Field Pilot Test of Thermal Stimulation of Rubble Reservoir
Field Pilot Test of Thermal Stimulation of Rubble Reservoir
Using Down Hole Induction Heaters
V.C.Babu Sivakumar, Bahrain Petroleum Company
Abstract
Production of the viscous heavy oil from the shallow Rubble
reservoir (about 1500-1600’) in the Bahrain field has been a
challenge given its low gravity (12-18º API) and high viscosity
(350 cps at 40º C). It was planned to carry out a trial test of
thermally stimulating the formation to improve the production
rates from this reservoir using a downhole heater. Since the
greatest pressure drop in a producing well is always near the
well bore application of heat at the well bore to reduce the oil
viscosity can result in dramatic increases in oil production.
The trial test showed an increase in oil production from < 10
bopd to about 70 bopd initially and later stabilized at about 40
bopd. Even a modest increase in downhole temperature was
seen to have a significant effect on both the water cuts and
production rates.
Of the various types of downhole heaters available it was
chosen to install an induction type of heater in view of the
various inherent advantages. Primarily, these heaters could be
installed in wells with no special completion requirements.
The Inductor tool assembly utilizes the electrical energy to
inductively heat the reservoir through the Ferro-magnetic
casing. Efficient three-phase power delivery through an ESP
cable using high voltage and low current minimizes resistive
power losses and as much as 80-85% of the delivered power
can be converted into heat.
The paper deals with the trial installation, operation problems
encountered and the production performance of the well on
trial. Based on the success of the pilot additional installations
are being planned for trials in two more wells with heavier and
more viscous oil quality.
Background
The “Rubble Limestone” is a name given to the massive
limestone unit of the Mishrif formation, middle Cretaceous in
age. It is overlain by the Blue Shale member of the Aruma
formation and underlain by the Ostracod member of Rumaila,
a formation in the upper Wasia group. Because of the Wasia-
Aruma unconformity the Rubble has been truncated from the
crestal area of the Bahrain anticline and is present only in the
flank position. Low on the flank where there has been little
erosion the Rubble can be subdivided in two units – Upper and
Lower Rubble. The porosity from core data ranges from 24%
in upper Rubble to 19% in lower Rubble. Most production
tests have indicated that the Rubble crude is heavy (12-18º
API) and viscous (estimated in-situ viscosity of 85 –150 cp).
Core data indicates a permeability of less than 10 md.
However production tests show the effective permeability to
be higher. The Bahrain Review Team (BRT)1, in its study
indicated oil in place of about 1 billion STB (880 million in
lower Rubble and 130 million in upper Rubble). However,
subsequent revisions and recent reservoir estimates (1999)
place the recoverable reserve figure at about 100 million STB.
In view of the high reservoir oil viscosity the primary
recovery in the Rubble zone is expected to be low. Initial
laboratory tests with the Rubble crude have shown favorable
decreases in viscosity with increased temperature. The low
gravity combined with high viscosity suggested that thermal oil
methods would be effective for the Rubble zone oil recovery.
A study was commissioned to design a suitable thermal
displacement process for the zone. The thermal method
recommended for the field was a Combined Thermal Drive. It
was proposed to inject a combination of water-air to advance
the hot flood front (in-situ combustion) through the oil
reservoir in a 5 spot pattern with a well spacing of 2.5 acres. It
was suggested that as the injected salt water-air mixture
reaches the 1,000°F-burning front, steam would be generated
and the steam zone, which would be at a temperature of 400-
500°F, will increase the displacement rate.
Injection of 1,000 MSCFD of air per 5-spot pattern (total
of 4 patterns) was recommended. The total HP required for the
project works out to 1,200 HP. The CTD process was designed
to operate at a “Water to Air” ratio of about 200 bbls/MMSCF.
The produced water can be used for the injection with the only
requirement being filtration. Special Units would however be
required to ignite and generate high temperature reservoir
combustion.
The costs of production are usually higher because of the
problems associated with higher well bore velocities due to
higher GORs, higher well bore temperatures as the front
reaches the producing wells, acid concentrates in the produced
water etc. All of the above problems dictate the necessity of a
more elaborate planning and field pilot trials before putting in
elaborate thermal displacement and production handling
facilities in place.
