Techniques of Controlling Water Coning in Oil Reservoirs
S I Onwukwe*
Abstract
Water coning in oil reservoirs pose serious hindrance to optimizing oil
production as unwanted fluids tend to replace the oil in the production stream,
which invariably limits the ultimate oil recovery. The coning of water into
production wells is caused by pressure gradients established around the wellbore
by the production of fluids from the well. These pressure gradients can raise the
water-oil contact near the well where the gradients are dominant. Many methods
have been published for evaluating and combating coning tendencies in the
wellbore, yet water coning are still a major issue in many oil fields all over the
world. This article examines the various approaches of controlling coning of
unwanted fluid in the wellbore thereby optimizing oil production from the
reservoir.
Keywords: Water coning, Downhole water sink, Downhole water loop,
Horizontal well, Radial drilling.
Introduction
Oil recovery from reservoir with oil zone underlying
a gas cap, overlying bottom water, or sandwiched
between gas and water may face the challenge of
excessive water and/ or gas production due to coning
phenomena. Coning is a term used to describe the
mechanism underlying the upward movement of
water and/ or the down movement of gas into the
perforations of a producing well, thereby resulting to
high water cut (BS&W) and high gas-oil-ratio
(GOR) respectively, leading to losses in ultimate oil
recovery (Fig. 1).
Coning occurs when the pressure drawdown created
by a well is large enough to overcome the
gravitational resistance of nearby water or gas
causing these fluid(s) to be drawn into the well. This
phenomenon is affected by the characteristics of the
fluids (density and viscosity), rock properties
(vertical and horizontal permeability) and geometry
of the reservoir and well type and location. Coning
can result to excessive water and/ or gas production
from oil reservoir, thereby significantly reducing
reservoir energy.
*
Occurrence of coning are common in mature oil
fields or the later years of a productive life of a well,
when the oil-water-contact (OWC) and the gas-oilcontact (GOC) have moved up and down
respectively into the oil column.
A review of many Niger Delta oil reservoirs shows
that most of the reservoirs are at its mature stage and
therefore vulnerable to coning problems, which can
have a detrimental effect on the ultimate oil
recovery, and hence the project economics.12 Due to
high coning occurrences in the Niger-Delta, most
wells have been shut-in and a lot of recompletions
are being made in order to combat the problem of
coning.20 Many methods have been used to control
coning tendencies in oil wells. Different prevention
strategies may require specific operating procedures
to accommodate different production system.
Coning management strategy can be improved upon
as more options are known to handle specific
reservoir conditions and development concept being
applied. This article therefore discusses the various
aspect of technology relative to the control of coning
in oil wells in order to optimize oil production.
Department of Petroleum Engineering, Federal University of Technology, Owerri, Nigeria.
E-mail Id: [email protected]
© ADR Journals 2015. All Rights Reserved.
9
Onwukwe SI
Figure 1.Typical Effect of Coning21
Coning Phenomenon
Experimental Approach
One of the main reasons for coning is pressure
drawdown. A vertical well exhibit a large pressure
drawdown in the vicinity of wellbore, unlike
horizontal wells which provide an option whereby
pressure drawdown is minimized and high
production rate sustained.
The pioneering work on water coning problem
focused on coning mechanisms and experimental
studies. Muskat and Wyckoff published the first
article to analyze water coning in oil production
theoretically.19 They defined the critical oil
production rate as the maximum allowable oil flow
rate that can be imposed on the well to avoid a cone
breakthrough. The critical rate would correspond to
the development of a stable cone to an elevation just
below the bottom of the perforated interval in an oilwater reservoir.
For reservoirs with gas cap and bottom water,
coning will never occur if piston-like displacement
is maintained between the oil and gas, and/ or oil
and water interfaces. However, a non piston-like
displacement can occur as production progresses. In
such situation, there is coning particularly when the
viscous forces are much higher than the gravity
forces. Thus gas and water will make their way to
the wellbore. Coning tendencies are inversely
proportional to density difference and are directly
proportional to viscosity.11 The density difference
between gas and oil is normally larger than the
density difference between water and oil. Hence gas
has a less tendency to cone than water. However gas
viscosity is much lower than the water viscosity, and
therefore, for the same pressure drawdown in the
given reservoir, the gas flow rate will be higher than
the water flow rate. Thus, density and viscosity
difference between water and gas tend to balance
each other.
