Page 361 - Fundamentals of Gas Shale Reservoirs

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WETTABILITY OF GAS SHALE RESERVOIRS

Hassan Dehghanpour, Mingxiang Xu and Ali Habibi
School of Mining and Petroleum Engineering, University of Alberta, Edmonton, Alberta, Canada

16.1 INTRODUCTION and Alam, 2008). The pore‐scale distribution of reservoir
fluids strongly depends on the wettability state, which in
The rapid increase of energy demand has shifted the focus turn depends on various factors such as rock mineralogy
of petroleum industry toward abundant unconventional and the properties of the materials coating the rock surface
resources worldwide. Organic shales have become an impor- (Anderson, 1987; Hamon, 2000; Mohammadlou and Mork,
tant hydrocarbon source in North America, and are being 2012; Rao and Girard, 1996). Rock wettability strongly
explored as a resource in other continents as well. From influences the capillary pressure and relative permeability,
2000 to 2012, the contribution of shale gas to the total natural which depend on pore‐scale positioning of reservoir fluids.
gas production increased from 1% in the United States and Therefore, selection of relevant capillary pressure and relative
Canada, to 39% in the United States and 15% in Canada permeability curves for reservoir engineering calculations
(Stevens, 2012; US EIA, 2013). The abundant hydrocarbon of unconventional reservoirs requires characterizing the
resources in tight formations are now technically accessible wetting state of the reservoir rock. Furthermore, the interac-
due to advances in horizontal drilling and multistage tion of fracturing and treatment fluids with the shale matrix
hydraulic fracturing. However, measurement and modeling strongly depends on shale wettability, which is poorly under-
of petrophysical properties required for reserve estimation stood. In general, wettability characterization of organic
and reservoir‐engineering calculations are among the shales is important for (i) selecting fracturing and treatment
remaining challenges for the development of tight forma- fluids, (ii) investigating residual phase saturation and its
tions. In particular, characterizing wettability (wetting pore‐scale topology, (iii) investigating the occurrence of
affinity) of tight rocks and shales is challenging due to their water blockage at fracture face, and (iv) selecting relevant
complex pore structure, which can be either in hydrophobic capillary pressure and relative permeability models for
organic materials or in hydrophilic inorganic materials. reservoir engineering calculations.
The wetting state of a reservoir rock can be identified
by measuring the equilibrium contact angle (Johnson and
16.2 WETTABILITY Dettre, 1964), the Amott wettability index (Amott, 1959), the
United States Bureau of Mines (USBM) wettability index
Wettability is the measure of the preferential tendency of a (Donaldson et al., 1969), spontaneous imbibition rate/volume
fluid to wet the rock surface in the presence of the other (Kathel and Mohanty, 2013; Morrow, 1990; Ma et al., 1999;
fluid(s) (Agbalaka et al., 2008). Four general states of wetta- Olafuyi et al., 2007; Zhou et al., 2000), hysteresis of the
bility have been recognized as (i) water‐wet, (ii) fractional‐ relative permeability curves (Jones and Roszelle, 1978), and
wettability, (iii) mixed‐wettability, and (iv) oil‐wet (Donaldson nuclear magnetic relaxation (Brown and Fatt, 1956).

Fundamentals of Gas Shale Reservoirs, First Edition. Edited by Reza Rezaee.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.