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Petrophysics Equations

Browse 132 petrophysics petroleum engineering equations with variables, units, source references, and calculator links.

Petrophysics calculators support porosity, saturation, resistivity, formation factor, shale correction, SP-log, nuclear-log, and core-log interpretation workflows.

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PetrophysicsRock Properties

Acoustic Transit Time

Δt=106v\Delta t=\frac{10^6}{v}
View formula
PetrophysicsRock Properties

Adhesion Tension from Water-Oil Interfacial Tension

τ=σwocos⁡θ\tau=\sigma_{wo}\cos\theta
View formula
PetrophysicsRock Properties

Air Density for de Nouy Upper Phase Correction

ρair=4.324×10−2PT\rho_{air}=4.324\times10^{-2}\frac{P}{T}
View formula
PetrophysicsRock Properties

Amott-Harvey Wettability Index

IAH=Iw−IoI_{AH}=I_w-I_o
View formula
PetrophysicsRock Properties

Amplitude Transmission Coefficient in Seismic Reflection and Refraction

T=2Z1Z2+Z1T=\frac{2Z_1}{Z_2+Z_1}
View formula
PetrophysicsRock Properties

Apparent Gamma Ray Recorder Intensity

Ja(t)=J1+(J2−J1)(1−e−t/rc)J_a(t)=J_1+(J_2-J_1)(1-e^{-t/r_c})
View formula
PetrophysicsRock Properties

Apparent Interfacial Tension - de Nouy Ring Method

S=mg2lS=\frac{mg}{2l}
View formula
PetrophysicsElectrical Properties

Apparent Resistivity

R=GtΔV12IR=G_t\frac{\Delta V_{12}}{I}
View formula
PetrophysicsElectrical Properties

Archie Cementation Exponent from Formation Factor

m=ln⁡(F/a)ln⁡(1/ϕ)m=\frac{\ln(F/a)}{\ln(1/\phi)}
View formula
PetrophysicsResistivity Logs

Archie Flushed-Zone Water Saturation from Shallow Resistivity

Sxo=(aRmfϕmRxo)1/nS_{xo}=\left(\frac{aR_{mf}}{\phi^mR_{xo}}\right)^{1/n}
View formula
PetrophysicsElectrical Properties

Archie Formation Factor from Porosity

F=1ϕmF = \frac{1}{\phi^m}
View formula
PetrophysicsElectrical Properties

Archie Resistivity Index from Water Saturation

RI=Sw−nRI = S_w^{-n}
View formula
PetrophysicsElectrical Properties

Archie True Formation Resistivity

Rt=RwϕmSwnR_t = \frac{R_w}{\phi^m S_w^n}
View formula
PetrophysicsElectrical Properties

Archie Water Saturation from Formation Factor

Sw=(FRwRt)1/nS_w = \left(\frac{FR_w}{R_t}\right)^{1/n}
View formula
PetrophysicsResistivity Logs

Archie Water Saturation from Resistivity Logs

Sw=[(aϕm)(RwRt)]1/nS_w = \left[\left(\frac{a}{\phi^m}\right)\left(\frac{R_w}{R_t}\right)\right]^{1/n}
View formula
PetrophysicsResistivity Logs

Arps Formation Water Resistivity Temperature Correction

Rw2=Rw1T1F+6.77T2F+6.77R_{w2}=R_{w1}\frac{T_{1F}+6.77}{T_{2F}+6.77}
View formula
PetrophysicsRock Properties

Atlas Wireline Neutron Lifetime Log

Σ=3.15τ\Sigma=\frac{3.15}{\tau}
View formula
PetrophysicsResistivity Logs

Bulk Volume Water from Porosity and Water Saturation

BVW=ϕSwBVW=\phi S_w
View formula
PetrophysicsResistivity Logs

Carothers Formation Factor - Calcareous Sands

F=1.45ϕ1.70F=\frac{1.45}{\phi^{1.70}}
View formula
PetrophysicsResistivity Logs

Carothers Formation Factor - Shaly Sands

F=1.65ϕ1.33F=\frac{1.65}{\phi^{1.33}}
View formula
PetrophysicsResistivity Logs

Chalky Carbonate Formation Factor from Porosity

Fc=1ϕ2F_c=\frac{1}{\phi^2}
View formula
PetrophysicsResistivity Logs

Chevron Formation Factor from Porosity

F=1.13ϕ1.73F=\frac{1.13}{\phi^{1.73}}
View formula
PetrophysicsRock Properties

Clavier Shale Volume from Gamma Ray Index

Vsh=1.7−3.38−(IGR+0.7)2V_{sh}=1.7-\sqrt{3.38-(I_{GR}+0.7)^2}
View formula
PetrophysicsRock Properties

