paper, the shallow formation around the borehole is divided into plastic zone and elastic zone considering it under nonuniform in-situ stress. The collapse pressure formula of shallow formation is derived by taking the shrinkage rate of the borehole as the instability criterion. The fracturing pressure calculation model of shallow sediment under non-uniform in-situ stress is derived by combining the theory of excess pore pressure and hydraulic fracturing.
Elasto-plastic analysis of collapse pressure for deep coal seam drilling based on Hoek-Brown criterion
Elasto-plastic characteristics of deep coal seam at Pingding region in Qinshui Basin was determined under the effect of high confining pressure using triaxial compression test.Taking Hoek-Brown criterion as the limit equilibrium condition,elasto-plastic calculation method of collapse pressure for deep coal seam was derived based on plastic zone radius,and influence factors which control wellbore collapse were obtained,including Geologic Strength Index(GSI),non-uniform stress coefficient and engineering allowable plastic zone radius.Taking CBM well PD-095 in Pingding region as an example,the elasto-plastic results of collapse pressure were compared with elastic results,and the relationship between collapse pressure and influence factors was analyzed.The results show that the elasto-plastic results of collapse pressure decrease 6.52%,4.66% than the elastic results of Hoek-Brown criterion and Mohr-Coulomb criterion when the plastic zone radius is 1.5 times of wellbore radius;the elasto-plastic results of collapse pressure decrease with the increment of plastic zone radius and the descendent of the stress non-uniform;minus power function relationship between the collapse pressure and the GSI value is presented.
A New General Model of Fluid Loss in Hydraulic Fracturing
Presentation d'un nouveau modele general de la perte de fluide dans la fracturation hydraulique. Verification experimentale de ce modele par des donnees de laboratoire pour des fluides non newtoniens et des fels reticules