Summary
This paper presents a comprehensive set of experimental data for the
membrane efficiency of four shales when interacting with different water-based
and oil-based muds. Pressure transmission tests were used to measure the
membrane efficiency using three different cations and two different anions at
different concentrations (water activities).
It was found that the measured membrane efficiencies of shales, when exposed
to salt solutions, were low—ranging from 0.18% to 4.23%. Useful correlations
are presented between the membrane efficiency and other shale properties.
Results suggest that the membrane efficiency of shales is directly proportional
to the ratio of the cation exchange capacity and permeability of shales. Higher
cation exchange capacities and lower permeabilities correlate very well with
higher membrane efficiencies. Moreover, the ratio of the hydrated solute (ion)
size to shale pore throat determines a shale's ability to restrict solutes from
entering the pore space and controls its membrane efficiency. Cations and
anions with large hydrated radii yielded higher membrane efficiencies, compared
to ions with small hydrated diameters. Thus, the formulation of drilling fluids
must take into account the types of cation and anion in the water-based
fluid.
It was also found that the membrane efficiency of oil-based muds was high,
however, these membrane efficiencies were not 100 % as postulated by many
researchers.
Background and Past Work
Shale interaction with drilling fluids differs from sandstone and carbonates
interactions caused by the absence of mud cake formation, and that is
attributed to the fact that the shale permeability is so low. Unlike drilling
sandstones and carbonates, mud cakes, which normally act as semipermeable
membranes do not form while drilling shales because the shale permeability is
much lower than the mud cake permeability. Therefore, it is widely accepted
that the shale itself could act as a semipermeable membrane sustaining osmotic
flow.
Osmosis has long been recognized as a means to extract water out of a shale
when the water activity of the shale is higher than that of the drilling fluid.
In the absence of a hydraulic pressure gradient, the movement of mud filtrate
into shale is mainly governed by the chemical potential difference between the
pore fluid and the mud, and this results in the osmotic transport of water,
(Ewy and Stankovich 2000). However, it has recently been shown that the osmotic
potential generated between shale and drilling fluid is greatly influenced by
the flow of ions into or out of shale caused by the ionic concentration
imbalances (Zhang et al. 2004). Therefore, the actual osmotic effect is often
less than the osmotic potential. This has spurred much interest to quantify the
impact of ionic flow on the osmotic potential and that, in turn, has led to
introducing the concept of shale membrane efficiency. The membrane efficiency
describes the ability of a shale to hinder ion movement when interacting with
drilling fluids. If the shale completely stops ionic flow, the shale is said to
act as a perfect semipermeable membrane with a membrane efficiency of unity. If
the shale lets ions flow freely, the shale is said to act as a nonselective
membrane with a membrane efficiency of zero.
Staverman (1952) was one of the first researchers to investigate the
membrane efficiency of shale. He presented a model to estimate the reflection
coefficient (i.e., the membrane efficiency) of shale membranes. He showed that
the measured osmotic pressure obtained using a nonideal membrane is different
from the thermodynamically predicted value. Furthermore, this measured osmotic
pressure is highly dependent on the permeability of the membrane to the
solutes. Following Staverman, Low, and Anderson (1958), Fritz and Marine
(1983), and Ballard et al. (1994), presented theories that suggested osmosis as
a mechanism for swelling pressures generated by shales. These studies all
showed that a shale could act as a leaky semipermeable membrane, because it did
not completely stop the flux of ions.
While the previously mentioned studies focused on verifying osmotic
transport in shale and gave a qualitative measure of its membrane efficiency,
these studies did not quantitatively measure the membrane efficiency of shale.
Therefore, the next phase of experimental work by various researchers focused
solely on quantitatively estimating the membrane efficiency of shale. van Oort
et al. (1996), Ewy and Stankovich (2000), Mody et al. (2002), and Schlemmer et
al. (2003) conducted pressure transmission tests to measure the membrane
efficiency of shales. In addition, they studied the transport of water and ions
in shales and the impact on shale stability to facilitate the improvement of
water-based muds as shale drilling fluids. Their results showed that
low-permeability shales could act as leaky semipermeable membranes that sustain
osmotic flow of water. Also, they showed that the membrane efficiency of shales
was low and ranged from 1% to 10%. They argued that shale cation exchange
capacity and permeability could be responsible for the membrane behavior of
shales. Based on their results, they concluded that the ability of shales to
act as osmotic membranes could provide a powerful new means for stabilizing
shale rocks when exposed to water-based drilling fluids.
While all these studies gave a general idea of the membrane efficiency of
shale when interacting with drilling fluids, none of these studies presented a
clear picture of the relationship between the membrane efficiency of shale and
the properties of shale and drilling fluids. In this work, the membrane
efficiency of shale, when interacting with water-based and oil-based muds, has
been estimated using pressure transmission tests. The dependence of the
membrane efficiency of shale on ion type and concentration in the drilling
fluid is fully explored using different cations and anions over a range of
concentrations. The influence of shale permeability and cation exchange
capacity on the membrane efficiency is also investigated using four different
shales.
© 2008. Society of Petroleum Engineers
View full textPDF
(
2,018 KB
)
History
- Original manuscript received:
5 August 2006
- Meeting paper published:
5 December 2006
- Revised manuscript received:
16 October 2007
- Manuscript approved:
13 November 2007
- Version of record:
20 June 2008
View Cart
