INTRODUCTION This paper describes a numerical Many gas wells exhibit pressure model for analyzing gas-well tests test behavior which is difficult if and predicting long-term deliverability. not impossible to interpret using Field applications presented include conventional methods of analysis. an interpretation of a gas well test Difficulty of interpretation is frein a tight sand leading to an accurate quently encountered in low permeability long-term deliverability projection. reservoirs and in layered reservoirs The model presented numerically simuwith limited or incomplete crossflow. lates two-dimensional (r-z) gas flow In these cases, assumptions in conand accounts for effects of turbulence, ventional analysis methods, such as skin, afterflow, partial penetration, complete (or no) crossflow and pressure-dependent permeability and negligible effects of afterflow or any degree of crossflow ranging from interlayer recirculation through the complete to none. Through a novel wellbore, are frequently invalid. treatment of the equations describing reservoir flow, skin and afterflow, This paper describes a numerical the model simulates shutin at the wellmodel which accounts for many factors head and then calculates afterflow and which are neglected in conventional any subsequent circulation of gas methods of analysis. The model through the wellbore from some layers numerically simulates two-dimensional to others. (r-z), transient gas fiow in a cyiinder References and illustrations at end representing the drainage volume of a of paper. single well. The calculations account 2 ANALYSIS AND PREDICTION for effects of turbulence, skin, afterflow, partial penetration, pressuredependent permeability and any degree of crossflow ranging from complete to none. Equations describing gas flow in the reservoir, skin effect and afterflow are combined in a manner which allows simulation of shutin at wellhead rather than bottomhole; the model calculates afterflow and any recirculation of gas through the wellbore from some layers to others. Thus, the calculated results show the effects of afterflow and recirculation on shape of the pressure buildup curve. Field applications presented illustrate use of the model to predict the long-term flow characteristics of gas “w”ells prim to ~~hnse+inn tQ Z3 ““....-----pipeline. The wells selected for illustration have been tested with both short and long-term tests to indicate the reliability of the method. An additional field application shows use of the model to explain and reproduce long-term (up to 600 days) gas well buildups. The method presented is equally applicable to simulation of oil well tests and performance and the slightly modified equations for that case are given in the Appendix. BRIEF DESCRIPTION OF THE MODEL Equations comprising the model are described in detail in the Appendix. Only a brief outline of the method is presented here. The basic equation of the model is eq. (1) describing transient, two-dimensional (r-z) gas flow in a cylindrical drainage volume*: