Abstract

The study of fracturing in carbonate formations has become a critical factor for the exploitation policies for the main oil and gas producing reservoirs in Mexico, since a great percentage of hydrocarbons production is associated with this kind of reservoirs.

In this paper a methodology that integrates diverse sources of information at different spatial scales is proposed, the range of spatial scales goes from the microscopic level study of thin section of rocks and cores to the megascopic level where fracturing is determined with seismic information.

To define reservoirs fracturing it has been utilized information derived from cuttings samples, cores, well logging, wellbore images, well testing and seismic interpretations.

Given that the fractures are not isolated phenomena in nature, the studies herein described, target to know their origin, in this way, fractures and their spatial relations are identified with the purpose of defining fracture system, principal stresses and their association with major tectonic features such as folds and faults; letting know the fracturing pattern and its association with the geologic model of the area.

Fracture parameters are estimated by means of core analysis and well log information using indices derived mainly from resistivity and porosity logs, on the other hand, fracture orientation is determined from dipmeter information, wellbore images and its correlation with cores.

The application of this method and its results in two fields of Mexico are shown, these results allowed to establish models at well and reservoir levels. Also from these it was possible to estimate preferential fracture orientations in areas not yet drilled, which is of great importance for the optimum field development.

Introduction

A big part of Mexico's oil reserves come from reservoirs located in the country's SE region, in these fields naturally fractured carbonate reservoirs abound, and their geological ages range from Earlier Paleocene to Late Jurassic, the units found there, show sequences of partially dolomitized limestones with low content of clays. Porosity varies from 3 to 23%, and a great part of it is due to fractures and vugs, an example of this type of formations is given in figure 1. The estimated permeabilities for these units are greater than 2 darcies which explains the high productivities of some wells completed in these units, some wells reached productions greater than 30,000 bpd.

Fractures are discontinuities due to a rock's lost cohesion, they also represent a volume that can be occupied by fluids and that can serve as preferential flow paths. There are many models to explain the origin of fractures resulting from underground stresses, in general fractures can be classified in 1, 2, 3.

  • Gravitational: due to compaction of large volumes of rock and decompaction of clays.

  • Tectonic: due to folding, faulting and diapirism.

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