SUBSURFACE
ENVIRONMENTS
Instructor: Nawarat Intarapanich
Course : Basic Petroleum Geology
Talk Outlines
Subsurface Water
Subsurface Temperature
Subsurface Pressure
Subsurface Fluid Dynamics
Subsurface Water
Free Water
Free to move in or out
of pores in response
to pressure
differential.
Interstitial or
Irreducible water
Bonded to mineral
grains
Can’t be removed
during HC production
Subsurface Water
Chemistry of
Subsurface waters
Eh & pH
Subsurface Water
Chemistry of
Subsurface waters
ppm
mg / liter Concentration
Salinity;total of
density dissolved solids
Sea Water has
approximately 35,000
ppm.
Connate water salinity
in sand generally
increase with depth
Subsurface Water
Chemistry of Subsurface waters
Contain varying
Composition
concentrations of
inorganic salts, traces
of organic compounds
including hydrocarbon
Regional mapping of its
composition can lead
to locating the
migration pathways
and new HC fields
Subsurface Temperature
BHT = Bottom Hole
Temperature
True stabilized
temperature from
Horner Plot (Fertl
& Wichmann (1977)
t
t t
T=number of hours since circulation & logging
Delta t = hours of mud circulation at that depth
Geothermal Gradient
Plot corrected BHT’s
versus Depth at
various depth
Range from 1.8 to
5.5 degree C/100m
Global Average 2.6
degree C/100m
Y a x b
You can find temperature at any depth
Geothermal Gradient & Heat flow
Heat flow = Geothermal Gradient x thermal conductivity of the rock
Local Thermal Variations
Controlling factors
1. Nonplanar geometry
of sediments
2. Movement of Fluids
3. Regional variations
of Heat Flow
Regional Thermal Variations
Controlled by Affect petroleum
tectonics generation
Subsurface Pressure
Measurement while
drilling
Drilling (d) exponent logR / 60N
plot by Jordan & dexponent
log12W / 106 D
Shirley, 1966) R = rate of penetration (ft/hr)
Aim to detect N = rotary speed (rpm)
W= weight on bit (lb)
abnormal pressure D = diameter of borehole (inches)
Practical in drilling
D exponent Plot
Some overpressure indicators
Basic Principles
Force per unit area
acting on a surface
Terzakhi’s Law
(kg/cm2 or psi) s=p+o
Overburden s =Overburden Pressure
p = Lithostatic Pressure
pressure
o = Fluid Pressure
Lithostaticpressure
Fluid pressure
Hydrostatic
hydrodynamic
Fluid Pressure
In oil industry fluid pressure is In fact, there are 2
generally calculated as types of fluid
follows:
pressure
Hydrostatic
P = 0.052 x wt x D
Hydrodynamic
P = Hydrostatic Pressure (psi)
wt = mud weight (lb/gal)
D = Depth (feet)
2 types of fluid pressure
Hydrostatic pressure is Hydrodynamic
imposed by a column of pressure gradient
fluid at rest caused by fluid flow
Fresh water or density = Potentiometric or
1, the hydrostatic
gradient is 0.433 psi/ft
piezometric level is
Brine water
calculated as
(55,000ppm), the follows:
hydrostatic gradient is Elevation to potentiometric level
P
D E
0.45 psi/ft W
P= bottom hole pressure (psi)
Temperature dependent W= weight of fluid (psi/ft)
D= Depth (ft)
E= Elevation of kelly bushing above sea level (ft)
Hydrodynamic pressure
Hydrodynamic pressure
Hydrostatic Pressure
If a 10,000ft hole contains Assume a formation at 10,000ft has
drilling fluid with a weight of a known hydrostatic pressure of
11.5 ppg (pound/gallon), the 6292 psi. The mud weight
needed to drill this formation
hydrostatic pressure at the
“balanced” (hydrostatic pressure
bottom hole is equal to formation pressure) is
calculated as follows:
P=0.052 x MW x D MW= P/(0.052 x D)
P=0.052 x 11.5 x MW= 6292/(0.052 x
10,000 10,000)
P=5,980 psi MW = 12.1 ppg.
Killing weight of mud
Assume a well has taken a kick at 10,000 ft
while drilling with 11.5 ppg mud. The well
shut-in and the drill pipe pressure reads 312
psi. The excess in ppg mud weight equivalent
is calculated as follows:
MW = 312/(0.052 x 10,000) = 0.6 ppg
To kill the well you need MW = 11.5 + 0.6 ppg
=12.1 ppg or greater to overcome that kick.
Further study
Find more detail in the reference texts
You will see more in Petroleum Engineering
Class next 2-3 weeks!
