Study on Damage by Drilling Fluid of Fractured Tighten Reservoirs in Bozhong Sag

The evaluation of reservoir damage is important to reservoir protection and oil and gas yield improvement. Reservoirs in the Bozhong sag of the Bohai Bay Basin are naturally fractured tight reservoirs, coring in these reservoirs is difficult and many different methods of evaluating the reservoir damage are in use, resulting in difficulties in implementing effective reservoir protection measures. To deal with this problem, combined with the actual situation of reservoirs , design and manufacture 3D printed fractured cores with transparent and visible interior and reservoir damage evaluation devices. In evaluating reservoir damage by different drill-in fluids used in Bozhong, the flow-rate damage rate instead of the permeability damage rate is used. Experimental results show that the drill-in fluid EZFLOW, which is presently in field application, has stable rheology and good filtration property. Compared with 3% bentonite drilling fluid, the EZFLOW drill-in fluid imposes smaller damage to the reservoirs, the rates of reservoir damage imposed by the EZFLOW drill-in fluid are between 11.7% and 26.2%, which are weak reservoir damage. The rate of damage to the cores is decreasing with increase in the opening degrees of the cores; for different fractured cores, the longer the cores and the wider the fractures, the higher the damage rate.