ABSTRACT

The Deep Cretaceous carbonate reservoirs of Lake Maracaibo, Venezuela, produce mainly from open fractures. Any improvement in production rate requires optimal access of the wellbore to open fractures. Well tracks with maximum rates of open fracture interception have been calculated using computer models for a structure defined by the Icotea fault in Block IX. Optimum well tracks are towards azimuth 330° with deviation 60° in the West Flank and towards azimuth 030° with deviation 60° in the East Flank.

The open fracture networks consist of incompletely cemented or leached fractures which contain a "channel and island" structure of interconnected porosity. This structure, in combination with the high rock matrix strength and the subparallel strike of the open fractures relative to the E-W trend of the present-day maximum horizontal stress, indicate that it is unlikely that the conductivity of individual fracture will be reduced drastically upon pressure depletion.

The reservoir geological model established for Block IX is consistent with data obtained from the West Flank of Block I, some 40 km to the north and therefore has regional validity.

INTRODUCTION

The Cretaceous carbonate reservoirs in Lake Maracaibo are found beneath the productive Tertiary clastic reservoirs and are referred to as the "Deep Cretaceous", since they lie at a depth in excess of 10,000 ft (approx. 3000 m). Production of hydrocarbons from these Deep Cretaceous carbonate reservoirs is controlled by open fractures that connect the matrix and stylolite porosity with the wellbore1. Any improvement in production rate requires effective access to the open, hydraulically conductive fractures and knowledge of their occurrence with respect to stratigraphic architecture. This will help to:

  1. optimize the interception rate of the hydraulically conductive fractures2;

  2. predict the pressure sensitivity of the reservoir; and

  3. establish the regional validity of the reservoir geological model.

This contribution intends to show how a detailed description of the fracture network, based on core observations and the relationship of open-fracture development relative to the stratigraphic architecture, helps to optimize field development. Rock mechanical tests have been performed to assess the effect of reservoir pressure decline on the hydraulic conductivity of the open fractures.

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