_Well Log Analysis for the Interpretation of Hydrocarbon Evidence

Thirteenth International Congress of the Brazilian Geophysical Society

Well log analysis for the interpretation of hydroca rbon evidence: Marginal accumulation in sandstone reservoir - Recôncavo Bas in, Brazil Dean Pereira de Melo* PETROBRAS, Juliana Rodrigues Almeida UNIGRANRIO, Gabriel Pinheiro Carneiro PETROBRAS Akihisa Motoki UERJ, Eliane da Costa Alves UFF Copyright 2013, SBGf - Sociedade Brasileira de Geofísica

This paper was prepared for presentation during the 13th International Congress of the Brazilian Geophysical Society held in Rio de Janeiro, Brazil, August 26-29, 2013.

Contents of this paper were reviewed by the Technical Committee of the 13th International Congress of the Brazilian Geophysical Society and do not necessarily represent any position of the SBGf, its officers or members. Electronic reproduction or storage of any part of this paper for commercial purposes without the written consent of the Brazilian Geophysical Society is prohibited. ____________________________________________________________________

Abstract This paper shows the well log analyses of 1FGA0001BA of Recôncavo Basin, State of Bahia, Brazil, highlighted on saturation water (Sw) determination. Six potential sandstone reservoir intervals have obtained positive results. One has g as and other five contain oil and/or fresh water. Two models are analyzed: Rw obtained from resistivity l og (ILD) and Rw measured on water formation samples collected from Recôncavo Basin, where the Sw has been determined for both. The Rw values of the ILD profile are coherent in comparison with those the R w obtained from samples. Two equations for Sw determination have been used: Archie and Simandoux. Two most favorable intervals are suggest with good porosity (19%), lower values of Vsh (9%) and lower Sw (39%). Introduction The Recôncavo Basin presents economic and scientific importance about oil and gas production in Brazil. After the beginning of Petrobras activities on 1953, the Reconcavo and Sergipe-Alagoas Basin had sustained the national production until 1960 (Milani e Araújo, 2003). According to ANP (National Agency of Petroleum), 5626 wells was drilled in Recôncavo Basin up to february 2002. The stratigraphic evolution of the above-mentioned sedimentary basin is represented by the following formations: Afligidos (Permian); Alianca, Sergi (Jurassic - Dom João local stage); Itaparica, Água Grande, Candeias, Marfim (Cretaceous - Rio da Serra local stage); Pojuca, Taquipe (Cretaceous - Aratu local stage); Salvador, Maracangalha (Cretaceous - Rio da Serra and Aratu local stage); Sao Sebastião (Cretaceous - Buracica and Jiquiá local stage), Marizal (Cretaceous - Alagoas local stage); Sabia, and Barreiras (Neogene). The formations Aliança, Sergi, Itaparica are representative of pre-rift phase where fluvial and eolian sedimentation were dominant, with some lacustrine influence. The other formations, with the exception of the Marizal, Sabia and Barreiras, are representative of the syn-rift phase (Silva et al., 2007). Early syn-rift stage was

featured by lacustrine environment followed by main deltaic sequences of the sedimentation of Aratu local stage. Late syn-rift stage was characterized by fluvial facies. The post-rift phase was made up mainly of alluvial depositional systems with sandstones, conglomerates, shale and carbonate layers (Silva et al., 2007). The well 1FGA0001BA, which is the target of the present paper, is located in Fazenda Gameleira field, which is about 85 km to the north-norteast of the city of Salvador (Figure 1).

Figure 1. Well location map Method Gamma ray (GR), caliper (CALI), resistivity (ILD), density (RHOB), and neutron (NPHI) have been adopted to carry out the study proposal. These measurements were obtained using water-based mud. The applied methods are according to Krygowsky (2003) and Girão Nery (2004). The main parameter for Sw determinations are presented on Table 1.

