Abstract:
Wellbore instability, kick and loss of circulation through fractures are the most
significant problems observed during petroleum drilling. Selection of an appropriate mud weight is
the key solution for these problems. To select a safe range of mud weights, the knowledge of
formation pore pressure and fracture gradient of the drilling area is essential. This paper aims to find
the most suitable fracture pressure gradient and pore-pressure models for Mannar and Cauvery
basins in Sri Lanka. Three pore-pressure prediction strategies, namely, Hottman and Johnson ,1965 [4]
method, Ben Eaton ,1975 [9]method and D-exponent method-Eaton ,1975 [9]and three fracture
pressure gradient prediction methods, namely, Hubbert and Willis ,1957 [10] method, Matthews and
Kelly (1967) method and Ben Eaton (1969 [6] method, were used in this research. Petro-physical data
of four wells previously drilled in the Mannar basin and Cauvery basin in Sri Lanka were used to
predict pore pressure and fracture pressure gradient using above methods. Then the predicted values
were compared with the actual pressure values obtained from well log records of same wells. Area
specific variables, including Poisson ratio and Matrix Stress Coefficient were determined prior to the
fracture pressure gradient prediction. After determining the suitable prediction methods, a basin
analysis software (Petrel), (Schlumberger, 2010.1) software was used to visually interpret and analyse
the results. The results of the Pore-pressure analysis suggested that a combination of Hottman and
Johnson method and D-exponent-Eaton method are suitable pore pressure prediction models for the
study area. The analysis of fracture pressure gradient strategies found that the Ben Eaton method is
the most suitable method to predict the fracture pressure gradient of the area.
