Stress state in the Cuu Long and Nam Con Son basins, offshore Vietnam, implications for borehole stability analysis

Abstract

We analysed in situ stress and pore pressure fields in the Cuu Long and Nam Con Son basins using data from petroleum exploration and production wells to evaluate the stress state. Then we used the obtained in-situ stress data to analyse the stability of two planned wells. The data were obtained from the sea floor to 4300 m burial depth and include both hydrostatic and overpressured sections. Seven data points are located in sediments, and seventeen data points in igneous basement rocks at depths greater than 3000 m. The estimated magnitudes of SH are 70-110% of the Sv magnitudes. Considering the errors in the stress magnitude estimates, their relative magnitudes suggest that a borderline normal/strike-slip stress regime presently exists in normally pressured sequences of the Nam Con Son and Cuu Long basins. Of the twenty-four data points, twenty have effective stress ratios at a critical stress state for faulting on the assumption that there are faults present that are optimally oriented for failure with friction coefficients of 0.5. The results suggest that the stress state in these basins is generally critical. Borehole stability analysis showed that for drilling deviated wells in the highly fractured granite basement in the Cuu Long Basin, mud pressure must be not less than 3 MPa below the pore pressure to avoid breakout and borehole collapse. For drilling in the overpressured and fractured carbonate section in the Nam Con Son Basin, the mud pressure must not exceed the pore pressure by more than 2MPa in order to minimize the risk of blowout or inducing fractures that may lead to mud losses and abandonment of the well.