Page 315 - Fundamentals of Gas Shale Reservoirs

SPECIALTY SHORT-TERM TESTS 295
The left‐hand side represents desorption mass transfer or an equivalent amount of gas depending on the DST con-
from the pore wall surfaces into the matrix pore body, and figuration. Conventional DST tool is not suitable for use in
the right‐hand side presents gas desorption rate. unconventional shale reservoirs because shale permeability
The following terms are specific to the above formula- is extremely low, and during shut‐in not enough formation
tion, which we present for clarity only. fluid enters the drill stem column to create an adequate
pressure build‐up response. For unconventional shale reser-
D * c D c (13.55) voirs, mini‐DST should be used. Mini‐DST consists of a 2‐l
chamber, a constant flow pump, and a pressure gauge
K
bk (Kurtoglu et al., 2013). The early‐time flow regime toward
D c K* 10 3 c (13.56) the mini‐DST perforation is spherical, which eventually
becomes radial flow at larger times. Joseph and Koederitz
.
b c K 0 795 k 04 . for N 2 (13.57) (1985) derived Equation 13.58, 13.60, and 13.63 of the next
section for the spherical flow regime. Kurtoglu et al. (2013)
a = Gas adsorption quantity, SCF/ton
c successfully used these equations to analyze a Bakken mini‐
*
D = Molecular diffusion coefficient for component c, DST test.
c
2
corrected for porosity and tortuosity, cm /s
D = Molecular diffusion coefficient for component 13.6.1.1 Spherical Flow Regime Joseph and Koederitz
c
c, cm /s (1985) derived the following equations, which are suitable
2
2
D = Knudsen effective diffusion coefficient, cm /s for analyzing spherical flow regime taking place near the
Κ *
K
b = Knudsen slippage coefficient for component c, atm mini‐DST perforation:
c
y = Mole fraction of component c in gas phase,
c
fraction p 70 6 . qB 1 ( s) 2453 qB c 1 (13.58)
t
/
ξ = Gas molar density, lb‐mol/ft , p/(z RT) kr k 32 t
3
g g sp sw sp
where
13.6 SPECIALTY SHORT-TERM TESTS
p p pt() (13.59)
In previous sections, we presented the rate‐normalized i w
pressure versus square root of time technique for pro- 2453 qB c t
ducing wells, which is specifically suitable for unconven- m sp k 32 (13.60)
/
tional reservoirs. Another technique involves conducting a sp
long pressure build‐up test and analyzing the pressure– k 3 k kk 3 kk (13.61)
2
time data. Both approaches rely on data gathered during sp xy z r v
long periods. In the specialty short‐term tests, the produc- 05 h
.
tion time duration is short. For instance, a short‐duration r sw p (13.62)
DST, or mini‐DST, requires 1500 seconds of flow period ln hr / w
p
followed by 1 h of pressure buildup. This test produces
formation fluids at constant rate from a single perforation r sw pseudo spherical wellboreradiusin miniDST
into a small chamber, thus the wellbore storage effect is h theopen holeintervalbetween thepackersinminiDST
near zero (Kurtoglu, 2013). For engineering applications, p
we model the flow regime as spherical flow, which we At sufficiently large flow times, the spherical flow regime
can readily analyze for permeability using appropriate becomes radial flow because streamlines become parallel to
equations in the following section. For greater accuracy, the formation boundaries. When radial flow regime becomes
we use numerical modeling to capture various flow regimes dominant, we can use the radial flow equations to calculate
more accurately. formation radial permeability for comparison with the early‐
time spherical flow permeability. Comparing radial and
spherical flow permeability is of value in determining
13.6.1 Mini-DST
whether vertical permeability anisotropy is significant.
A conventional DST is a short‐term production and shut‐in Kurtoglu et al. (2013) analyzed both the spherical and radial
test, conducted in the drill stem, in conventional higher per- flow pressure transient data for a Bakken well.
meability reservoirs. Form the analysis of the pressure build‐ For pressure build‐up analysis, we use the following
up data of the DST, we calculate formation permeability, approximate equation for spherical flow in mini‐DST anal-
formation damage skin factor, and reservoir static pressure. ysis when pressure buildup is sufficiently long (Stewart and
In conventional DST, we can produce several barrels of oil Wittmann, 1979):