The La Liga champions have been linked with the 26-year-old all summer and are understood to have finally made their interest formal on Wednesday.
But Blues boss Jose Mourinho has insisted that Luiz is central to his plans and the bid, which is thought to be in the region of £35million, is expected to be rejected.
Speaking after his side's 2-0 pre-season win over AC Milan, Mourinho said: "David has come back with us fantastically and we have to use his quality.
"He has big self-confidence and self-esteem, especially when he comes up from the back with the ball."
Luiz signed a new five-year deal with Chelsea last September.