Table 1. Parameters obtained from well log

Parameter Log Method Porosity (φ) RHOB and

NPHI From log and Gaymard

equation (gas) Rw ILD Minimum Rwa Sw ---- Archie and Simandoux

equation Vsh GR IGR

Suta Vijaya

Typewritten Text

http://sys2.sbgf.org.br/congresso/abstracts/trabalhos/sbgf_4268.pdf

Well log analysis in sandstones ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________


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The potential sandstones reservoir intervals have been recognized through low gamma ray relative values and RHOB porosity larger than 12%. All intervals are evaluated for quality control according to caliper log. The zones larger than the bit size generate anomalies on logs. They must be noted and in some cases should be discarded.

The average porosity was read from RHOB log. For the gas bearing layers, the porosity is calculated using Gaymard equation (Gaymard & Poupon, 1968). The Eq. 1 shows the adopted correction form of the gas on porosity.

2)()( 22

ndc

φφφ += (eq. 1)

φc – porosity corrected; φd – density porosity; φn – neutron porosity.

The Vsh is calculated using equation 2 and 3 (Girão Nery, 2004):

minmax

minlog

GRGR

GRGRIGR

−−= (eq. 2)

IGRAA

IGRVsh

⋅−−=

)1( (eq. 3)

Where IGR is gamma-ray index.

To determine the Rw value, the minimum Rwa method is used. The three orientation follow: a) Existence of water layer where Rwa is the same of Rw; b) In case of very low porosity, the Rwa value should be discarded; c) Compare with values of Rw from the field or basin to avoid moving away from reality (Girão Nery, 2004).

In order to saturation water evaluation, the Archie law has been considered (eq. 4):

nm Sw

RwaRt

⋅⋅=

φ (eq. 4)

Where Rt is resisitivity of rock on uninvaded zone; a is tortuosity factor; m is cementation or porosity exponent; n is saturation exponent.

This is a simple model that predicts the amount of water in a reservoir, where Rt, Rw and porosity must be know. This model considers that the rock is free from conductive minerals. In addition Archie Law is appropriate for Vsh=0 when the influence of shales in reservoir is really low.

For shaly reservoir the Simandoux equation for Sw determination is a classical approach (Worthington & Johnson, 1987). A basic form of Simandoux equation is:

SwRsh

Vsh

Rwa

Sw

Rt

nm

+⋅⋅= φ1

There are many variations of the Simandoux equation. The equation used by authors follows Crain (1986) which seems to work well in several areas of the world. Results There are 6 intervals identified as potentials reservoirs for the well 1FGA0001BA. Petrophysical log interpretation become possible the obtained of thickness of porous layer, porosity (average recognized from RHOB log), VSh (shale volume) and fluid content (Table 2).

Table 2 – Reservoir Informations

N Reservoir (m)

Thickness of porous

layer (m)

Average Porosity -

φa (%)

Vsh (%)

Interpreted Fluids

1 735 - 752 15 22 16 saline water

2 892 – 907 14 22 9 Gas 3 1145-

1158 10 19 9 Oil and/or

fresh water 4 1177 –

1187 8 19,5 9 Oil and/or

fresh water 5 1312,5-

1332 18,5 16,5 5 Oil and/or

fresh water 6 1380 -

1408 13 15 23 Oil and/or

fresh water

For the interval number 1, saline water is interpreted, but low resistivity oil is possibly. The electric resistivity is 0,7 Ωm and the gamma ray shows clay influence.

According to the ANP report, the fluid sampled from the interval number 2 was gas. There are no fluid samples for other intervals. The average resistivity is 1,8Ωm and the difference between neutron porosity and density porosity is 2.5%. The Vsh is low compared with the former.

The interval number 3 is interpreted to be saturated by oil and/or fresh water. The resistivity is 2,5Ωm and difference between neutron porosity and density porosity is 4%. The VSh is 9%. The caliper log indicates that the interval is larger than bit size, which is 2”.

Reservoir number 4 is similar to the 3, with the same caliper problem.