One of the major causes of coning is large pressure
drawdown from excessive production. One can have
severe coning in spite of high reservoir permeability.
This happens with the fact that bottom water and top
gas travel through high-permeability (vertical)
formations.
J. Adv. Res. Petrol. Tech. Mgmt. 2015; 1(1): 8-16.
Meyer and Garder derived a correlation for the
critical oil rate required to achieve a stable water
cone.16 They found that the critical rate for a well
was determined by: the length of well penetration,
density difference of oil and water and the oil zone
thickness. Their correlation for critical oil rate is
expressed as:
(1)
Many other authors developed working correlations
and curves to determine the critical oil rate.3,5,6,8,14
Numerical Simulation Approach
With the increase of computing power and
improvement of simulation technology, several
computer simulators make it possible to simulate
more complex coning problems. The first numerical
simulation research on coning problem was carried
out by Welge and Weber.33 They applied two-phase,
two-dimensional model using the alternating
Onwukwe SI
10
direction implicit procedure (ADIP) in the gas and
water coning simulation. Their simulation results
matched the producing wells experiencing water or
free gas production by coning. They suggested that
the average horizontal and vertical permeability
ratio, (kh/kv) is critical parameters in the coning
study.
Schols developed an empirical formula for critical
oil rate based on results obtained from numerical
simulator and laboratory experiments as:26
(2)
Miller and Rogers presented detailed coning
simulation approach which was suitable to evaluate
water coning problem for a single well in a reservoir
with bottom water.17 They simulated a single well
using radial coordinates and a grid system which
could be used to determine the most important
parameters in water coning on both short term and
long term production. Numerous researchers have
shown the effectiveness of computer simulation in
generating mathematical model for specific coning
management.4,15,18,25
Water Coning Control Methods
Several practical solutions have been developed to
delay water breakthrough time and minimize the
severity of water coning in oil wells. The basic
methods included:




Separating oil and water in the OWC using
horizontal impermeable barriers,
Controlling the fluids mobility in the reservoir,
Producing oil and water separately by
Downhole water sink (DWS) wells,
Minimizing drawdown around wellbore through
horizontal well technology.
Some of the methods are briefly described as
followings:
Horizontal impermeable barriers
Creating and placing horizontal barriers to
separating oil and water in the OWC for controlling
water coning has been considered. Karp studied
different designs of the horizontal barriers, by
establishing an experimental apparatus to test the
effects of different cement barriers and choose the
right materials for different reservoirs.13 They found
that reservoirs with high-density or high viscosity
crude oils, low permeability or thin oil-zone
thickness were not suitable to use this technology.
Pirson and Mehta found that the horizontal barrier
just delayed the water breakthrough time while it did
not provide absolute remedy to the water-coning
problem.25 Water would overpass the barrier and
breakthrough to the production interval when the
cone radius became greater than the barrier. It is
useful only where the horizontal fracture is available
to form such an impermeable barrier. On the other
hand, this technology might impede the water drive
from the field point of view.
Downhole Oil-water Separation (DOWS)
Technology
In recent years, downhole oil-water separation
(DOWS) technology, as a technique of separating
water downhole to reduce surface water production
has been developed. This technique allows water to
be separated in the wellbore and injected into a
suitable injection zone downhole while oil is
produced to the surface. Shortly after the
introduction of the DOWS technology to the oil
industry in the 1990’s, considerable research work
has been done and several trial applications have
been undertaken to test the technology.10
One of the main reasons of water coning in oil well
is the pressure drop caused by the oil production in
oil zone. If an equal pressure drop in the aquifer is
applied, water will not rise up and water coning can
be controlled, then the drained water can either be
lifted to the surface or be injected into the same
aquifer at a deeper depth.