Coates NMR Permeability from Free-Fluid Ratio

k=(ϕNMRC)4(FFIBVI)2k=\left(\frac{\phi_{NMR}}{C}\right)^4\left(\frac{\mathrm{FFI}}{\mathrm{BVI}}\right)^2
View formula
PetrophysicsResistivity Logs

Compacted Carbonate Formation Factor from Porosity

FcO=1ϕaF_{cO}=\frac{1}{\phi^a}
View formula
PetrophysicsRock Properties

Composite Capture Cross Section of the Formation Schlumberger Thermal Decay Time Tool

Σ=4.55τ\Sigma=\frac{4.55}{\tau}
View formula
PetrophysicsRock Properties

Contact Angle from Interfacial Tensions

θ=cos⁡−1(σso−σswσwo)\theta=\cos^{-1}\left(\frac{\sigma_{so}-\sigma_{sw}}{\sigma_{wo}}\right)
View formula
PetrophysicsRock Properties

Corrected Interfacial Tension - de Nouy Ring Method

σ=SC\sigma=SC
View formula
PetrophysicsRock Properties

de Nouy Ring Interfacial Tension Correction Factor

C=0.7250+0.01452Sl2(ρD−ρd)+0.04534−1.679rRC=0.7250+\sqrt{\frac{0.01452S}{l^2(\rho_D-\rho_d)}+0.04534-\frac{1.679r}{R}}
View formula
PetrophysicsRock Properties

Density Log Porosity from Bulk Density

Φ=ρma−ρbρma−ρf\Phi=\frac{\rho_{ma}-\rho_b}{\rho_{ma}-\rho_f}
View formula
PetrophysicsElectrical Properties

Diffuse-Layer Thickness from Salt Concentration

xd=3n−1/2x_d=3n^{-1/2}
View formula
PetrophysicsElectrical Properties

Dual-Water Brine-Saturated Rock Conductivity

Co=1Fo(Cwf+fdlvQQv(Cwb−Cwf))C_o=\frac{1}{F_o}\left(C_{wf}+f_{dl}v_QQ_v(C_{wb}-C_{wf})\right)
View formula
PetrophysicsElectrical Properties

Effect of Clay on Conductivity

Co=CwF+CsC_o=\frac{C_w}{F}+C_s
View formula
PetrophysicsRock Properties

Effective Photoelectric Absorption Cross Section Index

Pe=(Z10)3.6P_e=\left(\frac{Z}{10}\right)^{3.6}
View formula
PetrophysicsElectrical Properties

Electric Resistance to Radial Current from a Wellbore

Re=ln⁡(re/rw)2πhσR_e=\frac{\ln(r_e/r_w)}{2\pi h\sigma}
View formula
PetrophysicsElectrical Properties

Electrical Conductivity from Resistivity

σ=1ρe\sigma = \frac{1}{\rho_e}
View formula
PetrophysicsElectrical Properties

Electrical Resistivity from Resistance Geometry

R=rALR = r \frac{A}{L}
View formula
PetrophysicsElectrical Properties

Electrochemical Potential - SP Log

Ec=2tClRTFln⁡(a1a2)E_c=2t_{Cl}\frac{RT}{F}\ln\left(\frac{a_1}{a_2}\right)
View formula
PetrophysicsElectrical Properties

Electrode Array Geometric Coefficient

Gt=4πr1r2r2−r1G_t=4\pi r_1\frac{r_2}{r_2-r_1}
View formula
PetrophysicsElectrical Properties

Electrokinetic Potential Across Mud Cake

Ek=xpyE_k=xp^y
View formula
PetrophysicsRock Properties

Electron Density Index for Gamma-Gamma Absorption Logging

ρe=2NeNA\rho_e=\frac{2N_e}{N_A}
View formula
PetrophysicsRock Properties

Epithermal Neutron Diffusion Coefficient

De=ξLe2ΣeD_e=\xi L_e^2\Sigma_e
View formula
PetrophysicsRock Properties

Epithermal Neutron Flux Distribution

ψe=NN4πDee−r/Ler\psi_e=\frac{N_N}{4\pi D_e}\frac{e^{-r/L_e}}{r}
View formula
PetrophysicsResistivity Logs

Equivalent Formation Water Resistivity from SP

Rwe=Rmfe10−SSP/K,K=61+0.133TFR_{we}=\frac{R_{mfe}}{10^{-SSP/K}},\quad K=61+0.133T_F
View formula
PetrophysicsRock Properties

Error Percentage of Porosity Measurements

Eϕ=ϕm−ϕpsϕm100E_\phi=\frac{\phi_m-\phi_{ps}}{\phi_m}100
View formula
PetrophysicsResistivity Logs

Fertl-Hammack Shaly Sand Water Saturation

Sw=(FRwRt)0.5−VshRw0.4ϕeRshS_w=\left(\frac{FR_w}{R_t}\right)^{0.5}-\frac{V_{sh}R_w}{0.4\phi_eR_{sh}}
View formula
PetrophysicsResistivity Logs

Flushed-Zone Apparent Formation Factor from Apparent Saturation

Fxo=FsSxo2F_{xo}=\frac{F_s}{S_{xo}^2}
View formula
PetrophysicsResistivity Logs

Flushed-Zone Water Saturation from Formation Factor

Sxo=(FRmfRxo)1/nS_{xo}=\left(\frac{FR_{mf}}{R_{xo}}\right)^{1/n}
View formula
PetrophysicsResistivity Logs

Formation Factor - Archie's Equation

F=TϕF = \frac{T}{\phi}
View formula
PetrophysicsElectrical Properties

Formation Factor from Water-Filled Rock Resistivity

F=RoRwF = \frac{R_o}{R_w}
View formula
PetrophysicsResistivity Logs

Formation Resistivity and Permeability - Carothers Limestones

k=4×108F3.65k=\frac{4\times10^8}{F^{3.65}}
View formula
PetrophysicsResistivity Logs

Formation Resistivity and Permeability - Carothers Sandstones

k=7×108F4.5k=\frac{7\times10^8}{F^{4.5}}
View formula
PetrophysicsElectrical Properties

Formation Water Resistivity from Resistivity Ratio Method

Rw=RtRxoRmfR_w=\frac{R_t}{R_{xo}}R_{mf}
View formula
PetrophysicsRock Properties

Fraction of Total Porosity Occupied by Clays

q=ΦS−ΦDΦSq=\frac{\Phi_S-\Phi_D}{\Phi_S}
View formula
PetrophysicsRock Properties

Fresh Water-Filled Porosity - Limestones

Φ=ρls−ρbρls−ρw\Phi=\frac{\rho_{ls}-\rho_b}{\rho_{ls}-\rho_w}
View formula
PetrophysicsRock Properties

Gamma Ray Log Shale Index

IGR=GR−GRclGRsh−GRclI_{GR}=\frac{GR-GR_{cl}}{GR_{sh}-GR_{cl}}
View formula
PetrophysicsRock Properties

Gas-Effect Corrected Porosity from Neutron-Density Logs

Φcorr=(ΦD2+ΦN22)0.5\Phi_{corr}=\left(\frac{\Phi_D^2+\Phi_N^2}{2}\right)^{0.5}
View formula
PetrophysicsElectrical Properties

Generalized Archie Formation Factor with Lithology Coefficient

F=aϕmF=\frac{a}{\phi^m}
View formula
PetrophysicsRock Properties

Half Thickness Value

h1/2=0.693αlh_{1/2}=\frac{0.693}{\alpha_l}
View formula
PetrophysicsResistivity Logs

Hingle Crossplot True Resistivity

Rt=[ϕ(SwnaRw)1/m]−mR_t=\left[\phi\left(\frac{S_w^n}{aR_w}\right)^{1/m}\right]^{-m}
View formula
PetrophysicsResistivity Logs

