Problems with I2C connection on ESP8266 - 12F, which pins should I use?
Unlike most Arduinos, the ESP8266 doesn't have a hardware TWI, so I²C is bit-banged in software. This means that you can use any GPIO pins. By default, Wire.begin() initializes pins 4 (SDA) and 5 (SCL), but you can specify other pins using Wire.begin(int sda, int scl).

Documentation

(Note: the datasheet specifies GPIO2 as SDA and GPIO14 as SCL, but that doesn't matter, AFAIK, it's just a convention.)