Module 14
Determination of Net Pay in Fluvial/Deltaic Environments
Glossary Terms:
bulk volume water (BVW): BVW is the percentage of the total rock volume that is
occupied by water. It compares to the more commonly used water saturation term in
that water saturation is the percent of the total pore space occupied by water. It is a
critical input to estimating fluid mobility in the reservoir.
bulk volume water irreducible (BVWirr): BVWirr is the percentage of the total rock
volume that is occupied by water that is bound to the rock matrix by capillary forces. It
should not be confused with water that is bound to the shale molecules. It is a critical
input in the estimation of permeability.
Coates-Denoo technique: This technique was presented by Coates and Denoo (1981)
to relate permeability to porosity and BVWirr. It was an adaptation of the original Coates
and Dumanoir (1973) work that related permeability to porosity and Swirr. It provided a
reasonable correlation between core permeability to air and log based porosity and
BVWirr. It was later modified by Barba and Darling (1991) to include a variable
calibration factor that can be used to match effective permeability to reservoir fluids.
drawdown: the amount of pressure drop across the sand face in a producing reservoir.
If there is no difference between the pressure in the wellbore and the formation
pressure or if the pressure in the wellbore is higher a zone will not flow. If the pressure
in the wellbore is less than the reservoir pressure flow will occur. Drawdown is the
amount of this difference.
Klinkenberg corrections: A correlation technique that corrects core permeability to air
to core permeability to gas.
MRIL version of the NMR tool type: There are two types of nuclear magnetic
resonance tools widely available in the industry today. Halliburton Energy Services has
the Magnetic Resonance Imaging Log (MRIL) while Schlumberger has the Combinable
Magnetic Resonance log. Both tools provide measurements of BVWirr, while only the
MRIL can provide effective porosity.
neutron-density combination: The most common porosity tools run on wells today are
the compensated neutron and formation density logs. The compensated neutron log
responds to the volume of hydrogen in the rock while the density log responds to the
electron density. In combination the two tools usually provide a reasonable estimate of
total porosity (uncorrected for shale volume).
nuclear magnetic resonance (NMR) log data: The NMR log measures the time it
takes for protons to relax after being excited by a magnetic field. It provides an estimate
of the amount of water that is bound to the rock matrix (BVWirr) and thus a key input to
permeability.
Module 14
Determination of Net Pay in Fluvial/Deltaic Environments
permeability: a measurement of fluid mobility, usually expressed in one thousandths of
a Darcy or millidarcy. It is a key component of net pay as wells with low or no
permeability will not produce economic quantities of hydrocarbons.
Simandoux water saturation equation: Conventional wireline logs cannot measure
the volume of hydrocarbons directly. They measure the volume of salt water in the
rock, and from this infer an oil volume from the difference between the pore volume in
the rock and the volume of water. When formations contain no clays the water
saturation can be obtained directly from the porosity and resistivity logs. Virtually all
sands in fluvial/deltaic environments contain clay, however, and an adjustment needs to
be made to the conventional water saturation relationships to account for the effect of
clay resistivity. The Simandoux model is one of the most widely used in this area.
skin damage: During the drilling phase of well construction the primary objective is to
plug off the well to keep it from blowing out. Large volumes of water and mud invade
the formation in this process. When the casing is cemented large volumes of water are
also lost. All of this material creates obstacles to the flow of reservoir fluids once the
production phase starts. Skin is a measure of the degree of damage to flow that has
occurred.
Sw (Water Saturation): The volume of water in the pore space expressed as a
percentage.
T2 value: A parameter used in nuclear magnetic resonance logging that determines the
criteria for irreducible water.
underbalanced drilling: As discussed in the “skin damage” section above, fluids often
damage the formation and impede reservoir fluid flow. Underbalanced drilling occurs
when the fluid pressure in the wellbore is less than the formation pressure. It helps
minimize skin damage.
viscosity: A measure of fluid resistance to flow. Oil has a much higher viscosity than
gas and flows less readily.
watercut: The percentage of water in the production stream.
XRD data: X-ray diffraction data. These data are the key calibration value for the
volume of shale. They measure the bulk volume of shale as a percentage of total rock
volume.
Equations Used In Exercise:
GRC (Gamma Ray Corrected) =GR*((MudWt/3000)* (MudWt*(1-.056*3.625)+10)*
(CALI-3.625)+0.785)
GR=raw gamma ray tool reading from field expressed in API units
Module 14
Determination of Net Pay in Fluvial/Deltaic Environments
MudWt= density of mud in lb/gal
Cali=hole size measurement in inches
GI (GR Index) =(GRC-GRMIN)/(GRMAX-GRMIN)
GRC = Gamma ray value in 100% sand
GRMAX=Gamma ray value in 100% shale
Vclay (Volume Clay) = 0.33*[2^(2*GI)-1] (consolidated ss)
Vclay (Volume Clay) = .083*[2^(3.7*GI) -1] (unconsolidated ss)
DPHI (Density Porosity) =(RHOMA-RHOB)/ (RHOMA-RHOF)
RHOMA=2.65 & RHOF=1.0
RHOMA = bulk density of matrix rock in g/cc
RHOF = bulk density of mud filtrate in g/cc
PHIE (DPHI corrected for shale volume) =DPHI*(Vclay*DPHISH)
DPHISH (DPHI shale)=0.035
Sw (Archie Water Saturation) = (0.81*Rw/ILD)/PHIE
Rw = 0.058 ohm-m
ILD = deep resistivity measurement ohm-m
Rw = water resistivity ohm-m
Sw (Modified Simandoux Equation)
=(0.5*Rw/PHIE^m)*((4*PHIE^m)/(Rw*ILD)+(Vclay/Rshale)^2)^(1/n)-Vclay/Rshale
m=2.08, n=1.44
Rshale=1.8 ohm-m
m=cementation exponent
n=Saturation exponent
Rshale = shale resistivity ohm-m
Permeability =(4*PHIE^2*((PHIE-BVW)/BVW)))^2
BVW = bulk volume water v/v
References:
Coates, G.R. and Dumanoir, J.L.:”A New Approach to log Derived Permeability,:
Transactions SPWLA 14th Annual :Logging Convention, Lafayette, 1973.
Coates, G.R., and Denoo, S.:” The Producibility Answer Product,” Schlumberger
“Technical Review,” 29 No. 2 June 1981 pp 55-63
Module 14
Determination of Net Pay in Fluvial/Deltaic Environments
Barba, R.E. and Darling, H: “Recent Advances in Log Derived Permeability,” presented
at the 1991 Schlumberger Gulf Coast Interpretation Symposium, New Orleans.