Permeability Estimation for Large-Scale Potential CO2 Leakage Paths in Wells Using a Sustained-Casing-Pressure Model

Abstract

Large-scale geological storage of CO2 is likely to bring CO2 plumes into contact with a large number of existing wellbores. Estimating the flux of CO2 along a leaking wellbore requires a model of fluid properties and of transport along the leakage pathway. Wells that exhibit sustained casing pressure (SCP) in an intermediate annulus have a leakage path along a cement/steel interface, or within the cement in the annulus. The former path is analogous to a leakage path along a cement/earth interface outside the casing. The latter path can occur in cement outside the casing. Thus the likely magnitude of the permeability of leakage paths outside the well can be estimated from the permeability of these analog paths. A sustained casing pressure (SCP) model yields information about effective permeability of the pathway. By choosing reasonable ranges for other well construction parameters, we apply the SCP model to obtain a range of effective permeabilities for a well based on a measured casing pressure build up history. We illustrate the approach with several field examples. For a relatively slow pressure build up (several psi/day), the permeability of the leakage path is in the range of microdarcy to hundreds of microdarcy. Fast pressure build up (thousands psi/day) indicates permeabilities in the range of tens of millidarcy to hundreds of millidarcy. To account for the uncertainty in wellbore construction parameters, we calculate the distribution of effective permeability of a leaky well using Monte-Carlo simulation. The resulting permeability shows an approximately log-normal distribution skewed toward the maximum possible value. The expected value and a confidence interval are obtained for each well, which represents the most probable permeability of the well for a given pressure build up. For the wells studied here the expected values range from 10 microdarcy to 100 millidarcy. The expected leakage path permeability correlates reasonably well with pressure build up rate. This is to be expected from Darcy’s law, and thus a strong correlation between leakage path permeability and other characteristics of the wells in this sample does not exist. Obtaining the statistics of the expected leakage path permeability will thus require more observations of SCP wells. The effective permeability of a leaky well is essential in calculating the CO2 leakage flux. Under the assumption that a leaky well encountered by a CO2 plume has a leakage pathway with the similar properties to an SCP well, we calculate the CO2 flux for the best, worst and most probable scenarios for the example wells. In the most probable scenario of CO2 flux, we calculate the expected CO2 leakage rate. Slow leakage yields a 0.1 kg/y CO2 rate while fast leakage could have a rate of 1000 kg/y.

Probability Distributions for Effective Permeability of Potentially Leaking Wells at CO₂ Sequestration Sites

“…Watson and Bachu (2007): review of sustained casing pressure in Alberta, Canada and contains frequency of failures but without leak rate 3. Tao et al (2010b): uses sustained casing pressure data in Gulf of Mexico to determine permeability and provides leak rates but not frequency of failure 4. Bourgoyne et al (2000); Xu and Wojtanowicz (2001): review of sustained casing pressure in the Gulf of Mexico provides frequency and two calculations of permeability Using the approach described in Section 2, six different bi-modal, log-normal probability distributions were created using Equations 1 and 2.…”

“…The second source is observations of sustained casing pressure (SCP), see Figure 2 (Bourgoyne et al, 2000;Watson and Bachu, 2007;Tao et al, 2010bTao et al, , 2011. These are observations of gas leakage occurring within one or more internal annuli of a well ( Figure 3) and are monitored by pressure gauges or vent lines on the annuli.…”

Analytical and Experimental Investigation of the Critical Length in Casing–Liner Overlap

“…The effective cement permeability was transformed to geometries of leakage pathways, such as microannulus and gas channels. This model was also used by Tao et al [37]. The authors modified the SCP model to adapt to the CO 2 leakage model.…”

Frequency Distributions of Effective Permeability and Gas Flux in Leaky Wellbores

“…A value of permeability that provides a good fit to the measurement (SCP buildup or SCVF flow rate) is obtained for each set of input parameters. From the resulting frequency distribution of permeability the expected value for the leaky wellbore can be obtained (Tao et al, 2010b).…”