Humble Formation Factor from Porosity

F=0.62ϕ2.15F=\frac{0.62}{\phi^{2.15}}
View formula
PetrophysicsResistivity Logs

Indonesian Water Saturation from Shaly Sand Logs

Sw=[1/RtVsh1−0.5VshRsh+ϕemaRw]2/nS_w=\left[\frac{\sqrt{1/R_t}}{\frac{V_{sh}^{1-0.5V_{sh}}}{\sqrt{R_{sh}}}+\sqrt{\frac{\phi_e^m}{aR_w}}}\right]^{2/n}
View formula
PetrophysicsRock Properties

Integrated Radial Geometric Factor

G(r)=1−e−r/hˉG(r)=1-e^{-r/\bar{h}}
View formula
PetrophysicsRock Properties

Larionov Older-Rock Shale Volume from Gamma Ray Index

Vsh=0.33(22IGR−1)V_{sh}=0.33\left(2^{2I_{GR}}-1\right)
View formula
PetrophysicsRock Properties

Larionov Tertiary-Rock Shale Volume from Gamma Ray Index

Vsh=0.083(23.7IGR−1)V_{sh}=0.083\left(2^{3.7I_{GR}}-1\right)
View formula
PetrophysicsElectrical Properties

Lateral Device Geometric Coefficient

GL=4πAMANMNG_L=4\pi AM\frac{AN}{MN}
View formula
PetrophysicsRock Properties

Linear Absorption Attenuation Coefficient

αl=σN\alpha_l=\sigma N
View formula
PetrophysicsRock Properties

Liquid Permeability - Permeameter Lab Measurement

k=μVLAPtk = \frac{\mu V L}{A P t}
View formula
PetrophysicsRock Properties

Mean Free Path Photon Absorption

h=1αlh=\frac{1}{\alpha_l}
View formula
PetrophysicsElectrical Properties

Membrane Potential for SP Log

Em=RTFln⁡(a1a2)E_m=\frac{RT}{F}\ln\left(\frac{a_1}{a_2}\right)
View formula
PetrophysicsResistivity Logs

Movable Hydrocarbon Saturation from Flushed Zone

Shm=Sxo−SwS_{hm}=S_{xo}-S_w
View formula
PetrophysicsResistivity Logs

Mud Cake Resistivity from Mud and Filtrate Resistivity

Rmc=0.69Rmf(RmRmf)2.65R_{mc}=0.69R_{mf}\left(\frac{R_m}{R_{mf}}\right)^{2.65}
View formula
PetrophysicsResistivity Logs

Mud Filtrate Resistivity from Mud Resistivity

Rmf=KmRm1.07R_{mf}=K_mR_m^{1.07}
View formula
PetrophysicsRock Properties

Neutron Lethargy Logarithmic Energy Decrement

u=ln⁡(EoE)u=\ln\left(\frac{E_o}{E}\right)
View formula
PetrophysicsRock Properties

Neutron Porosity in a Shaly Formation

ΦN=ΦT+VshΦNsh\Phi_N=\Phi_T+V_{sh}\Phi_{Nsh}
View formula
PetrophysicsRock Properties

NMR Free-Fluid Index from Porosity Partition

FFI=MPHI−BVI\mathrm{FFI}=\mathrm{MPHI}-\mathrm{BVI}
View formula
PetrophysicsElectrical Properties

Non-Ideal Shale Membrane Efficiency from Shale Resistivity

meff=0.47+0.3Rshm_{eff}=0.47+0.3R_{sh}
View formula
PetrophysicsElectrical Properties

Non-NaCl Static Self Potential from Ion Activities

Essp=−Klog⁡10(aNa+(aCa+aMg)0.5aNam+(aCam+aMgm)0.5)E_{ssp}=-K\log_{10}\left(\frac{a_{Na}+(a_{Ca}+a_{Mg})^{0.5}}{a_{Nam}+(a_{Cam}+a_{Mgm})^{0.5}}\right)
View formula
PetrophysicsElectrical Properties

