Performance evaluation of water injector wells is usually done by injection and fall-off tests. However, the well test analysis of fall-off tests is often challenging because of the interaction of two immiscible fluids in the reservoir.
The paper presents a field example where six different fall-off tests, separated by large time intervals in a long injection sequence were analysed for a well injecting water in an oil reservoir. In order to reproduce the fluid interaction in the reservoir, it was essential to consider the relevant impact of temperature on the injected fluid viscosity in the near-wellbore zone due to the cold-water injection. At the wellbore scale, a temperature reduction of around 95 degree Celsius was observed with respect to the virgin reservoir temperature, which in turn led to an increment of viscosity of the injected fluid by approximately four orders of magnitude. The analysis of each fall-off showed a radial composite behaviour, with the two radial flow stabilizations depending upon the different mobility of oil and water, while the radius of interface depending upon the cumulative volume of injected water. The interpretation provided the key reservoir characteristics (formation capacity in the water-invaded and virgin oil zone, transient injectivity index, static reservoir pressure, skin, etc.) and allowed an assessment of the well injection capability and its change with time in the reservoir. Within Eni, repeated fall-off analysis in a well is becoming a powerful and cheap tool for efficient waterflood management.
Besides providing key parameters for reservoir characterization and a dynamic picture of near wellbore region, the paper highlights the peculiarities of a fall-off test and the way reliable outputs can be achieved. It demonstrates how it is crucial to consider temperature related viscosity variations and how it is the key driver for achieving accurate well test results.
The injection of fluids in hydrocarbon reservoirs is a widely used method to achieve higher recovery factors and increase recoverable reserves. Particularly, water injection in oil reservoirs represents an important secondary recovery method in deepwater oilfields operated by Eni due to the low associated costs.