Novel Lab Method to Detect Methane or CO_2 Leakage from Damaged Cement in Unconventional Oil and Gas Wells

摘要

Subsurface fluid leakage is a primary concern from unconventional oil and gas development as well as carbon sequestration and gas storage wells. During the past thirty years, methane leakage from said wells has received significant attention from industry, government and the public because of environmental and human health concerns. Recent studies have increased not only public awareness but also concern because methane is a powerful greenhouse gas and a known precursor to the formation of ozone, an air pollutant that is regulated. In a typical well steel casing is installed and the annular space between this casing and the drilled rock is filled with cement. Ideally, this cement provides a hydraulic seal in the casing- rock annulus. If this cement becomes damaged it can allow fugitive methane or CO_2 emissions to aquifers or to the surface. Current emission estimates from surface facilities, cemented annuli and the surrounding rock are thought to be somewhere between 1 and 11% of the actually produced gas. Methane leakage can be viewed as short-, medium- and long-term. Proper cement system design and good operational practices can easily mitigate short-term leakage that is often the result of wrong cement weights. Since medium-term leakage has received significant attention from industry the assumption of this research is that proper cementing techniques will mitigate this type of leakage. Long-term leakage usually relates to wellbore integrity issues and can occur weeks to years after initiation of production - substantial leakage may also occur after a well has been plugged and abandoned. Controlling or preventing long-term leakage has not been prioritized by industry because their commercial interest is with production or with developing products and services that mitigate short- and medium-term leakage. However, more research has been directed toward long-term leakage recently because of concern with atmospheric methane and CO_2, and the costs associated with remediating a leaky well. Mechanisms accelerating cement seal degradation include pressure cycling from hydraulic fracturing and other well operations. It is currently uncertain if conventional cementing can prevent this kind of leakage. This research presents a novel method to detect leakage along the length of a wellbore at the lab scale from pressure cycling. Pressure is applied in a cyclic manner to pseudo casing and permeability along the length of the cement is monitored. This method can detect not only the time that the cement becomes damaged but also the location. CT images are also taken before and after to visualize damage. It is hoped that in better understanding how cement damage occurs that methods can be developed to prevent damage in future wells and repair damage in current wells.