CN115749707A - Method for exploiting super-heavy oil by using multi-branch horizontal well to assist SAGD (steam assisted gravity drainage) - Google Patents
Method for exploiting super-heavy oil by using multi-branch horizontal well to assist SAGD (steam assisted gravity drainage) Download PDFInfo
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Abstract
The invention belongs to the technical field of thickened oil exploitation, and particularly relates to a method for assisting SAGD (steam assisted gravity drainage) exploitation of super-thickened oil by a multi-branch horizontal well. The exploitation mode can greatly improve the oil exploitation speed, the oil recovery rate and the oil-gas ratio of the super heavy oil. Firstly, selecting a super heavy oil reservoir suitable for SAGD exploitation of a double-horizontal well, drilling a secondary branch horizontal well between two adjacent SAGD well groups, firstly performing reservoir transformation on the secondary branch horizontal well, and then injecting a solvent and soaking; meanwhile, after SAGD production is carried out for 2-3 years, a steam cavity develops to the top of an oil reservoir, and a secondary branch horizontal well is started in a huff-puff mode; after the thermal communication is established between the secondary branch horizontal well and the SAGD steam cavity, the secondary branch horizontal well is continuously produced; and finally, adjusting the injection and production parameters of the SAGD well group and the secondary branch horizontal well to form injection and production balance, and continuously producing until the production is finished. The method can greatly improve the oil extraction speed, the recovery ratio and the oil-gas ratio of the super heavy oil.
Description
Technical Field
The invention belongs to the technical field of thickened oil exploitation, and particularly relates to a method for exploiting super-thickened oil by using auxiliary SAGD of a multi-branch horizontal well.
Background
The main principle of the double-horizontal-well SAGD (steam assisted gravity drainage) is that 2 horizontal wells which are longitudinally parallel are arranged at the bottom of an oil layer, steam is continuously injected into the oil layer from an upper horizontal well to heat the oil layer and crude oil, a steam cavity is continuously expanded to exchange heat with the crude oil in the oil layer, the heated and viscosity-reduced crude oil and steam condensate water flow downwards under the action of gravity, and the crude oil and the steam condensate water are extracted from a horizontal production horizontal well at the lower part of the oil layer.
For example, chinese patent application CN104265253A discloses an SAGD exploitation method for heavy oil reservoirs, which comprises the steps of opening a first SAGD well group on the heavy oil reservoir, preheating the first SAGD well group, and injecting pure steam or a steam composition to form a steam cavity for production; when the steam cavity of the first SAGD well group rises to the top of the oil layer, a second SAGD well group is arranged; alternately injecting pure non-condensable gas and a slug of a steam composition into the first SAGD well group when the steam cavity of the first SAGD well group extends to a position in the middle of the well; injecting pure steam or a steam composition into a second SAGD well group to form a steam cavity for production when the steam cavity of the first SAGD well group is in a descent phase; when the production of the first SAGD well group is finished and the steam cavity of the second SAGD well group is expanded to the middle position between wells, pure non-condensable gas and steam composition slugs are injected into the second SAGD well group alternately to control the pressure in the steam cavity.
In addition, chinese patent application CN111364961A provides an extra heavy oil SAGD mining method, which includes the following steps: s10, arranging a steam injection well and a production well, wherein the steam injection well and the production well both extend to an ultra-heavy oil reservoir; s20, introducing steam into the super heavy oil reservoir through a steam injection well and/or a production well to form a steam cavity in the super heavy oil reservoir; step S30, determining the position of an interlayer in the super heavy oil reservoir, setting a tool lowering well, and extending the tool lowering well to the position of the interlayer; s40, lowering the jetting tool into the well to the position of the interlayer through the tool; and S50, spraying the sand-containing fluid to the interlayer through a spraying tool so as to form cracks in the interlayer and make the steam cavity break through the interlayer through the cracks.
Under the injection and production mode, crude oil is drained mainly under the action of gravity, the driving force of lateral expansion is weak, particularly for reservoirs with strong reservoir non-mean values, and a triangular cold oil retention area is easily formed between adjacent steam cavities in the later period of SAGD production and cannot be exploited. Meanwhile, the conventional SAGD production cycle is generally about 15 years, the oil yield in the later production period is low, the steam consumption is high, and the accumulated oil-gas ratio in the whole SAGD production process is low and the economy is poor.