Normal Sonde Geometric Coefficient

GN=4πAMG_N=4\pi AM
View formula
PetrophysicsResistivity Logs

Oil Saturation from IE and CDN Resistivity Logs

So=1−RoRtS_o=1-\sqrt{\frac{R_o}{R_t}}
View formula
PetrophysicsRock Properties

Pair Production Gamma Ray Interactions

Ee=Ec−EE_e=E_c-E
View formula
PetrophysicsResistivity Logs

Partially Saturated Shaly Sand Resistivity Vsh Model

Rt=1(Vsh/Rsh)Sw+(1/F)Sw2/RwR_t=\frac{1}{(V_{sh}/R_{sh})S_w+(1/F)S_w^2/R_w}
View formula
PetrophysicsResistivity Logs

Phillips Formation Factor - Average Sands

F=1.45ϕ1.54F=\frac{1.45}{\phi^{1.54}}
View formula
PetrophysicsRock Properties

Photoelectric Absorption Cross Sectional Area

αpe=12.1Z3.6Eγ3.15\alpha_{pe}=\frac{12.1Z^{3.6}}{E_\gamma^{3.15}}
View formula
PetrophysicsResistivity Logs

Pickett Crossplot Log True Resistivity

log⁡10Rt=−mlog⁡10(ϕ)+log⁡10(Rw)−nlog⁡10(Sw)\log_{10}R_t=-m\log_{10}(\phi)+\log_{10}(R_w)-n\log_{10}(S_w)
View formula
PetrophysicsRock Properties

Poissons Ratio Seismic Arrival Time Method

ν=Vp2−2Vs22(Vp2−Vs2)\nu=\frac{V_p^2-2V_s^2}{2\left(V_p^2-V_s^2\right)}
View formula
PetrophysicsRock Properties

Porosity from Neutron Flux Response

ϕ=α−βlog⁡10(N)\phi=\alpha-\beta\log_{10}(N)
View formula
PetrophysicsResistivity Logs

Porter-Carothers Formation Factor - California Pliocene Logs

Fg=2.45ϕ1.08F_g=\frac{2.45}{\phi^{1.08}}
View formula
PetrophysicsResistivity Logs

Porter-Carothers Formation Factor - Gulf Coast Miocene Logs

Fc=1.97ϕ1.29F_c=\frac{1.97}{\phi^{1.29}}
View formula
PetrophysicsRock Properties

Pycnometer Volume Correction

Vg=Vk[1+Ω(Tt−Tk)]V_g=V_k\left[1+\Omega(T_t-T_k)\right]
View formula
PetrophysicsRock Properties

Rate of Radioactive Decay

N=Noe−CdtN=N_o e^{-C_d t}
View formula
PetrophysicsRock Properties

Raymer-Hunt Transform - Porosity Transit Time Relationship

Δt=((1−ϕ)2Δtma+ϕΔtf)−1\Delta t=\left(\frac{(1-\phi)^2}{\Delta t_{ma}}+\frac{\phi}{\Delta t_f}\right)^{-1}
View formula
PetrophysicsRock Properties

Relation Between Concentration of K Th or U and Recorded Total Gamma Ray Signal

γ=4CTh+8CU+CK\gamma=4C_{Th}+8C_U+C_K
View formula
PetrophysicsElectrical Properties

Relationship Between SSP and Rw - NaCl Predominant

SSP=−Ksplog⁡10(RmfeqRweq)SSP=-K_{sp}\log_{10}\left(\frac{R_{mfeq}}{R_{weq}}\right)
View formula
PetrophysicsRock Properties

Relative Centrifugal Force

RCF=(RPM265)2dRCF=\left(\frac{RPM}{265}\right)^2d
View formula
PetrophysicsRock Properties

Reservoir Wettability Characterization Rise-in-Core Method

cos⁡θ12=μ1ρ22−μ2ρ12ρ12ρ22CγL2L1m2t\cos\theta_{12}=\frac{\mu_1\rho_2^2-\mu_2\rho_1^2}{\rho_1^2\rho_2^2C\gamma_{L2L1}}\frac{m^2}{t}
View formula
PetrophysicsResistivity Logs

Residual Hydrocarbon Saturation from Flushed Zone

Shr=1−SxoS_{hr}=1-S_{xo}
View formula
PetrophysicsElectrical Properties

Resistivity Bed Reflection Coefficient

CR=R1−R2R1+R2C_R=\frac{R_1-R_2}{R_1+R_2}
View formula
PetrophysicsResistivity Logs

