Geoscience BC's Phase 1 Horn River Basin Aquifer Characterization Project (PRCL, 2010) concluded that the Mississippian Debolt-Rundle carbonate platform demonstrates the best potential to act as a productive water source and sink for the completions activities of producers in the Horn River Basin. Phase 2 of the study reinforces this conclusion and adds data from new wells drilled since completion of the Phase 1 report. Resulting map revisions increase the overall enhanced reservoir volume, particularly in the centre of the basin, where well control had been sparse.
The Debolt-Rundle carbonate platform has been subdivided into four units that are mappable across the basin: the lower, middle and upper Rundle, capped by the Debolt. At the top of the upper Rundle and Debolt, reservoir quality is enhanced by leaching and dolomitization beneath the pre-Cretaceous unconformity, forming a unit referred to as the "Detrital Zone". The highest-quality and most continuous water-bearing reservoirs thus occur within the upper Rundle and Debolt where they subcrop beneath the pre-Cretaceous unconformity in the eastern part of the Horn River Basin.
Reservoir quality mapping was focused on the Debolt-Rundle succession. Net porous reservoir and porosity-thickness maps from the Phase 1 study were updated using sample cuttings observations and well logs from Phase 2 wells. As in Phase 1, approximations regarding reservoir quality had to be applied because of the highly heterogeneous nature of the "Detrital Zone". These were kept as consistent as possible with the Phase 1 project to ensure continuity. The updates show local increases and decreases in the porosity thickness within the Mississippian detrital zone and the lower enhanced reservoir zone, but there was an overall increase in "confirmed‟ porosity thickness and net porous reservoir.
No new fluid and test data were available for the Phase 2 study, but hydrogeological work in the Phase 1 study concluded that more than 10 billion m3 of water is present in the enhanced reservoir portion of Mississippian carbonates, and far greater volumes are present in the lower quality reservoirs. Core and test analyses demonstrate that the good quality reservoirs, in particular in the east of the basin, could support long term high-rate water pumping and injection, but that the regional poorer quality reservoir strata could not support high rates.
Data from the Phase 1 study, and from the new Apache/Encana Debolt Water Plant show total dissolved solids within the Debolt-Rundle to be between 15,000 and 40,000 mg/l, and H2S concentrations to be up to 65 mg/L. These values indicate that the water is non-potable, but is suitable for hydraulic fracturing. It should be noted that it was necessary for Apache/Encana to remove the H2S found in the water before use as a hydraulic fracturing fluid.
Upper Mississippian Mattson sandstones and basal Cretaceous Gething, Bluesky, and Chinkeh sandstones are considered to have good aquifer potential locally, but their distributions are limited, and test data are scanty. Few wells added for the Phase 2 study contributed significantly to the Phase 1 data already compiled for these units, and therefore, no updates were undertaken for them.
The first water production and treatment plant of its kind -- the Apache/Encana Debolt Water Plant -- is currently producing water from the Debolt aquifer at a rate of 16,000 m3 per day and is predicted to supply at least 90% of the water needs for drilling and completions in the area in 2011 (Waterman, 2011). This plant has provided a successful, economic and environmentally responsible water source, and proves the potential of the Debolt reservoir.