Wellbore integrity analysis of a natural CO2 producer

The long-term integrity of wellbores in a CO2-rich environment is a complex function of material properties and reservoir conditions including brine and rock compositions, CO2 pressure, and formation pressure and temperature gradients. Laboratory experiments can provide essential information on rates of material reaction with CO2. However, field data are essential for assessing the integrated effect of these factors in subsurface conditions to provide a basis for validation of numerical models of wellbore behavior. We present a comprehensive study and conclusions from an investigation of a 30-year old well from a natural CO2 production reservoir. The wellbore was exposed to a 96% CO2 fluid from the time of cement placement. This site is unique for two reasons: it represents a higher, sustained concentration of CO2 compared to enhanced oil recovery fields and both the reservoir and caprock are clastic rocks that may possess less buffering capacity than carbonate reservoirs. A sampling program resulted in the recovery of 10 side-wall cement cores extending from the reservoir through the caprock. The hydrologic, mineralogical and mechanical properties of these samples were measured and those results were combined with an in-situ pressure-response test to investigate cement integrity over a range of length scales. Fluid sampling was conducted with pressure and temperature measurements for geochemical analysis of the cemented annulus and the adjacent formation. These combined data sets provide an assessment of well integrity including original cement seal and the impacts of CO2. Cement evaluation wireline surveys indicate good coverage and bonding, consistent with observations from sidewall cement core samples that have tight interfaces with the casing and formation. Although alteration of the cement samples is present in all cores in varying degrees, hydraulic isolation has prevented leakage based on the pressure gradient measured between the caprock and CO2 formation as well as lack of corrosion and no casing pressure history. Simulation of a hydraulic isolation test (Vertical Interference Test) indicates the best match for effective permeability of the wellbore system is approximately 1-10 millidarcies which suggests cement interfaces are a more significant potential migration pathway as compared with the cement matrix. Effective placement of the Portland-fly ash cement system was a key element in the observed performance of the barrier system that provides hydraulic isolation. The types of information collected in this survey permit analysis of individual components (casing, cement and reservoir fluid and pressure measurements) for comparison to the larger scale system including the interfaces. The results will be used as part of the CO2 Capture Project's effort to develop a long-term predictive simulation tool to assess wellbore integrity performance in CO2 storage sites.