The first method is known as Downhole Water Sink
(DWS) technology which has been studied and
applied for many years, while the second one is the
Downhole Water Loop (DWL) technology which is
relatively new comparing to DWS but showing
beneficial advantages and potential to improve oil
production).
Downhole Water Sink (DWS) Technology
Widmyer introduced and patented a novel coning
control idea to the petroleum industry known as the
downhole water sink (DWS) technology.34 In his
patent, he used two separated completions in one
well to control water coning: one produced oil from
the oil zone and the other drained water in the
aquifer. Thus, the water coning could be controlled
by the two opposite pressure drawdown.
The interest of the oil industry was drawn to the
DWS technology after Wojtanowicz and Xu
improved the technology to a more workable and
successful method when they simulated a dual
completion using a “tailpipe water sink” as shown in
Fig. 2.35 First, an oil well is drilled through the oil-
J. Adv. Res. Petrol. Tech. Mgmt. 2015; 1(1): 8-16.
11
Onwukwe SI
bearing zone to the underlying aquifer. Then, the
well is dually completed both in the oil and water
zones. A packer separates the oil and water
perforations. During production, oil flows into the
upper completion being produced up the annulus
between the tubing and the casing, while water is
drained through the lowermost completion through
perforations in the casing and then lifted up through
the open tubing below the initial OWC. As a result,
the produced oil is water free and the drained water
is oil free.
Until now, DWS completion has been field tested in
numerous reservoirs all over the world with good
results.22,28,30 The drawback of this technology is
that, it brings large amount of water to the surface
which requires more water processing facility and
adds the production costs.
Figure 2.Downhole Water Sink (DWS) Well Completion27
Downhole Water Loop (DWL) Technology
In order to overcome this disadvantage of DWS
system, Wojtanowicz and Xu proposed a concept of
Downhole Water Loop (DWL) technique to cut back
the volume of formation water produced by an oil
well from a hydrocarbon reservoir underlain by a
water zone.35 The method employed dual
completion of the well inside the water zone, below
the OWC to install the water loop equipment
(separated by a packer) in addition to the
conventional completion in the oil zone (above the
OWC). The water loop installation included a
submersible pump, the upper (water sink)
perforations and the lower (water source)
perforations. A submersible pump would drain the
J. Adv. Res. Petrol. Tech. Mgmt. 2015; 1(1): 8-16.
formation water around the well from the water sink,
and then would reinject the same water back to the
water zone through the water source perforations
(Fig. 3).
It is possible to use the flow potential theory to
develop expressions for the streamlines and
isopotential lines for a number of cases of 2D fluid
flow in the DWL well system as shown in Fig. 4. By
reinjecting the produced water far away from the
production interval, pressure interference is avoided
and the water displacement mechanism is
maintained. Also, supplementing the produced water
reinjection with external water, in order to enhance
the water displacement may additionally improve oil
recovery.29
Onwukwe SI
12
Figure 3.Downhole Water Loop (DWL) Well Completion
Figure 4.Flow Streamlines from Uniform Source and Point Source to Point Sink in 2D System
Application of Horizontal Well Technology
Horizontal Well Completion with Stinger
Several researchers have recommended horizontal
well technology as a solution for the development of
reservoirs with water coning problems.1,31,32
Pressure losses along the horizontal wellbore can be
redistributed by inserting a piece of pipe of a smaller
diameter into the completion/ production liner (Fig.
5). This smaller piece of pipe is typically called “the
stinger”. Permadi et al. studied this approach both
numerically and with a Hele-Shaw physical model
and concluded that a stinger length of 0.25 times the
producing length of the horizontal wellbore is
adequate to redistribute the pressure drawdown and
have a more uniform flux along the wellbore.24 They
also reported that the rate reduction due to the
insertion of the un-perforated stinger is relatively
small and is overcome by the much longer delay in
the time to water breakthrough.For most of the pipe
length studied, they observed that inserting the
stinger was capable of delaying the time to water
breakthrough in horizontal wells by as much as four
times. Thus, the completion of the horizontal well
with stinger creates improved sand face pressure
profile by controlling the inflow of fluid along the
wellbore.
