Sicilians tend to lead to sharp conflicts with one color attacking on the queen's flank and the other color attacking on the king's flank. This one is atypical, with both sides skirmishing on the queen side.
Preliminary assessment: a broad outline is already clear. Black has an isolani on the a-file; White will concentrate his fire on that pawn, forcing Black to defend it. Black can ill afford to let it go without a fight, as that would give White a connected passer. Black's B on b7 is biting on granite, but in compensation, he has two center pawns.
12. Bc1e3 Nb8d7
White's development is all but complete now, whereas Black is behind. White's bishops and knights are admirably placed to support further actions on the queen's flank, his king can castle into safety. Black must make some sort of counter-demonstration in the center soon, or he will be overrun on the queen side, he must get his KB onto a good line, and he must get his K to safety.
13. O-O a5
Every pawn move creates a weakness. This is a case in point. Black just lost control of b5; this will ultimately prove to be his undoing. Still, it is understandable why Black should have played this move now: 1. he wants to drive the White knight away from its excellent post on b4. 2. He wants to be able to play the QR to c8--which he could not do so long as the pawn was double-attacked on a6.
This temporary retreat of the knight is not serious. It will soon rejoin the fray.
Perhaps Black expected to get a powerful crossfire going through the open center with the double fianchetto. However, given that Black's development is lagging, this is too slow. It effectively hands White two extra tempi. Probably better were either a) ... Be7 and ... 0-0, or b) ... d5 with the dark-squared B to follow somewhere along the a3-e7 diagonal.
15. Na2c3 Ra8c8 16. Be2b5
b5 was crying out to be occupied; this pin will prove to be troublesome for Black.
This is consistent with move 14. ... g6. But I wonder whether Black really thought it through. Two possibilities: 1. He was counting on Qxd6, whereupon Black might try to entrap the White Q by continuing ... Nd5 and ... Be5. Or 2. he simply overlooked that the d6 pawn hangs. Either way, I was quite prepared to charge into the breach.
17. Qd1xd6 Nf6d5 18. Nb3c5
The pressure on d7 is reaching critical mass; spontaneous combustion is imminent.