Application Of A New Method For Determining Flow Characteristics Of Fractured Gas Wells In Tight Sands
Authors: Scott J.O. 
Published: December 2007

Flow characteristics of fractured gas wells in low permeability sands of the Green River Basin of Wyoming show behavior similar to models for limited flow capacity fractures. Analysis methods are shown to analyze pressure buildup tests to identify relative fracture conductivity and length.

Gas production histories of wells have been curve fitted and are shown to conform to limited conductivity type curves. Pressure buildup tests before and after long production periods indicate low initial fracture conductivity improving with time. Calculated effective fracture lengths are smaller than expected.

Introduction

In a previous paper, I described analysis techniques that can be used to evaluate pressure transient tests and flow behavior in fractured wells in low permeability formations. The techniques are based on permeability formations. The techniques are based on analytical expressions that closely fit the type curves of Cinco et. al. and Agarwal et. al.

These techniques have been used to analyze the behavior of gas wells that have received large volume fracture treatments. Tn most of these wells the early transient pressure buildup and flow behavior matches the theoretical models for vertical fractures of limited conductivity. In such wells the characteristic type curves demonstrate log-log slopes of less than the slope of one-half that has been found for vertical fractures of high conductivity.

In the work by Cinco and Samaniego type curves for low conductivity fractures are shown to have two characteristic slopes. At very early dimensionless time the log-log slope is about one-fourth, for all values of dimensionless fracture conductivity, then changes to various slopes dependent on relative conductivity for several time cycles preceeding radial flow. This very early slope of one-fourth has been described as bilinear flow. It is normally not interpretable by curve matching unless the transient goes beyond this period into the pre-radial regime.

One of the limitations of type curve matching of transients of fractured wells is that the methods depend on matching only one transient. This condition would be obtained during a sustained flow period following complete pressure stabilization of a well. It is also practically obtained during the early part of a pressure buildup test when the shut-in time is small compared to the flow time.

However, during the testing period of wells in tight formations these conditions may not be obtained. The condition that is likely to be obtained is a period of erratic flow rate followed by a pressure period of erratic flow rate followed by a pressure buildup test of the same or longer duration. Because of the erratic flow rate, the most interpretable transient is the pressure buildup. However, this transient is not directly interpretable by type curve matching because it is affected by the pressure drawdown effect of the production transient. This changes the characteristic shape of the transient curve and if matched to type curves for single transients will result in gross misinterpretations.

ANALYTICAL EXPRESSIONS

Difficulties with type curve matching led me to develop equations that can be used in accordance with superposition principles to match single or multiple transient behavior of fractured wells in the preradial period. preradial period. P. 201