The Pattern That Was Not Balanced: Voidage Replacement Below the Bubble Point
A 5-spot waterflood in a 22% porosity sandstone (Bo 1.25 rb/STB, Bw 1.02) has been injecting for 18 months. The single injector takes 6,200 BWPD at 1,900 psi wellhead pressure, which is 3,600 psi bottomhole at 4,800 ft TVD; the fracture gradient is 0.75 psi/ft. The four producers total 5,000 BFPD at 62% water cut, average GOR has climbed from 1,050 to 1,800 scf/STB, and static pressure has fallen from 3,300 to 3,000 psia. At current conditions the solution GOR is 1,100 scf/STB and Bg is 0.0054 rb/scf; the bubble point is 3,050 psia. The monthly report states "voidage replacement ratio 1.14 - pattern is balanced." Two producers are approaching their gas-lift unloading point and you are asked to sign off on flood performance.
1. Reproduce the voidage replacement ratio the report used, then compute the correct value. Is the pattern balanced?
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What the report did (liquids only)
- Oil: 5,000 x 0.38 = 1,900 STB/d x 1.25 = 2,375 RB/d
- Water: 3,100 bbl/d x 1.02 = 3,162 RB/d
- Total liquid voidage = 5,537 RB/d
- Injection: 6,200 x 1.02 = 6,324 RB/d
- VRR = 6,324 / 5,537 = 1.14
What it omitted. Static pressure (3,000 psia) is now below the 3,050 psia bubble point, so free gas occupies pore space and is being produced:
- Free gas rate = (1,800 - 1,100) scf/STB x 1,900 STB/d = 1.33 MMscf/d
- Reservoir volume = 1.33e6 x 0.0054 = 7,180 RB/d
Corrected VRR
- Total voidage = 2,375 + 3,162 + 7,180 = 12,717 RB/d
- VRR = 6,324 / 12,717 = 0.50
Conclusion. The pattern is replacing roughly half of the reservoir volume it is withdrawing. The 1.14 figure is an artefact of ignoring the free gas, which is the largest single component of voidage once the pressure is below the bubble point. The report is not conservative - it is wrong by a factor of more than two.
2. Why is reservoir pressure still falling and GOR still rising? What does the injector bottomhole pressure tell you?
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Pressure and GOR are the same event. When static pressure crossed below 3,050 psia, solution gas began to evolve in the pore space. At 3,000 psia a thousand standard cubic feet occupies about 5.4 reservoir barrels, so a small pressure drop creates a large gas saturation. That gas:
- consumes pore volume that the injected water must fill before pressure can be restored;
- lowers oil relative permeability and therefore oil rate at a given drawdown;
- is produced as the GOR rise (1,050 to 1,800 scf/STB).
Because only liquid voidage was being replaced, the gas saturation has been allowed to grow unchecked - the flood is being operated as a partial-pressure-maintenance scheme without anyone realising it.
The injector is at parting pressure. BHP = 3,600 psi at 4,800 ft TVD = 0.75 psi/ft, which equals the stated fracture gradient exactly. Injection at the fracture gradient means:
- part of the 6,200 BWPD may be going out of zone (upward along the fracture or into non-pay) rather than into the oil column;
- conformance and sweep are unverified;
- the effective VRR in the oil column is likely below the already poor 0.50.
This also explains why a high injection rate has not arrested the pressure decline.
3. List the actions you would take in the next 90 days, in priority order, with the reasoning for each.
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Step-rate test on the injector plus a pressure fall-off. Establish parting pressure and fracture closure, and confirm how much of the injected volume is going into a fracture rather than matrix. Injection pressure should be pulled back below parting (target roughly 3,300-3,400 psi BHP) unless the fracture is a deliberate design.
Injection profile log (spinner/temperature). Confirm which intervals take water and whether any is leaving the oil zone. Without this, every VRR discussion is speculative.
Re-baseline the surveillance. Recompute VRR monthly including free gas using measured Bg/Rs, and report BHP on the injector alongside rate. A VRR quoted on liquids only should be removed from the report template.
Fix the production side before the two wells die. With gas lift near the unloading point, a well that loads up loses its rate entirely. Optimise lift gas on those wells, lower drawdown where gas is being cycled, and consider installing plunger assist or converting to a more robust lift.
Plan for additional injection capacity. If injection must be held below parting pressure, a single injector almost certainly cannot deliver the 12,700 RB/d of voidage the pattern needs. Adding a second injector, or reperforating into intervals with better conformance, is a capital decision that should be scoped now rather than after pressure reaches the oil-saturation penalty.