Source of H2S in Surface Vent Casing Flows of Thermal Recovery Oil Sands Wells
The origin of H2S detected in surface casing vent flows (SCVF) of several SAGD wells was established using stable carbon and sulfur isotopes as well as gas and water compositions. An extensive fluids sampling program to establish the source of the SCVF H2S (reduced sulfur) using stable sulfur isotope compositions, major and minor element water analysis, chemical and isotopic chemical gas compositions and sulfate reducing bacteria (SRB) analysis. This analysis program showed that only one well SCVF contains H2S in detectable quantities as analyzed by GC and that all three sampled SCVF wells contain reduced sulfur species (e.g., ethyl mercaptan), which are commonly the products of very early diagenesis of immature organic matter. The SCVF bulk δ34S values range from -9.5 to -1.5% CDT with associated δ13Cmethane of -60 to -72% PDB, characteristic of methanogenesis from immature organic matter. Quaternary water sampled beneath SAGD wellpad is similar in chemical and isotopic composition to published data, with δ34Ssulfate values ranging from -13 to -2%. No SRB were detected in these waters consistent with the low δ34Ssulfate values. The local formation waters in the non-bitumen areas also have heavy δ34S values, consistent with recharge of near surface water dissolving Paleozoic and the active SRB community in these distal waters. In comparison, the produced H2S from the SAGD operation has δ34S values of 7 to 8% CDT, consistent with high temperature cracking of S-bearing hydrocarbons (δ34Sbitumen = 4 to 4.5%) and possible interaction with minor sulfide minerals (δ34Smineral = 3 to 6%) in the production formation. The produced gases have δ13Cmethane (-53 to -51% PDB) typical of natural gas associated with petroleum biodegradation, found in oil sands reservoirs. These chemical data show that the H2S (and other reduced sulfur species) chemistry in the SCVF is distinct from produced SAGD fluids. The SCVF H2S is the product of near well bore heating of Quaternary sediments to temperatures are in excess of 150C, which has caused exsolution of dissolved and sorbed gases and accelerated diagenesis of immature organic matter which releases H2S, thiophene, methyl and ethyl mercaptan, COS and other volatiles from organics.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California