RU2477788C1 - Method for underground gasification - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: method includes well drilling from ground surface into treated interval of formation, electrodes allocation in wells, voltage supply to electrodes, electric current supply and formation heating. In order to provide maximum output of heat generation in the formation the current frequency is chosen depending upon maximum of angle tangent of dielectric losses of the rock and electric current of chosen frequency is fed through the formation. Heating is done due to dielectric and resistant losses in the formation. In the course of formation heating there performed is a gradual changing of electric current frequency according to the change of maximum of angle tangent of dielectric losses of the rock. Note that wells for electrodes location are drilled by grid-intersection coordinates in points of square network, and gas draw-off wells in the centre of network squares.

EFFECT: simplification of gasification method, reduction of economic expenses in the course of preparation for gasification, increase of formation heating speed.

2 cl, 3 dwg

Description

The invention relates to mining, in particular, to methods for underground gasification of solid fossil fuels and can be used to obtain a gaseous energy carrier (combustible gas) from coal or shale at the site of occurrence.

A known method of underground gasification, including drilling wells, their failure, ignition, supply of blast and removal of productive gas [RF Patent No. 2385412, IPC ЕВВ 43/295, publ. 03/27/2010].

The disadvantage of this method is the low energy intensity (calorific value) of the produced commercial gas due to the presence in it of a large amount of ballast gas resulting from the burning of part of the organic mass in the chamber of the underground gas generator.

A known method of processing a subterranean formation containing a solid organic substance selected as a prototype, comprising providing at least one well extending into the treatment interval in the subterranean formation, creating at least one fracture from at least one well, which crosses at least one well, the placement of the electrically conductive material in the gap, the contact of the two electrodes with the electrically conductive material, the application of voltage to the two electrodes to pass e rupture of an electric current so that an electric current passes through at least a portion of the electrically conductive material and sufficient heat is generated by the electrical resistivity of the electrically conductive material to pyrolyze at least a portion of the solid organic matter into recoverable hydrocarbons [RF Patent No. 2349745, IPC Е21В 43/24, publ. 03/20/2009].

The disadvantage of the prototype is the generation of heat sufficient for the pyrolysis of the specific electrical resistance of the electrically conductive material, which is associated with a large number of preparatory work, consisting in creating a fracture and placing the electrically conductive material in the gap.

The objective of the invention is the creation of a method of underground gasification, which reduces the amount of preparatory work due to heat generation by dielectric and resistive losses in the reservoir.

The technical result achieved by using the invention is to simplify the method of gasification, reduce economic costs in the preparation of gasification, increase the heating rate of the formation.

The problem is solved due to the fact that the method of underground gasification, as well as in the prototype, includes drilling wells from the surface of the earth passing in the processed interval in the underground formation, placing electrodes in the wells, applying voltage to the electrodes, transmitting current, heating the formation, tap gas through gas wells. According to the invention, the method of underground gasification includes selecting a current frequency based on the maximum tangent of the rock loss angle, passing a current of the selected frequency through the formation, heating the formation due to resistive and dielectric losses in the formation, smoothly changing the frequency of the electric current in accordance with the change in the maximum of the tangent of the dielectric loss angle breed.

To reduce the loss of thermal energy due to dissipation into the surrounding space, it is advisable to drill wells to place electrodes along grid coordinates at the nodes of the square grid, and wells to collect gas in the center of the grid squares.

The invention is illustrated by illustrations, in which Fig. 1 shows a functional diagram of the implementation of the underground gasification method, Fig. 2 shows a typical dependence of the loss tangent on the frequency of the applied voltage, and Fig. 3 shows the layout of the wells.

The method of underground gasification involves drilling wells 1 from the surface of the soil, passing into the treated interval in the underground formation 2 of solid fossil fuels and placing electrodes 3 inside them, connected by high-quality cables to a ground AC source 4 (Fig. 1). To reduce energy loss due to scattering into the surrounding space, the electrodes 3 are placed along the grid coordinates in such a way as to ensure maximum density of the electromagnetic field (Fig.3). In this case, the electrodes 3 are placed in the wells 1 for the electrodes. The withdrawal of commercial gas is carried out through gas wells 5.