The invention aims to provide an improved SAGD method for rapidly exploiting an ultra-heavy oil reservoir by combining two modes of a two-stage multi-branch horizontal well and an SAGD well, wherein the exploitation mode can use a triangular cold oil region between traditional SAGD well groups through an auxiliary branch well, solves the problems that the oil extraction speed is low and the triangular cold oil retention region between the well groups cannot be used in the prior art, and greatly improves the oil extraction speed, the oil recovery rate and the oil-gas ratio of the ultra-heavy oil.
Disclosure of Invention
In order to overcome the technical problems, the invention provides a method for exploiting ultra-thick oil by using multi-branch horizontal well assisted SAGD, and the exploitation mode can greatly improve the oil extraction speed, the oil recovery rate and the oil-gas ratio of the ultra-thick oil.
In order to achieve the above purpose, the technical scheme provided by the invention is as follows:
a method for assisting SAGD in exploiting super heavy oil by using a multi-branch horizontal well comprises the following steps:
(1) Selecting an ultra-heavy oil reservoir;
(2) Drilling a secondary branch horizontal well between two adjacent SAGD well groups;
(3) After reservoir transformation is carried out on the second-level branch horizontal well, a solvent is injected and soaked;
(4) After SAGD production is carried out for 2-3 years, the steam cavity is developed to the top of the oil reservoir, and the second-level branch horizontal well is started in a huff-puff mode;
(5) After the second-level branch horizontal well is in thermal communication with the SAGD steam cavity, the second-level branch horizontal well continues to produce;
(6) And adjusting injection and production parameters of the SAGD well group and the secondary branch horizontal well to form injection and production balance, and continuously producing until the production is finished.
Preferably, in the step (1), the continuous oil layer thickness of the super heavy oil reservoir is greater than 10m, the oil layer porosity is greater than 0.2, the horizontal permeability is greater than 500mD, the ratio of the vertical permeability to the horizontal permeability is greater than 0.5, the oil saturation is greater than 0.40, and continuous mudstone and shale interlayers are not developed in the oil layer.
Preferably, in the step (2), the SAGD well consists of two horizontal wells which are parallel up and down, the upper part is a steam injection horizontal well, the lower part is a production horizontal well, the SAGD well distance is 70-80m, the production horizontal well distance is 1-2m from the bottom of an oil layer, the distance between the steam injection horizontal well and the production horizontal well is 4-6m, the horizontal plane of the main well hole of the secondary branch horizontal well is positioned between the two SAGD wells, the main well hole position of the secondary branch horizontal well in the longitudinal direction is parallel to the SAGD production horizontal well, and the length of the horizontal section of the main well section of the horizontal well is the same as that of the SAGD well and is 400-600m.
Preferably, in the step (2), the secondary branch horizontal well is formed by communicating a plurality of primary steam injection well branches on the horizontal section of the auxiliary horizontal well, each primary branch is communicated with at least one secondary steam injection well branch, the primary steam injection well branches are located on two sides of a main well bore of the steam injection horizontal well and extend obliquely upward at an angle of 0-90 degrees with the main well bore, the length of the horizontal section of the main well bore is 400-600m, the length of each primary branch is 50-100m, the maximum offset distance of the branch section on the horizontal plane of the primary steam injection well branches from the main well bore is 15-25m, and the maximum offset distance of the branch section on the vertical direction from the main well bore is 3-8m.
Preferably, in the step (2), the branches of the secondary steam injection well and the branches of the primary steam injection well extend along the main well hole at an angle of 0-90 degrees, the length of each branch of the secondary steam injection well is 50-100m, the maximum offset distance between the branch of the secondary steam injection well and the branch section on the horizontal plane of the main well hole is 20m, the maximum vertical distance between the branch of the secondary steam injection well and the main well hole is 5m, and the branch of the secondary steam injection well and the branch section are parallel to the main well hole; the main well is completed by a sieve tube, and the first-stage steam injection well branch and the second-stage steam injection well branch are both completed by open holes.