While vertical wells act like point source
concentrating all the pressure drawdown around the
bottom of the wellbore, horizontal wells act more
like a line sink and so distribute the pressure
drawdown over the entire length of the wellbore.
The result is reduced pressure drawdown around the
wellbore.
Peng and Yeh have showed that the use of
horizontal wells is a proven technology for reducing
coning problems and improving recovery in
reservoirs underlain by water.23
Various methods have recently been recommended
by researchers to improve on the productivity of
horizontal wells with regards to combating water
coning problems.
J. Adv. Res. Petrol. Tech. Mgmt. 2015; 1(1): 8-16.
13
Onwukwe SI
Figure 5.Horizontal Well Completion with a Stinger
Variation of Perforation Density
Another scheme commonly recommended for
controlling water coning in horizontal wells is
variation of perforation-density to uniformly
distribute the influx of fluid. The perforation density
in each segment is varied (either by varying size of
Heel
perforation or by varying the number of perforation
for the same hole sizes) until the optimum
distribution is achieved.2 In other words, the
perforation density is less at the heel than at the toe
of the horizontal well (Fig. 6). For the case of water
coning control, the optimum distribution is when
each segment has the same inflow.
Toe
Figure 6.Schematic of Typical Variable Perforation Density
Application of Radial Drilling Technology
Radial Drilling Technology is a hydraulic jetting
method to penetrate the reservoir formation
perpendicularly from the existing cased or non cased
well bore. The Technology enables the drilling of
several micro horizontal wells with diameter of 2550mm into reservoir at an exact depth perpendicular
to the mother-well conveyed by coiled tubing
(Fig.7).9 The micro horizontal well can penetration
the formation (100m) over conventional perforating
(<1.3m) and 4 laterals can be jetted on one horizon
(Fig. 8). Radial Drilling Technology therefore
provides reservoir intervention means to drill several
micro horizontal wells through vertically drilled well
to control coning by significantly reducing the
pressure drawdown created around the wellbore.
The technique is equivalent to reforming a single
well to small-scale horizontal well group which can
effectively reduce coning tendencies by leveraging
on the significantly reduced drawdown and also
provide access to the residual oil between existing
wells of the mature fields.
Figure 7.Schematic of Radial Drilling downhole system7
J. Adv. Res. Petrol. Tech. Mgmt. 2015; 1(1): 8-16.
Onwukwe SI
14
Figure 8.Radial Drilling penetration compared to conventional perforation
Conclusions
Water coning (influx) into an oil well is caused by
the high pressure gradient around the wellbore
during oil production. In order to avoid this problem
of water influx, the oil well should be produced
below the critical oil rates. However, these critical
oil rates are usually too low for any economic
reason. Consequently, several solutions have been
developed to minimize the impact of the concurrent
production of unwanted water in oil wells.
Downhole Water Sink technology is one industry
technique that allows for production rates above the
critical oil rates at the oil zone with reduced
contamination with water. It involves segregated
production of the oil and water with a dual
completion string with zonal isolation. However,
most of the analytical models developed to evaluate
the performance of oil wells subject to water coning
have made several simplifying assumptions to
reduce the complexity of their solutions.
Another industry technique for developing
reservoirs subject to water coning is horizontal
wells. However, horizontal wells are themselves not
free from water conning problems. Like the vertical
well counterpart, typical critical oil rates to avoid
water coning into horizontal wells are too low for
economic purpose. However, the stinger completion
method, in combination with reduced perforation
density, could provide significant increase in well
productivity. Also radial drilling technology
provides reservoir intervention means to drill several
micro horizontal wells through vertically drilled well
to control coning occurrence via reduced pressure
drawdown around the borehole.
