Liquid Removal From Hydraulically Fractured Gas Wells
Authors: Tannich J.D. 
Published: December 2007

Hydraulic fracturing can restrict the productivity of a gas well by reducing the relative permeability to gas in the region invaded by the liquid, and by increasing the producing bottom-hole pressure. A numerical model of the cleanup process is described and the transient behavior of tight gas sands after massive fracturing treatments is illustrated.

Introduction

Hydraulic fracturing fluid can restrict the productivity of a gas well after a treatment by reducing the relative permeability to gas in the region invaded by liquid. permeability to gas in the region invaded by liquid. Until this fluid is at least partially removed from the formation and the fracture, deliverability will remain impaired. Additional reductions in productivity can result because dense liquid is held up in the wellbore, thereby increasing the producing bottom-hole pressure, and because turbulence (nondarcy flow) in the fracture reduces the effective fracture conductivity. This paper discusses gas well productivity during cleanup.

The process of liquid removal from a fractured gas well is illustrated in Fig. 1 and is described below.Immediately after the well has been fractured, the gas saturation in invaded zones on either side of the fracture is reduced to the irreducible saturation by flow of the fracturing fluid (time t1). Viscous fluid will completely saturate the propped fracture and the tubing will contain as much liquid as can be supported by reservoir pressure. pressure.When the well is placed on production (time t2), flow will be initiated near the wellbore first. At this time, the flow resistance in the fracture and the formation is controlled by the viscosity of the fracturing fluid and often is large relative to the resistance for singlephase gas flow.Gas eventually will break through to the fracture, normally at the wellbore, and cleanup then proceeds gradually out along the fracture (time t3). The flow resistance decreases rapidly as liquid is removed and the gas saturation increases. The productivity of the fractured well will increase with time as fluid is removed because the effective fracture length is increasing. Productivities of idealized (ignoring cleanup) and actual Productivities of idealized (ignoring cleanup) and actual (including cleanup) fractured wells are compared with the productivity of an unfractured well in Fig. 2. The idealized well reaches maximum productivity immediately after the fracturing treatment, but when cleanup effects are considered, the well requires several days to attain maximum productivity.

Fracturing fluid must be lifted through the tubing as well as removed from the formation. Liquids in the wellbore reduce productivity by increasing hydrostatic pressure gradients and by increasing friction losses. pressure gradients and by increasing friction losses. The fact that swabbing may be required to initiate production from low-pressure formations is one consequence of liquid in the wellbore.

Turbulence reduces fracture conductivity whenever the gas flow velocity in the fracture exceeds a critical value, which in most instances occurs soon after gas first enters the fracture. Non-Darcy flow limitations are particularly important during cleanup because the particularly important during cleanup because the presence of liquid in the fracture confines the gas to a presence of liquid in the fracture confines the gas to a fraction of the total area available for flow.

Several studies have shown that the ideal productivity increase after fracturing will exceed steady-state estimates.

JPT

P. 1309