Page 50 - Handbook of Natural Gas Transmission and Processing Principles and Practices

For conventional gas wells, the deliverability equation can, for all practical purposes, be
expressed as (ERCB, 1975):

(1.34)

where q is the flow rate; C is a constant that encompasses permeability and completion
effectiveness and other gas properties such as gravity, viscosity, and temperature; P is the
R
reservoir pressure; P is the pressure at the well; and n is an exponent (between 0.5 and 1) that
wf
accounts for the Darcy and non-Darcy flow in the reservoir (commonly referred to as the laminar
and turbulent flow).
The constants C and n are determined from a flow test (often called Absolute Open Flow–AOF).
The P depends on surface facility restrictions, and usually reflects the pressure at which the gas is
wf
to be delivered to the transportation system or to the gas plant.
The material balance equation relates the reservoir pressure to the amount of gas produced:

(1.35)

where Z is the gas deviation factor (compressibility factor); G is the gas produced; and G is the
p
Original-Gas-In-Place. The subscript “R” represents the current reservoir pressure, and the
subscript “i” represents the initial reservoir pressure.
The previously mentioned two equations appear to be independent of time. However, that is not
so. In Eq. (1.35), G changes with time. Hence, the reservoir pressure changes with time. This, in
p
turn, causes the flow rate calculated in Eq. (1.35) to decrease as the reservoir depletes.
Combining Eqs. (1.34) and (1.35) and superimposing operating restrictions such as contract rates
and facilities limitations (compression/pipeline/processing) result in a forecast of production rate
versus time. Production stops when the flow rate becomes uneconomic. The total gas that will have
been produced at the time of abandonment is called the Expected Ultimate Recovery (EUR).
The Original-Gas-In-Place (G) is determined from geological estimates of the areal extent
and thickness of the reservoir, as well as its porosity and saturation, or from analysis of production
data.
The recovery factor is the ratio of the Expected Ultimate Recovery to the Original-Gas-In-Place
(EUR/G). It depends primarily on the reservoir pressure at the time of abandonment. The
abandonment pressure itself is directly related to the delivery pressure and to the distance of
transmission. As a consequence the recovery factor can range from 50% (for remote locations) to
90% for wells adjacent to fully developed transportation systems.

1.8.2. Unconventional Gas
1.8.2.1. Exploration
Unlike conventional gas, the location of unconventional resources is generally known, often as an
indirect result of conventional mining or oilfield activity. These unconventional gas resources are
generally laterally extensive, and therefore, less exploration is required to locate them.

1.8.2.2. Drilling
Whereas most conventional gas wells are vertical, unconventional gas wells are either horizontal
wells or multiple-well pads (Fig. 1.7). There are two principal reasons for this:

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