General estimate shows the operating cost to be roughly
three times that of a normal well including emulsion treatment,
water disposal, gas metering etc.
One well, Well 374, was drilled in the Rubble Zone as the
candidate steam injection pilot well. On evaluation it was seen
that with the very poor injectivity the well couldn’t be used for
steam injection. Lack of injectivity to steam as expected, was
the most serious obstacle to proceeding with a steam flood
pilot. The project was thereafter put off until further studies
could be undertaken and the thermal mechanism for the
recovery from this zone firmed up. It was concluded that a
dowwnhole electric heating system, which would require much
lower capital and operating expenses would be much more cost
effective, for a pilot test, than the cyclic steam injection which
would require substantial rotating surface equipment such as
steam generators, sweet water source including treatment
facilities for boiler feed, pumps, tankage, pipelines etc.
Introduction
It was thereafter decided to test the response of the
formation to stimulation with downhole heating and the
consequent increases in well productivities. Such a trial before
embarking on the major investment would help in: a)
establishing a suitable mode of recovery for the viscous and
heavy oils prior to the thermal flood, b) pre-heating the
formation and developing steam injectivity in the formation
and c) thermal stimulation of reservoirs affected by emulsion
blockage, asphaltenes, paraffin etc.
The pilot project would study the response of Rubble crude
to in-situ heat application and thus the feasibility of the
implementation of thermal displacement methods for the
recovery of viscous oil from the Rubble zone. Installation of a
suitable down-hole heater was planned to enable the evaluation
of response as suggested above by heating the area in the
proximity of the well bore. This pilot study would form the
basis for proceeding to the next step of carrying out a pilot
steam flood or a CTD, as suggested, for recovering the Rubble
Zone Oil reserves.
A survey was thereafter initiated to establish a suitable
system for installation in our candidate Rubble well no. 480. It
was found that many of the existing downhole heaters such as
the electrical heating systems, electromagnetic heating systems
and the RF (radio frequency) heating systems etc. required
special completions such as fiber glass tubing's and were
therefore not suitable for our application.
Triflux Heating System
Finally, we identified the Madis system4, which is based on
induction heating using high voltage three phase AC power
delivery to a downhole transformer that converts the power to
a low frequency high current power. The proposed system can
be run without any special completion requirement. The down
hole temperature can be regulated and controlled from the
surface through the Power Conditioning Unit. The system
consists of an Induction tool assembly comprising of three
sections run in hole in tandem with the production tubing
system and placed opposite to the formation. The induction
tool assembly is powered through a three phase armoured ESP
cable run along with it strapped to the tubing assembly. The
Inductor tool assembly utilizes the electrical energy to
inductively heat the reservoir through the Ferro-magnetic
casing. Efficient three-phase power delivery through an ESP
cable using high voltage and low current minimizes resistive
power losses and as much as 60% of the delivered power can
be converted into heat. The three sections of the induction tool
assembly can be heated to varying temperatures as desired
with the help of the power-conditioning unit.
The proposed heating system, electromagnetic induction
heating, is intended to stimulate the productivity of the oil well
by improving the fluid flow conditions in the near well bore
region of the well to a radial distance of about 3 to 4 meters.
This heating is expected to be very effective in stimulating
reservoirs where the limitation to the well productivity is
expected to be in this region. This is frequently the case in
high viscosity wells such as the Rubble zone wells. The near
well bore heating acts in the following ways to improve
productivity:
- The viscosity of oil is reduced markedly in the heated
zone thereby reducing the well bore pressure drop, which is a
likely limit on the productivity.
- Viscous oil wells typically exhibit a substantially higher
liquid phase viscosity in the near well bore region during the
production caused by the emergence of light hydrocarbons
from the liquid phase as the pressure is reduced from the
reservoir pressure to the well bore pressure. Heating of the
well bore eliminates this dynamic skin effect.
- Permanent reservoir skin damage sometimes exists
because of the effects of drilling fluids on the reservoir face
and because of the deposition of asphaltenes and paraffin that
occur during the production process. Near well bore heating
can remove this skin altogether or, at least, mitigate its effect
on the productivity.
