A tussle between QPR's Josh Scowen and Pompey's Ronan Curtis led to a mass mele involving all 22 players.
As a result Lee Brown and Curtis of QPR Joel Lynch of Ponpey were booked for the brawl that took place 14 minutes from time.
BBC Radio 5Live report:"There was a nasty fight. Arms and fists flying everywhere. It was a mass bundle involving most of the players."
Before the mele Pompey took the lead in comical fashion in the 63rd minute when Rangers defender Joel Lynch sliced a Lee Brown cross into his own net.
But Wells ensured a pulsating fourth-round tie went to a replay when he crashed home from close range 11 minutes later after Luke Freeman's corner had caused panic in the home defence.
Rangers were hoping to book a place in the fifth round for the first time since Trevor Sinclair scored his famous bicycle kick goal against Barnsley in 1997 and had the better of the opening exchanges.
Pompey goalkeeper Craig MacGillivray was called into action for the first time when Darnell Furlong flicked a header into his arms after ghosting in at the near post to meet Freeman's corner.
It took until the 28th minute for Steve McClaren's side to create their first meaningful chance.
Freeman floated in a free-kick from the left and Lynch found himself unmarked only to head tamely into the gloves of a grateful MacGillivray from eight yards.
Pompey captain Brett Pitman did have the ball in the net moments later but the goal was ruled out after referee Gavin Ward spotted a foul on Rangers defender Toni Leistner.
The south coast side almost took the lead on the brink of half-time when Louis Dennis' dipping 20-yard volley was acrobatically tipped over the crossbar by Joe Lumley.
Pompey, who beat Championship promotion contenders Norwich in round three, created another great chance four minutes after the restart.
The home crowd exploded in joy when Kenny Jackett's men took the lead in fortuitous fashion three minutes past the hour mark.
Republic of Ireland winger Ronan Curtis rolled the ball forward for the overlapping Brown down the left and his cross was turned into his own net by the hapless Lynch.
The busy Dennis floated the ball over the top of the Rangers defence but could only lift his shot over the crossbar with just a helpless Lumley to beat.
Rangers hit back when Wells found the roof of the net to claim his seventh goal of the season and despite an open conclusion to the game, neither side could find a winner.
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