Applied Organic Geochemistry and Best Practices to Address a Surface Casing Vent Flow - Lessons from Remediation Work of a Shale Gas Well in Quebec
Organic geochemistry has proven to be an ideal tool to
identify the source of and remediate a surface casing vent flow in Quebec.
A large dataset of gas carbon isotopes from seven
wells in the Quebec Lowlands gives well defined isotopic profiles across the
2000 meter thick shale succession. The carbon isotope signatures of ethane,
propane and methane carbon are burial history dependent. Three carbon isotope
domains are distinguished for both ethane and propane based on trend
inversions. These unequivocal signatures are compared to carbon isotope
signatures from surface casing vent (SCV) gas and used to pinpoint the source
depth of gas observed at the vent. Methane carbon isotopes have proved to be
much less reliable and less diagnostic except for biogenic gas.
At the Leclercville 1 well, gas from hydraulically fractured Utica shales sampled at the flowline and SCV gas were analysed for carbon isotopes. The entirely different carbon isotope signatures indicate that SCV gas does not originate from the fractured Utica interval. Bond and noise-temperature logs were also run and interpreted to identify all potential sources of gas behind the casing. The bond log was not sufficiently clear to be useful in this case.
SCVF gas at the Leclercville 1 well has an ethane carbon isotope composition that ranges from δ13C -32.12 to -32.54 and propane from δ13C -25.16 to -26.70; this clearly points to a gas source located within the Lorraine Formation at either a depth of 1 or 1.5 km. In contrast, the hydraulically fractured Utica shale gas, located at a depth of about 2 km, has much more negative isotopic values (ethane δ13C -39.27, and propane δ13C -34.85). Noise-temperature logs indicated that the most likely zone sourcing the SCV gas was located at around 1.1 km.
Following best practices, cement squeeze operations were conducted in three different zones from bottom to top: 1) in the existing Upper Utica perforations, 2) in the Lorraine at about 1500 m and 3) at about 1000 m. Although remedial works in the first two zones were operational successes, the flow problem remained. Significant flow reduction and pressure drop were observed at the vent after remedial operations above the 1100 metre interval outlined by both isotopes and wireline logs.
Ethane and propane isotopes are vital complements to methane isotopes and can be successfully used to satisfactorily identify the depth of origin of surface casing vent gas.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California