Thermal field operations and performance

Ethan's second month in the thermal group begins in Bakersfield. Tom Ashworth, the operations superintendent at Tamarisk Hill steamflood, meets him at the central plant and hands him a hard hat and a printout of the monthly field report. 'Paul taught you the theory,' Tom says. 'Out here we manage it one pattern and one well at a time. Every morning we look at three numbers: the steam we put in, the oil we got out and the temperature at every producer. If those are right, the rest usually follows. And because the board wants us to bid on that SAGD project in Alberta, I will show you how their operators run a pad too.'

This lesson follows the fluids through a thermal operation: from water treatment and steam generation, through the wells and the reservoir, to the production facility where oil, water and gas are separated and the water is recycled. It explains the steam-oil ratio and the other key performance measures, the subcool control that keeps a SAGD producer working, artificial lift at high temperature and the problems that most often limit production.

The steam-oil ratio

The most important number in any thermal project is the steam-oil ratio, SOR: the volume of steam injected, measured as the cold water equivalent volume of the water used to make it, divided by the volume of oil produced over the same period. An SOR of 3 means three volumes of water were turned into steam for every volume of oil produced. The cumulative SOR, CSOR, uses the totals since the start of the project or of a period, and smooths out the day-to-day noise.

The SOR drives most of the costs. Fuel to make steam is usually the largest operating cost, and in most thermal projects it is also the largest source of greenhouse gas emissions. Every unit of steam needs water treatment and boiler capacity, and every unit of produced water must be separated and handled. Typical SAGD projects in Alberta operate with SORs between about 2 and 4; the best projects in clean, thick pay run below 2.5, while thin or heterogeneous reservoirs may run above 4. Steamfloods typically run between about 3 and 8 over their life, rising as the floods mature. Some operators also report the inverse, the oil-steam ratio, OSR.

A related measure is the energy intensity of production, in gigajoules of fuel per cubic meter of oil, or million Btu per barrel. It accounts for the steam quality and boiler efficiency as well as the SOR, and it converts directly into fuel cost and emissions, which is why lesson 11 uses it.

# SAGD pad performance: steam-oil ratio, steam quality and water balance
SI = False  
oil = [310, 300, 295, 280, 260, 255]           # m3/d bitumen, last six months, pad of eight well pairs
cwe = [930, 915, 930, 900, 880, 895]           # m3/d steam as cold water equivalent
for i, (o, s) in enumerate(zip(oil, cwe), 1):
    if SI: print(f"month {i}: oil {o} m3/d, steam {s} m3/d CWE, SOR {s / o:.2f}")
    else:  print(f"month {i}: oil {o * 6.2898:,.0f} bbl/d, steam {s * 6.2898:,.0f} bbl/d CWE, SOR {s / o:.2f}")
csor = sum(cwe) / sum(oil)
print(f"cumulative SOR for the period: {csor:.2f}")
# water balance: produced water returns about 1.0 times the steam; some is lost to blowdown and the reservoir
steam = sum(cwe) / len(cwe)
prod_water = 0.95 * steam
blowdown = 0.10 * steam                        # OTSG blowdown, part recycled after evaporation in some plants
recycle = prod_water - 0.05 * prod_water       # treatment losses about 5 %
makeup = steam + blowdown - recycle
print(f"produced water {prod_water:.0f}, recycled {recycle:.0f}, make-up {makeup:.0f} m3/d ({makeup / steam:.0%} of steam)" if SI
      else f"produced water {prod_water * 6.2898:,.0f}, recycled {recycle * 6.2898:,.0f}, make-up {makeup * 6.2898:,.0f} bbl/d ({makeup / steam:.0%} of steam)")
# subcool: steam temperature at the producer pressure minus fluid temperature at the producer
for T_sat, T_prod in ((215, 210), (215, 195), (215, 214)):
    sc = T_sat - T_prod
    note = "steam may break through to the pump" if sc < 3 else ("liquid pool is high; rate may be held back" if sc > 15 else "inside the target band")
    print(f"subcool {sc if SI else sc * 1.8:4.0f} {'C' if SI else 'F'}: {note}")

month 1: oil 1,950 bbl/d, steam 5,850 bbl/d CWE, SOR 3.00
month 2: oil 1,887 bbl/d, steam 5,755 bbl/d CWE, SOR 3.05
month 3: oil 1,855 bbl/d, steam 5,850 bbl/d CWE, SOR 3.15
month 4: oil 1,761 bbl/d, steam 5,661 bbl/d CWE, SOR 3.21
month 5: oil 1,635 bbl/d, steam 5,535 bbl/d CWE, SOR 3.38
month 6: oil 1,604 bbl/d, steam 5,629 bbl/d CWE, SOR 3.51
cumulative SOR for the period: 3.21
produced water 5,428, recycled 5,156, make-up 1,128 bbl/d (20% of steam)
subcool    9 F: inside the target band
subcool   36 F: liquid pool is high; rate may be held back
subcool    2 F: steam may break through to the pump

The Alberta pad Tom uses as an example shows the SOR rising from 3.0 to 3.5 over six months while the oil rate falls. He is not alarmed by the trend itself. 'Those chambers have reached the cap rock and are spreading,' he says. 'A rising SOR is normal at that stage, and our steamflood patterns at Tamarisk Hill do the same as they mature. What I watch for is a sudden jump, which usually means steam is going somewhere it should not: into a thief zone, through a casing leak or straight into a producer.'

