Abstract

Fracturing fluid that remains in the fracture and formation after a hydraulic fracture treatment can decrease the productivity of a gas well by reducing the relative permeability to gas in the region invaded by this fluid. This fluid can block the gas flow into the fracture, thus reducing the effective fracture length. Pressure transient tests performed on hydraulically fractured wells often reveal that the effective fracture half-lengths are substantially less than the designed length from fracture stimulation.

In this work we used reservoir simulation to determine the relationship between fracture fluid production, effective fracture length, and gas productivity. While the effective fracture length is affected by such factors as non-Darcy flow, it is related directly to fracture cleanup, and increases with time. From this study we found that the rate of fracture fluid production is affected significantly by the conductivity of the fracture. Greater dimensionless fracture conductivity results in more effective well cleanup, longer effective fracture lengths versus time, and greater effective stimulation of the well.

The results of this study provide a better understanding of the gas production behavior from wells hydraulically fractured using water-based fracturing fluids. The relationships between fracture conductivity, effective fracture length, and gas productivity presented in this paper can be used in economic calculations to balance the costs of higher fracture conductivity against the additional revenue resulting from longer effective fracture lengths. Results presented will allow operators to better design optimal fracture lengths for typical gas reservoirs.

Introduction

The productivity of hydraulically fractured gas wells is often not optimal because of the presence of fracturing fluid in the fracture and formation around the fracture. This fluid reduces the relative permeability to gas in the invaded zone around the fracture and, in some cases, may damage the formation resulting in a significant reduction in formation permeability at the face of the fracture.^1,2 If the fracturing fluid remains in the fracture or formation, the effective fracture length of the well can be significantly lower than the designed length.

Effective fracture half-lengths (and fracture conductivities) determined from pressure buildup tests are typically low. Lee and Holditch³ presented the results of pressure transient analysis from hydraulically-fractured, low permeability gas reservoirs. The results indicate that fracture half-lengths calculated based on achieving pseudoradial flow average only 5% to 11% of the designed lengths, while fracture lengths determined from reservoir simulation history matching average about 68% of the designed lengths.

Alvarez, et. al.⁴ recently conducted a study that showed the effects of non-Darcy flow on pressure transient analysis of hydraulically fractured gas wells. This study revealed that calculated fracture half-lengths and fracture conductivities can be reduced by over 90% due to non-Darcy flow effects. These authors suggested that the best estimates of formation permeability, fracture half-length, and fracture conductivity can be obtained using a reservoir simulator that is capable of handling non-Darcy flow and fracture closure effects.

The relationship between fracture fluid production and effective fracture length has not been thoroughly discussed in the literature. The objectives of our work were to further investigate this relationship and, in particular, to determine the effect of fracture conductivity on fracture fluid cleanup and effective fracture length. We have used reservoir simulation of hydraulically fractured wells to determine the relationships between reservoir and fracture properties and well performance. In this paper we determine effective fracture lengths by both direct observation of simulation results and by analysis of simulation pressure transient data for a range of reservoir and fracture properties.