General Equation

gwe Dqsrr

akhC

75.0ln

2

)]p(p)p(p[Cq wfpRpg

qg = surface gas ratea = units conversion factor (see next slide)k = permeabilityh = total thicknessre = external radiusrw = wellbore radiuss = steady-state total skinD = rate-dependent skin coefficientpwf = wellbore flowing pressurepR = volumetric average reservoir pressure = gas viscosityZ = gas Z factorBg = gas formation volume factor

dpZ

pp

p

p

p

ref

2 dp

Bp

p

pg

p

ref

1

or

Pseudopressure Function

Gas Rate Equation in Field Units

dp

BDqsrr

khq

Rp

wfpggwe

g

1

75.0ln

2001127.0

qg = surface gas rate, scf/dk = permeability, mdh = total thickness, ftre = external radius, ftrw = wellbore radius, fts = steady-state total skinD = rate-dependent skin coefficient, 1/(scf/d)TR = reservoir temperature, deg Rpwf = wellbore flowing pressure, psipR = volumetric average reservoir pressure, psi = gas viscosity, cpZ = gas Z factorBg = gas formation volume factor, res.bbl/scf

dp

Z

P

TDqsrr

khq

Rp

wfpRgwe

g

2

75.0ln

703.0

221

75.0ln

703.0wfR

Rgweg pp

ZTDqsrr

khq

Pseudopressure integral

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

200000

0 2000 4000 6000 8000 10000

pressure (psia)

p/u

Z,

psi

/cp

PrPwf

Gas rate is proportional to area

under curve