This project addressed wastewater disposal knowledge gaps in the Montney Play region identified by the Scientific Hydraulic Fracturing Review Panel, operators, and the regulator (BC Oil and Gas Commission [BCOGC]). It integrated hydrogeology, geomechanics, reservoir characterization (including petrophysics), and structural geology to evaluate suitable wastewater disposal zones below the base of usable groundwater.
Recovered wastewater is a mix of hydraulic fracturing fluid and produced water. After it is recovered, it is often pumped into disposal zones via a disposal well. Disposal zones are porous and permeable zones within formations and may include depleted pools where natural gas or oil was previously extracted, or deep saline aquifers.
During natural gas development, wells are drilled and completed. Completion is the process of finishing a drilled well with downhole equipment, for the production of natural gas (or oil), and any methods required to improve flow from the formation, including hydraulic fracturing. Hydraulic fracturing involves pumping a mixture of sand, water and additives into rocks, causing them to fracture and improve the flow of natural gas or oil. The well is then flowed-back, a process that recovers fluid and additives and allows natural gas or oil to flow up the well. Once the fracturing fluid has been recovered, additional water may flow back up the well during the production of the natural gas; this is called produced water. Produced water could be residual water from the producing zone, or from underlying saline aquifers or a combination of both.
Deciding where to place a disposal well is a complex process, especially in a maturing natural gas production area like the Montney Play region where existing disposal zones can be near capacity.
This project fits under Geoscience BC’s Strategic Objective of Advancing Science and Innovative Geoscience Technologies and our goal to:
- Improve economic competitiveness of the Montney Play region through advanced science and innovative geoscience technologies.
Specifically, this project set out to:
- Consult with operators, service companies and BCOGC staff to best understand disposal requirements and operational experiences.
- Interpret performance of existing disposal wells.
- Complete current maps on disposal aquifer distribution, quality and fluid composition.
- Map and assess structural elements across the Montney Play region to identify discontinuities related to containment risks, and to help understand disposal reservoir geomechanical settings.
- Integrate well test, production and reservoir pressure data with geomechanical data to characterize in-situ stresses and rock properties, which can influence disposal aquifer compartmentalization and containment risks.
- Develop practical conclusions and recommendations to guide operators and the BCOGC in decisions around development and regulation of disposal capacity and containment across the entire Montney Play region.
Benefits of this project include:
- More efficient, continued natural gas operations in the Montney Play region.
- Reduced costs for exploration of wastewater disposal zones and reduced wastewater disposal costs.
- Improved knowledge of wastewater disposal zones within the Montney Play region.
- New, peer-reviewed literature to guide future operations and research.
The Montney Play region is in BC’s Northeast Region, in the territories of Treaty 8 First Nations.
What was found?
The final project report contains 10 conclusions, which can be summarized as:
- Of the 13 formations assessed, there are seven that demonstrate good regional aquifer quality. Each of these seven formations has been mapped to highlight areas where there is sufficient net porous reservoir present to support wastewater disposal.
- There are a variety of structural influences across the Montney Play region which create complex structural and stress conditions which can influence disposal zone compartmentalization and potential for induced seismicity. Local study is required for the siting and risk assessment for each new disposal well.
- Structural modeling identifies numerous faults that appear to be critically stressed. Minor variations in pressure / stress conditions – induced, for example, by wastewater disposal – can cause movement and seismicity along these faults.
- Reservoir engineering analysis of historical reservoir pressures can assist in understanding disposal zone behaviour. Remaining capacity in an active disposal well can be predicted (semi-quantitatively).
- Cretaceous aquifers generally contain low- to moderate-salinity waters, demonstrating active flow and recharge from the west.
- Water compatibility studies were beyond the scope of this project. However, regional mapping of water chemistries provided in the report can be useful in assessing compatibility issues for each disposal wellbore.
- Pressure Depth Ratios (PDR) highlight relatively under-pressured areas where more disposal capacity may be available. Conversely, areas with high PDRs may have reduced disposal capacity.
- 15 minimum horizontal in situ stress (SHmin) determinations were compiled for 12 wells in the project area. This is a very small amount of data and limits the scope for regional analysis.
- Disposal favourability maps have been created by combining all the analyses in the study. Disposal capacity is extremely variable across the Montney Play region.
- Researchers recommend that operators considering disposal at a particular location should use additional detailed or proprietary data to review each zone mapped as favourable to judge which would best meet their specific disposal needs.
In the final project report make several recommendations, which can be summarized as:
- Operators and the BCOGC should use the regional disposal favourability maps from this project as part of the information to inform prioritization of disposal zones. Site specific assessments should be done, using local mapping and analysis.
- Where disposal wells are, or are planned to be, close to faults mapped as critically stressed, the BCOGC should consider mandating seismic arrays and monitoring.
- The rationale for using virgin reservoir pressure to calculate maximum allowable fill-up pressure assumes that seal strength is proportional to virgin pressure. That is, the reservoir has been charged to its leak-off pressure. Further technical work on this issue is warranted.
- Regarding operator-submitted pressure data, short shut-in times in an injection well will tend to read higher than actual reservoir pressure. It is therefore in the operator’s best interest to obtain the most representative (i.e., lowest justifiable) pressure possible. Long shut-ins and careful extrapolation are worthwhile. It may be useful to remind operators to conduct appropriate pressure testing.
- Maximum injection pressure limits for wastewater injection wells to preclude the initiation and propagation of hydraulic fractures, or the extension of natural fractures, should be Diagnostic Fracture Injection Test (DFIT) derived, small volume fracture pressure tests to determine an appropriate “fracture gradient”.
- Several horizontal wells have been purpose-drilled for disposal, particularly in the Baldonnel, Cadomin and Peace River formations. While some of these have performed well, there have been induced seismicity and hole integrity issues. Use of horizontal wellbores for disposal should be evaluated on a case-by-case basis.
- Based on limited data, the authors did not see significantly better disposal well performance in depleted gas pools versus regional aquifers. They do not recommend delaying abandonment of depleted reservoirs in the hope that they can be reused as disposal wells.
- Detailed evaluation of the Bluesky aquifer should be undertaken to better quantify its value as a disposal zone.