Experimental Investigation of Well Cement Integrity in Decommissioned Oil and Gas Wells – The Effect of Water and Oil Contamination

Abstract During the cement plug placement phase of well decommissioning, the cement slurry is at risk of intermixing with the wellbore fluids, which may impact the performance and isolation capabilities of the resulting cement plug. In this study, we investigated the sealing performance of the cement plugs, which are contaminated with different wellbore fluids. Two cement blends were used: neat API class G cement and an engineered API Class G cement with additives for optimal performance. Two types of contaminants were investigated: water to mimic fluids used by decommissioning rigs and light crude oil to mimic residual produced hydrocarbons. The contaminants were added to the slurry at varying levels (12.5% and 25% v/v for water and 2.5% v/v for light crude oil). The contaminated blends were cured inside a bench-scale wellbore simulator under ambient temperature and 6.89 MPa for 7 days. The isolation performance of the contaminated plug was assessed by conducting water flow tests. For the neat API Class G cement plug, the water flow rates through the 25% water contaminated plug were an order of magnitude higher than the flow rates through the uncontaminated plug. Visual inspection of the possible flow paths by Nitrogen injection revealed that Nitrogen gas was flowing at the cement-casing interface. For the API class G cement plug with additives, water flow rates through 25% water contaminated plug were 2 to 3 orders of magnitude higher than that of the uncontaminated cement plug. Contamination with water in this blend created a different failure mode with Nitrogen flowing through the matrix of the cement. An oil contamination of 2.5% of both the neat API class G and API class G+ additives cement had little to no effect on their isolation capabilities as long as the fluid injection pressure and confining pressure are the same and kept at the level of curing condition (i.e. 6.89 MPa, no change in effective stress). When the effective stress on the cement plug is changed, (by reducing the confining pressure down to atmospheric pressure, while keeping the injection pressure constant at 6.89 MPa), sealing ability of both the original and the uncontaminated cement plugs deteriorated. The contaminated plugs lost more of their sealing capabilities when compared to the uncontaminated plug. These results indicate that the effective stress (not the oil contamination) was the main controlling factor for the deterioration of cement plug sealing performance with oil contamination only having a small effect.