A pressure transient test (injection/falloff test) was conducted on a horizontal well in a gas storage reservoir to obtain a reservoir description for injection and production analysis. In our analysis of the test data, we used currently available analytical solutions for a horizontal well in a closed, rectangular reservoir. We found that it was difficult to reach a high level of confidence in the reservoir description obtained from analysis of the test data using only analytical solutions because of uncertainties associated with identification of the various flow regimes. Therefore, we used a reservoir simulator to validate and improve the accuracy of the reservoir description derived from the analytical solutions. The improved reservoir description led to an accurate forecast of the horizontal well’s injectivity and productivity, further verifying the utility of this approach to test analysis and of the inherent value of the idealized analytical solutions.