THE APPLICATION OF MODIFIED CAMBRIDGE MODEL FOR THE WELLBORE STABILITY ANALYSIS OF DEEPWATER SHALLOW SEDIMENTS IN PETROLEUM DRILLING1)¹⁾

Deepwater shallow sediments are weakly structured and unconsolidated. The deformation and failure is one of the key points in the study of deepwater shallow sediments. The modified Cambridge model was widely used in the studies of deepwater shallow sediments except for the borehole stability analysis, in which deepwater shallow sediments are still regarded as ideal elastic plastic materials. The formation is divided into three parts: the elastic zone, the flow zone and the plastic zone. In the elastic zone, the formation is linear elastic. In the flow zone, the modified Cambridge model is applied. In the plastic zone, the formation obeys the Mohr-Coulomb criterion. In this paper, the stress distribution of each region is solved, and on the basis of the solution, combined with the excess pore pressure, the borehole stability analysis model of deepwater shallow sediments is established. The calculation results agree with actual data, which verifies the proposed model.