Abstract

This study presents simulation results of fracture fluid cleanup and gas production behavior for hydraulically fractured gas wells. Isotropic and anisotropic, and single- and multi-layer reservoirs were studied. Systematic investigation revealed that formation damage near the fracture is a key factor affecting fracture fluid cleanup and gas production. Also, improper simplification of a multilayer reservoir may lead to overestimates of gas production.

Introduction

In low permeability gas reservoirs, hydraulic fracturing is often necessary to increase gas recovery and to provide economic gas flow rates. The fluid injected during the fracture treatment will leak off into the formation and will reduce the relative permeability to gas in the invaded region. In some cases, the injected fracture fluid may reduce the formation permeability in the invaded zone. Such damage can be caused by clay swelling, precipitation of solids, or migration of released fines.

Several studies have investigated fracture fluid cleanup and gas production behavior. These investigations were for isotropic, single-layer reservoirs.

Tannich studied the transient productivity of gas wells following hydraulic fracturing treatments to evaluate the effects of relative permeability reduction, liquid holdup in well tubulars, turbulence, and other factors that influence fracture fluid cleanup from the formation. The effects of fracture length, formation permeability, and formation flow capacity on fracture fluid cleanup and gas production were discussed in detail.

Holditch studied the combined effects of formation permeability damage, relative permeability damage, and capillary pressure increase in the invaded zone of a hydraulically fractured gas well. He concluded that the reservoir properties such as capillary pressure, change of capillary pressure in damaged zones, and relative permeability in low-permeability gas reservoirs are extremely important. These properties are primary factors in determining the behavior of a fractured well during cleanup. He also pointed out the conditions under which a water block to gas flow might become a serious problem.

Montgomery et al. and Sherman et al. studied how fracture fluid invades the formation near the fracture and the factors that affect fracture fluid cleanup. Both studies considered the effects of fracture conductivity, formation damage to the fracture face, and relative permeability hysteresis in the invaded zone. In addition, Ref. 4 also presented the effects of operating procedures on ultimate gas recovery where the effect of fracture closure stress was included.

We made several hundred computer runs to investigate further the effects of various factors on fracture fluid cleanup and gas production behavior for single- and multi-layer gas reservoirs. For single-layer gas reservoirs, the factors investigated were initial reservoir pressure, initial water saturation, formation porosity, formation permeability, formation anisotropy, injected fracture fluid volume, injection time, propped fracture length, propped-created fracture length ratio, fracture conductivity, formation damage near the fracture, flowing bottom-hole pressure, relative permeability, fracture fluid viscosity, and capillary pressure.

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