The reservoir number 5 presents the largest thickness with high porosity, with high contents of oil and/or fresh water. The resistivity is 1,4Ωm. The difference between neutron porosity and density porosity is 3%.

In the reservoir number 6, the presence of oil and/or fresh water is expected. This interval shows high Vsh (23%) and low porosity (15%). The resistivity is 2,7Ωm with deflection to 2.0Ωm toward the base of sandstone. The difference between neutron porosity and density porosity is 2%. This evidences leads the authors to interpret cementation of upper portion of this interval.

The ambiguous interpretation of oil and/or fresh water is due to the use of ILD log without microresistivity data.



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Formation water resistivity determination (Rw) The formation water resistivity has been calculated using Rwa minimum method. Four Rwa (apparent resistivity) have been calculated (Table 3).

Table 3 - Rwa values

Depth (m) Rt Rwa 745 0,7 0,042 858 0,7 0,045

1312 1,3 0,035 1380 1,1 0,040

The representative Rw value is 0,042Ωm suggesting that at 745m the interpreted fluid is probably saline water and this value is close to minimorum Rwa. The Rwa at 1312m can be influenced by low porosity. The m value is 1,8, being adequate for sandstone (LaTorraca & Hall, 1991). The considered a value is 0,62 and n value is 2 (Crain, 1989). The second value of Rw used here was published by Teles et al. (2010). This paper reports the electric conductivity measured from samples of formation water. 13 wells were sampled. The Figure 2 shows resistivity vs. depth diagram.

Figure 2 – Values of resistivity obtained by electrical conductivity measured in water formation samples (Teles el at., 2010)

The most resistivity values range between 0,0072Ωm and 0,008Ωm. The resistivity obtained from Rwa minimum method is quite different from those measured from water samples (Figure 3).

Figure 3 – Values of resistivity obtained by electrical conductivity measured in water formation samples (Teles el at., 2010) with resistivity obtained from log analysis (red point).

Although the distance between the well where the water sample was collected in relation of the well 1FGA0001BA have been unknown, the measure of resistivity from samples is a rare data and should be highlighted. Indeed, for the Sw calculation two models are proposed using Archie equation: 1) Consider the resistivity 0,042Ωm; 2) Considering the median of resistivity obtained from formation water samples 0,0078Ωm (Teles et al., 2010). Supposing Rw=0,042Ωm (method of Rwa minimum), the value of Sw is relatively high (Table 4).

Table 4 – Water saturation, porosity, and apparent resistivity. Model for Rw=0,042Ωm using Archie equation.

Reservoir (m) φa Rwa Sw

735 - 752 0,22 0,046 0,75 892 – 907 0,22 0,118 0,47 1145-1158 0,19 0,131 0,45

1177 – 1187 0,19 0,132 0,44 1312,5-1332 0,16 0,051 0,72 1380 - 1408 0,15 0,036 0,85

For the second model the value of Rw will be 0,0078Ωm (median of thirteen resistivity data) obtain in samples (Figure 2).



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Table 5 – Values of water saturation. The porosity and apparent resistivity are the same of Table 4. Model for

Rw=0,0078Ωm using Archie equation.

Reservoir (m) Sw 735 - 752 0,32 892 – 907 0,20 1145-1158 0,19

1177 – 1187 0,19 1312,5-1332 0,31 1380 – 1405 0,37

The Sw values of the model 2 are very low and optimistic for hydrocarbon contents. The authors propose the following two hypotheses: 1) The water formation of the well 1FGA0001BA is not as saline as the water sampled by the other wells reported by Teles et al. (2010); 2) The resistivity of ILD measures the invaded zone with lower saline than water formation.

Considering that all of the six sandstone intervals which have influence of clays. It is interesting to apply the Simandoux equation which comprises the Vsh. The results of Sw with Rw=0,042Ωm is present on Table 6.

