Part five — the two-way traffic
A reservoir is not a bottomless tank — it is a pressurised sponge, and every barrel you take out leaves a hole behind. Producer wells let fluids out; injector wells push fluid back in so the pressure holds and the oil keeps moving. This page follows one cubic metre of injection water from the sea, through the platform and down the well, across the rock, and back to the surface again.
A well opened so oil, gas and water flow up from the reservoir to the platform, driven by the reservoir's own pressure. Its wellhead has a choke that controls how fast it is allowed to flow.
A well used in reverse: treated water (or gas) is pumped down it and out into the rock. It refills the space the producers empty, holds pressure up, and sweeps oil toward them.
Ten stages, from the sea to the producer and back. Tap a numbered badge on the drawing (or a stage button) to see what happens there. The animation shows water being treated, pumped down the injector, sweeping across the reservoir, and finally breaking through at the producer.
On a phone, swipe the drawing sideways to see it all.
Both are steel-lined holes about 3 km deep and look alike from the outside. The difference is which way the fluid moves — and that changes the equipment, the pressure and the risks.
| Producer | Injector | |
|---|---|---|
| Job | Bring oil, gas and water to the surface. | Put water or gas into the reservoir to hold pressure and sweep oil. |
| Fluid | Hot, gassy, sometimes sandy crude — plus produced water. | Clean, treated, oxygen-free water (or gas / CO₂). Cooler than the rock. |
| Wellhead pressure | Starts high, falls as the reservoir depletes; a choke holds it back. | Often ~100–250 bar, set by injection pumps. Must stay below fracture pressure. |
| Flow control | Production choke; sometimes gas lift or pumps once natural flow fades. | Injection choke and flow meter so each well takes its planned share. |
| Tubing | Carbon steel or alloy; sand screens if the rock is loose. | Often corrosion-resistant or plastic-lined; the packer keeps casing out of the pressure path. |
| Main risks | Sand, wax, scale, water breaking through, falling rates. | Plugging, scale, souring, corrosion, over-pressure that fractures the rock. |
Placement decides how evenly the water sweeps the oil. Engineers choose a pattern from the shape of the field, the rock and the cost of drilling.
Four injectors surround every producer — an even, repeating grid used on onshore fields with many cheap wells.
A row of injectors pushes a straight front toward a row of producers — good where wells can be lined up.
Injectors ring the outside, in or near the aquifer, squeezing the oil inward — typical of offshore fields with few wells.
Gas goes in at the top (crest), water at the flank; the producer sits in the oil between them.
Two quick calculations engineers do every day. The values are illustrative; move the sliders and watch what changes.
The goal is voidage replacement: put back, at reservoir conditions, the volume that production takes out.
Bottom-hole pressure = wellhead pressure + the weight of the water column. It must beat the reservoir — but not crack the rock.
Water is the workhorse, but injector wells are used for much more.
Treated seawater or aquifer water. Cheap, plentiful and the most widely used way to hold pressure and sweep oil.
Water that came up with the oil is cleaned and sent back down. It saves fresh water and avoids discharging to the sea — and has been standard practice for decades.
Gas goes into the gas cap at the crest to keep pressure up and store the gas for later sale instead of flaring it. Gas can also dissolve into oil and make it flow more easily.
Water-alternating-gas: slugs of water and gas take turns. Gas reaches oil the water bypassed; water stops the gas racing ahead.
Thickened water (with polymer) or adjusted-salinity water fingers less and can free oil clinging to rock. Results depend heavily on the reservoir.
Injectors also lock CO₂ underground — like the captured CO₂ Kasawari sends into a depleted reservoir (see Part 3) — or dispose of produced water and drill cuttings.
Most injection problems come back to one of three things: the water was dirty, the chemistry didn't match the rock, or the pressure was too high. Tap one to see cause and cure.
Engineers track rate and pressure continuously, run tracer chemicals that show up at producers, log flow with downhole tools, and repeat 4D seismic (Part 1's survey, repeated years apart) to see the water front move through the rock.
Injection refills the voidage so pressure — the reservoir's engine — doesn't run down.
Filtering, de-sulphating and de-oxygenating are cheap next to a plugged or soured reservoir.
Above reservoir pressure the water goes in; above fracture pressure the rock breaks. Stay between.
Water comes back with the oil, is cleaned, and goes down again. Late in life most of what comes up is water.