I don't think Chelsea have a lot of chances against Real Madrid, even if they play well in defense and there is a draw in London, Real will not miss the opportunity to beat Chelsea in Madrid.
They just luckily won over Dortmund and went to quarter finals. They are worse than in the past season when they mostly won big against Real Madrid in a second leg match but Real Madrid with their champion mentality pulled the match back and did not allow Chelsea to make a big shock.
In this season both clubs have decreasing performances but more severity is seen in Chelsea than in Real Madrid that makes Real Madrid again is a likely a winner.
Chelsea have a small chance if they play well in defence, which they do well, and will be able to take chances in the counter attack, but I think that these chances will be few and everyone will need to be treated very carefully.
It's reasonable for a weaker team play defensive against a stronger team but unfortunately Chelsea defense is not actually good in this season. I don't think they will be able to keep their goal clean after two matches against Real Madrid. With poor performance from their attacking players, it's super hard for them to score goals when Real Madrid have Courtois.