Field Notes · Flare maths
Why associated gas rises, then falls
Associated gas rate is oil rate times gas-oil ratio, and the product is usually a hump. Why a plant sized to day-one gas flares or idles.
The catch with flare gas is that the rate does not stay put. Sizing a plant well starts with understanding why.
Two curves, multiplied
Associated gas rate is simply oil rate times gas-oil ratio (GOR).
- Oil rate usually declines as the field is produced.
- GOR sits flat while reservoir pressure is above bubble point, because the gas stays dissolved in the oil. Once pressure falls below bubble point, gas comes out of solution and free gas flows more easily to the wells, so the ratio climbs. Late in field life, as pressure and gas run low, it eases off again.
Multiply the two and gas production makes a hump: up, then down. Pressure maintenance, such as water injection, can keep the GOR near flat, so the shape depends on the field.
Why it matters for the plant
A plant sized to day-one gas flares the excess while the rate climbs, then runs below capacity for much of its life once it falls. Neither is a good use of capital.
Staged modular blocks, 5 to 50 MW at the wellhead or plant fence, can follow the profile instead: one step up as gas rises, one step down as it falls, each step with a little spare capacity.
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