Sanding Risk Assessment for Multiple Depleted and Overpressured Sandstone Reservoirs

Abstract The study field, situated in the Gulf of Thailand in a water-depth of about 60 metres, is a shallow-water Gas Field characterised by multiple stacked sandstone reservoirs interspersed with shales. All current wells use cased and perforated completions without downhole sand control. Recently, several poorly consolidated Deep sandstones have experienced sand production at various depletion levels during well start-up operations. To investigate the underlying causes of sand production, a comprehensive geomechanical study and analytical sanding assessment was carried out using field and laboratory data for calibration. The analyses helped in identifying sand-prone perforated intervals and the estimation of the critical drawdown and depletion levels that could trigger sanding. A geomechanical model was built for the Gas Field using well logs and was calibrated with drilling data, pressure measurements, leak-off tests and core data. Laboratory tests on sandstone core samples, including thick-walled cylinder (TWC) tests, were conducted to analyse rock mechanical properties. An analytical poro-elastic shear failure model was employed to predict sand failure under varying pressure conditions. Calibration of the sanding model was performed using field observations from production data to determine the occurrence (onset) of sanding. Initially, no significant sanding was detected from the Deep reservoirs so an effective strength factor (ESF) range of 1.75 – 2.3 was determined to correspond with the absence of sanding and the minimal sand observed in the DSTs. More recently, sanding was detected in a well that is deviated 25° during start-up operations, particularly originating from the Deep sandstone interval, with arbitrary perforation orientations. This observation of sand prompted a recalibration of the sanding model. Analytical sand production prediction was conducted for various reservoirs and depletion scenarios. Critical drawdown analysis shows that sanding may occur from the least favorable perforation orientations near the top of the well. The study predicts a medium to high risk of sanding when reservoir pressure depletion ranges from 2000 to 3000 psi, depending on various factors such as well trajectory, TWC strength, the stress path factor, and ESF. The sanding evaluation revealed that certain sandstone intervals are prone to sanding, while most zones may remain sand-free during the initial depletion phase. Individual well and zone assessments highlighted the need for production monitoring and drawdown management to address sanding issues in existing wells in the Gas Field. For future wells, a selective perforation strategy can help to prevent or delay sanding by avoiding perforation in the riskiest sand-prone zones. Long-term sand control may require active downhole sand control measures to be implemented.