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SPE 169394
Pore Pressure Analysis in Chicontepec Basin: Presidente Aleman
Field Felipe de Jess Martnez-Estrella, Weatherford; Daniel Ibarra,
Weatherford; and David Velzquez-Cruz, SPE member
Copyright 2014, Society of Petroleum Engineers This paper was
prepared for presentation at the SPE Latin American and Caribbean
Petroleum Engineering Conference held in Maracaibo, Venezuela, 2123
May 2014. This paper was selected for presentation by an SPE
program committee following review of information contained in an
abstract submitted by the author(s). Contents of the paper have not
been reviewed by the Society of Petroleum Engineers and are subject
to correction by the author(s). The material does not necessar ily
reflect any position of the Society of Petroleum Engineers, its
officers, or members. Electronic reproduction, distribution, or
storage of any part of this paper without the written consent of
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contain conspicuous acknowledgment of SPE copyright.
Abstract
Pore pressure analysis is a key issue during the well drilling
planning stage. Usually, the well geopressure analysis is
developed in one dimension (1D), that is, only on the well
location along its whole depth. However, we can increase the
number of dimension to visualize spatially the pressure behavior
with respect to time, depending on information available.
The increment in dimensions allows us to include the geological
characteristics of the area to improve drilling well planning.
The paper describes the geopressure analysis experiences
performed in the Presidente Aleman Field in Chicontepec Basin.
This field is located one-kilometer south-west of Papantla,
Veracruz, Mxico and it has an area of 206.9 km2. In recent
drilling campaigns, more than 160 wells were drilled, however,
to develop the present work we gather information from 14
scattered wells around the field. These types of wells were
termed template-wells, because they were the first drilled into the
template with all sort of logs, tests and samples. The behavior of
earth-pressures (overburden pressure, pore pressure and
fracture pressure) was outlined using shale compaction behavior
with depth. The analysis depicts geological characteristics of
the Chicontepec basin and Presidente Aleman Field. Then, the
origin of abnormal pore pressure, depth of fluid retention,
behavior of normal compaction trends around the field is
discussed. In addition, we present the variability of rock density
and
its effect over the overburden stress together with fracture
pressure distribution and drilling experiences.
Introduction
The Chicontepec Paleochannel is located in Tampico Misantla
basin (figure 1) and is situated in the oriental margin of
Mxico in Mexican Golf coast plain. It has an area of 3,800 km2
and is considered the most important oil reserve of Mxico
with around of 18,000 million of BOE, 40% of Mxico oil reserves.
The Chicontepec Paleochannel is flanked at the orient by
Atoln de la faja de oro (Golden Lane reef) and at occident by
Sierra Madre Oriental (figure 2).
Figure 1. Tampico-Misantla Basin Scheme (modified from Pemex,
2009)
Figure 2. Chicontepec Paleochannel.
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The Chicontepec project was divided in three zones: North,
Center and South (figure 3). The north zone includes sectors 1,
2,
3 with fields like Aragn, Coyotes, Soledad, Pastoria, etc. The
center portion is comprised by sectors 4, 5, 6, and 7 with
fields
like Coyol, Miquetla, Humapa, Coyutla, Agua Fria, etc. The south
part with fields Furbero, Presidente Aleman and Remolino
is the sector 8.
The Presidente Aleman Field is one kilometer at south-west of
Papantla, Veracruz, Mxico (figure 4). It has an area of 206.9
km2, and was discovered by Presidente Aleman 1 exploratory well.
The well was completed in January of 1950 with an oil
production of 226 BPD in an interval between 2705-2721 m. The
producer formation was Tamabra, which it is a breccia
limestone. With the formation Tamabra development in the
Presidente Aleman field at the beginning of 50s, the sandstones of
tertiary age called Chicontepec Canal were discovered above
formation Tamabra and began oil production.
Figure 3. Chicontepec divided by sectors.
Figure 4. Presidente Aleman Field.
ATG Project I, II and IV
From 2008 to 2012, Weatherford developed three drilling and
completion integral projects to PEMEX called "Aceite
Terciario del Golfo" (ATG). The projects I and II with 516
wells, and project IV with 131 wells. To face the challenge,
Weatherford deployed 42 drilling rigs around all eight sectors
of Chicontepec basin. Figure 5 and 6 show the number of wells
drilled according to the project. The Presidente Aleman field
was the most drilled field during the drilling campaign of ATG
I, II and IV projects. The tertiary deposits located at the
Paleochannel of Chicontepec are made up by lithic sands of fine
grain that shows permeability lower than one milidarcy. This
fact represents a technological challenge for its profitable
exploitation. To solve the challenge, a detailed geological and
reservoir characterization have been performed including
geopressure analysis to reduce the drilling well cost.
Figure 5. Number of wells drilled by Weatherford in ATG
projects
Figure 6. Wells drilled by field developed by Weatherford in ATG
projects
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SPE 169394 3
Presidente Aleman Field Geopressure Analysis
To develop Presidente Aleman pore pressure analysis we
considered 14 vertical wells in different drilling pads in the
field.
First, the analysis was developed in 1D and then interpolated to
2D and 3D to figure out the behavior of geopressures
spatially. The study includes the identification of abnormal
pore pressure zones, normal compaction trends, overburden
stress
behavior, pore pressure magnitudes and fracture gradient
distribution. Figure 7 shows the 14 wells selected and a line
that
describes the 2D correlation developed.
