Abstract: A New Approach to Evaluate and Characterize Secondary Porosity in Carbonate Reservoirs of South Mexico.

Villavicencio, Antonio - Pemex; Frass, Manfred* - Schlumberger W&T; Casco, Rene - Schlumberger Geoquest; Martinez, Justino; Sanchez José; and Molina, Javier - Pemex

In the south México Mesozoic reservoirs, the main oil production is related to secondary porosity, which include fractures and vuggy porosity. The development of fractures are fault related. Major producing wells are those located close to the main faults systems were the fracture aperture is enhanced. Vugs are developed by dissolution.

Due to the complexity of the pore size distribution in carbonates, evaluating the pay zones is more difficult. Macro pores are easily filled with oil above the free water level, while micro pores have retained water after oil migration into the reservoir. This can result in high irreducible water saturation values and misinterpretation could occur, if only conventional logs are used.

The use of ultrasonic borehole imaging logs, combined with magnetic resonance logs, has proved to be an alternative way to improve the fracture characterization and/or vuggy porosity evaluation in wells drilled with oil based mud. From the ultrasonic images a fracture orientation and frequency can be obtained, as well as the vuggy carbonate intervals defined; and from the magnetic resonance log, a porosity measurement discriminating capillary, primary and secondary porosity is achieved.

This interpretation is based on the T2 distribution where long T2 values are observed in intervals and where the ultrasonic images show the presence of fractures and/or vugs. Applying a cut off to these distributions, a porosity is quantified which can be related to a secondary porosity . Using the same methodology the primary porosity is also obtained and the irreducible water saturation is measured. Based on these results a combined evaluation plotted at the same scale and integrated with the rest of the logs is generated.

Well lride - 1168, located in one of the major fields in south México, is an example where only fractures are observed in the ultrasonic images over the limestone-dolomite interval, with values ranging from 2% to 4% total porosity, and are determined by logs and from core analysis. Values between 0.3% up to 0.8% of secondary porosity are measured by the magnetic resonance tool.

In wells lride - 128D and Cactus - 67, intervals with fracture and vuggy porosity are identified at cores and correlated with the ultrasonic images, and values of 0.1% up to 4% of secondary porosity are measured by the magnetic resonance tool.

Well Cactus - 67 was the first one of these examples where a production of 1350 BOPD is obtained, based on this log interpretation.

In conclusion, this combination of logs can generate a fast and reliable evaluation in carbonate reservoirs with primary and secondary porosities.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil