Method of determining in-situ pore pressure in chemically active formations
Methods for determining an in-situ pore pressure in a formation rock may include acquiring well log data, calculating a cation-exchange capacity, calculating a mole fraction of fixed charges using the cation-exchange capacity, determining a formation temperature, calculating the in-situ pore pressure, outputting the in-situ pore pressure of the formation rock, and using the in-situ pore pressure in a wellbore stability model to monitor and maintain stability of the formation rock. Corresponding electronic devices may include a computer processor coupled to at least one logging tool, wherein the logging tool is configured to acquire well log data in a formation rock. Non-transitory computer storage mediums may include program instructions, that when executed, may be configured to perform the methods for determining an in-situ pore pressure in a formation rock.
1. A method for determining an in-situ pore pressure and maintaining stability of a formation rock during wellbore operations, the method comprising:
acquiring well log data from at least one logging tool, wherein well log data comprises elemental capture spectroscopy log data and/or X-ray diffraction mineralogy log data;
calculating a cation-exchange capacity using a volume fraction and a cation-exchange capacity for each mineral in the formation rock using the well log data;
calculating a mole fraction of fixed charges using the cation-exchange capacity, a porosity of the formation rock, a grain density of the formation rock, and a water molar volume;
determining a formation temperature;
calculating the in-situ pore pressure at a plurality of depths using the mole fraction of fixed charges, the formation temperature, a surface temperature, a hydrostatic pressure, an overburden stress, and an in-situ water activity;
outputting the in-situ pore pressure of the formation rock; and
using the in-situ pore pressure in a wellbore stability model to monitor and maintain stability of the formation rock, wherein monitoring and maintaining stability of the formation rock using the in-situ pore pressure comprises adjusting one or more of a composition of a wellbore fluid, a weight of the wellbore fluid, a type of wellbore fluid, an injection pressure of the wellbore fluid, a rotary speed of a drill bit, and a rate of penetration of a drill bit into the formation,
wherein the calculating the in-situ pore pressure is conducted by a formula represented by:
p
insitu
=
p
hyd
+
(
σ
v
-
p
hyd
)
T
T
surface
[
(
m
f
c
)
2
+
(
1
-
a
insitu
)
2
-
(
1
-
a
insitu
)
]
n
where p insintu is the in-situ pore pressure, p hyd is the hydrostatic pressure, σ v is the overburden stress, T is the formation temperature, T surface is the surface temperature, m fc is the mole fraction fixed charges, α insitu is the in-situ water activity, and n is a parameter determined by field data.
2. The method according to claim 1 , wherein determining the formation temperature comprises calculating the formation temperature and/or measuring the formation temperature.
3. The method according to claim 1 , wherein the well log data comprises gamma ray log data and wherein the method comprises using the gamma ray log data to determine a clay content of the formation rock.
4. The method according to claim 3 , wherein calculating the cation-exchange capacity is performed using the clay content of the rock formation.
5. The method according to claim 1 , wherein the method comprises determining mineral types and volume fractions for the minerals in the formation rock using the capture spectroscopy log data and/or the X-ray diffraction mineralogy log data.
6. The method according to claim 1 , wherein the well log data comprises photoelectric log data, density log data, and neutron porosity log data.
7. The method according to claim 6 , wherein the method comprises determining mineral types and volume fractions for the minerals in the formation rock using the photoelectric log data, the density log data, and/or the neutron porosity log data.
8. The method according to claim 1 , wherein calculating the cation-exchange capacity comprises multiplying the volume fraction with the cation-exchange capacity separately for each of the minerals in the formation to obtain a cation-exchange product for each of the minerals in the formation rock and adding each product.
9. The method according to claim 1 , wherein calculating the mole fraction of fixed charges comprises multiplying the cation-exchange capacity by (1−the porosity of the formation rock) by the grain density of the formation rock and by the water molar volume to obtain a mole fraction product, and dividing the mole fraction product by the porosity.
10. The method according to claim 1 , wherein determining the formation temperature at a depth comprises calculating the formation temperature by multiplying the surface temperature of the formation rock by the depth and by a geothermal gradient.
11. The method according to claim 1 , wherein calculating the in-situ pore pressure comprises adding a hydrostatic pore pressure and a factor calculated from the mole fraction of fixed charges, the formation temperature, the surface temperature, the hydrostatic pressure, the overburden stress, and the in-situ water activity.