Figure 7. Well considered in the study
Typical geological setting
We developed a 2D section from NW-SE (figure 8) that shows how
geologic formations gradually increase their deep in 200
meters. The typical geologic column of Presidente Aleman field
is integrated by formations: upper and lower Palma Real
(tertiary shales and shaly sandstones), Tantoyuca (tertiary
sandstones and conglomerates), Guayabal (tertiary shales),
Discordance C (tertiary shale-sand sequence), Discordance B
(tertiary calcareous shales) and Discordance A (cretaceous
limestone).
Figure 8. NW-SE 2D section in Presidente Aleman Field
projects
Figure 9. Typical geological setting
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Abnormal pressure zones and normal compaction trends
The fluid retention depth (FRD) of Presidente Aleman field
ranges from 950 to 1200 TVD meters. The FRD is the depth that
marks the change from normal to abnormal pore pressure (figure
10). The field has a normal pore pressure of 1.02 g/cc
according with the Chicontepec basin behavior. The main
mechanism of overpressure in the field is due to overburden
pressure (compaction disequilibrium). The Normal Compaction
Trend of President Aleman field is described by an
exponential equation (Velzquez-Cruz, 2008), which was analyzed
for each of the wells in the study resulting in the values
shown in figure 11. The form of equation is (Athy, 1930):
..............................................................................................................................................
(1)
Where:
Rn = Normal trend for resistivity
Ro = Resistivity at surface
c = normal compaction behavior index
Z = Depth
Figure 10. Normal and abnormal pressure zones
Figure 11. Exponential normal compaction trend coefficients
Overburden gradient behavior
To develop the overburden analysis we use the bulk density from
density logs and synthetic density using transit time and
Gardner equation (Gardner et al, 1974). Then we integrate the
density with respect of the depth to obtain the overburden
gradient (Mouchet et al, 1989). The overburden gradient from
each well was correlated and were developed a 2D section and
a 3D cube to the field (figure 12). The overburden stress
increases steadily from NW-SE according to the field deepness.
Figure 12. Overburden gradient behavior of the field
Figure 13. Pore pressure cube at Presidente Aleman field
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SPE 169394 5
Pore pressure analysis
From previous works in pore pressure prediction (Velzquez Cruz
et al, 2008) we found that original Eaton's equation for
resistivity and transit time (Eaton, 1975) overestimates pore
pressure than real results obtained from well's measures. Hence, we
proposed adjustments to Eatons model in order to obtain results
more likely to real measures of pore pressure in
wells drilled at the Presidente Aleman field. According with
Eaton, the alpha exponent () was a great question mark until
was evaluated with much data. The fitting factor of Eatons
equation () was evaluated with wells drilled in Chicontepec basin
for resistivity and sonic data. The results revealed that the alpha
exponent is smaller than its original value for both
logs. As soon as the pore pressure model was adjusted, the pore
pressure analysis was developed to each well along with a
2D correlation and a 3D cube. Due to overburden gradient, the
pore pressure increases from NW to SE in Presidente Aleman
field. In the field, there are two thresholds of pore pressure:
one from 1.05 to 1.16 g/cc from surface to the top of Guayabal
formation and then the pore pressure increases to 1.33 g/cc from
Guayabal to the targets. Figure 13 shows the pore pressure
behavior in the field.
Fracture gradient prediction
Fracture gradient analysis shows that Guayabal formation has the
higher values (larger than 2.00 g/cc) due to its shaly plastic
properties. In sandy Chicontepec formation, the fracture
gradient exhibits a reduction from 1.85 to 1.90 g/cc in the
target
zone. The 3D cube shows the fracture gradient variation in the
field (figure 14). To use fracture gradient equation (Eaton,
1969), we developed an equation to estimate dynamical mechanical
properties from shear and compressional sonic data
(figure 15).
Figure 14. Fracture gradient prediction
Figure 15. Dynamical mechanical properties
Conclusions
The normal and abnormal pore pressure zones were defined with a
main source mechanism due to compaction disequilibrium.
Normal compactions trends for Presidente Aleman field were
defined and that allowed improving de pore pressure prognosis.
The traditional resistivity pore pressure model was adjusted to
Presidente Aleman field compaction disequilibrium condition.
The 3D model allows us to follow the behavior of geopressures in
the field and its variability.
Nomenclature
BOE, Barrel of Oil Equivalent
BPD, Barrel Per Day
TVD, True Vertical Depth
FRD, Fluid Retention Depth
g/cc, grams per cubic centimeter
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References
1. Athy, L. F. (1930), Density, Porosity, and Compaction Of
Sedimentary Rocks, AAPG Bulletin, v. 14, p. 1-23, 1930. 2. Eaton
(1969), Fracture Gradient Prediction and Its Application in
Oilfield Operations, SPE-2163 3. Eaton, B. (1975), The Equation for
Geopressure Prediction from Well Logs, SPE 5544. 4. Gardner, G.H.F.
et al (1974), Formation velocity and density the diagnostic basics
for stratigraphic traps, Geophysics, 1974 5. Mouchet, J. P. and
Mitchell, A. (1989), Abnormal Pressure While Drilling,
Elf-Aquitaine, Boussens, France, Technical Manual
No. 2, 255 p. 1989.
6. PEMEX (2009), Las reservas de hidrocarburos en Mxico,
Petrleos Mexicanos, 2009. 7. Velzquez-Cruz, David, Lpez-Sols, Vctor
Manuel, Daz Viera, Martn Alberto (2008), Prediccin de Presiones
Anormales
para la planeacin de la Perforacin de Pozos Marinos en Mxico, VI
International Seminar Exploration and Production of Oil and Gas
INGEPET 2008, Lima, Per, October 13-17, 2008, EXPL-2-DV-53.