The conception of improved water-based drilling fluids that stabilise boreholes in shales by reducing pore pressure transmission

In the early 1990's two Shell researchers, John Downs and Dr. Eric van Oort, conceived and developed improved water-based drilling fluids that stabilised boreholes in shales by reducing pore pressure transmission. They used clouding alcohol alkoxylates (TAME) and a Jeffamine ED600 diamine as the active shale stabilising ingredients in conventional water-based muds.

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A Retrospec�ve Look at the Concep�on of Water-Based Drilling Fluids That Reduce Pore
Pressure Transmission in Shales
John Downs and Eric van Oort

Thirty years ago, Shell presented a paper at the SPE Offshore Europe conference describing their in-
house development and field testing of a novel type of water -based shale drilling fluid
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.

What made Shell’s new shale drilling fluid different from anything developed previously was that it
was designed specifically to maintain pressure isolation between a wellbore at overbalanced pressure
containing a water-based fluid and any shale formations surrounding the wellbore.

Pressure leakage

This novel approach recognized that the main source of wellbore instability while drilling shales with
water-based fluids is the gradual loss of the initial sharp pressure differential between the wellbore
and shale over time.

The pressure differential or overbalance is lost because the aqueous drilling fluid filtrate being
squeezed into the shale propagates a pressure front that advances far out into the shale ahead of
the filtrate front
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. The gradual loss of overbalance support on the borehole wall because of such
pore pressure transmission leaves the shale susceptible to failure and collapse into the wellbore (see
Figure 1).



Figure 1 – The initially stable stress state of a shale formation (solid black lines) at the wellbore wall
deteriorates when near-wellbore pore pressure is increased over time, leading to a reduction of the normal
effective stresses σ (defined as applied stresses S minus pore pressure P pore, i.e. σ = S - P pore ) that drives the
stress state towards failure (dotted black lines), as indicated by the orange arrow. Shale failure envelope is
represented by the red line in this shear stress vs. normal stress view. Once the stress state touches the
failure envelope, shale failure of the shale material will ensue .

Oil-based muds generally do not invade shales due to capillary entry pressures unless the
overbalance pressure is very high, the shale is fractured or is oil-wet. They are highly effective at
preventing pore pressure transmission. but they are not the best solution from an environmental
perspective.

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A cloudy solution

Shell’s answer to this problem was to formulate an aqueous drilling fluid with benign water-soluble
and biodegradable alcohol alkoxylate compounds that clouded out when heated to form a fine
water-insoluble emulsion as the fluid filtrate leaked away into the hot shale surrounding the
wellbore
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.


This Thermally Activated Mud Emulsion or TAME is remarkably effective at preventing or slowing
pore pressure transmission in shales under downhole conditions. The results of pressure
transmission tests with aqueous solutions of alcohol alkoxylate Surdyne L110, designated AA2 in SPE
paper 26699, showed that at concentrations of 5% v/v or higher the alkoxylate can significantly
reduce the rate of pore pressure penetration over time in shale (Figure 2 ).

Figure 2 – Rates of pore pressure transmission through shale in a pressure transmission tester with
water and aqueous solutions of a TAME clouding alcohol alkoxylate Surdyne L110 from Shell Chemicals.


As an added benefit the clouding of the alcohol alkoxylate is reversible, meaning that as the drilled
shale cuttings are carried up the wellbore annulus the alkoxylate will revert to its water-soluble
state. The environmental impact and persistence of biodegradable low-toxicity water-soluble
alkoxylates attached to drilled cuttings is likely to be lower than cuttings coated with oil and oil-
wetting surfactants.

Downhole polymeriza�on
Following the field validation
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of the TAME shale drilling fluid concept in 1992, the Shell researchers
began looking for even more effective water-based fluids that could maintain pressure isolation
between an overbalanced wellbore and any shales surrounding that wellbore.

Their vision was to design a water-based drilling fluid with two or more benign components that
would react at downhole temperature and pressure to form a polymeric reaction product on the
shale surface, sealing the shale pores with a thin water-impermeable polymer film. Such reactions
would proceed best in a non- aqueous medium and it was hoped that the fine droplets of alcohol

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alkoxylate in a clouded TAME system might make a) an effective reaction medium, and b) a
convenient vehicle for transporting the reaction products to the shale surface.

They thought at the time that interfacial polymerization, with one reactant in droplets of a non -
aqueous phase and the other in the bulk continuous aqueous phase, was a synthesis strategy used in
the chemical industry that they might exploit to manufacture pressure- isolating polymers downhole.

Polyoxyethylene diamine
Shell’s initial screen of candidate reactants suggested that water -soluble diamines might allow the
synthesis of polyamide or polyurea films under downhole conditions of elevated temperature and
pressure (Figure 3).


Figure 3 – Condensation reaction between a dicarboxylic acid and a diamine to form a polyamide.

In a Reported Invention form filed with Shell’s Patents & Licensing Division on 26
th
April 1993 two
Shell researchers, John Downs and Eric van Oort, reported that their pressure transmission testing of
candidate reactants had surprisingly revealed that a polyoxyethylene diamine of molecular weight
600 (tradename Jeffamine ED600) dissolved in water was capable of significantly reducing hydraulic
pressure transmission in shales by itself , i.e., without the need for further reaction (see example in
Figure 4)

Figure 4 – Pressure transmission of water and a 15% v/v aqueous solution of Jeffamine ED600 through a
shale sample at 50
o
C in a pressure transmission tester

This was an unexpected and non -obvious finding, given that at room temperature the Jeffamine
ED600 had no observable cloud point in the fresh water used as the solvent. Jeffamine ED 600 was
environmentally attractive from a biodegradability standpoint, having an inherently biodegradable
polyoxyethylene backbone capped with propylene oxide and tipped with amine groups (Figure 5).

