Frequency Distributions of Effective Permeability and Gas Flux in Leaky Wellbores

Abstract

Using a previously described model, we estimate the effective permeabilities of about 300 wellbores in six different fields that exhibit sustained casing pressure (SCP) or surface casing vent flow (SCVF). Uncertain parameters that affect the estimated permeability, such as the location of the leak source, are accounted for by a Monte-Carlo simulation approach, yielding an expected value of leakage path permeability for each measurement of SCP or SCVF. Characterizing leakage paths can be useful in diagnosing and remediating poor zonal isolation. This can be particularly valuable in quantifying the likelihood that shallow groundwater is being contaminated by operations in deeper formations, such as shale gas production. Analogous leakage paths are a primary risk factor for large-scale geologic storage of anthropogenic CO 2 . Being able to estimate CO 2 fluxes along existing wellbores is required for quantitative risk assessment.The expected values of estimated permeabilities along most of the leaky wellbores are between 10 d to 10 md. Using the wellbore permeabilities to estimate the aperture of the leakage pathways, we estimate the capillary pressure and hence the minimum CO 2 plume heights required for CO 2 to enter the leakage paths. Almost all leakage pathways require very modest plume heights, so capillary effects are unlikely to prevent leakage. We compute worst-case steady CO 2 leakage fluxes, finding that over 90% of the fluxes are less than 0.1 ton/m 2 /yr. The frequency distribution of the effective permeabilities of leaky wellbores is believed to be the most comprehensive compiled to date. It enables quantitative assessment of leakage rates in a risk assessment context, in particular for the hazard of contamination of shallow resources (e.g. groundwater) by CO 2 or natural gas.

Comment on “Numerical investigation of the potential contamination of a shallow aquifer in producing coalbed methane” by Xianbo Su, Fengde Zhou, and Stephen Tyson

“…For comparison, the effective permeabilities of actual "leaky" wells determined by Tao et al (2013) are shown in Figure 1. These permeabilities are typically in the range of 0.01-10 mD, with the highest values around 1 D, or 1% of the value used by Su et al in their model.…”

“…To maintain an air of reasonableness, this scenario would require (i) to be Figure 1. Effective permeabilities of actual "leaky" wells from Tao et al (2013). Note the logarithmic scale, and that the maximum value on the scale is 10 D. 100 D (as used in the models created by Su et al) would sit outside the plot area.…”

“…For comparison, the effective permeabilities of actual “leaky” wells determined by Tao et al. (2013) are shown in Figure 1.…”