Experimental Study of the Impact of Drilling Fluid Contamination on the Integrity of Cement-Formation Interface

Primary cementing is performed during drilling and completion of wells mainly to provide zonal isolation. Ideally, 100% drilling fluid displacement should be achieved during cementing. This is difficult to achieve and some mud is left on the wellbore walls. This study investigates the effect of the undisplaced mud on the integrity of the cement-formation interface. Flow-through experiments were conducted at 14.48 MPa (2100 psi) overburden pressure and temperature of 22° C (72° F) with cement-sandstone composite cores and brine at a flow rate of 1 ml/min. The cement-sandstone composite cores had 0% and 10% drilling mud contamination respectively. Variations in the permeability of the composite cores were recorded throughout the flow-through experiments by measuring the pressure drop across the composite cores. The composite cores were characterized before and after the flow-through experiments to delineate the alterations in the composite cores due to the flow-through experiments. Higher pH values were observed in the effluent brine from the 10% mud contaminated core than the 0% mud contaminated core due to increased dissolution of cement. Microtomography revealed higher porosity at the interface zone of the 10% mud contaminated core. These show that mud contamination has a deleterious effect on the cement-sandstone interface and may create pathways for inter-zonal communication.