**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/ft ^{3}**Converting 0.52 psi/ft to Mud weight lb/ft

^{3 }= 0.52/0069 = 74.8 lb/ft

^{3}.

Mud gradient (**lb/ft ^{3}**) = 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/ft ^{3}** 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/ft^{3}.

**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/ft^{3}]gh(ft)

Where g = acceleration due to gravity

It can be further reduced as

P = 7.48 density of mud x gh [(lbm/ ft^{2}) = 7.48 density of mud x h (ft^{2}/144in^{2})=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 501 Solved Problems for Drilling Operations.