Engineering Reference • Textbook Grounded
Petroleum Engineering Formula Library
Mathematical formulations, assumptions, and governing equations powering the AshToolbox Oil & Gas Calculation Suite.
1. API Gravity & Specific Gravity
API = (141.5 / SG_60/60) - 131.5 ⇔ SG_60/60 = 141.5 / (API + 131.5)
Reference water density at 60°F = 62.366 lb/ft³ (999.016 kg/m³). ASTM D1250-04 volume correction factor:
VCF = exp(-α_60 × ΔT × (1 + 0.8 × α_60 × ΔT))
2. Real Gas Equation of State (Z-Factor)
P × V = z × n × R × T, &quad; B_g = 0.028279 × (z × T) / P (rcf/scf)
Dranchuk-Abou-Kassem (DAK, 1975) 11-constant reduced-density BWR equation:
F(ρ_r) = 1 + R_1 ρ_r + R_3 ρ_r² - R_4 ρ_r&sup5; + R_5 ρ_r² (1 + A_11 ρ_r²) exp(-A_11 ρ_r²) - R_2 / ρ_r = 0
3. Pipe Hydraulics & Colebrook-White
ΔP_f = f × (L / D) × (ρ × v² / 2g_c)
1 / √f = -2 × log10[ (ε/D) / 3.7 + 2.51 / (Re × √f) ]
4. Drilling Annular Pressure & ECD
ECD = MW + ΔP_annular / (0.052 × TVD)
Bingham Plastic: τ = YP + PV × γ. Herschel-Bulkley: τ = τ_0 + K × γ^n.
5. Arps (1945) Production Decline Equations
q(t) = q_i / (1 + b × D_i × t)^(1/b), &quad; Q(t) = [q_i / (D_i(1 - b))] × [1 - (1 + b × D_i × t)^(1 - 1/b)]
6. Reservoir Volumetrics (STOIIP & OGIP)
STOIIP = 7758 × A × h × NTG × [φ × (1 - S_w)] / B_oi (STB)
OGIP = 43560 × A × h × NTG × [φ × (1 - S_w)] / B_g (scf)
7. Discounted Cash Flow & Petroleum Economics
NPV = ∑ [ NCF_t / (1 + r)^(t - 0.5) ] - CAPEX, &quad; IRR ⇒ NPV(r) = 0