References
1. Al Kaioumi QM, Nassar A, Hendi A. Oil Rim
Development Using Horizontal Wells Off Shore
2.
3.
4.
5.
6.
7.
8.
9.
10.
Abu Dhabi. SPE 36299. 7th Abu Dhabi
International Petroleum Exhibition and
Conference, Abu Dhabi, UAE, 13-16 Oct, 1996.
Asheim H, Oudeman P. Determination of
Perforation Schemes to Control Production and
Injection Profiles along Horizontal Wells. Paper
SPE 29275, 1997.
Bournazel C, Jeanson B. Fast Water Coning
Evaluation. SPE Paper 3628. SPE 46th Annual
Fall Meeting, New Orleans, Oct 3-6, 1971.
Byrne WB Jr, Morse RA. The Effects of
Various Reservoir and Well Parameters on
Water Coning Performance. SPE 4287.
Proceedings of the 3rd Numerical Simulation of
Reservoir Performance Symposium of SPE in
Houston, TX, Jan 10-12, 1973.
Chaney PE, Noble MD, Henson WL et al. How
to Perforate Your Well to Prevent Water and
Gas Coning. OGJ May 1956: 108.
Chaperon I. Theoretical Study of Coning
Toward Horizontal and Vertical Wells in
Anisotropic Formations: Subcritical and Critical
Rates. SPE Paper 15377. SPE 61st Annual Fall
Meeting, New Orleans, LA, 5-8 Oct, 1986.
Huanpeng C, Gensheng L, Zhongwei H et al.
Extending Ability of Micro-Hole Radial
Horizontal Well Drilled by High-Pressure
Water jet. 2013 WJTA-IMCA Conference and
Expo, Sep 9-11, 2013, Houston, Texas.
Chierici GL, Ciucci GM, Pizzi G. A Systematic
Study of Gas and Water Coning by
Potentiometric Models. JPT Aug 1964: 923.
Dickinson W, Dickenson RW. Horizontal
Radial Drilling System. SPE 13949. SPE 1985
California Regional Meeting, California, 1985.
Inikori SO. Numerical Study of Water Coning
Control with Downhole Water Sink (DWS)
Well Completions in Vertical and Horizontal
Wells. A PhD Dissertation, Department of
Petroleum Engineering, Louisiana State
University, Aug 2002.
J. Adv. Res. Petrol. Tech. Mgmt. 2015; 1(1): 8-16.
15
11. Joshi SD. Horizontal Well Technology. Tulsa,
Oklahoma: Pennwell Publishing Company,
1990.
12. Kabir CS, Agamini M, Holgiun RA. Production
Strategy For Thin-Oil Column with Saturated
Reservoir. SPE Paper 89755. SPE Annual
Conference and Exhibition, Houston, 26-29
Sep, 2004.
13. Karp JC. Horizontal Barriers for Controlling
Water Coning. JPT Jul 1962: 783-90.
14. Khan AR. A scaled model study of water
coning. Trans AIME 1970; 1249: 771-76.
15. Letkeman JP, Ridings RL. A Numerical Coning
Model. SPEJ Dec1970; 10(4).
16. Meyer HI, Garder AO. Mechanics of Two
Immiscible Fluids in Porous Media. J Applied
Phys Nov 1954; 11: 25.
17. Miller RT, Rogers WL. Performance of Oil
Wells in Bottom Water Drive Reservoirs. SPE
4633. Proceedings of the 48th Annual Fall
Meeting of the Society of Petroleum Engineers
of AIME, Las Vegas, NV, Sep 30-Oct 3, 1973.
18. Mungan N. A Theoretical and Experimental
Coning Study. SPEJ Jun 1975; 15(3): 132.
19. Muskat M, Wyckoff RD. An Approximate
Theory of Water-Coning in Oil Production.
AIME Transaction 1935; 114: 144-63.
20. Obah B, Chukwu O. A Generalized Approach
to Coning Problem. OIL GAS European
Magazine, Mar 2000: 26-29.
