Friction is one of the most important topics in contact mechanics and the real contact area plays a significant role in determining the emergent friction properties. We study the detachment of the original contact clusters during the surface sliding. The exponential decay curves of remaining contact area for different size of clusters are approximately converged after the normalization. Most of the clusters are fully detached after the surface sliding of one unit length of cluster. A BEM-based model is also developed to calculate the real contact area between rough surfaces in presence of trapped fluid zones. The model has good performance on several surfaces and we also apply the model on lubricated aluminum surface to simulate the friction coefficient under different external pressure.