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.