Troubleshooting
Losing returns: a decision tree for lost circulation
Match the treatment to the size of what you are losing fluid into, and stop throwing product at a hole that needs casing.
4 min read
Lost circulation is expensive in three currencies at once: the fluid and product you pump away, the rig time you spend fighting it, and the hole you eventually lose if you fight it badly. The single most useful thing you can do when returns drop is work out how much you are losing, because that number selects the treatment.
Classify the loss before you treat it
| Loss type | What it looks like | What it usually is |
|---|---|---|
| Seepage | Returns slightly down, sump level falling slowly | Permeable, porous formation — sand, gravel, weathered rock |
| Partial loss | Reduced but continuous returns at surface | Open natural fractures, or a coarse permeable zone |
| Severe loss | Little or no return, but the hole stays full | Large fractures, rubble zones, fault gouge |
| Total loss | No return and the fluid level in the hole drops | Open voids, cavernous ground, old workings |
The distinction matters because lost circulation material works by bridging. A bridging agent sized for seepage will pass straight through a fracture, and a coarse agent aimed at a fracture will plug the bit and the annulus on the way to a seepage zone it was never going to help.
Seepage: fix the fluid, not the formation
- First, check the fluid properties. A thin, low-viscosity system with no filter cake will seep into ground that a properly built mud would seal without any special product at all.
- Build the cake. This is the case where bentonite does the work polymer cannot.
- Add fine bridging material — fine granular or fibrous LCM — to seal the pore throats.
- Reduce pump pressure and annular velocity. Pushing harder against a seeping formation makes it seep faster.
Partial and severe loss: bridge the fracture
Here you are trying to build a plug inside an opening you cannot see. A blend works better than any single material, because a mix of particle sizes and shapes forms a mat where uniform particles wash through.
- Granular — nut plug, ground walnut shell, graded calcium carbonate — provides the structural bridge across the opening.
- Fibrous — cellulose and similar — mats across the granular bridge and holds it together.
- Flaked — mica, cellophane — lays over the mat and seals the remaining gaps.
- Pump the blend as a spotted pill placed across the loss zone, not mixed through the whole active system where most of it will never reach the problem.
Know your bit and your smallest passage
Coarse LCM has to get through the pump, the swivel, the rod bore and the bit waterways before it reaches the formation. In small-diameter coring, that limits what you can realistically pump — which is why a fine, high-fluid-loss squeeze often beats a coarse blend on a wireline rig.
Total loss: stop pumping product at it
When the fluid level in the hole is falling, you are pumping into an opening large enough that no reasonable amount of LCM will bridge it. Programs lose real money at this point by continuing to try.
- Consider drilling blind through the zone with water, if the hole is stable and the cuttings do not need to come back — sometimes the cheapest answer is to accept the loss and get past it.
- Set a cement plug across the zone, let it cure properly, and drill it out. Curing time is not the place to save an hour.
- Case it off. This is what casing exists for, and reaching for it early is usually cheaper than the three days of LCM that precede it.
- If the hole cannot be saved, plug and re-collar with a plan that cases the zone from the start.
The prevention that actually works
- Know where the loss zones are before you drill into them. Adjacent holes, old workings and mapped fault traces are all on file somewhere.
- Keep annular velocity and pump pressure no higher than the hole needs. Equivalent circulating density is what breaks weak formations open.
- Carry LCM on site before you need it. The zone will be found at 2am, and the nearest stock is usually a day away.
- Plan the casing telescope with the loss zones in it, so the option to case off exists when it is needed.
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