Spontaneous Potential Log or SP

Français

Le Log de Potentiel Spontané : Un Outil Simple aux Informations Puissantes

Le Log de Potentiel Spontané (SP), un incontournable de la diagraphie depuis les années 1930, est un outil d'une simplicité trompeuse qui a le pouvoir de révéler des informations cruciales sur les formations souterraines. C'est l'une des techniques de diagraphie les plus anciennes et les plus simples, mesurant la différence de tension naturelle entre la formation et le fluide de forage dans le puits.

Fonctionnement :

Le log SP repose sur un principe fondamental : les différences de salinité. Lorsque le fluide de forage, souvent une solution saline, entre en contact avec les fluides de formation (qui peuvent être salés, d'eau douce, ou même des hydrocarbures), un potentiel électrochimique est créé. Cette différence de potentiel, mesurée par l'outil SP, est enregistrée le long du puits, produisant une courbe SP caractéristique.

Applications du Log SP :

Le log SP, malgré sa simplicité, offre une mine d'informations sur la formation. Il peut être utilisé pour :

Estimation Qualitative de la Perméabilité : Les formations à plus forte perméabilité tendent à montrer des déviations SP plus fortes, car elles permettent un échange de fluides plus important entre la formation et le puits.
Évaluation de la Qualité du Réservoir : La courbe SP aide à identifier les zones de réservoir potentielles en ciblant les zones à forte perméabilité et porosité.
Calculs de la Résistivité de l'Eau (Rw) : La combinaison des données du log SP avec d'autres logs, tels que les logs de résistivité, permet une estimation précise de la résistivité de l'eau de formation (Rw), un paramètre crucial pour la caractérisation du réservoir.
Estimation de la Shalinité de la Zone : Les réponses du log SP peuvent différencier les formations argileuses des formations propres. Les zones argileuses ont tendance à montrer des déviations SP supprimées ou déformées en raison de la présence de minéraux argileux.

Limitations :

Le log SP n'est pas sans limites :

Profondeur d'Investigation : Sa pénétration est limitée à la zone proche du puits, ce qui le rend insensible aux caractéristiques plus profondes de la formation.
Influence de la Salinité de la Boue : La réponse du log SP est considérablement affectée par la salinité de la boue de forage, nécessitant une interprétation et une calibration minutieuses.
Applicabilité Limitée dans Certaines Formations : Son efficacité est compromise dans les formations à faible perméabilité ou lorsque les fluides de formation ont une salinité similaire à celle du fluide de forage.

Conclusion :

Malgré ses limites, le log SP reste un outil précieux pour les géologues et les géophysiciens. Sa capacité à fournir des estimations qualitatives de la perméabilité, des informations sur la qualité du réservoir et des informations sur la shalinité en fait un élément crucial de l'analyse des logs de puits. Le log SP, bien qu'une technique relativement simple, continue de jouer un rôle important dans la compréhension de la géologie complexe du sous-sol.

Test Your Knowledge

Quiz: The Spontaneous Potential Log

Instructions: Choose the best answer for each question.

1. What is the fundamental principle upon which the SP log operates?

a) Differences in pressure between the formation and the wellbore b) Differences in temperature between the formation and the wellbore c) Differences in salinity between the formation and the wellbore d) Differences in magnetic properties between the formation and the wellbore

Answer

c) Differences in salinity between the formation and the wellbore

2. Which of the following is NOT a common application of the SP log?

a) Estimating the permeability of a formation b) Identifying potential reservoir zones c) Determining the exact depth of a fault d) Evaluating the water resistivity of a formation

Answer

c) Determining the exact depth of a fault

3. What is a limitation of the SP log?

a) It can be used to identify hydrocarbon deposits with high accuracy. b) Its penetration is limited to the near-wellbore region. c) It is unaffected by the salinity of the drilling mud. d) It is highly effective in formations with very low permeability.

Answer

b) Its penetration is limited to the near-wellbore region.

4. How does the SP log differentiate between shaly and clean formations?

a) Shaly formations show stronger SP deflections. b) Clean formations show stronger SP deflections. c) Shaly formations show suppressed or distorted SP deflections. d) Clean formations show suppressed or distorted SP deflections.

Answer

c) Shaly formations show suppressed or distorted SP deflections.

5. What is the primary reason the SP log remains a valuable tool in well log analysis despite its limitations?

a) It is the cheapest and easiest logging technique. b) It can identify specific types of hydrocarbons. c) It provides valuable qualitative information about permeability, reservoir quality, and shaliness. d) It is highly accurate in estimating the depth of formations.

Answer

c) It provides valuable qualitative information about permeability, reservoir quality, and shaliness.

Exercise: SP Log Interpretation

Scenario: You are a geologist working on a well log analysis. The SP log shows a strong negative deflection at a depth of 1000 meters. This deflection is followed by a gradual increase in the SP reading towards a more positive value. The formation is suspected to be a sandstone reservoir.

Task:

What does the strong negative SP deflection indicate about the formation at 1000 meters?
How does the gradual increase in SP reading after the deflection suggest a change in the formation?
Based on the SP log information and the suspected formation type, what is the likely nature of the reservoir?

Exercice Correction

The strong negative SP deflection at 1000 meters indicates a high permeability zone. This is because the difference in salinity between the formation fluids and the drilling mud is significant, leading to a stronger electrochemical potential.
The gradual increase in SP reading after the deflection suggests a transition to a lower permeability zone. This could be due to a change in the lithology, porosity, or the type of formation fluids present.
Based on the SP log information and the suspected sandstone formation, the reservoir is likely a porous and permeable sandstone with good reservoir quality. The strong negative deflection at 1000 meters indicates a zone of high permeability, which is a desirable characteristic for a reservoir.

Books

"Well Logging for Petroleum Engineers" by B.H. Dolman: A comprehensive text covering various well logging techniques, including detailed discussions on the SP log.
"Log Interpretation Principles and Applications" by O. Serra: Another comprehensive book on well logging with extensive sections dedicated to understanding and interpreting SP logs.
"Applied Geophysics for Petroleum Exploration and Production" by W.M. Telford et al.: Offers a solid understanding of the geophysical principles behind the SP log and its application in reservoir characterization.

Articles

"The Spontaneous Potential Log: Theory and Interpretation" by G.V. Chilingarian: An insightful article discussing the theoretical basis of the SP log and practical guidelines for interpreting its data.
"A Review of Spontaneous Potential Log Interpretation" by R.F. Meyer: Summarizes the history, principles, and applications of the SP log, highlighting its limitations and areas of improvement.
"The Use of Spontaneous Potential Logs in Reservoir Characterization" by J.D. Schlumberger: Focuses on the role of the SP log in characterizing reservoirs, emphasizing its contribution to permeability and porosity estimations.

Online Resources

Schlumberger Oilfield Glossary: https://www.slb.com/resources/oilfield-glossary/sp This resource provides a concise definition of the SP log and explains its function in well logging.
Society of Petrophysicists and Well Log Analysts (SPWLA): https://www.spwla.org/ This professional organization offers a wealth of resources on well logging, including articles, publications, and presentations related to the SP log.
GeoScienceWorld: https://www.geoscienceworld.org/ This platform hosts a wide range of peer-reviewed articles on various geological and geophysical topics, including research on the SP log.

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