The method of underground gasification is as follows.

Gasification is carried out by heating the formation to a gas temperature of solid fuel (300-500 ° C). Heating is carried out by passing a high-frequency current from a ground source through the reservoir due to dielectric and resistive losses. The power allocated in this case in the formation should be sufficient to create and maintain the required temperature. When the formation is heated above the gas evolution temperature, combustible gases are discharged through gas wells 5.

The power of heat in the reservoir is determined by the formula:

;

;

σ is the specific conductivity of the shale, Ohm · m;

E _cf - field strength, V / m;

k is a numerical coefficient;

ε is the relative dielectric constant of the medium;

ε ₀ is the dielectric constant of vacuum;

f is the frequency, Hz;

tanδ is the dielectric loss tangent;

S is the distance between the electrodes, m

T is the temperature;

t is time.

To increase heat generation in the formation, it is necessary to select the current frequency at which the dielectric loss power will be maximum. A typical dependence of the loss tangent on the frequency has a relaxation maximum ω _m , the frequency of which depends on the type of rock (figure 2). In order to increase the dielectric loss power in the formation, the generator operating frequency is chosen equal to the frequency of the maximum dielectric loss tangent for a gasified rock.

With increasing temperature, the rock characteristics change, as a result of which the frequency of the maximum tangent of the dielectric loss angle changes. The frequency of the transmitted current should be changed in such a way as to correspond to the frequency of the maximum dielectric loss at a given temperature.

The power allocated in the formation will be spent on heating the formation and heat removal by thermal conductivity, as a result of which part of the energy will be dissipated into the surrounding space. To reduce the energy loss due to dissipation into the surrounding space, it is more advantageous to use the electrode system, placing the electrodes 3 along the grid coordinates. In this case, the wells 1 for placement of the electrodes 3 are drilled in the nodes of a square grid located at a distance S from each other, and gas wells 5 in the center of the grid squares (Fig. 3).

The distance between the electrodes S affects the power of heat generation in the formation and, as a consequence, the time the formation is heated to the temperature of gas evolution. The distance S should be as large as possible, but its increase also increases the heating time. Thus, the distance S is chosen experimentally based on the optimal time of heating the formation.

Example.

The method was tested in an experimental installation with a fragment of a reservoir of oil shale, which ensures the adequacy of the experiment to actual conditions. Four electrodes were used, located at the nodes of the square with a side S = 0.4 m. A generator with an output voltage of 10 kV was connected to the electrodes. The generator operating frequency at the initial stage was 70 kHz as the frequency corresponding to the maximum dielectric loss for a given rock at a given temperature. In 2 hours, the temperature in the interelectrode space of the sample reached 300 ° C. During heating, the frequency of the generator was maintained corresponding to the maximum dielectric loss and by the end of the experiment changed to 60 kHz. The results obtained allow us to conclude that the method is industrially applicable in real deposits.

The method provides the maximum power of heat in the treated formation due to heat generation by dielectric and resistive losses in the formation and allows to reduce the number of preparatory work during gasification and increase the heating rate of the formation.

Claims (2)

1. The method of underground gasification, including drilling wells from the surface of the earth in the processing interval in the underground formation, placing electrodes in the wells, applying voltage to the electrodes, transmitting electric current, heating the formation and removing gases through gas wells, characterized in that for maximum power heat generation in the formation, the current frequency is selected based on the maximum of the tangent of the dielectric loss angle of the rock, an electric current of the selected frequency is transmitted through the formation and heating occurs due to dielectric and resistive losses in the formation, then during the heating of the formation, a smooth change in the frequency of the electric current is made in accordance with a change in the maximum tangent of the dielectric loss angle of the rock. 2. The method according to claim 1, characterized in that the wells for placement of the electrodes are drilled along the grid coordinates in the nodes of the square grid, and gas wells in the center of the grid squares.

Images