Injection wells are widely used in the petroleum industry to support sustained oil production. Water is injected into the reservoir for filling in the void space left by produced oil and to maintain the reservoir pressure. These injectors are frequently evaluated with fall-off tests to assess their injectivity with time. However, most fall-off tests on these wells are unexpectedly dominated by prolonged storage effects with a long unit-slope line on the log-log plot. As a result, the radial flow regime on the pressure derivative plot gets a little chance to develop within the stipulated test period. A high skin damage factor with an abnormally-high storage coefficient (e.g., 1 to 3 bbl/psi, which is one or two orders of magnitude larger than the typical values in regular wellbores) have to be introduced to the well test models to match the pressure behavior from these wells. This anomalous behavior suggests that the wellbore is connected to additional storage volume due to an induced fracture system around the well, having been intersected by a section of it. Apparently, these fractures close gradually after shutting in the well for a pressure fall-off test, manifesting as a prolonged storage effect in the data. This can be explained with the fact that most of the injectors inject water under fracturing conditions, which induce large fractures around the wellbore over time. The fracture system might play a role in taking the damage further inside the reservoir and complicate subsequent remedial work to restore well's injectivity.
In this paper, we will show and discuss several fall-off tests from water injection wells in giant oil fields in the Middle East. The results of these tests, conducted over the years, are to show how the entire test duration is progressively dominated by larger wellbore storage with increasing fracture volume around the wellbore. A changing wellbore storage model (Spivey and Lee) is useful in illustrating the phenomenon of dual storage effect – one due to the wellbore, followed by the other one due to the fracture volume. The model is also used to identify the damage conditions of two regions – one just around the wellbore and the other is located slightly deeper into the reservoir.