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.
The Inca Occupation of the South Coast of Peru
like the others, however; it forks into several nearly parallel ravines of which the largest are Huayuri, Rio Grande-Palpa, Ingenio (called Collao in ancient times), Nasca proper (the ancient Caxamarca), Taruga, and Poroma. Due to peculiarities of the local topography, the irrigated areas in Ica and the ravines of Nasca are located inland, at the foot of the mountains, rather than at the mouths of the rivers as is more usual in Peruvian coastal valleys. No sharp natural or cultural boundary separates the valley of Chincha from the valleys of Cafiete (ancient Huarco and Runahuana), Asia, and Mala, the next valleys going up the coast to the northwest. At the other end of the area we are considering, however, there is a sharp break; beyond Yauca the Andean foothills reach out to the sea, natural conditions are quite different, and there are important differences in the archaeological picture as well. The historical sources which mention the south coast are not particularly helpful, except in so far as they provide us with a few dates. To begin with, no chronicler gives an explicit account of the political organization of the south coast at the time it came into contact with the Incas. It can, perhaps, be inferred from Cabello Balboa's statements about common policy toward the Inca invaders that there was some sort of political connection between the valleys of Mala, Cafiete, and Chincha, but the connection need have been no more than a temporary military alliance. Cabello makes no mention at all of Pisco. The valleys of Ica, Nasca, and 125