SPE Members 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. 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 pretest 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 minimum 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 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. Introduction The Lisburne reservoir is located within the Lisburne Participating Area of the Prudhoe Bay Unit on the North Slope of Participating Area of the Prudhoe Bay Unit on the North Slope of Alaska, as shown in Fig. 1. Discovered in 1968 during the drilling of Prudhoe Bay State No. 1, development of the Lisburne oil accumulation was deferred until early 1984 with field startup in late 1986. Because of the complexity of this carbonate reservoir, a five month Lisburne Interference Test (LIT) was planned for completion prior to field startup to further evaluate the reservoir for future development planning. The objectives of the test included quantifying the areal permeability and continuity within the pattern area and to evaluate the degree of reservoir anisotropy. The test was designed using the anisotropic form of the exponential integral solution to the diffusivity equation, since the Lisburne reservoir would likely exhibit anisotropic behavior. Likewise, it was intended to incorporate the anisotropic interference test models for the LIT analysis. However, the interference analysis, following the LIT, proved to be less than straightforward when only two of the four observation wells could be analyzed using the dual porosity model, due to reservoir heterogeneities. This data limitation resulted in the development of a modified approach to the conventional interference model in order to achieve a complete analytical solution, as presented in the following sections. INTERPRETATION THEORY Interference analysis is a pressure transient technique which provides information about the formation encompassing the provides information about the formation encompassing the active and observation wells. Since the pressure measurements are made in a non-producing well, the impact of skin and storage at both the active and observing wells are minimized and negligible for the LIT. P. 303