Injectivity Monitoring & Evolution for Water Injectors in a Deepwater Turbidite Field

Water injection is one of the key improved recovery techniques used for pressure maintenance and sweeping. Throughout the life of a field, changes in injectivity can have an effect on reservoir pressure management and sweep efficiency which both have a direct impact on production. This study aims to present an original methodology to analyse the performance of water injectors in a deepwater turbidite field and evaluate the evolution of their injectivity over time. An injectivity monitoring tool was developed by incorporating injection flowrate and pressure data with the following analytical techniques: (i) Instantaneous Injectivity Index, (ii) Hearn Plot or Reciprocal Injectivity Index, (iii) Hall Plot, (iv) Derivative Hall Plot and (v) Pressure Transient Analysis. The injectivity monitoring tool was able to capture subtle changes in injectivity and demonstrate the long term trend of stable injectivity in this field, even in situations where only wellhead pressure and injection flowrate were available. The resulting analysis showed that there is very good injectivity for all water injectors in this field with little or no degradation over time. One of the key drivers for the good injectivity is the water injection process philosophy in this field. This process consists of injecting deaerated seawater with biocides in order to prevent bacterial growth which causes near wellbore plugging. Another contributing factor to the good injection performance is the presence of injection valves which enable each injector to perform downhole shut-ins to stop migration of fines and curtail any possible water hammer effect when intermittent injection shut-ins required. It was furthermore found that there was no significant difference in injectivity that could be associated to deviation angle or completion type (Stand Alone Screens (SAS) or Expandable Sand Screens (ESS)). Injectors completed with SAS however appeared to exhibit increased injectivity with increasing screen length but no such correlation was apparent between screen length and injectivity for injectors with ESS.