Method for heating oil shale subsurface in-situ

Abstract

A method for heating oil shale underground in situ. Shale oil and fuel gas can be obtained from an underground oil shale seam in situ, and the fuel gas can also be obtained from an underground coal seam in situ. Wells are drilled downwardly reaching an operation region of an underground oil shale ore bed. Electricity for partial discharge of the ore bed is conducted into electrodes, and a plasma channel is formed in the ore bed and subjected to breakdown by the electricity; after the resistance of each of two electrode regions is lowered, the two electrodes are used for conducting currents into the plasma channel in the oil shale ore bed; the oil shale ore bed is heated under the resistance heating function of the plasma channel; and released heat is used for realizing thermal cracking and gasification of fixed organic carbon in the oil shale ore bed.

Claims

1. A method for heating oil shale subsurface in-situ, which comprises: drilling at least two wells downwardly from a ground surface until depths of the at least two wells reach working areas of an underground oil shale seam; putting an electrode in each of the at least two wells; firstly applying an alternating current voltage having a magnitude selected to cause a partial electric discharge to the electrodes, thus forming plasma channels resulting from an electrothermal breakdown in the oil shale seam; after the plasma channels from the electrothermal breakdown are formed, conducting electrical current into the plasma channels in the oil shale seam through the electrodes in two of the at least two wells, after resistance between the electrodes in the two of the at least two wells is reduced; wherein the conducting comprises applying a direct current or an alternating current between the electrodes in the two of the at least two wells having a current magnitude selected to cause resistance heating of the oil shale seam through a resistance heating function of the plasma channels; and realizing pyrolysis and gasification of organic carbon in the oil shale seam by the heating.

2. The method for heating oil shale subsurface in-situ as claimed in claim 1, characterized in that the applying an alternating current voltage comprises applying a 1-10 kV alternating current voltage per meter of distance between the electrodes to lead to the partial electric discharge; and the heating, after the plasma channels from the electrothermal breakdown are formed, comprises applying a voltage in a range of 10-100 V per meter of distance between the electrodes and current in a range of 10-100 A.

3. A method for heating a coal mine seam subsurface in-situ, which comprises: drilling at least two wells downwardly from a ground surface until depths of the at least two wells reach working areas of an underground coal mine seam; putting an electrode in each of the at least two wells; firstly applying an alternating current voltage having a magnitude selected to cause a partial electric discharge to the electrodes, thus forming plasma channels resulting from an electrothermal breakdown in the coal mine seam; after the plasma channels from the electrothermal breakdown are formed, conducting electrical current into the plasma channels in the coal mine seam through the electrodes in two of the at least two wells, after a resistance between the electrodes in the two of the at least two wells is reduced; wherein the conducting comprises applying a direct current or an alternating current between the electrodes in the two of the at least two wells having a current magnitude selected to cause resistance heating of the coal mine seam through a resistance heating function of the plasma channels; and realizing pyrolysis and gasification of organic carbon in the coal mine seam by the heating.

4. The method for heating a coal mine seam subsurface in-situ as claimed in claim 3, characterized in that the applying an alternating current voltage comprises applying a 1-10 kV alternating current voltage per meter of distance between the electrodes to lead to the partial electric discharge; and the heating, after the plasma channels from the electrothermal breakdown are formed, comprises applying a voltage in a range of 10-100 V per meter of distance between the electrodes and current in a range of 10-100 A.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic diagram of the present invention.

MODE OF CARRYING OUT THE INVENTION

(2) Referring to FIG. 1, the method of the present invention comprises the following: drilling two wells 1 downwardly from the ground surface until the depth of the wells reaches the working areas of the underground oil shale seam 2; putting electrodes 3 in the wells; connecting the electrodes 3 to a power supply 5 on the ground via cables 4; firstly applying a high voltage that is enough to lead to the partial electric discharge to the electrodes 3, thus forming plasma channels 6 resulting from the breakdown of the high electricity in the oil shale seam 2; conducting electrical current, through the two electrodes 3, into the plasma channels 6 in the oil shale seam 2, after the resistance between the two electrode 3 areas is reduced; heating the oil shale seam 2 through the resistance heating function of the plasma channels 6; and realizing pyrolysis and gasification of given organic carbon in the oil shale seam 2 by using the heat released.

(3) Here are the principles of the present invention.

