Drilling guides › Weighting up mud with barite
Weighting up mud with barite
Raising mud weight, for example to the kill weight from a kill sheet, means adding a dense solid, usually barite (specific gravity about 4.2, or 35 ppg).
Weighting up happens in two quite different situations. Most of the time it is planned: the mud weight is stepped up as the well goes deeper and pore pressure rises, and the mud engineer has hours to do it. During a well control event it is urgent: the kill weight has to be mixed in the active system while the well is shut in, and every hour spent mixing is an hour for gas to migrate. The arithmetic is the same in both cases; what changes is how much the logistics matter.
Where the formulas come from
Both formulas below come from a simple mass balance. The weight of the old mud plus the weight of the barite equals the weight of the new mud, and the volumes add up:
Vb is the volume of barite added per barrel of starting mud. Multiplying it by 1,470 lb per barrel of barite turns it into pounds. The 35 − W2 at the bottom explains why very heavy muds need so much barite: as the target weight approaches the density of barite, each extra 0.1 ppg costs more and more.
Barite needed
W1 is the current mud weight and W2 the target, both in ppg. 1,470 is the weight of one barrel of barite in pounds (35 ppg × 42 gal).
Volume increase
Worked example
Raise 500 bbl of mud from 10.5 ppg to 11.6 ppg:
Make sure the pits can hold the extra 23.5 bbl, or dump some mud first.
If the rig can add bulk barite at about 20,000 lb an hour, this weight-up takes roughly 1.7 hours of mixing, before the time needed to circulate the new weight evenly.
Starting with a fixed final volume
If the final volume is fixed (for example, the pits are full), the starting volume of light mud is:
For 500 bbl of 11.6 ppg mud from 10.5 ppg: 500 × 23.4 ÷ 24.5 ≈ 478 bbl of the old mud, plus the barite.
Lower-grade barite
Not all barite is 4.2 sg. With 4.1 sg barite the "35" becomes 4.1 × 8.345 ≈ 34.2 ppg and the "1,470" becomes about 1,437 lb per barrel.
Worked example: 4.1 sg barite
The same 500 bbl weight-up from 10.5 to 11.6 ppg:
The difference is small at this weight but grows quickly in heavy muds, so use the real density from the supplier's certificate.
Metric
Worked example: metric
80 m³ of 1.20 sg mud to 1.32 sg with 4.2 sg barite:
Cutting mud weight with water
The same mass balance works in reverse. To bring the weight down by dilution with fresh water (8.33 ppg):
Example: taking 400 bbl from 12.0 to 11.5 ppg needs 400 × 0.5 ÷ 3.17 ≈ 63 bbl of water. Dilution also thins the mud chemistry, so the mud engineer will normally add products along with the water.
Worked example: kill mud for the whole system
A kill sheet calls for 11.6 ppg in a well whose string, annulus and working pit volume add up to 900 bbl of 10.5 ppg mud.
That is nearly twice the barite of the 500 bbl example, and the pits must find room for 42 bbl more. Checking the barite stock against a number like this before drilling into a pressure transition is far easier than finding out during a shut-in.
Questions people ask
Why not use something heavier than barite?
Hematite (iron oxide, around 5 sg) is used when very high mud weights are needed or when the solids content must be kept low. It weighs up with less volume, but it is more abrasive on pumps and bits. The formulas are the same with 5.0 × 8.345 in place of 35.
What about weighting up across a reservoir?
In a reservoir section the mud is often weighted with ground calcium carbonate instead, because it can be removed later with acid and does less damage to the producing rock. At about 2.7 sg it adds weight slowly, so it is only used for modest increases.
Why does the weight fall again after mixing?
Usually because air was whipped into the mud at the hopper, or because barite that never wetted properly has settled in a quiet corner of the pit. Weigh the mud with a pressurised balance and check the pit bottoms before blaming the calculation.
Common mistakes
- Forgetting the volume increase, then overflowing the pits during a kill.
- Calculating for the active pits only, when the hole and the surface lines also need to be filled with kill mud. Kill mud volume is at least the string plus annulus plus a working surface volume.
- Adding barite faster than the hopper and agitators can wet it, leaving lumps that settle in the pits instead of raising the weight.
- Weighing samples before the new weight is mixed through the whole system.
Good practice
- Know how much barite is on location and how fast it can be mixed, and keep enough to raise the active system by the planned kick margin.
- Watch the rheology as weight goes up. Heavier muds carry more solids, and the mud engineer may need to thin the system to keep ECD and pump pressure in check.
- In deviated wells, watch for barite sag: barite settling on the low side of the hole when the mud is static, which leaves light and heavy mud in the hole after a trip.