Abstract

Actual field data show linear flow in a large number of tight gas wells. Sometimes this transient linear flow lasts for many 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; thus it is controlled in these cases by the natural occurring reservoir properties. In this study, long-term linear flow caused by the presence of natural parallel fractures is investigated. An analytical matrix-parallel fracture flow model is presented. A systematic procedure to analyze linear flow in tight gas wells is described. Application of this methodology to production analysis from three tight gas wells and validation of the results by using numerical simulation is also shown.

Introduction

Linear flow is characterized by√t behavior 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 tips1,2. However, this paper refers to observed well behavior 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 wells3–12 because of the extremely low permeability.

A half unit slope of 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 behavior has been reported in tight gas wells which did not have particularly large fracture treatments7,9,11. The reason for linear flow may not be known for a particular well. But several papers discuss physical scenarios which may cause linear flow5,7,11,13,14, including the occurrence of natural fractures. Tectonic stresses determine the direction of both hydraulic fracture and natural fractures. These natural fractures would tend to be parallel to the hydraulic fracture plane and would cause linear flow even if the 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 fractures15.

In this paper, we show how parallel natural fractures lead to permeability anisotropy which causes 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 papers15–25 and books26–31. Permeability anisotropy may occur for a variety of reasons. One of the most important reasons is the presence of parallel natural fracturing.

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