Drilling guides › Drilling fluid properties: mud weight, PV, YP and gels
Drilling fluid properties: mud weight, PV, YP and gels
The mud report is filled in at least once a tour on most rigs, and it is easy to treat it as a form. Each number on it answers a specific question, though: can the mud hold back the formation, can it lift cuttings, will it suspend barite when the pumps stop, is it building a thin wall cake, and are drilled solids taking over. This guide goes through the standard field tests for water-based mud in the order they usually appear on the report, explains what each one tells you, and shows how the viscometer readings are turned into the rheology numbers used in hydraulics.
Mud weight
Measured with a mud balance (a pressurised balance gives a truer reading when the mud holds entrained air or gas). It sets the hydrostatic pressure, so it is the primary well control barrier while drilling:
A 12.0 ppg mud is 1.44 sg (about 1,440 kg/m³) with a gradient of 0.624 psi/ft. Check the weight going in and coming out of the hole: lighter mud returning can mean gas or formation fluid cutting the mud, and should be reported straight away.
Funnel viscosity
The Marsh funnel time is the number of seconds one quart of mud takes to drain from a full funnel. Fresh water at about 70 °F runs in roughly 26 seconds. It is a quick trend check on the shaker, not a rheology measurement: two muds with the same funnel time can behave very differently downhole. Use it to spot a sudden change, then confirm with the viscometer.
Plastic viscosity and yield point
The rotational viscometer (often called a Fann) shears the mud at set speeds and reads the resulting dial deflection. The 600 and 300 rpm readings give the two Bingham plastic numbers on every mud report:
PV reflects mechanical friction between the solid particles and the liquid in the mud, so it rises with total solids, especially fine drilled solids. Keep it as low as the mud weight allows: high PV raises circulating pressure and ECD without helping hole cleaning. YP reflects attractive forces between particles and polymers, and is what helps carry cuttings in the annulus. A rising YP with a steady PV usually points to chemical causes such as contamination or flocculation; a rising PV with steady YP points to solids. The low-shear yield point, from the 6 and 3 rpm readings, matters most for hole cleaning in high-angle wells and for barite sag. In SI units, 1 cP = 1 mPa·s and 1 lb/100 ft² = 0.4788 Pa.
Power-law n and K
Bingham numbers describe the mud well at high shear but overestimate stress at low shear. Hydraulics programs therefore use the power-law (or the Herschel-Bulkley) model, with a flow behaviour index n and a consistency index K:
n = 1 is a Newtonian fluid like water. The lower n is, the more shear-thinning the mud: thin where shear is high (in the bit and drill pipe) and thick where shear is low (in a wide annulus), which is what you want. K is a measure of how thick the mud is overall.
Worked example 1: PV, YP, n and K from a mud check
A 12.0 ppg water-based mud gives θ600 = 58, θ300 = 36, θ100 = 18, θ6 = 8, θ3 = 6.
The low n_a shows the mud is strongly shear-thinning at annular shear rates, which is good for cuttings transport. The YP/PV ratio of about 0.64 is typical of a reasonably well-treated polymer or bentonite mud.
Worked example 2: reading a trend
Twenty-four hours later, with no change in mud weight, the same mud gives θ600 = 72 and θ300 = 42.
PV has risen by 8 cP while YP has hardly moved, and n has risen, so the mud is less shear-thinning. That pattern points to fine drilled solids building up rather than a chemical problem. The response is solids control (check shaker screens, run the centrifuge, dilute) rather than adding thinner. Confirm with the retort and methylene blue test before acting.
Gel strengths
Gels are the maximum 3 rpm reading after the mud has stood still for 10 seconds and for 10 minutes (sometimes 30 minutes too). They show how well the mud suspends cuttings and barite when circulation stops. Low, flat gels (for example 4 and 6) are easy to break and keep surge and swab pressures down. Gels that climb steeply with time ("progressive" gels) take high pump pressure to break and can cause losses when circulation restarts. Gels that are too low risk barite sag and cuttings settling during connections.
Fluid loss and filter cake
The API filter press pushes mud through filter paper at 100 psi for 30 minutes at room temperature and reports the filtrate volume in millilitres and the cake thickness. The HTHP test runs at higher pressure and temperature (500 psi differential and a temperature near bottom-hole is common) and its volume is doubled because the cell's filter area is half the standard one. Low fluid loss with a thin, tough cake reduces differential sticking risk, formation damage and swelling of reactive shales. A thick, soft cake is a warning even if the filtrate volume looks acceptable.
Solids, sand and pH
- Retort: distils a sample to give the water, oil and total solids by volume. Combined with mud weight it lets the mud engineer split solids into barite and low-gravity (drilled) solids. Low-gravity solids are commonly kept below a few per cent by volume in weighted muds.
- Sand content: the volume per cent of particles that do not pass a 200-mesh screen. Sand is abrasive to pumps and BHA components; many programmes ask for it to stay below about 0.5 to 1%.
- Methylene blue test (MBT): measures reactive clay. A rising MBT without bentonite additions means drilled clays are dispersing into the mud.
- pH: most water-based muds are run alkaline, often around 9 to 10.5, to control corrosion, keep certain additives working and help neutralise acid gases. A sudden fall in pH can be the first sign of CO₂ or H₂S influx.
Good practice
- Run viscometer tests at a consistent, recorded temperature (often 120 °F or 150 °F for water-based mud) so readings can be compared tour to tour.
- Take samples from the same point each time, normally the flow line for returns and the suction pit for mud going in.
- Look at trends across several reports, not single values.
- Change one thing at a time when treating the mud, and re-test before the next treatment.
- Report any unexpected drop in mud weight or pH, or a rise in chlorides, to the driller and company representative at once.