Water treatment and steam generation

A thermal project is, to a large extent, a water-handling business. Each unit of oil produced comes with several units of produced water, the condensed steam, at high temperature. That water is separated from the oil, cooled in heat exchangers that recover its heat, de-oiled by skim tanks, gas flotation and filters, and then softened or otherwise treated before it is used again to make steam.

Most SAGD and steamflood projects in North America use once-through steam generators, OTSGs, which can tolerate water with more dissolved solids than drum boilers. An OTSG passes the water once through a long tube coil and turns about 75 to 80 percent of it into steam, giving wet steam with a quality of about 0.75 to 0.80. In SAGD plants the remaining liquid is separated in a high-pressure separator and sent to blowdown, so that only dry steam goes to the wells; part of that blowdown can be recycled after evaporation. Some newer plants use evaporators to make distilled water for drum boilers, and many recover heat from the boiler exhaust. Cogeneration, in which a gas turbine makes electricity and its hot exhaust makes steam, is used at several large projects and lowers the combined emissions of power and steam.

Water use is regulated too. California's steamfloods recycle much of their produced water into steam, but they also produce far more water than they need, so the rest is reinjected into disposal or injection wells or, in a few fields, treated and supplied for crop irrigation. Water disposal and injection wells are permitted by CalGEM under its underground injection control program. In Alberta, Directive 081 limits the use of fresh and brackish make-up water at in situ projects, and many SAGD plants recycle more than 90 percent of their produced water.

Subcool control

In SAGD the producer must collect bitumen and condensate without letting live steam into it. If steam breaks into the producer, it wastes heat, reduces the pressure difference that moves the liquid, erodes the screens and can damage the pump. The control variable is the subcool: the difference between the saturation temperature of steam at the producer pressure and the actual fluid temperature at the producer. A positive subcool means the fluid at the producer is liquid below its boiling point, which implies a liquid pool sitting above the well.

Operators target a subcool band, often roughly 5 to 15 °C (9 to 27 °F) at surface or measured with fiber-optic or thermocouple strings along the well. Too little subcool risks steam breakthrough; too much means a deep liquid pool that floods the bottom of the chamber, reduces the drainage area and lowers the rate. The producer's pump speed is the usual control: speeding up the pump lowers the liquid level and the subcool, slowing it raises them. Because temperature can vary along a 900 m (3,000 ft) well, operators use the temperature profile to see which parts of the well are draining well, and use dual tubing strings, flow control devices or steam splits between heel and toe to even out the chamber.

In steamfloods the equivalent concern is steam breakthrough at the producers. When a producer's temperature rises toward the steam temperature and its gas or steam rate climbs, operators may restrict it, change the injection pattern, or convert it to an injector.

Artificial lift at high temperature

Thermal wells produce hot fluids that cannot flow to surface on their own, so nearly all need artificial lift. Sucker-rod pumps are widely used, particularly in steamfloods and in many SAGD wells, because they tolerate high temperature and some sand. Electric submersible pumps designed for high temperature, up to about 250 °C (480 °F), allow higher rates and lower operating pressures in SAGD and are now common in Alberta. Progressive cavity pumps are used in cold heavy oil wells but their elastomers limit them in hot service. In the early life of SAGD projects run at high pressure, gas lift or natural steam lift was sometimes used.

Thermal wells also need strong mechanical design. Casing heated from about 10 °C to over 200 °C tries to expand and, being cemented, is put under large compressive stress, which may exceed its yield strength. Thermal casing designs use higher grades, premium connections, thermal cement and controlled heating procedures. Wellhead and surface piping use expansion loops for the same reason.

Common production problems

Several problems recur at thermal projects. Sand production through damaged screens fills wells and erodes equipment, and may need workovers or remedial liners. Emulsions of water and heavy oil can be difficult to break and require chemicals, heat and long residence times. Scaling and fouling in steam generators and wellbores result from silica, calcium and iron in the water. Hydrogen sulfide forms by aquathermolysis, the reaction of steam with sulfur compounds in the bitumen, and must be managed for safety. Geological problems include thief zones, top water or gas that drain heat, and mud layers that stop chambers rising. Most can be managed, but each tends to raise the SOR.

Turnarounds, reliability and the daily routine

A thermal facility runs around the clock, and its performance depends as much on reliability as on reservoir engineering. Steam generators are taken offline in rotation for tube inspection and cleaning, and major plant turnarounds are scheduled every few years, during which steam injection falls and the reservoir begins to cool. Operators plan these carefully, often injecting extra steam before a turnaround and ramping back up gradually afterward to protect well integrity. Daily work includes well tests on each producer, usually through a test separator, to allocate production back to wells; chemical programs for emulsion breaking, scale and corrosion; and surveillance of temperatures, pressures and pump performance from a central control room. Small improvements across many wells, a slightly better subcool here, a faster pump repair there, add up to a lower SOR and a higher field rate.

Over a year, those daily decisions are what separate a well-run project from an average one, and the steam-oil ratio is where the difference shows first.