Mud Weight

The term “weight” is used in connection with mud more often than “density”, even though density is a more correct term.

Most tests of mud density are reported in pounds per gallon (ppg)

Density can also be reported in pounds per cubic foot(lb/cu ft), grams per cc (gr/cc), specific gravity (SG), or pressure gradient (psi/ft).

Conversion

To convert specific gravity to pounds per gallon (ppg)

Mud weight in ppg = specific gravity (SG) x 8.33

Converting 1.198 to mud weight in ppg = 1.198 x 8.33=10 ppg

To convert pounds per gallon (ppg) to pressure gradient (psi/ft)

Mud gradient (psi/ft) = mud weight in ppg x 0.052 Converting 10 ppg to psi/ft =0.052×10=0.52 psi/ft.

To convert pounds per gallon (ppg) to specific gravity

Mud weight in SG = mud weight in ppg / 8.33 Converting 10 ppg to specific gravity (SG) = 10/8.33 =1.20

To convert pressure gradient to mud weight in lb/ft3Converting 0.52 psi/ft to Mud weight lb/ft3 = 0.52/0069 = 74.8 lb/ft3.

Mud gradient (lb/ft3) = mud gradient in psi/ft /0.0069

To convert pressure gradient to mud weight in SGConverting 0.519 psi/ft to Mud weight in SG = 0.52/0.433 = 1.198.

Mud weight in SG = pressure gradient in psi/ft /0.433

To convert pressure gradient to mud weight in ppgConverting 0.52 psi/ft to mud weight in ppg = 0.52/0.052=10 ppg.

Mud weight in ppg = mud gradient in psi/ft /0.052

To convert specific gravity to mud weight in lb/ft3 Mud weight in lb/ft3 = specific gravity x 62.4 

Converting 1.198 to mud weight in ppg = 1.198 x 62.4=74.8 lb/ft3.

To convert specific gravity to pressure gradient in psi/ft Pressure gradient in psi/ft = specific gravity (SG) x 0.433 Converting 1.198 to mud weight in ppg = 1.198 x 0.433=0.52 psi/ft

Example

Prove that when density of drilling fluid is expressed in lbm/gal and depth in h ft, the pressure at the bottom of the wellbore can be expressed as P = 0.052 x density of mud (lbm/gal) x depth (h) psi

Pressure = density of mud [(lbm/gal) x 7.48(gal/ft3]gh(ft)

Where g = acceleration due to gravity

It can be further reduced as

P = 7.48 density of mud x gh [(lbm/ ft2) = 7.48 density of mud x h (ft2/144in2)=0.052 x density of mud (lbm/gal) x depth (h) psi

Equivalent Mud weight

The pressure (P –psi) in the wellbore or formation can be expressed in terms of equivalent mud weight (EMW) and is given as

EMW = Pressure/ (0.052 x Length (true vertical depth, ft)

Other References

More details can be obtained from Drilling Calculations501 Solved Problems for Drilling Operations.

Glossary of Drilling Operations

Conversion Factors

General Terms

Gel Strength

Formation Pressures

Well Control Calculations