Page 274 - Fundamentals of Gas Shale Reservoirs

254 GAS TRANSPORT PROCESSES IN SHALE

(a)
10 2

10 0 P = 10kPa
P = 100kPa
10 –2
P = 1MPa
K app /K D max 10 –4 P = 10MPa

10 –6
T = 400K, M = 16g/mol

10 –8

10 –10
10 –10 10 –8 10 –6 10 –4
Mean throat radius (m)

(b)
10 2
P = 10kPa
10 0 P = 100kPa

P = 1MPa
10 –2
P = 10MPa
K app /K D max 10 –4

–6
10
10 –8 T = 400K, M = 16g/mol

10 –10

10 –12
10 –10 10 –8 10 –6 10 –4
Mean throat radius (m)
FIGURE 11.10 Effect of throat sizes and pressure on normalized gas permeability K app / K D max in; (a) a single scale network with a con-
nectivity of 4 and (b) a dual‐length scale network with a connectivity of 4 (ECM, f = 0.5).

become a part of the kerogen in the form of a single phase. interstitial pore spaces expands first; then, adsorbed gas on
The controlling mass transport process of the dissolved gas the surfaces of the pores in kerogen desorbs to the pore
is molecular diffusion. Depending on the geochemistry of network. At this stage, the concentration of gas molecules on
the organic materials (thermal maturity, organic source, the pore inner surface decreases and creates a concentration
etc.), different gas solubility could be expected. The contri- gradient in the bulk of the kerogen, thereby triggering gas
bution of dissolved gas to gas‐in‐place and ultimate recovery diffusion (Etminan et al., 2014; Javadpour et al., 2007).
of a shale reservoir could be significant; hence, evaluation of Etminan et al. (2014) developed batch pressure decay
the gas‐diffusion process into kerogen becomes important. (BPD) technique to accurately measure the contributions of
In addition to the total contribution of each process, the onset different storage processes to the total gas‐in‐place. With the
time of each process during production is critical. Once same BPD test, they also measure gas molecular diffusion in
production starts from a reservoir, the compressed gas in kerogen. The method is robust and accurate and can save