Through collaborations with colleagues in industry, academia, and government, the Scherer and Celia groups are using tools and models developed at Princeton to characterize wells in the field.
Teapot Dome Samples
The actual impact of CO2 injection on well integrity depends on whether cements in existing wells are more or less susceptible to attack than those used in the laboratory. In collaboration with Dr. Barbara Kutchko at NETL, the researchers have continued to examine cement samples obtained in 2004 from a 19 year-old well at the Rocky Mountain Oil Testing Center in Wyoming. Some of the samples show extensive sulfate attack, apparently from reaction with the brine. Clearly, the durability of well cement will be affected by brine concentration, so this factor will have to be taken into account in predicting the risk of leakage. A manuscript is in preparation regarding the condition of the samples from Teapot Dome.
Cement from a Natural CO2 Deposit
In an effort led by Walter Crow of BP, samples of cement were recovered by taking sidewall cores from a 30-year-old well that had been placed into a natural CO2 deposit in Colorado. The cement cores showed a degree of carbonation that decreased with increasing distance from the boundary with the CO2 deposit. This was interpreted to mean that a leak must have been present to bring the CO2 into contact with the cement. Calculations showed, however, that the pores of the shale cap rock would have been saturated with CO2 owing to contact with the deposit. As soon as the well was constructed, CO2 would have diffused from the shale toward the cement, where it could have created the observed carbonation layer in the absence of a true leak from the CO2 deposit. Carbonation of cement near natural deposits must thus be interpreted with caution, as cement far above the reservoir may be carbonated without a leak along the well.
A series of calculations was run to evaluate the risk of creating leaks in a reservoir as the pressure of injected CO2 deflects the cap rock: since the stiffness of the rock differs from that of the cement plugs that pass through it, cracks might form in the cement that would permit leakage. Simulations done by Jan Prévost and Zhihua Wang using Dynaflow, indicate that the bending stress in the cap rock is negligible. However, as the cap rock deflects upward, shear stresses are created along the well, if it is anchored in the reservoir. Preliminary results indicate that leakage paths could be created in this way, but further calculations are needed to test the sensitivity of the results to the boundary conditions.
Vertical Interference Test Data
Over the past year, the Celia Group continued to work with Walter Crow, Brian Williams, and others at BP to analyze and interpret results from the Vertical Interference Tests (VIT) that BP has been leading at several old wells. Walter has led the effort to re-enter the old wells and perform the tests, and CEE alumnus Sarah Gasda has been using the code she developed while at Princeton to analyze the results and infer plausible ranges of effective permeability for the cement outside of casing. The test is described in Gasda et al. (2008) and some of the results are described in Crow et al. (2008a,b). In particular, the most recent analysis involves 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. 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 have been measured and those results are combined with an in-situ pressure-response test to investigate cement integrity over larger length scales.
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 test data indicates the best match for effective permeability is approximately 1-10 millidarcies which suggests cement interfaces as the primary path for potential migration rather than through the cement matrix.