Analysis of Injectivity Decline in Some Offshore Water Injectors

Abstract

Abstract The challenges and risks relating to injectivity decline due to injection of particulate-laden water are well documented in the petroleum industry and elsewhere. Although different theories have been advanced to rationalize this problem, the modeling aspects remain largely unresolved, putting huge investments at risk. By combining the fractional-flow and deep-bed filtration (DBF) theories, this work formulates a new model for describing reservoir impairments due to suspension transport by injection water. The critical settling velocity of the suspended particulates is determined from fractional-flow theory and used to create a condition at which particle settling will occur, dependent on its size. The particle settling velocity as obtained from Stokes law for laminar flow (NRe ≪1) is compared to the critical settling velocity, which is a function of the residence time of the particles, to obtain the optimum transported particle size profile in the formation. Furthermore, the average size of transportable particulates and their total volumes are then obtained and used to determine the volume of deposits which is in turn used to develop a new injectivity decline model for predicting permeability impairment due to the deposition of suspended particles in the porous media. The resulting models are validated by analyzing reported dataset from two water injectors offshore Niger Delta. Overall, the results are reasonable as the impairment model rationalized available field dataset satisfactorily. The developed models should find relevance in the following field applications: (1) quantifying formation damage; (2) optimising water treatment facilities design; (3) implementing an effective and safe backflow operation.