Is this "Arcane Wormhole" spell based on Portal, balanced?

Question

Directly inspired by the portals from the videogame Portal. This spell is designed to create two identical wormholes on a flat surface that allow instantaneous, adjacent teleportation between them regardless of distance. Note I did copy some of the wording and the spell's level from Arcane Gate as the spells are very similar.

Arcane Wormhole

6th level Conjuration

Casting Time: 1 Action

Range: 120 feet

Components: V, S, M (Jasper and Aquamarine dust worth at least 200gp each, which the spell consumes)

Duration: Concentration, up to 10 minutes

Class: Bard, Sorcerer, Wizard

You create two small linked teleportation portals that remain open for the duration. Choose two flat surfaces (of any orientation, such as a wall, floor or ceiling) that you can see within range. An elliptical portal will appear of it’s surface that’s 7 ft tall and 3 ft wide, if the surface cannot accommodate this space the spell ends.

The portals are two-dimensional glowing rings of orange and blue flame respectively (the flames do not deal damage or light fires) that is attached, and causes no damage to the surface. If the surface is destroyed, moved or otherwise disturbed the spell ends.

Any creature, object or spell entering the portal exits from the other portal as if the two were adjacent to each other, maintaining their current momentum provided their speed does not exceed the range of the spell per round and ignoring the distance between the portals. If the creature is unwilling to travel through a portal the must succeed a Dexterity saving throw to catch themselves on the edge of the portal. It is possible to see and attack through one portal to the other side.

As a bonus action the caster can change the location of either of the two portals to a different surface, so long as it is within range and line of sight of the caster, but not through a portal created by this spell.

If the spell ends while a creature or object is traveling through a portal the object is adjected to the nearest side and not harmed.

At Higher Levels: When you cast this spell using a spell slot of 7th level or Higher, the spells distance in increased by 120ft for each spell slot above 6th.

I attempted to make the physics calculations easier by adding "provided their speed does not exceed the range of the spell per round" so at level 1 that 120ft per 6 seconds. That way the "infinite loop catapult" (placing a portal above and below and repeated falling through to gain momentum before changing the location of the top portal and launching and extreme speeds) or jumping through a portal from great height is restricted by the level of the spell. EDIT: I understand that momentum isn't really a thing (mechanically speaking) in dnd 5e, so I might be fixing a problem that noone has, I was just trying to answer the theoretically question "with this spell, how far would my players travel with the infinite loop catapult" without having to pull out a calculator and perform advanced trigonometry every time. I was not trying to create a new mechanic out of nowhere.

To expand upon the word "Balanced"

• Does providing the above restriction prevent potential "infinite speed" exploits or anything similar?
• Is there anything inherently "game breaking" about the concept that makes balancing impossible?
• The way it's worded, are there any unintentional exploits beyond the capabilities of the spell/ the mechanics presented in Portal?
• Does the wording make sense or does it appear too "ramble-y"?
• Does the spell's potential power match it's spell level or should it be higher or lower?
• Is there any problems I've missed or scenarios I haven't considered that makes this spell unbalanced?

Answer 1

Probably no, but you can't say without actual playtesting

A standard way of estimating homebrew content without playtesting is to compare it with similar (official ones, hence, already playtested) spells in terms of cost and effects.

However, the Arcane Wormhole spell effects are mostly narrative:

Any creature, object or spell entering the portal exits from the other portal as if the two were adjacent to each other, maintaining their current momentum

In its current state, it has little to no clear mechanical details, aside from the 120ft teleporting portal, which looks like a lesser nevertheless more expensive version of the Arcane Gate.

For example, since there are no rules for momentum, the "infinite speed" problem emerges from a loophole in the non-existent rule. It might be a loophole in a DM's adjudication, but completely depends on the DM.

The possibility of changing the portal location seems interesting, however, it allows the whole party to travel over great distances, since the caster can reposition the portals about 100 times. So the effective distance becomes ~2.3 miles, not sure if it was intended.

The Arcane Gate can deal up to 12d6 falling damage, which can be completely prevented by a DEX saving throw. It is not so much for a 6th level spell (a 3rd level Fireball deals 8d6, and half on save). Yes, you can open a portal once, and then move it with a bonus action, but you still have only one bonus action per round.

The spell also allows you to make melee attacks at distance, or throw objects through the portal, but these are brand new mechanics so it is quite hard to estimate their impact without actual playtesting.

You should pay more attention to typical scenarios

I guess the spell intent was to add a spell with a lot of creative uses. However, you can make things easier for the DM by describing a couple of specific applications and giving a hint about mechanics, which they could use to adjudicate the consequences.

• a caster opens the portal under a creature's feet, and the other one on a wall — how this should be resolved in combat?
• a caster opens the portal under a creature's feet, and the other one on a ceiling right above — how this should be resolved in combat?
• a creature attacks through the portal and then goes away — does that trigger an opportunity attack?
• a creature grapples another one and tries to pull it through the portal — does the Dex save rule still apply?

Still, the spell effect description are too open-ended and unclear

There are a lot of questions remain. Does a pillar fall into the portal, if you open it just underneath the pillar, or the pillar is "the surface"? Can you collapse a building that way? Can you pull a chain into the portal and what happens when the portal is closed? Do liquids go through the portal? The open-ended nature of the spell description leaves a lot for the DM's adjudication, and DMs differ, so it would be extremely hard to balance.