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Electrical Heating

Present By:Saeid Javidi

Course Instructor:Dr.B.Sedaee Sola

Autumn 2015

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Introduction

Electrical Heating for heavy-oil recovery is not a new idea but the commercialization and wider application of this technique require detailed analyses to determine optimal application conditions.

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Introduction

• Electrical Heating is a thermal process which can be applied to a well to increase its productivity. The productivity increase is substantial and comes about because of the removal of thermal adaptable skin effects(Visco-Skin for Example) and the reduction of oil viscosity in the vicinity of the wellbore.• DHEH allows production enhancement and thus improvement of

recovery factor, with significantly lower investment costs when compared with those typically associated with the implementation of thermal technologies such as steam injection

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Conventional steam injection candidates such as steam injection and hot water injection are limited to relatively shallow, thick, permeable, and homogenous sands that are onshore.

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Figure 1 describes how the process works.

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The essential components of an electrical heating system are:• power supply,• power delivery system,• Electrode assembly,• ground return.

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This technology uses a three-phase system designed to provide a defined wattage according to different application and type of cables.The heat section is set downhole and is connected to the surface with a power cable. It generates heat to near wellbore region, decreasing viscosity and friction and consequently increasing oil mobility.

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Salient features of the process are:• It is a continuous, not a cycle process. Electrical Heating

occurs simultaneously with production of fluids.• Low frequency Power(not microwave frequency) is used.•All the downhole equipment can be contained within a single

wellbore.

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The variable frequency(2 to 60 Hz), power supply(Isted,1992), is capable of delivering up to 300 kW of power. The power delivery system may consist of tubing , cables or a combination of both. The electrode assembly consist of bare casing pipe with fiberglass electrical isolation joints attached to the ends.The length of electrode and location in the reservoir is a matter of engineering design.The current return or ground can be the casing string above the fiberglass insulation.

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Current leaves the power supply and is conducted down the power delivery system to the electrode assembly. The electrode is in electrical contact with reservoir formation. From the electrode, the current is forced to flow through the reservoir and return to the power supply up the casing.The electrical path in the reservoir is primarily electrolytic because the conducting path is through the connate water in the reservoir. The connate water is heated by electrical losses and the remaining fluids and rock are heated by thermal conduction.The heated radius, the distance at which the oil viscosity is much reduced, can be three to seven meters(Vermeulen,1988)

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The amount of power to stimulate the well effectively is governed by the production rate as cooler fluids flow from the reservoir towards the well as the hot fluids are produced.Too much power can result in excessive temperatures and can damage the electrode assembly.The use of reservoir simulation to define operating power for a particular flow rate is therefore critically important.

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Visco-Skin

The visco-skin is a zone of high oil viscosity that develops in the low pressure region near the wellbore. It occurs in most naturally producing oil wells, but is especially prevalent in saturated heavy oils of 10 to 24° API gravity(McGee, 1989). Visco-Skin can best described by reference to figure 2.

13Figure 2

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Radially approaching the wellbore from the reservoir, the pressure decreases rapidly to the producing pressure. As the pressure drops, more and more gas evolves from the oil into the gaseous phase.A result of gas evolving from the oil is a viscosity profile like that shown in Figure 2. The oil viscosity reaches the maximum at the wellbore and decreases rapidly to the original oil viscosity in the Reservoir.The region of high oil viscosity usually extends only 1 to 2 meters into the reservoir.

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As a result, flow is impaired. The magnitude of the productivity decrease(Visco-Skin) depends on the ratio of oil viscosity at the wellbore to live oil viscosity,(viscosity parameter Pμ).In heavy oil reservoirs, Pμ is typically grater than 10 and productivity decrease caused by Visco-Skin is typically two or three times(McGee,1991).

