Evaluation of Microwave Methods for UKCS Heavy Oil Recovery

Nigel Brealey ( n.brealey@senergyltd.com ) of Senergy Ltd has been working on approaches to recovering heavy oil as part of the DTI's SHARP programme. He reports here on the potential for using microwave energy, but concludes that it is unlikely to make any significant difference to overall field decisions. However, microwave energy could give an attractive bonus in the North Sea and elsewhere if equipment were developed.

The Concept

The idea is primarily to boost production rates by heating oil and thus lowering viscosity in the vicinity of a producing well. The concept of using microwave energy is not in fact new. In [1] seven US patents are mentioned including one by an enterprising Mr Haagensen in 1965.

Microwaves are waves of energy, not heat. The amount of microwave energy absorbed by material depends on various factors including the dielectric and thermal properties. Depending upon the material, microwaves may be reflected, pass through, or be absorbed. There is common familiarity with "microwave transparent" materials such as glass, pottery, paper and plastics where the waves pass through unimpeded. Water, however, absorbs microwaves due to the dipole nature of the molecules which are set spinning by passing microwave energy.

The microwave heating mechanism is used in domestic or commercial microwave ovens. These ovens use a frequency of 2.45 GHz. Microwaves are generally taken by scientists as electromagnetic waves with frequencies ranging from 0.3 to 300 GHz. Relevant literature on the use of this type of technology actually covers frequencies from about 13 MHz to 2 GHz. If the frequency is too low, absorption may not be effective. If it is too high, then absorption is too high and the penetration depth is too shallow. These frequencies do not correspond exactly with the normal meaning of microwaves, but the concepts are so similar to general experience, it is proposed to still refer to this application as "microwaves" but could also include radio frequencies.

Nonetheless, different terminology does complicate literature searches. Abernethy [2] published an excellent foundation analysis in 1976, but refers to electromagnetic heating and does not use the word "microwave".

Projected Impacts

Abernethy's analysis referred to prior Russian work, but there is not much known about it. Fanchi [3] and Ovalles [1] are two other major references which build on Abernethy's work.

Ovalles made predictions of improvement based on laboratory measurements of microwave adsorption in reservoir rock. He simulated the effects of this absorption using CMG's advanced process and thermal reservoir simulator STARS. The resulting benefits appear very attractive on the basis of the energy consumed against the increased production. Unfortunately, the base well production rates they used are much lower than would be of interest for North Sea conditions.

Abernethy's method for predicting reservoir temperatures was used to extrapolate behaviour to North Sea conditions. The method can be simplified for steady state conditions to:

T = T0 + const ₁ * P / Q * exp (-const ₂ * S)

Where:

P is the power applied

Q is the flowrate

S is the reservoir penetration

The temperature at the wellbore is where "S" is zero and thus "const₂" has no effect. Abernethy made the assumption that the only heat loss was from the produced oil. The produced oil has the same temperature as the formation at the wellbore. Relating the power applied, P, to the energy produced enables "const₁" to be calculated independent of formation absorption characteristics. A value was calculated based on reasonable oil parameters.

The formation absorption characteristics are reflected in "const₂". The equation then gives the attenuation of the temperature as a function of distance from the wellbore. The value of "const₂" is also be dependent on the microwave frequency and would need to be determined by laboratory measurement for a specific application. In the absence of laboratory data, a value of "const₂" was selected to give similar temperature attenuation as in the Abernethy and Ovalles papers. This is illustrated in Figure 1 along with a case example of viscosity impacts assuming 40 kW of energy supplied subsurface.

Figure 1: Viscosity Impacts Assuming 40 kW of Subsurface Energy (Click for larger view)

The impact on production can then be calculated using a spreadsheet to add pressure drops over radial segments, Figure 2.

Figure 2: Comparison of Pressure Drops for Cold and Heated Flow (At Lower Rates) (Click for larger view)

As can be seen, if the base cold production rate is 20 bpd, then the heated rate is almost tripled for the same drawdown. This is comparable to the Abernethy and Ovalles projections. These rates are, however, far too low for North Sea conditions. To obtain the same percentage increment of production for a base cold production rate of 2000 bpd would take 4000 kW of microwave energy, which appears to be totally impractical. However, 40kW of microwave energy appears feasible. Indeed units of about this size have been used in pilot trials [4, 5].

The analysis was thus extended to higher rates, giving the outcome shown in Figure 3.

Figure 3: Comparison of Pressure Drops for Cold and Heated Flow (At Higher Rates) (Click for larger view)

The incremental benefit is about 300 bpd, which is as expected much lower in percentage terms, but is substantially higher in actual barrels. If the opportunity were attractive in the Abernethy and Ovalles examples, then it appears that it should be even more attractive for higher rate wells.

In fact the energy efficiency corresponds to about 10,000 Btu per bopd rate enhancement, which would be excellent. ("Huff and Puff" processes typically use more than 200,000 Btu per bopd rate enhancement).

There are, however, a number of limitations:

• There would likely be very considerable power losses in generating and transmitting microwaves.
• There has been very little experience of comparable applications. Historical applications of rather simpler electromagnetic resistive heating have had significant practical problems, even if they are now considered more robust [6].
• The temperature attenuation will increase considerably when water production occurs, which could be quite early in the North Sea . This will decrease the production impact.
• The percentage increases in production are low and unlikely to significantly impact overall development decisions given the apparent limits on microwave power application.

Nonetheless, there could be a useful bonus on production rates if the equipment were developed. Indeed, it is understood that designs exist for 1 megawatt transmitters for the LF-MF frequency range that were to be employed for downhole high power antenna systems in western oil shales [7]. Such application could extend outside the North Sea given the huge untapped resources of heavy oil worldwide.

References

1. "Opportunities of Downhole Dielectric Heating in Venezuela : Three Case Studies Involving Medium, Heavy and Extra-Heavy Crude Oil Reservoirs" SPE 78980. Ovalles et al. 2002
2. "Production Increase of Heavy Oils by Electromagnetic Heating" J. Cdn Pet. Tech 1976. E.R.Abernethy. This paper can be down loaded from the Petroleum Society website at http://www.petsoc.org/tbonline.html
3. "Feasibility of Reservoir Heating by Electromagnetic Irradiation" SPE 20483. 1990 Fanchi J.R
4. "Pilot Testing of a Radio Frequency Heating System for Enhanced Oil Recovery from Diatomaceous Earth" SPE 28619. Kasevich et al. 1994
5. "Recovery of Bitumen from Tar Sand with the Radio Frquency Process." SPE 10229. Sresty et al. 1986
6. "In Situ Electromagnetic Heating for Hydrocarbon Recovery and Environmental Remediation". Fred Vermeulen and Bruce McGee. Journal of Canadian Petroleum Technology. August 2000.
7. Personal communication. Ray Kasevich ( raykase@taconic.net ) of Kay Technology. He was the principal author of [4].

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