Preferably, the second branch starts from the middle of the first branch, and the second branch is parallel to the main borehole and extends in the same direction as the main borehole.
Preferably, in the step (3), in the reservoir stratum reconstruction process, injecting hot sewage into the secondary branch horizontal well to control pressure parameters and injection amount and improve bottom hole pressure so as to achieve the purposes of increasing water saturation around the well and improving physical uniformity around the well, wherein the maximum bottom hole pressure-the minimum principal stress is less than or equal to 0.5MPa; the injection quantity is increased to further increase the bottom hole pressure, so that the range of the expansion area around the pitshaft of the steam injection well and the production well is enlarged, wherein the fracture pressure-bottom hole pressure is more than or equal to 0.5MPa.
Preferably, after the reservoir of the second-level branch horizontal well is modified in the step (3), the well is washed by hot water to clean mud in the well, and the mud is injected with a solvent and soaked.
Preferably, the solvent is injected in an amount comparable to the wellbore volume, preferably 10-15 square;
preferably, the solvent is xylene or a mixed solvent of xylene and diesel oil; in the mixed solvent of the xylene and the diesel, the mass ratio of the xylene to the diesel is 3-5, preferably 5.
Preferably, in the step (4), before the SAGD production, the steam injection horizontal well and the production horizontal well are circularly preheated, and after communication is established between the SAGD steam injection well and the production well, the SAGD production is switched to; and after 2-3 years of production, the SAGD steam cavity grows to the top of the oil layer, and the secondary branch horizontal well is used for huff and puff production.
Preferably, in the step (4), the secondary branch horizontal well is produced in a throughput mode, the dryness of injected steam is more than 80%, the steam injection speed is 240t/d-300t/d, the steam injection strength is 8t/m-20t/m, the steam injection pressure is less than the fracture pressure of the stratum, the extraction-injection ratio is 0.7-1.2, and the well stewing time is 2-7 days.
Preferably, in the step (5), after the second-stage branch horizontal well is in thermal communication with the SAGD steam cavity, when the viscosity of crude oil between the SAGD steam cavity and the second-stage branch horizontal well is reduced to be below 1000mPa.s, the second-stage branch horizontal well is stopped to be huffled and puff, and the continuous production is converted.
Preferably, in the step (6), the SAGD well group and the secondary branch horizontal well injection and production parameters are adjusted, pressure balance is kept, and the injection and production ratio is controlled until production is finished.
Preferably, the injection-production parameters comprise steam injection amount, liquid production amount of the second-level branch horizontal well, operation pressure and extraction-injection ratio;
preferably, the steam injection range of the SAGD steam injection well is 80-150t/d;
preferably, the SAGD production well produces fluids in the range of 70-140t/d;
preferably, the liquid production amount of the second-level branch horizontal well is 20-80t/d;
preferably, the pressure of the secondary branch horizontal well and the SAGD production well synchronously fluctuate, and the pressure difference is less than 0.5MPa.
Preferably, the injection ratio is 1.1-1.4.
Compared with the prior art, the invention has the technical advantages that:
(1) According to the invention, the main well bore of the secondary branch horizontal well is longitudinally parallel to the SAGD production horizontal well, and a triangular cold oil retention area between adjacent steam cavities of the SAGD well group can be effectively used, so that the recovery ratio is improved.
(2) According to the invention, the branches of the two-stage multi-branch horizontal well extend obliquely upwards to the main well hole, so that the thermal communication between the two-stage multi-branch horizontal well and the SAGD steam injection horizontal well can be accelerated, meanwhile, the influence of an interlayer between wells on oil drainage is reduced, and the thermal efficiency and the oil extraction speed are improved.
(3) According to the invention, the two-stage multi-branch horizontal well is subjected to reservoir transformation and is soaked by the solvent, so that the inter-well group communication time can be shortened, and the oil extraction speed can be increased.