Resistivity Index - Archie's Law

RI=RtRoRI=\frac{R_t}{R_o}
View formula
PetrophysicsElectrical Properties

Resistivity Index from True and Water-Filled Resistivity

I=RtRoI=\frac{R_t}{R_o}
View formula
PetrophysicsElectrical Properties

Rock Conductivity for Clean Water-Bearing Rocks

Co=CwFC_o=\frac{C_w}{F}
View formula
PetrophysicsRock Properties

SDR NMR Permeability from T2 Geometric Mean

k=aϕNMR4T2gm2k=a\phi_{NMR}^4T_{2gm}^2
View formula
PetrophysicsResistivity Logs

Sethi Formation Factor - Clean Granular Formations

F=1ϕ2.05−ϕF=\frac{1}{\phi^{2.05-\phi}}
View formula
PetrophysicsRock Properties

Shale Index from Gamma Ray Spectrometry - Potassium

Ish,K=CK,log−CK,minCK,sh−CK,minI_{sh,K}=\frac{C_{K,log}-C_{K,min}}{C_{K,sh}-C_{K,min}}
View formula
PetrophysicsRock Properties

Shale Index from Gamma Ray Spectrometry - Thorium

Ish,Th=CTh,log−CTh,minCTh,sh−CTh,minI_{sh,Th}=\frac{C_{Th,log}-C_{Th,min}}{C_{Th,sh}-C_{Th,min}}
View formula
PetrophysicsRock Properties

Shale Index from Gamma Ray Spectrometry - Uranium-Free Gamma

Ish,Uf=GUf,log−GUf,minGUf,sh−GUf,minI_{sh,Uf}=\frac{G_{Uf,log}-G_{Uf,min}}{G_{Uf,sh}-G_{Uf,min}}
View formula
PetrophysicsResistivity Logs

Shell Formation Factor - Low-Porosity Carbonates

Fl=1ϕ1.87+0.019/ϕF_l=\frac{1}{\phi^{1.87+0.019/\phi}}
View formula
PetrophysicsResistivity Logs

Simandoux Total Shale Equation

Sw=0.4Rwϕe2[−VshRsh+(VshRsh)2+5ϕe2RwRt]S_w=\frac{0.4R_w}{\phi_e^2}\left[-\frac{V_{sh}}{R_{sh}}+\sqrt{\left(\frac{V_{sh}}{R_{sh}}\right)^2+\frac{5\phi_e^2}{R_wR_t}}\right]
View formula
PetrophysicsResistivity Logs

Simandoux Water Saturation from Shaly Sand Logs

Sw=−VshRsh+(VshRsh)2+4FRw(1−Vsh)Rt2FRw(1−Vsh)S_w=\frac{-\frac{V_{sh}}{R_{sh}}+\sqrt{\left(\frac{V_{sh}}{R_{sh}}\right)^2+\frac{4}{F R_w (1-V_{sh}) R_t}}}{\frac{2}{F R_w (1-V_{sh})}}
View formula
PetrophysicsRock Properties

Slowness of the Formation

Δt=tlLs−DbΔtm2(Ls2+Db2)−tl2Ls2+Db2\Delta t=\frac{t_lL_s-D_b\sqrt{\Delta t_m^2(L_s^2+D_b^2)-t_l^2}}{L_s^2+D_b^2}
View formula
PetrophysicsRock Properties

Sonic Compaction Correction Factor from Shale Transit Time

Cp=Δtshc100C_p=\Delta t_{sh}\frac{c}{100}
View formula
PetrophysicsRock Properties

Sonic First-Arrival Time in a Borehole

tlog=Lsvf+dh−(dt+2lc)vm1−(vmvf)2t_{log}=\frac{L_s}{v_f}+\frac{d_h-(d_t+2l_c)}{v_m}\sqrt{1-\left(\frac{v_m}{v_f}\right)^2}
View formula
PetrophysicsRock Properties

Sonic Porosity Raymer-Hunt-Gardner Method

Φsonic=CRHGΔt−ΔtmaΔt\Phi_{sonic}=C_{RHG}\frac{\Delta t-\Delta t_{ma}}{\Delta t}
View formula
PetrophysicsRock Properties

