Page 51 - Handbook of Natural Gas Transmission and Processing Principles and Practices

1. Either the productivity of the vertical well is too low; this is improved by using a horizontal
well (single or multilateral); or
2. The area of the reservoir that is drained by a vertical well is so small that it would require a
large number of wells to effectively drain the whole reservoir; this could be very intrusive
on the surface land activity and is resolved by drilling multiple directional wells from a
single pad, which significantly minimizes the surface footprint.

1.8.2.3. Completion
Typically, the permeability of unconventional gas reservoirs is extremely small. As a consequence,
with conventional completions, the deliverability per well can be uneconomic, in spite of the known
presence of large quantities of gas. The best way to improve productivity per well is to increase the
area of contact between the well and the reservoir. This is what underlies the success of
unconventional gas completions. The single most significant contribution to the commercialization
of “shale” gas and “tight” gas has been the multiple-stage hydraulic fracturing of horizontal wells
to create this increased contact area (Fig. 1.8).

1.8.2.4. Production
The deliverability of gas from unconventional gas wells is governed by a combination of Darcy's
law and Material Balance (just as for conventional reservoirs). However, Eqs. (1.34) and (1.35) are
not very useful, because they are only applicable after the flow from the reservoir has been
stabilized. When a well is first opened to flow, a pressure transient travels through the reservoir
(transient flow). It is only when this pressure transient reaches the boundaries of the reservoir
(Boundary Dominated Flow) that the flow is considered to be stabilized and Eqs. (1.34) and (1.35)
become applicable. The time to reach stabilization is a function of permeability, and in conventional

reservoirs, this time is, for all practical purposes, relatively short (<3 months). However, in
unconventional reservoirs, the permeability is so low that it can take many years before Eq. (1.34)
becomes applicable. Accordingly, more complex equations applicable during transient flow are
needed. Such equations will usually incorporate time explicitly, and typically require computer
programs. The form of such an equation is:

FIGURE 1.7 Different drilling patterns.

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