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Abstract: Defining Producing Zones with Open Hole Logging

Graterol, Jose and Ovidio Suarez - PDVSA

The information from production logging is very valuable in reservoir management, but the results sometimes are disappointing because a significant number of perforated intervals does not contribute to the well production. The characteristics of the producing zones have to be defined to correct this situation. Open Hole Logging analysis correlated with production logging results is the best way to identify permeable zones in absence of a continuous permeability survey.

This type of correlation was applied in an area, located in Lake Maracaibo, Western Venezuela, where several wells did not produce at all, although the sedimentological descriptions pointed to the presence of hydrocarbon facies in the perforated zones. The results of the correlation indicate productive zones as being characterized by a minimum relation of 1.6 between the corrected Deep (LLD) and Shallow (LLS) curves of the Dual Lateral log. The application of the method showed consistent results in the fifteen wells completed in the reservoir, and also indicated that the LLD/LLS relation in the perforated zones is directly correlatable with the initial well production (Fig. 1).

The success of the above described experience led to the proposal of defining producing zones based on correlations between production logs and open hole logging for another area, also in Lake Maracaibo, where four different Eocene-age sands are producing commingled. The production logs from four wells indicated that 66% of the perforated zones in the three lower sands, C-3, C-4 and C-5, did not contribute appreciably to the well production. The petrophysical analysis of the routine logs in these four wells showed the productive zones of the three lower sands should have total porosity values derived from the Formation Density log without any correction, of at least 17%, which equates to an approximate value of 100 mD of air permeability from core analysis. The consistency of this petrophysical characterization is confirmed by seven additional production logs and several production tests. The results of nine production logs in the C-2 sand showed the productive zones of this upper interval characterized by resistivity values from the Induction log greater than 40 ohms-m and Formation Density log readings less than 2.45 gr./cc. Additionally, the initial well production for the C-3 sand has been estimated using the product of True resistivity and thickness (Fig. 2).

The consistency of the producing zones definition has demonstrated that the reason for zones not contributing to well production is poorer rock quality rather than well zone damage as indicated by preliminary interpretations.

The application of these results has optimized well completions, redesigned well stimulations, redefined reserve calculations and modified the exploitation strategy for the area.

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