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).

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/ft3

• Converting 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 SG

• Converting 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 ppg

• Converting 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

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)