Table 6 –Water saturation value. The porosity and

apparent resistivity are the same of Table 4. Rw=0,042Ωm using Simandoux equation.

Reservoir (m) Sw

735 - 752 0,64 892 – 907 0,42 1145-1158 0,39

1177 – 1187 0,39 1312,5-1332 0,67 1380 – 1405 0,62

The values of Sw considering the resistivity of 0,0078Ωm calculated using Simandoux equation are present (Table 7).

Table 7 – Values of water saturation. The porosity and

apparent resistivity are the same of Table 4. Rw=0,0078Ωm, using Simandoux equation.

Reservoir (m) Sw

735 - 752 0,32 892 – 907 0,20 1145-1158 0,19

1177 – 1187 0,19 1312,5-1332 0,31 1380 – 1405 0,37

Conclusions

1 – There are six intervals of sandstones with potential reservoir characteristics.

2 – The most favorable intervals are 1145-1158m and 1177-1187m with good porosity (19%), lower values of Vsh (9% for both) and lower Sw.

3 –Two values of Rw for Sw determination are considered: Rw based on well log interpretation (Rwa minimum method) is 0,042Ωm; Rw based on values of water formation samples reported by Teles et al. (2010) is 0,0078Ωm (median). There is a significant difference between these values.

4 – The difference between the values of Rw can be explained by two hypothesis: 1) The water formation of the well 1FGA0001BA is not as saline as the water sampled by another wells reported by Teles et al. (2010); 2) The resistivity of ILD is measuring the invaded zone which is lower saline than water formation. It is notable that the value of 0,042Ωm is Rt (rock and fluid) and 0,0078Ωm is only water formation resistivity.

5 – The Sw has been obtained for the two values of Rw and consider Archie equation and Simandoux equation. The models for Rw 0,042 and Sw 39% for the intervals 1145-1158m and 1177-1187m (Simandoux equation), which are the most coherent.

6 – The reservoir intervals are shaly sandstones that induces low resistivity where may be expected higher resistivity compared with Rt. This characteristic leads us to prefer the Sw of Simondoux equation.

References ANP 2013. Agência Nacional do Petróleo. Área de Fazenda Gameleira. http://www.anp.gov.br/?dw=57781. Acess: Abril 22, 2013. Crain, E.R. 1986. The Log Analysis Hand Book; Penn-Well, Publ. Co., Tulsa, Oklahoma, U.S.A. Gaymard, R., Poupon, A., 1968, Response of neutron and formation density logs in hydrocarbon bearing formations, The Log Analyst, v. 9, No. 5, pp. 3 - 12. Girão-Nery, G. 2004. Perfilagem geofísica. Salvador: Hydrolog Serviços de Perfilagens LTDA. Krygowsky, A.D. 2003. Guide to petrophysical interpretation, pp. 147.

LaTorraca, G.A., Hall, C.G. 1991. Observations of rock fabric controls on the electrical properties of sandstones. Society of Core Analysts Conference, pp 1-16.

Milani, E.J., Araújo, L.M. Recursos Minerais Energéticos: Petróleo. 2003. In L.A. Bizzi, C. Schobbenhaus, R. M. Vidotti and J.H. Gonçalves (Eds.) Geologia, Tectônica e Recursos Minerais do Brasil. CPRM, Brasília, BR, p. 541-576. Teles, D.R.S., Azevedo, A.E.G., Santos, C.P.L. 2010. Caracterização isotópica de águas de formação hipersalinas de um campo de petróleo da Bacia do



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Recôncavo, Brasil. Rev. Bras. Geof. [online]. 2010, vol.28, n.2, pp. 291-301.

Worthington, P.F., Johnson, P.W. 1987. Quantitative Evaluation of Hydrocarbon Saturation in Shaly Freshwater Reservoirs. 16th Annual Convention Proceedings (Volume 2), pp. 127-152.

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_Well Log Analysis for the Interpretation of Hydrocarbon Evidence