Design and Analysis of a Nitrogen Step-Rate Test, Anschutz Ranch East Nitrogen Injection Project

Abstract

SUMMARY This paper presents the preparation, testing, and analysis of a nitrogen step rate test and the analysis of two falloff tests on a gas injection well in the Anschutz Ranch East Nitrogen Injection Project. Objectives of the tests were to determine the parting pressure, flow capacity, and relative influence of open fractures in the lower zone of the Nugget formation. The application of current technology in evaluating continuous gas injection wells is relatively undocumented. Gas pressure maintenance projects require many of the same operational considerations given to waterflood projects; i.e., good vertical sweep (wellbore conformance), and an injection pressure that is below formation parting pressure. Gas injection wells, however, in a retrograde condensate reservoir are inherently more difficult to analyze with conventional pressure transient methods because of changing gas compressibility factors and two-phase effects from condensing hydrocarbons. Therefore, extensive test planning and preparation must be completed for a gas injection well test to be successful. Because of the complicating factors involved, several different methods of analyses were used with the step rate and falloff test data. The step rate test was analyzed with Odeh's superposition technique and a 1-D radial gas model to obtain the zone flow parameters and confirm that a break occurred. The falloff test data were analyzed with conventional and multirate techniques to determine the influence of natural fractures and confirm the zone flow parameters derived from other methods. Type curve analysis was also performed to aid the interpretation of the test results. The step rate test results indicated that the test was successful. Conventional analysis indicated the parting of formation after the third step (of the seven performed) and the radial gas model also confirmed the change from radial to linear flow. A comparison of the reservoir properties derived from the various analysis methods (the radial model and multirate falloff analysis) and core data were in agreement. The test data analysis did not show the presence of an intense fracture network that would dominate the flow capacity of the zone.