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Figure 5 – Molecular formula of Jeffamine ED 600

It was also unusually benign for a polyether diamine, being only slightly irritating to the skin and eyes
and practically non-toxic to aquatic and marine organisms.

It is likely that the Jeffamine ED 600 has a useful cloud point window in brines, creating TAME
systems of high osmotic pressure with polymer membrane-forming capabilities if fed with the right
monomer co- reactants.

Unfortunately, budgetary constraints and staff reassignments in Shell Research at the time
prevented any further investigations and the details of Shell’s polyoxyethylene diamine discovery
reported internally in 1993 were not made public until 2022
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.


Polyoxypropylene diamine

Seven years later a group of M-I researchers filed a patent on 11
th
February 2000 claiming
the use of polyoxypropylene diamines, of the Jeffamine D type, as effective shale hydration
inhibitors in drilling fluid s
8
. According to the patent the most effective diamines had
molecular weights of 190-248 daltons. Interestingly, these water -soluble diamines were
later found to reduce pore pressure penetration in shales in a shale membrane tester
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.
Given the hydrophobic nature of their oxypropylene backbone t hey may have useful cloud-
points in drilling fluids containing elevated concentrations of salts.

This class of polyalkylene diamines, now commonly known in the oil industry as polyether
amines or polyamines, have been deployed as key active components in the very popular
water-based shale drilling fluids advertised as HPWBMs (High Performance Water-Based
Muds)
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. It is quite possible that when formulated in HPWBM with glycol ethers or other
clouding substances the polyamines will become involved in polymerization reactions under
hydrothermal downhole conditions that might enhance their wellbore pressure-isolating
properties.

Conclusion

Water-based drilling fluids that reduce the rate of pore pressure transmission in shales have
been used in well construction operations for over 30 years. Hopefully the next generation
of researchers wi ll advance the technology further by develop ing water-based drilling fluids
that actively synthesize pressure-isolating polymers downhole and in situ, perhaps using the
current clouding alkoxylates and polyamines as the reaction media and one of the reactants
respectively.

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References
1. Downs, J.D., van Oort, E., Redman, D.I., Ripley, D. and Rothmann, B.: “TAME: A
New Concept in Water-Based Drilling Fluids for Shales,” SPE 26699, presented at
the Offshore Europe Conference, Aberdeen, 7- 10
th
September 1993.
2. van Oort, E.: “A novel technique for the investigation of drilling fluid induced
borehole instability in shales”, SPE 28064 paper presented at the Rock Mechanics
in Petroleum Engineering conference, Delft, Netherlands, 29 -31st August 1994.
3. van Oort, E., Hale, A., Mody, F. and Roy, S., “Critical Parameters in Modelling the
Chemical Aspects of Borehole Stability in Shale and Designing Improved Water-
Based Shale Drilling Fluids”, SPE 28309, presented at the SPE 69th Annual
Technical Conference, New Orleans, La., 25- 28th September 1994.
4. van Oort, E., Hale, A.H., Mody, F.K. and Roy, S.: “Transport in Shales and the
Design of Improved Water-based Shale Drilling Fluids.” SPE 28309 paper , SPE
Drilling & Completion, 11, 03 (1996), p. 137–146.
5. van Oort, E.: “Physico- Chemical Stabilization of Shales”, SPE 37263, presented at
the SPE International Symposium on Oilfield Chemistry, Houston, Texas, 18-21st
February 1997.
6. van Oort, E., “On the Physical and Chemical Stability of Shales, J. Petr. Sci. Eng . 38
(2003), p.213-235.
7. Downs, J.D., Posting on LinkedIn 7
th
September 2022.
8. Patel, A.D, Stamatakis, E. and Davis, E.: “Shale hydration inhibition agent and
method of use”, Patent US 6,247,543 B1, published 19
th
June 2001.
9. Patel, A.D., Stamatakis E., Friedheim J. E., and Davis E.: “Highly Inhibitive Water-
Based Fluid System Provides Superior Chemical Stabilization of Reactive Shale
Formations”, paper AADE 01 -NC-HO-55 presented at AADE National Drilling
Technical Conference, Houston, Texas, 27-29th March 2001.
10. Patel, A., Stamatakis, E., Young, S., and Cliffe, S.: “Designing for the Future – A
Review of the Design, Development and Testing of a Novel, Inhibitive Water-
Based Drilling Fluid”, paper AADE-02-DFWM-HO-33, AADE Technical Conference,
Drilling & Completion Fluids and Waste Management, Houston, 2-3
rd
April 2002.
11. Patel, A.D., Stamatakis, E., Young, S., and Friedheim, J.: “Advances in Inhibitive
Water-Based Drilling Fluids—Can They Replace Oil- Base Muds?”, paper SPE
106476 presented at SPE International Symposium on Oilfield Chemistry,
Houston, Texas, 28th February–2nd March 2007