The greatest change in reservoir pressure gradient
will occur at the well bore from which gas is being produced. The pressure gradient front will gradually
extend to the surface outcrop, assuming that the porosity of the entire coal
seam is initially water saturated and that the coals at the outcrop are
hydraulically connected to the coals produced at the gas well(s).
Virgin (original) reservoir pressure approximates
hydrostatic head. “Over-pressured”
(similar to artesian) conditions are common at Basin-interior gas wells. The Fruitland coalbeds ascend from a depth
of 2500-3500 feet in the interior of the Basin to the surface at Basin rim
outcrops. At a “flexure” zone that lies
approximately one mile inside of the Basin rim coal exposures, the coalbeds
abruptly increase in dip angle from several degrees basinward to 20-50 degrees
at the outcrop. Coalbed exposures
around the Basin rim lie several thousand feet higher in elevation than the
locations of the Basin-interior coalbed gas wells. This geomorphologic condition allows Basin rim coalbeds to be
recharged by direct inflow from precipitation events, rivers crossing the
coalbeds, percolation, etc.
(Preliminary investigation suggests that precipitation events may
account for a mere 0.2% of the total coalbed recharge.) Connate water trapped in the coalbeds at the
time of deposition could be a factor in the saturation of coal beds elevated
above effective recharge areas. Groundwater
springs known to exist at numerous locations along the Basin rim coalbeds
during the pre-CBM production era indicate that the piezometric surface
(elevation of water-saturation) in these Basin-rim coals was well above the
elevation of the springs. This implies
that the coalbeds were effectively saturated below these elevations. The influence of the weight of a column of
water equal in height to the difference in elevation between the level of
water-saturation in the Basin rim coalbed and the elevation of the coalbed in
the gas well is reflected in the reservoir pressure observed within a
Basin-interior CBM well-bore. The
additional (over-pressure) exerted by
this hydrostatic head equals approximately 0.433 psi (the pressure exerted by a
one foot column of water) multiplied by the elevation difference between the
surface elevation of the gas well and the piezometric surface in the respective
coalbed expressed at the basin fringe.
The schematic stratigraphic cross section following illustrates the
foregoing discussion.
Land Surface SCHEMATIC OF STRATIGRAPHIC CROSS SECTION
ILLUSTRATING COALGAS WELL OVERPRESSURIZED BY HYDROSTATIC HEAD
Groundwater
Level In
Coal near Outcrop TOTAL HYDROSTATIC HEAD OBSERVED AT GAS WELL HYDROSTATIC
HEAD ABOVE ELEVATION OF GAS WELL
Fruitland
Coal Beds
Kirtland
Shale Coalgas
Well
Pictured
Cliffs Sandstone
As the coals are de-watered through production in the
Basin interior, outcrop wells would be susceptible to drawdown effects if the
coalbeds are continuous and relatively permeable between the producing area and
the outcrop. The coal isotherm predicts
that a lessening of hydrostatic pressure would be accompanied by an increase in
desorbed gas (Appendix C: Chart 7b). The Basin rim springs mentioned previously
have diminished. Some no longer
flow. Shallow water wells in the coal
outcrop (Houston water well, Henderson water well, and some Texas Creek wells)
initially showed water within the coal at very shallow depths (a few feet to a
few tens of feet). The Henderson water
well is now dry. The groundwater level
in at least one Texas Creek well has decreased substantially.