A new insight for reliable interpretation and design of injection tests

Injection/fall-off test is a viable alternative to the conventional production/build-up sequence since it eliminates surface emissions. However, the well test interpretation is complicated especially in oil reservoirs because of the presence of two mobile phases, the fluid originally in place (hydrocarbon) and the injected fluid (diesel, brine or nitrogen). Fluid saturations vary during the test and their distribution in time is governed by fluid mobilities and effective permeabilities; additionally, gravitational forces, thermal gradients and capillary pressures may also strongly affect the flow. As a consequence, the conventional analytical approach used to describe the pressure behavior is no longer applicable, and only numerical simulations can thoroughly describe the evolution of the saturation and pressure fields in the reservoir.A new near wellbore, axial simmetric numerical model was developed and implemented for properly designing and interpreting injection tests in both oil and gas reservoirs. The model accounts for all the aspects that can have an impact on fluid and pressure distribution. Simulation results are provided in terms of pressure, saturation and temperature profiles.The pressure and pressure derivative reservoir response obtained during injection tests were simulated for a large number of different scenarios. Simulation results demonstrated that capillarity and gravity forces can have a strong influence on the evolution of the saturation profile during the fall-off phase, but their impact on the test interpretation is negligible when a fully penetrating vertical well is considered. Conversely, the combined impact of thermal effects and of the shape of the permeability curves on the test interpretation results proved to be dramatic.

Research highlights
► A procedure for interpreting injection tests is given based on a new numerical model.
► An axial-symmetric near wellbore numerical model for two phase flow is presented.
► The model allows the evaluation of the mechanical skin and biphase skin.
► Capillarity and gravity impact proved negligible for fully penetrated vertical wells.
► Thermal exchange and relative permeability have a strong effect on reservoir response.