Interference Test in a Fractured Carbonate: A Lisburne Case History

Abstract

ABSTRACT A case history is presented for a 160-acre-five-spot interference test performed in the Wahoo formation, a fractured carbonate within the Lisburne reservoir located at the North Slope of Alaska. Several results are realized from this test. First, the Lisburne reservoir is a heterogeneous formation with the possibility for dual porosity and multi-layered behavior. Second, interference pressure data from two of the four observation wells can be analyzed utilizing a modified approach to the conventional anisotropic permeability technique. Third, even with considerable pre-test planning and design, the reservoir responded differently than expected. The Lisburne Interference Test preceded field startup in an effort to determine directional permeability and to evaluate vertical and lateral continuity within the reservoir with a rninimum amount of offset interference. The center well was cycled through a drawdown and build up while the four corner wells monitored pressure. Bottomhole pressure recorders were installed in the wellbores prior to the start of the test. During the interference test, the observed pressure responses in the observation wells were significantly more rapid and of a greater magnitude than expected. Concurrent pressure transient analysis of the data, made possible by surface readout equipment, resulted in a shorter, less expensive test. The test analysis provided effective reservoir permeability from dual porosity type curve matching and semi-log analysis of the active well data with good agreement between the drawdown and build up results. Analysis of the observation well pressure data was consistent with the active well data and was achieved using dual porosity transient interporosity flow type curves and semi-log analysis. Because of reservoir heterogenities, the pressure data from only two of the four observation wells were analyzable with the conventional anisotropic permeability analysis method. Generally this method requires pressure data from three uniquely-oriented observation wells to obtain a solution. However, the anisotropic permeability model can place limits on the feasible solutions using data from only two wells, and with the elimination of one of the four unknown values through core data, a complete solution of the Lisburne Interference Test was achieved.