Abstract Horizontal wells are becoming popular for primary and enhanced oil recovery operations because of unique advantages of horizontal wells in comparison to those for vertical wells. Monitoring the progress of a steam injection process requires a knowledge of the steamswept volume, as this provides a measure of the heat loss from the heated zone. Well testing offers a relatively quick and inexpensive way of estimating the steam-swept volume. The pseudosteady state method has been used to estimate the swept volume from pressure falloff testing of vertical wells, with a good degree of success. However, horizontal well testing is more complex than vertical well testing. To evaluate the accuracy and applicability of the pseudosteady state method in the estimation of swept volume for steam injection through a horizontal wel, a thermal numerical simulator is used to generate the pressure falloff data. Results of the study show that the pseudosteady state method may be used to estimate the swept volume for steam injection through a horizontal well. However, swept volume may be overestimated by 5 to 60 per cent. An investigation of the correct average pressure to use in calculating steam zone properties was also carried out. Pressure obtained by extrapolating the pseudosteady-state Cartesian straight line to zero time was found to be an appropriate average pressure. Injection time effects on the estimated swept volume are also studied. Results indicate that longer injection times prior to shut-in appear to have an adverse effect on the estimated swept volume for cases involving a pair of horizontal injection and production wells. However, for cases involving only a horizontal injection well, estimated swept volume seems to improve with longer injection times. Analyses of the well-test data show that steam chamber mobility can be reasonably estimated from pressure falloff tests of horizontal wells. Introduction The ability of horizontal wells to provide, among other things, large surface areas of contact with the reservoir makes them suitable for the efficient recovery of oil, especially from thin reservoirs. Oil recovery by steam injection requires a knowledge of the steam-swept pore volume, as this provides a measure of the heat loss from the heated zone. The pseudosteady state method, first proposed by Eggenschwiler et al.(1), and subsequently used by other investigators(2–7), has proven to be quite successful in estimating the swept volume from pressure falloff tests on vertical steam injection wells. This paper investigates the accuracy and applicability 0f the swept volume estimation by the pseudosteady state method, when steam is injected through a horizontal well. Simulated falloff test data have been used in this investigation. Steam chamber mobility and skin factor are also estimated from the well-test dala. To the best of our knowledge, a systematic analysis of simulated well tests for steam-injection horizontal wells has not been presented before in the petroleum Iiterature. A steam reservoir simulator is used to simulate steam injection into the reservoir until appreciable volumes are swept. Falloff tests are then simulated by shutting-in the injector and recording the wellbore gridblock pressure with time.