(4) The top well mode has a higher extraction effect on the thick oil extraction, and compared with a conventional horizontal well, the two-level branch horizontal well is adopted, so that the problems of long communication time between the conventional horizontal well and an SAGD well and low early oil extraction speed are solved, and the super thick oil can be more effectively extracted.
Drawings
FIG. 1: a flow chart of auxiliary SAGD operation of a branched horizontal well;
FIG. 2 is a schematic diagram: the method comprises the following steps of (1) schematically representing the auxiliary SAGD well mode of a branched horizontal well;
FIG. 3: the method is characterized in that the mining mechanism of the auxiliary SAGD stable production stage of the branched horizontal well is schematic.
The invention will now be further described with reference to the accompanying drawings and examples:
Detailed Description
The present invention will be described below with reference to specific examples to make the technical aspects of the present invention easier to understand and grasp, but the present invention is not limited thereto. The experimental methods described in the following examples are all conventional methods unless otherwise specified; the reagents and materials are commercially available, unless otherwise specified.
Example 1
(1) Performing coarse screening of oil reservoir
The selected oil reservoir is positioned in a Z1 well region of a Xinjiang Fenghu oil field, the oil reservoir is a fractured and cut monoclinic block oil reservoir, the dip angle of a stratum is 3-8 degrees, a target layer Jurassic system ancient group is a braided river sedimentary reservoir lithology which mainly comprises medium sandstone and fine sandstone, the buried depth of the oil reservoir is 580m, the thickness of a continuous oil layer is 20m, the porosity of the oil layer is 0.31, the horizontal permeability is 1400mD, the ratio of the vertical permeability to the horizontal permeability is 0.7, the oil saturation is 0.72, mudstone and shale interlayers which are not continuously developed in the oil layer are formed, and the viscosity of degassed crude oil at 50 ℃ is 4.5 million centipoise.
(2) The SAGD well distance of the oil reservoir is 70m, the horizontal section length is 500m, the production horizontal well is 2m away from the bottom of an oil layer, the steam injection horizontal well is 5m away from the production horizontal well, a secondary branch horizontal well is drilled between the two well groups, the main well hole is 2m away from the bottom of the oil layer and is parallel to the SAGD production horizontal well, 4 branches are symmetrically arranged in a staggered mode in the fishbone well, the first branch enters a target point and is 100m away from the A point of the main well hole, then a branch is designed in a crossed mode at an interval of 100m, the length of each branch is 100m, the maximum offset distance of the main well hole of the branch section on the plane is 15m, and the maximum offset distance of the main well hole of the branch section on the vertical direction is 5m; the second-stage branch of the steam injection well starts from the middle part of the first-stage branch, is parallel to the main well bore and has the same extending direction with the main well bore, the length of each second-stage branch is 80m, the maximum offset distance between the second-stage branch and the branch section on the horizontal plane of the main well bore is 20m, the maximum distance between the second-stage branch and the main well bore in the vertical direction is 5m, and the second-stage branch is parallel to the main well bore.
(3) Performing reservoir transformation on the second-level branch horizontal well, injecting a mixed solvent of xylene and diesel oil in a proportion of 15 (equivalent to the volume of a well bore) according to a mass ratio of 5;
(4) When SAGD is put into production for 3 years, the secondary branch horizontal well is put into production in a handling mode, the dryness of injected steam is larger than 90%, the steam injection speed is 220t/d, 230t/d, 240t/d and 250t/d, the steam injection strength is 10t/m, 11t/m, 12t/m and 13t/m, the steam injection pressure is smaller than the fracture pressure of the stratum, the extraction-injection ratio is controlled within the range of 1.0-1.1, and the well stewing time is 4 days.
(5) After 4 times of handling, establishing thermal communication between the SAGD steam cavity and the secondary branch horizontal well, converting the SAGD steam injection horizontal well into continuous steam injection, and continuously producing the SAGD production horizontal well and the secondary branch horizontal well;
(6) And adjusting injection-production parameters, and controlling the overall injection-production ratio to be about 1.2 until the end.
After the oil is extracted by the method of the embodiment, compared with the conventional SAGD, the peak oil production is increased from 25t/d to 45t/d, the peak oil extraction speed is increased by 80%, the final recovery ratio is increased by 8.2%, and the oil-gas ratio is increased by 0.04.
Example 2
(1) Performing coarse screening of oil reservoir
The selected oil reservoir is located in a z18 well area of an oil field in Xinjiang Fengcheng, the oil reservoir is a monoclinic oil reservoir controlled by lithology and structure, the dip angle of a stratum is 3-5 degrees, a target layer Jurassic system ancient-flush group is a braided river sedimentary reservoir lithology, medium sandstone and fine sandstone are taken as main materials, the buried depth of the oil reservoir is 650m, the thickness of a continuous oil layer is 15m, the porosity of the oil layer is 0.27, the horizontal permeability is 850mD, the ratio of the vertical permeability to the horizontal permeability is 0.58, the oil saturation is 0.65, mudstone and shale interlayers which do not continuously develop in the oil layer, and the viscosity of degassed crude oil is 11.5 kilo-centipoise at the temperature of 50 ℃.
(2) The SAGD well distance of the oil reservoir is 80m, the horizontal section length is 450m, the production horizontal well is 1m from the bottom of the oil layer, the steam injection horizontal well is 5m from the production horizontal well, a secondary branch horizontal well is drilled between the two well groups, the main well hole is 1m from the bottom of the oil layer and is parallel to the SAGD production horizontal well, 4 branches are symmetrically arranged in a fish bone well in a staggered mode, the first branch enters a target point and is 80m from the A point of the main well hole, then a branch is designed in a crossed mode at an interval of 80m, the branch section length is 80m, the maximum offset distance of the branch section on the plane from the main well hole is 12m, and the maximum offset distance of the vertical upper branch section from the main well hole is 5m; the second-stage branch of the steam injection well starts from the middle part of the first-stage branch, is parallel to the main well bore and has the same extending direction with the main well bore, the length of each second-stage branch is 60m, the maximum offset distance between the second-stage branch and the branch section on the horizontal plane of the main well bore is 12m, the maximum distance between the second-stage branch and the main well bore in the vertical direction is 3m, and the second-stage branch is parallel to the main well bore.
(3) Performing reservoir transformation on the second-level branch horizontal well, injecting a mixed solvent of xylene and diesel oil in a proportion of 14 (equivalent to the volume of a well bore) according to a mass ratio of 5;
(4) And after the SAGD is put into production for 4 years, the secondary branch horizontal well is put into production with the injected steam dryness being more than 90 percent, the steam injection speeds being respectively 220t/d, 230t/d, 240t/d, 250t/d and 250t/d, the steam injection strengths being 10t/m, 11t/m, 12t/m, 13t/m and 13t/m, the steam injection pressure being less than the formation fracture pressure, the production-injection ratio being controlled within the range of 1.0-1.1, and the soaking time being 3 days.
(5) After 5 times of handling, establishing thermal communication between the SAGD steam cavity and the secondary branch horizontal well, converting the SAGD steam injection horizontal well into continuous steam injection, and continuously producing the SAGD production horizontal well and the secondary branch horizontal well;
(6) And adjusting injection-production parameters, and controlling the overall injection-production ratio to be about 1.2 until the end.
After the method of the embodiment is adopted for oil extraction, compared with the conventional SAGD, the peak oil production is increased to 31t/d from 18t/d, the peak oil extraction speed is increased by 70%, the final recovery ratio is increased by 6.8%, and the oil-gas ratio is increased by 0.02.
Example 3
(1) Performing coarse screening of oil reservoir
The selected oil reservoir is located in a z1 well region of an oil field in Xinjiang Fengcheng, the oil reservoir is a fractured and cut monoclinic block oil reservoir, the dip angle of a stratum is 3-8 degrees, a target layer Jurassic group is a braided river sedimentary reservoir lithology which mainly comprises medium sandstone and fine sandstone, the buried depth of the oil reservoir is 320m, the thickness of a continuous oil layer is 28m, the porosity of the oil layer is 0.32, the horizontal permeability is 2200mD, the ratio of the vertical permeability to the horizontal permeability is 0.7, the oil saturation is 0.75, mudstone and shale interlayers which do not continuously develop in the oil layer, and the viscosity of degassed crude oil is 2.5 million centipoises at the temperature of 50 ℃.
(2) The SAGD well distance of the oil reservoir is 70m, the horizontal section length is 400m, the production horizontal well is 2m away from the bottom of an oil layer, the distance between a steam injection horizontal well and a production horizontal well is 5m, a secondary branch horizontal well is drilled between two well groups, a main well hole is 2m away from the bottom of the oil layer and is parallel to the SAGD production horizontal well, three branches are symmetrically arranged in a staggered mode in a fishbone well, the first branch enters a target point and is 100m away from a main well hole point A, then a branch is designed in a crossed mode at an interval of 100m, the branch section length on the plane is 100m, the maximum offset distance of the branch section on the plane from the main well hole is 20m, and the maximum offset distance of the vertical upper branch section from the main well hole is 8m; the second-stage branch of the steam injection well starts from the middle part of the first-stage branch, is parallel to the main well bore and has the same extending direction with the main well bore, the length of each second-stage branch is 80m, the maximum offset distance between the second-stage branch and the branch section on the horizontal plane of the main well bore is 20m, the maximum distance between the second-stage branch and the main well bore in the vertical direction is 8m, and the second-stage branch is parallel to the main well bore.
(3) Performing reservoir transformation on the second-level branch horizontal well, injecting a mixed solvent of xylene and diesel oil 13 (equivalent to the volume of a shaft) according to the mass ratio of 5;
(4) And after 3 years of SAGD operation, the secondary branch horizontal well is put into operation with the injected steam dryness being more than 90%, the steam injection speed being 220t/d and 230t/d, the steam injection intensity being 10t/m and 11t/m, the steam injection pressure being less than the formation fracture pressure, the production-injection ratio being controlled within the range of 1.0-1.1, and the soaking time being 3 days.
(5) After 2 times of handling, establishing thermal communication between the SAGD steam cavity and the secondary branch horizontal well, converting the SAGD steam injection horizontal well into continuous steam injection, and continuously producing the SAGD production horizontal well and the secondary branch horizontal well;
(6) And adjusting injection-production parameters, and controlling the overall injection-production ratio to be about 1.3 until the end.
After the oil is extracted by the method of the embodiment, compared with the conventional SAGD, the peak oil production is increased from 35t/d to 65t/d, the peak oil extraction speed is increased by 85%, the final recovery ratio is increased by 9.8%, and the oil-gas ratio is increased by 0.06.
The above detailed description is directed to one of the possible embodiments of the present invention, which is not intended to limit the scope of the invention, but rather the scope of the invention is intended to include all equivalent implementations or modifications without departing from the scope of the invention.
Claims (12)
1. A method for assisting SAGD in exploiting super heavy oil by using a multi-branch horizontal well comprises the following steps:
(1) Selecting an ultra-heavy oil reservoir;
(2) Drilling a secondary branch horizontal well between two adjacent SAGD well groups;
(3) After reservoir transformation is carried out on the second-level branch horizontal well, a solvent is injected and soaked;
(4) After SAGD production is carried out for 2-3 years, the steam cavity is developed to the top of the oil reservoir, and the second-level branch horizontal well is started in a huff-puff mode;
(5) After the second-level branch horizontal well is in thermal communication with the SAGD steam cavity, the second-level branch horizontal well continues to produce;
(6) And adjusting the injection and production parameters of the SAGD well group and the secondary branch horizontal well to form injection and production balance, and continuously producing until the production is finished.
2. The method of claim 1, wherein in the step (1), the continuous oil layer thickness of the super heavy oil reservoir is greater than 10m, the oil layer porosity is greater than 0.2, the horizontal permeability is greater than 500mD, the ratio of the vertical permeability to the horizontal permeability is greater than 0.5, the oil saturation is greater than 0.40, and continuous shale interlayers are not developed in the oil layer.
