Pore-pressure and fracture pressure gradient prediction model for few exploration blocks in mannar and cauvery basins, Sri Lanka

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.