(4) The given organic carbon has a very great resistance, i.e., 10.sup.8-10.sup.12 ohm/cm, and thus under the conventional situation, the resistance heat in the rock is very weak. High voltage alternating current is conducted between the electrodes 3, and heating is made via dielectric loss which gives rise to the partial electric discharge. An electrically-conductive region is formed between the discharge working sections, and the electrically-conductive region will be further extended and enlarged via the next discharge effect, so as to finally form a dendritic discharge structure, extending from one electrode to the other electrode in a dendritic structure. That is, the plasma channels resulting from the electrothermal breakdown are formed. During this phrase, it is necessary to apply the relatively high voltage to the electrodes so as to effect the partial electric discharge. The specific magnitude of the voltage depends on the distance between the electrodes and the type and structure of the rocks, and can be determined by means of experiments on the rock sample. When carrying out the experiments, the partial electric discharge can be observed by naked eyes or through the change in the electrical current on the ondoscope. Formation of the plasma channels resulting from the electrothermal breakdown can be determined via the decrease in the resistance between the electrodes. The voltage is approximately 1-10 KV/m, i.e., application of the voltage of 1-10 KV for every interval of one meter. The frequency of the current has no great effect on formation of the dendritic plasma channels resulting from the electrothermal breakdown, and the alternating current with industrial-frequency can be therefore used. After the plasma channels resulting from the electrothermal breakdown are formed, the linear resistance between the electrode regions will be decreased up to 10-100 ohm/cm. Formation of the plasma channels resulting from the electrothermal breakdown can be determined by monitoring the voltage and current between the electrodes.

(5) After the plasma channels from the electrothermal breakdown are formed, the electrodes should be connected to a DC power of high current or AC power of high current. That is, after the plasma channels from the electrothermal breakdown are formed, the power supply of the electrodes is switched into the DC power of high current or AC power of high current, and heating is made by using the resistance effect of the plasma channels resulting from the electrothermal breakdown, and the power supply under the heating mode has a voltage of 10-100V/m and current of 10-100 A.

(6) The oil shale seam may be replaced by coal mine seam, that is, the method of the present invention is applicable to heating coal mine seam subsurface in-situ.

Embodiment 1

(7) In the laboratory, the experiments are executed by using oil shale samples, and the distance between the electrodes is set to be 50 cm. Prior to the experiment, the resistance value between the electrodes is 250K ohm. When executing the experiment, alternating current having the frequency of 50 Hz and peak voltage of 5 KV is conducted into the electrodes. It can be found by visual observation that phenomena of the partial electric discharge occur at that voltage. The power consumption of the power supply is about 300 W. This process lasts for 30 minutes, and during the time period of 30 minutes, the plasma channels from the electrothermal breakdown are gradually formed. The resistance between the electrodes changes to 800 ohm. Subsequently, the current with the frequency of 50 Hz flows through the electrodes, and heating is made by using the resistance thermal effect of the low-resistance channels. At the beginning, the voltage is maintained at the level of several hectovolts, and its resistance is reduced to about 10 ohm with the continuous heating for the channels. At this moment, the voltage also is reduced to 100V in order to ensure the 1 KW power.

Embodiment 2

(8) In the laboratory, the experiments are executed by using lignite samples, and the distance between the electrodes is set to be 45 cm. Prior to the experiment, the resistance value between the electrodes is 150K ohm. When executing the experiment, alternating current having the frequency of 50 Hz and peak voltage of 8 KV is conducted into the electrodes. It can be found by visual observation that phenomena of the partial electric discharge occur at that voltage. The power consumption of the power supply is about 600 W. This process lasts for 15 minutes, and during the time period of 15 minutes, the plasma channels from the electrothermal breakdown are gradually formed. The resistance between the electrodes changes to 300 ohm. Subsequently, the current with the frequency of 50 Hz flows through the electrodes, and heating is made by using the resistance thermal effect of the low-resistance channels. At the beginning, the voltage is maintained at the level of several hectovolts, and its resistance is reduced to about 3-5 ohm with the continuous heating for the channels. At this moment, the voltage also is reduced to 60V in order to ensure the 1 KW power.

(9) It has been proved by the above experiments that the method of the present invention has the advantages of effectively reducing the construction quantities, no hydraulically fracturing the rock stratum, and avoiding use of the toxic conductive material.