Abstract:
Although methane leakage from oil and gas wells has received significant attention in the last thirty years, emission estimates still range between 1-9% of production. In more recent years methane leakage has been under scrutiny by industry, governments and the public. Methane leakage can be characterized as short, medium and long-term. Long-term methane leakage usually relates to wellbore integrity issues and only recently has it been suggested that it can occur through the formation near the wellbore from damage while drilling or pressure cycling, and that conventional cementing may not prevent it. The severity of long-term methane leakage from wells is currently unknown. Thus, this research is investigating long-term leakage with special attention paid to the effect of pressure cycling from hydraulic fracturing on cement and near wellbore rock integrity. An apparatus has been designed that is capable of testing cement under various cyclic pressure conditions: the numerous hydraulic fracturing stages and the inevitable shutdowns and start-ups during production. It is capable of measuring permeability changes along the length of the cement in order to determine when and where damage occurs during pressure cycling. Cement and microannular degradation will be assessed through observed changes in permeability, computerized tomographic (CT) evaluation and visual inspection. Knowing the main cause of leakage allows for specific remediation and/or solutions to prevent leakage without guesswork.
Introduction
During the last thirty years, methane leakage from hydrocarbon wells has received significant attention from industry, government and the public. Recent studies (Boothroyd et al. 2015; Caulton et al. 2014; Karion et al. 2013; Miller et al. 2013; Rivard et al. 2014; Osborn et al. 2011) have increased not only public awareness but also concern because methane is a powerful greenhouse gas (Myhre et al. 2013). In a typical well steel casing is installed and the annular space between this casing and the drilled rock is filled with cement (Figure 1). Ideally, this cement provides a hydraulic seal in the casing- rock annulus. If this cement becomes damaged it can allow fugitive methane emissions (King and King 2013). Current emission estimates from surface facilities, cemented annuli and the surrounding rock are thought to be somewhere between 1 and 9% of the actually produced gas (Howarth et al. 2011; Kirchgessner et al. 1997; Pétron et al. 2012; Ritter et al. 2013; Tollefson 2013). 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.
