Critical drawdown pressure of sanding onset for offshore depleted and water cut gas reservoirs: Modeling and application

Offshore gas reservoirs are generally exploited without energy supplement, reservoir pressure depletion will cause rock stress changes, and water cut will also weaken rock cementation, which results in an increasing likelihood of serious sand production in the unconsolidated sandstone reservoir. This paper aims at offshore unconsolidated sandstone gas reservoirs, analyzes rock stresses of the borehole wall with impact of reservoir pressure depletion on in-situ stresses and water cut on rock strength. Combining with fully-polyaxial rock failure criterion Mogi-Coulomb, a new sanding critical drawdown pressure (CDP) calculating model and a numerical calculating method for the model are developed. Furthermore, an applicable sanding prediction method for pressure depletion and water cut offshore gas reservoir in well whole life cycle is proposed. The model was applied in DF gas reservoir in South China Sea, the chart of sanding CDP for well DF-A in the whole production life cycle was plotted. The research results indicate: Initially, the sanding CDP is relatively large, with formation pressure depletion, CDP is reduced step by step until depleted to the critical reservoir pressure, and sand will be produced no matter how small the drawdown is. Once water cut occurs, sanding CDP is furtherly decreased by 15%-20%. The research outcomes are consistent with actual oilfield production situation, which can provide some theoretical supports for sand production gas wells.