Challenges in Interpreting Well Testing Data from Fractured Water Injection Wells with a Dual Storage Phenomenon

Abstract

Abstract Injection wells are widely used in the petroleum industry to support 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 tested by means of falloff tests to assess their injectivity. In most of the injectors, water is injected under fracturing conditions, which induce large fractures over time; however, most falloff tests on these wells do not exhibit any clear fracture behavior. Instead, prolonged storage effects with a long unit-slope line on the log-log plot are observed in the data. The observed storage coefficient values are found larger than the typical values in regular wellbores by one or two orders of magnitude. In this paper, we will show and discuss several falloff tests from water injection wells in giant oil fields in the Middle East. These tests were conducted over the years to show how the entire test duration is progressively dominated by larger wellbore storage with increasing fracture volume around the wellbore. As a result, the radial flow regime on the pressure derivative plot gets a little chance to develop within the stipulated test period. This abnormal behavior suggests that the wellbore is connected to additional storage volume due to a 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 falloff test, manifesting as a prolonged storage effect in the data. The examples with real data will also illustrate the phenomenon of dual storage effect – one due to the wellbore, followed by another one due to the fracture volume. We will show how to use the appropriate fracture storage coefficients to estimate fracture dimensions.