Value of Injection Testing as an Alternative to Conventional Well Testing: Field Experience in a Sour-Oil Reservoir

Abstract

Summary Environmental constraints and high costs, especially offshore, are making conventional-well testing less and less feasible and accepted by the public administration. New options were thoroughly evaluated to find a viable alternative to standard production tests for characterizing the well productivity without surface production. An accurate investigation demonstrated that injection tests could provide all the information needed to calculate the well productivity at reasonably low costs and with a good degree of reliability. On the basis of the results of laboratory and field pilot tests, it was proved that injectivity tests could be applied successfully to a real sour-oil field. Laboratory tests proved that brine could be a suitable injection fluid because there were no compatibility problems with the oil and the reservoir rock. It was verified that the interpretation of the pressure transients should be referred to the falloff period rather than to the injection phase. The formation permeability-thickness product (kh) could be identified correctly from the pressure-derivative analysis only if multiphase flow was assumed. The total skin value could also be obtained from the test interpretation. The total skin comprises two components: a mechanical component resulting from permeability damage and a biphase component resulting from fluid interaction in the reservoir. Except for a limited number of cases, the biphase skin can be evaluated only with numerical well testing, provided that the fluid relative permeability curves are available. It was also demonstrated that the biphase component depends mainly on the injection rate but is independent of the formation permeability. Then, the well-known transient equation was applied to determine the well productivity index (PI) based on the kh and the mechanical skin. PI values calculated from injection tests compared satisfactorily with PI values measured from six drillstem tests (DSTs) performed on appraisal wells.