| Popis: |
Production data from some gas fields show the presence of long-term linear flow for several years in a large number of wells, caused by the extremely low permeability in the reservoir. Long-term linear flow production may be generated and controlled in some cases by the reservoir geometry; in others it is controlled by the natural occurring reservoir properties. In this paper three physical scenarios that cause and control long-term linear flow in tight gas reservoirs are described. First, we analyze how parallel natural fractures lead to permeability anisotropy which causes long-term linear flow. An analytical matrix-parallel fracture model is presented. Second, we show how a fractured well in a tight gas reservoir causes linear flow perpendicular to the fracture. We focus on decline curve analysis methods for long-term linear flow in fractured tight gas wells. Third, we discuss long-term vertical linear flow in a high permeability streak in tight gas reservoirs. An analytical model to analyze long-term linear flow in a high permeability layer and 2D simulation results are shown. Formulas to estimate reservoir properties for both fractured tight gas wells with linear flow and vertical linear flow in a high permeability streak are shown. These formulas were developed for both infinite acting and closed reservoirs, under either constant bottomhole flowing pressure or constant gas rate conditions. A linear analysis can be applied to detect where the outer boundary effect occurs, by finding the slope of a [m(pi)-m(pwf)]/qg vs. t1/2 plot. Values of k1/2Ac, drainage area, and OGIP can be estimated without knowing prior porosity, permeability, and thickness. Application of this linear analysis to production analysis for conventional and tight gas wells and validation of the results by using numerical simulations are shown. |
