Abstract: Electrical and Ultrasonic Borehole Images of Venezuelan Wells: Reservoir Anisotropy Analysis and its Influence on Production

FRASS, MANFRED, LUIS RONDON, and JEAN C. CALTERO

Differential horizontal stresses create anisotropy in rock mechanical and fluid flow behavior which cannot be easily identified from conventional logs, but appear clearly on image logs. When fractures are induced, they develop along the maximum horizontal stress azimuth and the strike of those, created by a fracturing job will growth in the same azimuth, this knowledge reduces the risk of connecting water and oil bearing zones. Natural fractures are also stress related and control the fluid movement. The fracture frequency, length and aperture, are parameters which can be measured using borehole images.

In well J-479-X from East Venezuela, the porosity is about 10% and the permeability of the Naricual Formation, is about 100md, but 1800bopd are flowing, so only by the addition of fractures, this production could be explained. Looking at the ultrasonic images this is clear, the development of fractured intervals, related to a fault system, allowed a high production from this deep reservoir.

Hydrocarbon fluid flow, through clean sandstone, shaly sandstone, laminated sandstone, fractured sandstone or limestone, fractured basement, all exhibit different behavior. Therefore, to maximize oil recovery, a high resolution reservoir characterization is required, to resolve the influence of stratification and fracturing. Borehole images play an important role for geologist and reservoir engineers in this characterization process. Breakouts interpreted on microresistivity images from a pilot well (VLC-1184), in a horizontal well development in Lake Maracaibo, were related to the minimum horizontal stress orientation. The sonic anisotropy log indicated the same degree and orientation of anisotropy, but evidence of fracture or incipient failure was not observed on the oriented core. The minimum stress orientation from the logs was used to guide drilling trough a laminated sandstone reservoir sequence and resulted in, the first open hole completion in a horizontal well, in the Lake Maracaibo area, producing over 2000bopd.

From these examples, it is dear that, the integration of image logs and standard logs, is the key for the explanation of the well production behavior.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria