Heated fluid injection using multilateral wells

1. A well system comprising:

a main wellbore extending from a terranean surface toward a subterranean zone;

a first lateral wellbore extending from the main wellbore into the subterranean zone;

a second lateral wellbore extending from the main wellbore into the subterranean zone, the second lateral wellbore substantially parallel to the first lateral wellbore;

a liner junction device in the main wellbore having a first leg extending into the first lateral wellbore and a second leg extending downhole in the main wellbore;

a heated treatment fluid injection string that extends from in the main wellbore through the liner junction device and into the first lateral wellbore and terminates in the first lateral wellbore; and

a seal in the first lateral wellbore that seals against flow toward the main wellbore in an annulus adjacent an outer surface of the heated treatment fluid injection string.

2. The well system of claim 1 , further comprising a downhole fluid heater in the heated treatment fluid injection string.

3. The well system of claim 2 , wherein the downhole fluid heater is disposed in the first lateral wellbore.

4. The well system of claim 2 , wherein the seal seals between the downhole fluid heater and the first leg of the liner junction device.

5. The well system of claim 4 , wherein the seal comprises a polished bore receptacle.

6. The well system of claim 1 , wherein the heated treatment fluid injection string is coupled to a source of heated treatment fluid at the terranean surface.

7. The well system of claim 1 , wherein the seal seals between the heated treatment fluid injection string and the first leg of the liner junction device.

8. The well system of claim 7 , wherein the seal comprises a polished bore receptacle.

9. The well system of claim 1 , further comprising a second seal in the first lateral wellbore that seals against flow toward the main wellbore in an annulus adjacent the second leg and the first lateral wellbore.

10. The well system of claim 9 , wherein the second seal comprises a deposit of cement.

11. The well system of claim 1 , comprising a seal in the main wellbore that seals against axial flow in an annulus adjacent an outer surface of the liner junction device.

12. The well system of claim 1 , further comprising a liner residing in the first lateral wellbore and coupled to the first leg of the liner junction device, the liner comprising one or more joints of permeable tubing to allow flow of heated injection fluid from an interior of the liner into the subterranean zone.

13. The well system of claim 12 , wherein a portion of the heated treatment fluid injection string has apertures along a length coinciding with the portion residing in the liner, the apertures being of a selected size and spacing to distribute the heated injection fluid supplied through the heated treatment fluid injection string along a length of the liner.

14. The well system of claim 1 , further comprising a liner residing in the first lateral wellbore and being coupled to the first leg of the liner junction device.

15. The well system of claim 14 , further comprising a swivel connecting the liner junction device to the liner that allows the liner to rotate around its central axis.

16. The well system of claim 14 , wherein the first leg flexes to orient the first leg with the liner.

17. The well system of claim 1 , wherein:

the liner junction device comprises a body that extends from an upper seal assembly that is disposed in the main wellbore uphole of the liner junction device and engaged with a casing of the main wellbore to support the liner junction device,

the first leg extends from the body into the first lateral wellbore, and

the second leg extends from the body in a downhole direction in the main wellbore.

18. The well system of claim 1 , wherein the second leg is sealingly coupled to a lower lateral tieback and seal assembly disposed in the main wellbore downhole of the liner junction device.

19. A well system comprising:

a multilateral wellbore system having a main wellbore and a plurality of substantially parallel lateral wellbores that extend from the main wellbore;

a liner junction device residing in the main wellbore;

a liner residing in one of the lateral wellbores and coupled to the liner junction device;

a heated fluid injection string extending from in the main wellbore, through the liner junction device, and terminating in the liner; and

seals sealing against flow to the main wellbore from between the liner and the lateral wellbore and from between the heated fluid injection string and the liner.

20. The well system of claim 19 , wherein the seal sealing against flow to the main wellbore from between the heated fluid injection string and the liner comprises a polished bore receptacle.

21. The well system of claim 20 , wherein the polished bore receptacle resides in the liner junction device.

22. The well system of claim 19 , wherein the seal sealing against flow to the main wellbore from between the liner and the lateral wellbore comprises a deposit of cement in the lateral wellbore.

23. The well system of claim 19 , wherein the heated fluid injection string comprises a heated fluid generator.

24. The well system of claim 19 , wherein the liner junction device comprises a leg extending from the main wellbore into the one of the lateral wellbores and being coupled to the liner at a swivel joint.

25. A method comprising:

injecting a heated treatment fluid into a lateral injection wellbore extending from a main wellbore with a heated treatment fluid injection string that extends from the main wellbore, through a liner junction device residing in the main wellbore, into the lateral injection wellbore, and terminates in a liner residing in the lateral injection wellbore, the liner coupled to a leg of the liner junction device that extends from the main wellbore into the lateral injection wellbore;

sealing an annulus adjacent an outer surface of the heated treatment fluid injection string against flow toward the main wellbore; and

producing fluid from a production lateral wellbore extending from the main wellbore and spaced apart from the lateral injection wellbore.

26. The method of claim 25 , further comprising heating the heated treatment fluid using a downhole fluid heater.

27. The method of claim 25 , wherein sealing the annulus adjacent an outer surface of the heated treatment fluid injection string comprises sealing an annulus between the heated treatment fluid injection string and an adjacent tubular.

28. The method of claim 25 , wherein sealing the annulus adjacent an outer surface of the heated treatment fluid injection string comprises disposing cement in the lateral injection wellbore.

29. The method of claim 25 , wherein injecting the heated treatment fluid into an lateral injection wellbore comprises injecting the heated treatment fluid from a terranean surface.

30. The method of claim 25 , further comprising sealing the main wellbore above the lateral injection wellbore and below a wellhead.

31. The method of claim 25 , wherein the production lateral wellbore is substantially parallel to the lateral injection wellbore.