TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractActual field data show linear flow in a large number of tight gas wells. Sometimes this linear flow is transient for years. Linear flow is normally associated with hydraulic fractures. Short-term linear flow production analysis may characterize fracture treatments, but long-term linear flow production may be controlled in some cases by the reservoir geometry; in others, it is controlled by the natural occurring reservoir properties. In this paper, long-term linear flow caused by both the presence of natural parallel fractures and vertical flow in a high permeability streak is investigated. An analytical matrixparallel fracture model for a single-phase flow is presented. A systematic procedure to analyze linear flow in tight gas wells is described. Application of this methodology to production analysis from several tight gas wells and validation of the results by using numerical simulation are also shown.

Many tight gas wells show transient linear flow that lasts for many years. Linear flow is normally associated with hydraulic fractures, but tight gas reservoirs may contain geometrical effects that lead to linear flow behaviour. In this study, long-term linear flow caused by the presence of natural parallel fractures is investigated and a systematic procedure to analyze linear flow in tight gas wells is described. Application of this methodology to production analysis of three tight gas wells, and validation of the results by using numerical simulation, is described. Introduction Linear flow is characterized by t behaviour during transient flow. This is sometimes associated with hydraulically fractured wells with linear flow perpendicular to the fracture. At the end of linear flow, the pressure response (for a constant rate solution) of these wells flatten as flow enters from outside the fracture tips(1,2). However, this paper refers to observed well behaviour in which the pressure response becomes steeper at the end of linear flow, indicating an outer boundary effect. For these wells, there appears to be only linear flow during transient and outer boundary dominated flow. Actual field data shows long-term linear flow for years in a large number of wells(3–12) because of the extremely low permeability. A "half slope" (slope = 0.5) on a log-log plot of [m(pi) [m(pwf)]/ Qg vs. t for either constant gas rate production, qg, or constant bottomhole flowing pressure, Pwf, indicates linear flow. Long-term linear behaviour has been reported in tight gas wells which have no or not particularly large fracture treatments(7, 9, 11). The reason for linear flow may not be known for a particular well. But several papers discuss physical scenarios which may cause linear flow(5, 7, 11, 13, 14), including the occurrence of natural fractures. Tectonic stresses determine the direction of natural fractures. These natural fractures may tend to be parallel to the hydraulic fracture plane and cause linear flow even if the hydraulic fracture length was limited. However, if the tectonic stresses have changed since the formation of the natural fracturing, the hydraulic fracture could have a different orientation from the natural fractures(15). In this paper, we discuss how parallel natural fractures lead to permeability anisotropy and cause long-term linear flow. We show several field examples and outline a stepwise procedure for analyzing wells with long-term linear flow. Linear Flow Due to Anisotropy Parallel Natural Fracturing Long-term linear flow in tight gas wells may develop because of large permeability anisotropy ratios. Anisotropic permeability in porous medium has been examined in several papers(15–25) and books(26–31). One of the most important reasons for anisotropic permeability is parallel natural fracturing. Figure 1 shows a sketch of a well in a closed square with a parallel natural fracture system. In order to calculate the effect of the natural fractures on permeability, we assume that the natural fractures are continuous in the x direction and there is a regular spacing between fractures, dA, in the y direction.

Production analysis of actual data shows long-term transient flow geometries with boundaries in a large number of gas wells which are produced from tight gas reservoirs. In this paper, long-term linear and bilinear flows caused by the presence of natural fracturing are discussed. Linear and bilinear flows as a source of matrix block drainage are investigated. Systematic methodologies to analyze production data for estimation of reservoir properties and OGIP under linear, bilinear, and boundary dominated flows in tight gas wells are described. Application of these methodologies in various actual tight gas wells from industrial sources and validation of these procedures by using numerical simulation are shown. Long-term transient flows and short fracture half-length in various post hydraulic fractured wells suggest the convenience to develop tight gas fields with a tighter spacing between wells. Introduction Short-term linear and bilinear production data analysis may characterize fracture properties in a hydraulically fractured well1,2, but long-term linear and bilinear flow production may be generated and controlled by the reservoir geometry and/or by the natural occurring reservoir properties3,4. This paper deals with gas wells showing long-term linear and bilinear flows during the transient period and also with gas wells showing outer boundary dominated flow. Long-term linear and long-term bilinear transient behaviors have been detected in almost all-tight basins producing gas. Actual field data show transient flow for many years in a large number of wells because of their extremely low permeability5–14. Long-term linear7,13 and long-term bilinear behavior4 have been reported in tight gas wells that did not have particularly large fracture treatments. Several authors discussed the occurrence of bilinear flow regime in reservoirs7,12. Some of them presented models, solutions, and type curves under different conditions for both homogeneous and dual porosity reservoirs2,15–19. Some conditions causing bilinear flow are: a vertical well between two parallel leaky boundaries due to faulting or sedimentary process, a vertical well near a high conductivity infinite fault, a vertical well with a finite conductivity fracture20,21, a horizontal well in a fractured reservoir with transient dual porosity behavior during the intermediate linear flow period, a horizontal well in a layered reservoir with transient dual porosity behavior during the intermediate linear flow period and a linear reservoir with transient dual porosity behavior. In a previous paper22, long-term linear flow caused by both the presence of natural fractures and vertical linear flow due to high permeability streaks are shown. In this paper, linear and bilinear flow regimes as a source of the matrix block drainage are discussed. Afterwards, we show stepwise methodologies for linear, bilinear, and boundary dominated flows that can be used to analyze production data and estimate reservoir properties, pore volume, Vp, OGIP, and movable-reserves from gas wells in conventional and tight gas reservoirs. Later, gas rate forecasting, well spacing or infill drilling can be determined. Finally, we show several actual tight wells where linear, bilinear, and boundary dominated flows are detected and characterized. Matrix block drainage in a fracture network system Natural fractured reservoirs are often encountered in tight gas reservoirs. Five different models to characterize naturally fractured reservoirs including anisotropic models and dual porosity were discussed by Cinco-Ley23.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractProduction 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(p i )-m(p wf )]/q g vs. t 1/2 plot. Values of k 1/2 A c , 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.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractActual field data show linear flow in a large number of tight gas wells. Sometimes this linear flow is transient for years. Linear flow is normally associated with hydraulic fractures. Short-term linear flow production analysis may characterize fracture treatments, but long-term linear flow production may be controlled in some cases by the reservoir geometry; in others, it is controlled by the natural occurring reservoir properties. In this paper, long-term linear flow caused by both the presence of natural parallel fractures and vertical flow in a high permeability streak is investigated. An analytical matrixparallel fracture model for a single-phase flow is presented. A systematic procedure to analyze linear flow in tight gas wells is described. Application of this methodology to production analysis from several tight gas wells and validation of the results by using numerical simulation are also shown.