Data from a gas field shows the presence of long-term linear flow for severalyears in a large number of wells caused by the extremely low permeability inthe reservoir. Long-term linear flow production may be generated and controlledin some cases by the reservoir geometry, in others; it is controlled by thenatural occurring reservoir properties. In this paper three physical scenariosthat cause and control long-term linear flow in tight gas reservoirs aredescribed. First, we analyze how parallel natural fractures lead topermeability anisotropy which causes long-term linear flow. An analyticalmatrix-parallel fracture model is presented. An example to determine × and ypermeabilities in a square drainage area with natural fracturing and ahydraulically fractured well is described. Second, we show how a fractured wellin a tight gas reservoir causes linear flow perpendicular to the fracture. Wefocus on decline curve analysis methods for long-term linear flow in fracturetight gas wells. Third, we discuss long-term vertical linear flow in a highpermeability streak in a tight gas reservoir. An analytical model to analyzelong-term linear flow in a high permeability layer and 2D simulation resultsare shown. The drop of pressure in a higher permeability layer in a tightformation cause long-term vertical linear flow into the higher permeabilitylayer. Formulas to estimate reservoir properties for both fractured tight gaswells with linear flow and vertical linear flow in a high permeability streakare shown. These expressions were developed for both infinite acting and closedreservoirs under either constant bottomhole flowing pressure or constant gasrate conditions. A linear analysis can be applied to detect where the outerboundary effect occurs by finding the slope of a (m(pi)-m(pwf))/qg vs. sqrt(t)graph. Values of Ac*sqrt(k), drainage area, A, pore volume, Vp, and OGIP can beestimated without knowing prior porosity, permeability and thickness.Application of these models to production analysis for a tight gas well andvalidation of the results by using numerical simulations are shown.
Tight gas wells are present in reservoirs where the reservoir rock has lowpermeability (k = 0.1 md or less). Linear flow regime has been reported todominate in numerous situations and is characterized by t behavior duringtransient flow. Sometimes this linear flow lasts for many years and it isespecially an important flow regime associated with production from tight gasreservoirs.
In many tight gas wells, long-term linear flow occurs during depletion.Short-term linear flow production analysis may characterize fracture treatmentsin fractured wells, but long-term linear flow production may be controlled insome cases by the reservoir geometry, in others, it is controlled by thenatural occurring reservoir properties(1). Some causes that generate andcontrol longterm linear flow in tight gas formations are elongated reservoirs, anisotropy, natural fractured reservoirs, vertical flow in high permeabilitystreaks (layered reservoirs), and fractured wells (vertical wells interceptedby vertical, horizontal, or diagonal fractures, and horizontal wells withfractures). So, with these various causes of long-term linear flow a wrong ideaand misinterpretation of production and pressure data may originate.
