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 the Society of Petroleum Engineers is prohi bited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must 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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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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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.