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Field Case Comparison

The well was drilled into the sparky formation in the Frog Lake area and completed for electrical heating in June 1988. The oil there is heavy and oil can be produced under primary conditions. Figure 3(5) Shows the production history of the well. Peak production was 7.1 and declined to 3.0 before electrical heating. The well produced for 153 operating days and then was electrically stimulated immediately the production rate increased to over 12. The input power during stimulation averaged 15kW.

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Figure 3

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Field Case Comparison

The development and subsequent removal of the visco-skin in the near wellbore region is one explanation to account for the oil production during primary production and the rapid increase in production after a short period of thermal stimulation.

20Figure 4

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The extent of the visco-skin during primary production had to be calculated. The simulator was set to compositional mode and the oil viscosity distribution after 153 days was calculated and is shown in figure(4). As shown in the figure, the region of high oil viscosity is within one meter of the wellbore. The viscosity parameter is to be about ten, which is based on experimental work of Beal(Beal,1946).At the onset of electrical heating more than a threefold increase in production was observed in the field. This was achieved at initial power rates of less than 5kW.When the electrical heating option was turned on in the simulator, a production response match was attained. The simulator verified the removal of visco-skin as a mechanism of production stimulation.

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It is important to estimate the operating temperature of the electrode during electrical heating since the downhole equipment may fail at temperatures above 100°C.Figure(5) shows that the calculated temperature distribution in the reservoir around the electrode. These calculations are based on oil flow of 10 and input power of 30kW.

23Figure 5

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Since the flowrate changes during the life of the well, a curve showing the input power necessary for an electrode temperature of 100°C for various flowrates is required. This curve is shown in Figure 8, and is referred to as the P-Q Curve (Power Flowrate). Operating the system above the line will result in peak temperatures greater than 100°C.

25Figure 6

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A reservoir simulator , Tetrad, has been modified to incorporate the electrical heating equations. The simulator includes treatment of the electrical conductivity as a function of temperature, salinity and saturation. The simulator was validated against analytical calculations and field data. It has been used to design several electrode completions and assist in developing operating strategies for field implementation of the electrical heating process.The simulator verified the existence of a visco-skin in the near wellbore region of a heavy oil well and the subsequent response of the well to electrical heating and removal of the visco-skin.

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Field Case Comparison

“Tetrad” is a commercial numerical reservoir simulator that can operate in four main modes;a) Black Oilb) Multicomponentc) Thermald) GeothermalIt is the simulator which was modified to incorporate electrical heating.

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The simulations showed that the DHEH cable will increase the temperature of the surrounding fluid and that the heat transferred will increased production as a function of reducing the viscosity and allowing the fluid to flow better.In addition to the improved flow conditions and as a result of the reduced viscosity, bottomhole heating has been reported to produce several benefits, such as less friction inside the production tubing above the pump. This allows the pump to work more efficiently, with lower backpressure.Case studies have demonstrated that formation heating stimulates the mobility of oil by the thermal expansion experienced by gaseous phase of crude. The heated oil liberates dissolved gases in the solution. This process forms a layer of gas that, when heated, will expand, pushing the fluid upward. Likewise, water, when present in a limited amount in the reservoir, will be converted to steam, which in turns expands and increases bottomhole pressure, also acting as a pushing agent.

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It is important to note that flow rates have a significant impact on the temperature obtained. Oil that is static will absorb thermal energy, as opposed to oil that is moving away from the heat source. As a result, the higher the flowrate, the lower the amount of heat that is absorbed, and thus there is a smaller impact on production.The balance between heat input and oil produced is delicate.

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Conclusions

The calculations also indicate the electrical heating process can substantially increase production from a well. Because of the small heated radius, the process is more a wellbore stimulation process than a reservoir heating scheme. However, since the visco-skin is tightly bound to the wellbore, the process is effective in increasing productivity.

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References1. K.VINSOME, B.C.W.McGEE, F.E.VERMEULEN, F.S.CHUTE, Electrical

Heating, PETSOC-94-04-042. Downhole Electrical Heating for Enhanced Heavy-Oil Recovery, SPE-

0314-0132-JPT

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Thanks For Your Attention