Oil wells make gas too - the associated or casinghead gas that comes up dissolved in and alongside the oil. When several oil wells share a battery, their gas commingles just as their oil does, and the sales gas meter reports only a battery total. Gas-oil ratio back allocation is the standard way to split that commingled gas back to individual wells, using the relationship each well showed on test between how much oil it makes and how much gas comes with it. This guide explains how a tested GOR drives the split, why that ratio drifts over a well's life, and how continuous measurement keeps the allocation honest between tests.
GOR Back Allocation in one line: Gas-oil ratio back allocation assigns commingled associated gas to individual wells by multiplying each well's allocated or measured oil by its most recently tested gas-oil ratio. The resulting per-well theoretical gas volumes are then scaled so their sum matches the metered gas sales total, giving each well a share of the gas anchored to its own oil production.
The gas-oil ratio, or GOR, is simply how much gas a well produces per unit of oil, usually expressed as standard cubic feet of gas per barrel of oil. It is measured on a well test, where the well is isolated and both its oil rate and its gas rate are recorded at the same time; dividing gas by oil gives that well's GOR. Because the ratio ties gas to oil, it lets an operator estimate gas production from an oil number, which is exactly what is needed at a commingled battery where the oil per well is already being allocated but the gas is not directly measured per well.
The back-allocation runs in a clear sequence. First, oil is measured or allocated per well - often through the tank-battery allocation that splits the sales oil total among the wells. Second, each well's allocated oil is multiplied by its tested GOR to produce a theoretical gas volume for that well. Third, those theoretical gas volumes are summed and compared to the actual metered gas total, then scaled by an allocation factor so the well volumes reconcile exactly to what the gas meter recorded. The gas allocation therefore rides on the back of the oil allocation, which is why the technique is called back allocation.
This oil-anchored approach makes sense because at many oil batteries the oil is the more carefully measured stream and the associated gas is a byproduct that is metered only in total at the sales point. Rather than try to measure each well's gas directly, which would require gas metering on every well, the operator leverages the physical link between oil and gas that the GOR captures. As long as each well's GOR is reasonably current, multiplying oil by GOR gives a defensible estimate of that well's gas contribution.
The weakness of the method is that GOR is not a constant. It changes over a well's producing life, and it changes in ways that matter to the allocation. Early on, while reservoir pressure stays above the bubble point, gas remains dissolved in the oil and the produced GOR is relatively stable. As the reservoir depletes and pressure falls below the bubble point, gas begins to come out of solution in the reservoir, and the produced gas-oil ratio typically climbs, sometimes sharply, as more free gas is produced per barrel of oil. A GOR measured a year ago may badly understate the gas a well is making today.
Water and mechanical effects add more drift. As a well waters out, its oil rate falls and the gas that still comes up makes the apparent GOR rise. Artificial lift changes, gas breakthrough, and coning can all shift the ratio between one test and the next. Because a well test is a snapshot taken on one day, the GOR it produces is only exactly right on that day and grows less representative the longer it stands. An allocation that keeps multiplying today's oil by a stale GOR will systematically misallocate gas, crediting some wells with too much and others with too little.
The consequence is that the accuracy of GOR back allocation degrades with the age of the tests behind it. If one well in the group has drifted to a much higher GOR than its last test shows, the allocation factor absorbs some of the error but spreads it across every well, quietly distorting all of their gas volumes. This is why test frequency matters so much for gas allocation, and why a well showing signs of changing behavior deserves a fresh test sooner rather than later.
The way to limit the damage from drifting ratios is to shorten the blind interval between tests, and that is where continuous field measurement changes the picture. A cloud SCADA platform such as Merobix logs each well's oil-side runtime and production continuously and captures the battery gas-sales meter in real time, so the two inputs the allocation depends on - the oil per well and the total gas - are current rather than reconstructed at month-end. Instead of one GOR snapshot standing in for weeks of production, the operator has a live record against which to sanity-check whether the assumed ratios still hold.
SCADA also makes drift visible before it becomes a large allocation error. When metered total gas rises faster than the summed theoretical gas the GORs predict, the allocation factor moves away from one, and a platform that surfaces that factor over time turns a quiet accounting drift into an explicit signal that a well's GOR has changed and a test is due. Rather than discovering at an audit that the gas split had grown unreliable, the field team sees the divergence as it develops and can route the suspect well to test.
Where gas metering is available closer to individual wells or on a rotating test separator, feeding that data into the same platform tightens the allocation further, because a recent measured gas rate can update a well's GOR far more often than a periodic full well test. The general principle holds regardless of instrumentation: GOR back allocation is only as good as the ratios and totals behind it, and continuous SCADA measurement keeps both current so the gas assigned to each well stays defensible between well tests instead of drifting silently along with the reservoir.
Each well's oil is multiplied by its most recently tested gas-oil ratio to give a theoretical gas volume, those volumes are summed for the battery, and the sum is scaled by an allocation factor so it matches the metered gas sales total. Because gas is derived from each well's own oil and GOR, the split reflects the physical link between oil and associated gas rather than an arbitrary division.
GOR usually rises as a reservoir depletes. While pressure stays above the bubble point the ratio is fairly stable, but once pressure drops below it, gas comes out of solution in the reservoir and more free gas is produced per barrel of oil. Watering out, gas breakthrough, and lift changes push it further. A GOR measured months ago can badly understate the gas a well makes now, which is why old tests degrade allocation accuracy.
Frequently enough that the tested ratio still represents current production, which depends on how fast a given well's GOR is drifting. Wells past the bubble point or watering out change faster and need testing more often. Watching the allocation factor and the metered-versus-theoretical gas gap in a SCADA system flags when a well has drifted and a fresh test is due, so testing can follow the data instead of a fixed calendar alone.
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