U. Calgary analysis of energy balances and emissions of SAGD oil sands production finds need for improved processes; some operations not thermally efficient or net generators of energy
A team at the University of Calgary has assessed the thermal efficiencies, energy balances, and emissions of Steam-Assisted Gravity Drainage (SAGD)—both theoretically and as deployed at scale, using field data from the ERCB—for the production of bitumen from Athabasca oil sands reservoirs. In a paper in the journal Fuel, they report that current SAGD projects in Alberta show a very wide range of field performance.
Although optimized SAGD can yield “reasonably high” recovery factors, they found, the economic and environmental costs can be large given the amount of steam required. The data suggests that at the extreme, some operations are actually not net energy generating—i.e., the energy injected via steam exceeds the recovered chemical energy in the retrieved bitumen. The results suggest that in situ bitumen recovery processes need to advance well beyond current capabilities “if practical and sustainable energy balance and emissions scenarios are to be achieved,” they said.
One of the key challenges in producing bitumen and heavy oil is their high, variable viscosity. Heavy oil (between 10° and 20° API) has a dead oil viscosity ranging up to the thousands or tens of thousands of cP. Bitumen (<10° API) has viscosities ranging from the tens of thousands to more than 10 million cP at reservoir conditions.
However, when heated to steam temperatures, bitumen’s viscosity drops by several ordes of magnitude. For a typical Athabasca bitumen, the researchers note, the dead oil viscosity at 100 °C is equal roughly to 220 cP.
For a successful in situ oil sands bitumen recovery process, two requirements must be met: first, it is necessary to raise the oil mobility (often done by lowering its viscosity which results from raising its temperature) until it can be moved by natural forces such as gravity, and secondly, it is necessary to move the mobilized oil to a production wellbore so it can be produced to the surface.
Currently, commercial steam-based in situ processess used to recover bitumen from oil sands reservoirs are either one of Cyclic Steam Stimulation (CSS), or Steam-Assisted Gravity Drainage (SAGD). In this work, we will focus on SAGD although the analysis and results, conceptually, also apply to CSS.—Gates and Larter (2013)
|Operator experience is clearly an important factor but reservoir geology is king!|
—Gates and Larter
The geology of the reservoir is a key factor, the two researchers explain. Production reservoirs are “completely different” from the homogeneous sandstones with uniform fluids envisaged by the engineers that developed SAGD.
Geological heterogeneity impacts the recovery process through permeability changes of the reservoir sandstones within the oil column and the shale or mudstone barriers and baffles that prevent or retard fluid flow, respectively. The more laterally extensive the barrier, the longer it takes steam or production fluids to go around it and the longer it taks for mobilized oil to get to the production well. Also, non-productive reservoir within the oil column represents a heat sink which erodes the thermal efficiency of the provess. The main impact of fluid compositional heterogeneity is the due to effect of vertically and laterally varying oil phase viscosity...Thus, during SAGD production, both permeability and oil viscosity variations are important and adversely impact theoretical SAGD productivity.—Gates and Larter
Nonetheless, while average recoveries for cold heavy oil production range from 5% to 15%, average SAGD recovery factors are between 40% to 60%.
Gates and Larter calculated the theoretical steam to oil ratio (tSOR) required to mobilize the oil to its flow viscosity, given the porosity and fluid saturations. The lower the porosity, the higher the tSOR; the lower the oil saturation, the higher the tSOR.
They then used the tSOR to calculate the theoretical amount of carbon dioxide emitted per unit volume recovered bitumen (tCOR). Their results showed that the amount of CO2 emitted per unit volume oil produced is large; for a steam recovery process operating at about 3000 kPa, under ideal heating conditions, just more than 0.2 tonnes of carbon dioxide are emitted per cubic meter of bitumen recovered for 0.6 steam quality provided to the edge of the chamber.
Using public data, they then examined cumulative steam-to-oil (cSOR) ratios versus time for all major SAGD operations in Alberta by project and by field. They found that although there has been a reduction in the cSOR with time, the cSOR is leveling off for most operations at values above 2 m3/m3.
They found that in general, the best performing well pairs are from regions with better quality reservoir which have thick, highly oil saturated accumulations with few shale barriers and high vertical permeability throughout the reservoir.
They calculated that the energy breakeven point—the point at which the chemical energy output from combusting the bitumen to the energy input in the form of steam—is at cSOR values equal to around 11.5 m3/m3, for the recovery process alone. This means that above this cSOR, the SAGD process is a net energy consumer and thus is not an energy generation process.
After accounting for the energy required (and the bitumen lost) during the upgrading process to convert the bitumen to synthetic oil, and then refining into transportation fuels, they found that the overall breakeven point was equal to a cSOR of about 6.5 m3/m3.
Based on the cSOR field data...many operations exceed this value and thus are not net energy generation processes yet may be “economic”! With disconnected price markets for natural gas and bitumen, it is possible for bitumen recovery under these conditions to be economically viable today even though it makes no sense to pursue such an energy inefficient process when cSOR values are high.—Gates and Larter
In all cases, they found, carbon dioxide intensity is high and grows significantly as cSOR increases.
The analysis shows that although some SAGD operations are achieving good steam-to-oil ratios, many are not achieveing thermally efficient operation, with cumulative steam-to-oil ratios many times the theoretical vallue. This results from combinations of geological realities, operator decisions and the limitations of the SAGD process.
The results demonstrate that on an energy and carbon dioxide emissions basis, bitumen or bitumen-based energy recovery processes need to step well beyond the capabilities of current steam-based bitumen recovery process, such as SAGD, if practical and sustainable energy balance and emissions scenarios are to be achieved from the in situ oil sands operations.—Gates and Larter
Financial support for the work was provided by the Natural Sciences and Engineering Research Council of Canada (NSERC), Carbon Management Canada and the Canada Research Chairs program.
Ian D. Gates, Stephen R. Larter (2014) “Energy efficiency and emissions intensity of SAGD”, Fuel, Volume 115, Pages 706-713 doi: 10.1016/j.fuel.2013.07.073