Page 362 - Fundamentals of Gas Shale Reservoirs

342 WETTABILITY OF GAS SHALE RESERVOIRS

The conventional methods such as Amott and USBM can 16.3 IMBIBITION IN GAS SHALES
hardly be applied for measuring wettability of tight rocks
primarily due to their extremely low permeability and During spontaneous imbibition, the nonwetting phase ini-
porosity, complex pore structure, presence of organic mate- tially saturating the porous medium is naturally displaced by
rials and their mixed‐wet characteristics (Odusina et al., a wetting phase (Brownscombe and Dyes, 1952; Engelberts
2011; Sulucarnain et al., 2012). For example, Amott and and Klinkenberg, 1951; Leverett, 1940). This process can
USBM techniques require a forced displacement which may also occur at the field scale and may lead to significant con-
not be practical for shales due to their ultra‐low perme- sequences. During a hydraulic fracturing operation, frac-
ability. Recently, a technique has been applied based on turing fluid is pumped into the well to create fractures or
interpretation of nuclear magnetic resonance (NMR) signals fissures in the rock formation. The induced fracture network
to study the wettability of different shale formations such as produces a pathway for hydrocarbon flow toward the well-
Eagle Ford, Barnett, Floyd, and Woodford (Odusina et al., bore (Novlesky et al., 2011). However, a significant fraction
2011; Sulucarnain et al., 2012). These studies show that of injected fracturing fluid can imbibe into natural fractures
both brine and oil can wet the shale samples, which indi- and shale matrix, during and after fracking operations
cates their mixed‐wet characteristics due to the presence of (Dutta et al., 2012; Holditch, 1979; Odusina et al., 2011;
organic and inorganic materials. However, representative Roychaudhuri et al., 2011). Field data show that more than
interpretation of NMR signals from unconventional rocks is 50% of fracking water can remain in the reservoir even sev-
more challenging than those from conventional rocks eral months after opening the wells for flowback (Makhanov
(Washburn and Birdwell, 2013). In many situations, the et al., 2013). The imbibed water can reduce fracture conduc-
NMR signal from organic materials and clay‐bound water tivity, fracture effective length, and fracture face perme-
can influence the resulting response of saturating fluids. ability (Bahrami et al., 2012; Paktinat et al., 2006).
Moreover, as pore size decreases the error associated with On the other hand, recent simulation (Cheng, 2012),
NMR measurement increases (Prince et al., 2009). Finally, experimental (Dehghanpour et al., 2012, 2013), and field
invaded water‐based or oil‐based mud can influence NMR studies (Adefidipe et al., 2014; Ghanbari et al., 2013) show
results (Looyestijn and Hofman, 2006). In general, accurate that effective imbibition during shut‐in periods can accel-
measurement of liquid‐phase saturation in ultra‐low perme- erate early‐time gas production rate. Such observations
ability rocks using NMR is challenging due to the chal- has encouraged the industry to understand the physics of
lenges involved in NMR signal processing and also in spontaneous imbibition in gas shales during shut‐in periods,
calibration of the system. and to estimate the water loss versus soaking time (Lan
Spontaneous imbibition has been used as a reliable et al., 2014a). Moreover, spontaneous imbibition of water,
technique to quantify the wettability of reservoir rocks such brine, or surfactants has also been considered as an enhanced
as sandstones and carbonates (Akin et al., 2000; Ma et al., oil recovery method in shale reservoirs (Wang et al., 2011,
1999; Morrow and Mason, 2001; Ma et al., 1999; Takahashi 2012a). Therefore, understanding water–oil countercurrent
and Kovscek, 2010; Zhou et al., 2002). This technique is imbibition is critical for optimizing fracture and treatment
specifically attractive for tight rocks such as shales since a fluids for enhancing the transport of oil from the tight rock
forced displacement in such low‐permeability rocks requires matrix into the fracture network of oil shales.
a significant pressure drop, which may induce artificial Recently, extensive imbibition experiments have been
cracks. Therefore, measuring and interpreting spontaneous conducted on various shale samples from the Horn River
imbibition of oleic and aqueous phases can be an alternative Basin, which is located in Canada, and this has been
approach to quantify wettability of tight rocks such as identified as the third largest North American natural gas
shales. However, interpretation of imbibition data for char- accumulation discovered prior to 2010, with 110 Tcf (trillion
acterizing the wetting state is challenging due to the adsorp- cubic foot) recoverable gas in place (Johnson et al., 2011).
tion of water by clay minerals and oil by organic materials, The Horn River Basin consists of several subsurface pro-
and also due to the complexity of the rock pore structure ducing members including Evie (E), Otter Park (OP) and
(Clarkson et al., 2012, 2013). In general, liquid imbibition Muskwa (M), overlaid by Fort Simpson (FS) non-producing
in gas shales is controlled by many factors in addition to shale. The total organic carbon (TOC) of Horn River shales
wettability such as sample expansion due to clay swelling has been reported to be up to 5.9 wt.% (Chalmers et al.,
(Dehghanpour et al., 2012, 2013), depositional lamination 2012; Reynolds and Munn, 2010).
(Chalmers et al., 2012; Makhanov et al., 2014), osmosis Dehghanpour et al. (2012) characterized several shale
effect (Bai et al., 2008; Chen et al., 2010; Chenevert, 1989; samples from the Horn River Basin by measuring porosity,
Neuzil, 2000; Xu and Dehghanpour, 2014), water adsorp- contact angle, and mineralogy; then interpreting the well log
tion by clay minerals (Chenevert, 1970; Hensen and Smit, data and scanning electron microscopy (SEM) images.
2002), and the connectivity of pore network (Xu and Table 16.1 summarizes the mineralogy of example samples
Dehghanpour, 2014). from the Fort Simpson, Muskwa, and Otter Park shale