3. The method of claim 1, wherein in the step (2), the SAGD well consists of two horizontal wells which are parallel up and down, the upper part is a steam injection horizontal well, the lower part is a production horizontal well, the SAGD well distance is 70-80m, the production horizontal well is 1-2m away from the bottom of an oil layer, the steam injection horizontal well is 4-6m away from the production horizontal well, the horizontal plane of a main well hole of the secondary branch horizontal well is positioned between the two groups of SAGD wells, the main well hole of the secondary branch horizontal well in the longitudinal direction is level with the SAGD production horizontal well, and the horizontal section length of the main well section of the horizontal well is the same as that of the SAGD well and is 400-600m.
4. The method according to claim 1, wherein in the step (2), the secondary branch horizontal well is a horizontal section of the auxiliary horizontal well which is communicated with a plurality of primary steam injection well branches, each primary branch is communicated with at least one secondary steam injection well branch, the primary steam injection well branches are positioned on two sides of a main well bore of the steam injection horizontal well and extend obliquely upwards at an angle of 0-90 degrees with the main well bore, the length of the horizontal section of the main well bore is 400-600m, the length of each primary branch is 50-100m, the maximum distance of deviation of the branch section from the main well bore on the horizontal plane of the primary steam injection well branches is 15-25m, and the maximum distance of deviation of the branch section from the main well bore in the vertical direction is 3-8m.
5. The method of claim 4, wherein in step (2), the secondary steam injection well branches extend along the main wellbore at an angle of 0-90 degrees to the primary steam injection well branches, each secondary steam injection well branch has a length of 50-100m, the secondary steam injection well branches are offset from the branch sections on the horizontal plane of the main wellbore by a maximum distance of 20m, are vertically offset from the main wellbore by a maximum distance of 5m, and are parallel to the main wellbore; the main well is completed by a sieve tube, and the first-stage steam injection well branch and the second-stage steam injection well branch are both completed by open holes.
6. The method of claim 1, wherein in the step (3), in the reservoir transformation process, hot sewage is injected into the secondary branch horizontal well to control pressure parameters and injection quantity and improve bottom hole pressure so as to achieve the purposes of increasing water saturation around the well and improving physical property uniformity around the well, wherein the maximum bottom hole pressure-the minimum principal stress is less than or equal to 0.5MPa; the injection quantity is increased to further increase the bottom hole pressure, so that the range of the expansion area around the pitshaft of the steam injection well and the production well is enlarged, wherein the fracture pressure-bottom hole pressure is more than or equal to 0.5MPa.
7. The method of claim 1, wherein in step (3), after the second-stage branch horizontal well reservoir is modified, the well is washed with hot water to clean mud, injected with a solvent and soaked.
8. The method of claim 7, wherein the solvent is injected in an amount comparable to the wellbore volume, preferably 10-15 square; the solvent is xylene or a mixed solvent of the xylene and diesel oil; wherein in the mixed solvent of the xylene and the diesel, the mass ratio of the xylene to the diesel is 3-5, preferably 5.
9. The method of claim 1, wherein in step (4), before SAGD production, the steam injection horizontal well and the production horizontal well are circularly preheated, and after communication is established between the SAGD steam injection well and the production well, SAGD production is carried out; and after 2-3 years of production, the SAGD steam cavity develops to the top of the oil layer, and then the second-level branch horizontal well is utilized for huff-puff production.
10. The method of claim 1, wherein in step (4), the secondary horizontal offset well is produced in a huff and puff manner with a steam dryness of greater than 80%, a steam injection rate of 240t/d to 300t/d, a steam injection intensity of 8t/m to 20t/m, a steam injection pressure less than the formation fracture pressure, a production-injection ratio of 0.7 to 1.2, and an annealing time of 2 to 7 days.
11. The method of claim 1, wherein in step (5), after the secondary horizontal offset well is in thermal communication with the SAGD steam chamber, when the viscosity of crude oil between the SAGD steam chamber and the secondary horizontal offset well drops to less than 1000mPa.s, the secondary horizontal offset well is stopped from handling and is converted to continuous production.
12. The method of claim 1, wherein in step (6), SAGD well group and secondary branch horizontal well injection and production parameters are adjusted, pressure balance is maintained, and the injection and production ratio is controlled to be 1.1-1.4 until production is finished.
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