The hosts dominated the opening half by way of goalscoring chances but struggled to make the most of their advantage at the break.
Later in the half, Quincy Amarikwa had a good chance to open the scoring but Kwarasey was on hand to preserve parity once more.
Portland slowly but surely developed a foothold after the break, and Fanendo Adi struck the left post within two minutes of the restart.
The visitors continued to apply pressure and it seemed like their perseverance would pay off when Shaun Francis fouled Valeri to concede a penalty with 16 minutes left.
However, the Argentine rattled the crossbar from 12 yards as both sides shared the spoils in San Jose.