Spacing Between Transmitter and Receiver

(Ls)c=2soff1+Cmf1−Cmf(L_s)_c=2s_{off}\sqrt{\frac{1+C_{mf}}{1-C_{mf}}}
View formula
PetrophysicsElectrical Properties

Static Self Potential from NaCl Activities

Essp=−Klog⁡10(awamf)E_{ssp}=-K\log_{10}\left(\frac{a_w}{a_{mf}}\right)
View formula
PetrophysicsElectrical Properties

Static Spontaneous Potential from Equivalent Resistivities

SSP=−Klog⁡10(RmfeRwe)SSP=-K\log_{10}\left(\frac{R_{mfe}}{R_{we}}\right)
View formula
PetrophysicsRock Properties

Stieber Shale Volume from Gamma Ray Index

Vsh=IGR3−2IGRV_{sh}=\frac{I_{GR}}{3-2I_{GR}}
View formula
PetrophysicsResistivity Logs

Tortuosity from Resistivity Log Path Lengths

T=La2L2T=\frac{L_a^2}{L^2}
View formula
PetrophysicsRock Properties

Total Porosity from Pore Volumes

ϕ=Vi+VdVb\phi = \frac{V_i + V_d}{V_b}
View formula
PetrophysicsElectrical Properties

Total Rock Conductivity from Water Saturation

Ct=CwSwϕEC_t=C_wS_w\phi E
View formula
PetrophysicsRock Properties

True Sonic Porosity from Compaction Correction

Φt=ΦaCp\Phi_t=\frac{\Phi_a}{C_p}
View formula
PetrophysicsRock Properties

USBM Wettability Index

IU=log⁡10(A1A2)I_U=\log_{10}\left(\frac{A_1}{A_2}\right)
View formula
PetrophysicsRock Properties

Volumetric Photoelectric Absorption Cross Section

U=PeρeU=P_e\rho_e
View formula
PetrophysicsResistivity Logs

Water Salinity Index Ratio

Sw=CaCS_w=\frac{C_a}{C}
View formula
PetrophysicsResistivity Logs

Water Saturation from IE and CDN Resistivity Logs

Sw=RoRtS_w=\sqrt{\frac{R_o}{R_t}}
View formula
PetrophysicsResistivity Logs

Water Saturation from Neutron Capture Cross Section

Sw=(Σlog−Σma)−ϕ(Σh−Σma)ϕ(Σw−Σh)S_w=\frac{(\Sigma_{log}-\Sigma_{ma})-\phi(\Sigma_h-\Sigma_{ma})}{\phi(\Sigma_w-\Sigma_h)}
View formula
PetrophysicsResistivity Logs

Water-Saturated Shaly Sand Resistivity Vsh Model

Ro=1Vsh/Rsh+1/(FRw)R_o=\frac{1}{V_{sh}/R_{sh}+1/(FR_w)}
View formula
PetrophysicsRock Properties

Wavelength Equation

λ=Cf\lambda=\frac{C}{f}
View formula
PetrophysicsElectrical Properties

Waxman-Smits Clean-Sand Brine-Saturated Rock Conductivity

Co=1F(Ccl+Cw)C_o=\frac{1}{F}\left(C_{cl}+C_w\right)
View formula
PetrophysicsElectrical Properties

Wellbore Electric Voltage Generation

V=Ve,w−ΔVeln⁡(r/rw)ln⁡(re/rw)V=V_{e,w}-\Delta V_e\frac{\ln(r/r_w)}{\ln(r_e/r_w)}
View formula
PetrophysicsRock Properties

Wyllie Time-Average Sonic Porosity - Compacted Formations

Φs=Δt−ΔtmaΔtf−Δtma\Phi_s=\frac{\Delta t-\Delta t_{ma}}{\Delta t_f-\Delta t_{ma}}
View formula
PetrophysicsRock Properties

Wyllie Time-Average Sonic Porosity - Uncompacted Formations

Φs=(Δt−Δtma)/(Δtf−Δtma)Bcp\Phi_s=\frac{(\Delta t-\Delta t_{ma})/(\Delta t_f-\Delta t_{ma})}{B_{cp}}
View formula