- While the heated zone in `an electrically stimulated well
is not radially as large as the steam injection, the heat pervades
over the entire formation while steam will only flood through
the most permeable portions. Moreover with the poor
permeability of the Rubble zone it is suspected that sufficient
injectivity cannot be established in the zone to carry out a
steam flood project successfully.
- Since water viscosity is not changed appreciably as
compared to oil viscosity with increased temperature, the
relative permeability of the formation to oil increases and
would result in increased oil flow and a reduction of water
cuts.
Trial Installation Results
The Madis Downhole Induction heater was first installed in
the trial well 480 in March 1998. The downhole heating
system was commissioned and a production increase to about
70 bopd from about 10 bopd was soon seen. It was established
that turning off the heat would very soon cause drop in
production, which would be reinstated by turning on the
heaters. Apart from the increase in production the water cut in
the well also drastically dropped from 60% to about 30% soon
after the heater was started.
The system converts high voltage low current supply to
induction energy at the heating zone, which facilitates the
delivery of a large amount of heat to the formation at relatively
low component temperatures. The Power Conditioning Unit
(PCU) consists of a wide range of output voltage and current
settings, which can be adapted to suit the power requirements
of the zone. Basically, the downhole inductors in combination
with the PCU is designed to induce electrical current in the
casing to cause heating due to the steels resistance and to take
advantage of the additional heat generated by eddy currents
and hysterisis. With the triple inductor system it was possible
to direct and preferentially heat the sections across each of the
inductors by hooking up the three inductors independently to
the three electrical phases. The PCU therefore controls
electric supply to the Triflux Induction tool assembly and
helps optimize the location and quantity of heat released.
Trials of preferentially heating the upper section of the
formation showed marginal but perceptible changes in water
cut. This phenomenon was established by setting the
temperatures of the three Inductors at different temperatures,
the lowermost being at the lowest and the uppermost at the
highest temperatures. Reversing the settings was clearly
accompanied by an increase in water cuts.
During the first phase of the trials after a period of 8
months the trial was stalled due to development of leakage in
the tubing and a workover had to be commissioned for the
replacement of the tubing and downhole pump. The workover
was performed in November 1999 and the downhole heater put
back after a few days production without applying heat. As in
the earlier experience, it was once again seen that soon after
the formation sees the heat the production was jacked up from
>10 bopd to about 70 bopd. Gradually the production
stabilized to about 40 bopd and the well presently continues to
produce at that rate.
Soon after the workover it was seen that the lowermost
inductor failed to work. The cause, it was felt was due to the
failure of the cable jointing. However, with the upper two
inductors in working order it was decided to continue with the
pilot without pulling out the assembly. Thereafter it was
decided to just heat the upper part of the formation and the
heat applied with just the uppermost inductor. The element
has thereafter continued to work without any further problems.
In continuation of the trial it is now planned to install
similar induction type heater in two more wells with a poorer
quality oil and study the effect of heat on well bore
stimulation. Thereafter, it will be planned to proceed with a
full-fledged steam injection project for the recovery from the
Rubble Zone.
Conclusions
1. The trial has established that the Rubble Zone is
responsive to the application of heat.
2. The use of the Triflux Induction type downhole
heaters helped in ascertaining the response of sections
of the zone to stimulation with heat. It has been
observed that water cut has been substantially reduced
on application of heat preferentially to the upper
sections of the zone only.
3. Although the Induction heater as such has been seen to
have a fairly rugged construction, failures have been
caused by failures of tubing due to corrosion and cable
jointing.
4. Response of portions of the Rubble zone with heavier
and more viscous oil to stimulation by downhole
heating needs to be established before proceeding with
a full-fledged steam injection project.
5. The pilot has established the successful well bore
stimulation of the Rubble reservoir, however the result
cannot be taken as representative for the Rubble zone
until further thermal displacement trials are concluded.
Acknowledgments
I thank the authorities of The Bahrain PetroleumCompany
for permission to publish this paper. In particular I would like
to thank Mr.Faisal Al Mahroos, the Manager Petroleum
Engineering for all the help, advice and motivation during the
conduction of the pilot project and the preparation of this
paper. The contribution of Mr.Bob Isted of Madis
Engineering and Mr.Homer Spencer of Tesla Inc. towards the
project and this paper also requires special mention.