46th U.S. Rock Mechanics/Geomechanics Symposium,
2012. American Rock Mechanics Association
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Primary cementing is carried out during the drilling and completions of wells and the main objective is to provide zonal isolation. For effective cementing, the cement should completely displace the drilling mud (water-clay mixture). In practice, this is never achieved as some of the mud is not displaced and remains in the wellbore. This study investigates the effect of the residual mud on the hydraulic conductivity of the cement-formation interface. Flow-through experiments were conducted at 14.48 MPa (2100 psi) overburden pressure with cement-rock composite cores and brine at a flow rate of 1 ml/min. The cement-rock composite cores had 0% and 10% clay-rich fluid contamination respectively. The pressure drop across the composite cores was recorded throughout the flow-through experiments. Extensive micro-structural characterization of the cement-rock interface was carried out before and after the flow-through experiments. Higher pH values were recorded for the effluent brine from the mud contaminated core and the higher values indicate increased leaching of Ca2+. Micro-CT imaging revealed that the contaminated composite core possessed higher porosity at the interface zone. This shows that clay contamination of cement-rock interface degrades the interface zone and can provide a pathway for injected CO2 to escape from the intended storage zone.
One of the most promising technologies for controlling the amount of greenhouse gases in the atmosphere is large scale geological sequestration of CO2 [1, 2]. A key aspect of CO2 sequestration is ensuring that the sequestered CO2 stays within the intended storage zone permanently. The fact that more than 8000 wells in the Gulf of Mexico currently exhibit sustained casing pressure highlights the importance of investigations into cement-rock bond issues [6, 7]. Poor rock-cement bond is usually caused by poor primary cementing which includes inadequate mud displacement.
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