21. Mogbo O. Horizontal Well Simulation for Thin
Oil Rims Abundant in the Niger-Delta-A Case
Study. SPE Paper 133389. Trinidad and Tobago
Energy Resources Conference, Port of Spain,
Trinidad, 27-30 June, 2010.
22. Ould-amer Y. Attenuation of water coning
using dual completion technology. JPSE 2004;
45: 109-22.
23. Peng CP, Yeh N. Reservoir Engineering
Aspects of Horizontal Wells-Application to Oil
Reservoirs with Gas or Water Coning Problems/
Paper SPE 29958. International Meeting on
Petroleum Engineering, Beijing, PR China, Nov
14-17, 1995.
24. Permadi P, Wibowo W, Alamsyah Y et al.
Horizontal Well Completion With Stinger for
Reducing Water Coning Problems. SPE
Production Operations Symposium, Oklahoma
City, OK, Mar 9-11, 1997.
25. Pirson SJ, Mehta MM. A Study of Remedial
J. Adv. Res. Petrol. Tech. Mgmt. 2015; 1(1): 8-16.
Onwukwe SI
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
Measures for Water-Coning By Means of a
Two-Dimensional Simulator. SPE 1808. Fall
Meeting of the Society of Petroleum Engineers
of AIME, New Orleans, LA, Oct 1-4, 1967.
Schols RS. An Empirical Formula for the
Critical Oil Production Rate. Erdoel Erdgas A
Jan 1972; 88(1): 6-11.
Shirman EI. Experimental and Theoretical
Study of Dynamic Water Control in Oil Wells.
Ph.D dissertation, Louisiana State University,
Baton Rouge, 1998.
Siemek J, Stopa J. A Simplified SemiAnalytical Model for Water-Coning Control in
Oil Wells with Dual Completions System.
Transactions of the ASME Dec 2002; 124: 24652.
Singh K. Designing a Produced Water ReInjection Program in Bekapai Field. SPE
101938. Proceedings of Asia Pacific Oil and
Gas Conference, Melbourne, Australia, Oct
2002.
Utama FA. An Analytical Model to Predict
Segregated Flow in the Downhole Water Sink
Completion and Anisotropic Reservoir. SPE
120196. Proceedings of the SPE ATCE,
Denver, CO, Sept 21-24, 2008.
Vijah KS, Ashok KS, James JW. Strategy for
Control of Water and Gas Cresting in a Thin Oil
Column Using Horizontal Wells. SPE 39549.
SPE Indian Oil and Gas Conference and
Exhibition, New Delhi, India, 17-19 Feb, 1998.
Vo DT, Waryan S, Dharmawan A et al.
Lookback on Performance of 50 Horizontal
Wells Targeting Thin Oil Columns, Mahakam
Delta, East Kalimantan. Paper SPE 64385. SPE
Asia Pacific Oil and Gas Conference &
Exhibition, Brisbane, Australia, 16-19 Oct,
2000.
Welge HJ, Weber AG. Use of TwoDimensional Methods for Calculating Well
Coning Behavior. SPEJ Dec 1964; 4(4).
Widmyer RH. Producing Petroleum from
Underground Formations. U.S. Patent No.
2,855,047, Oct 3, 1955.
Wojtanowicz AK, Xu H. A New Method to
Minimize Oil well Production Water Cut Using
A Downhole Water Loop. CIM 92-13.
Proceedings of the 43rd Annual Technical
Meeting of the Petroleum Society of CIM,
Calgary, Canada, Jun 1992.
Onwukwe SI
16
APPENDIX
Nomenclature
Bo
Oil formation volume factor in rb/STB.
ho
Oil column thickness in ft.
hop
Oil zone perforation thickness in ft.
ko
Permeability of oil in md.
qoc
Oil critical flow rate.
re
Drainage radius in ft.
rw
Wellbore radius in ft.
µo
Viscosity of oil in cp.
∆ρ
Fluids densities difference in lb/ft3.
J. Adv. Res. Petrol. Tech. Mgmt. 2015; 1(1): 8-16.