FCL-a Computerized Well-log Interpretation Process For the Evaluation of Naturally Fractured Reservoirs
Authors: Aguilera Roberto , Acevedo Luis 
Published: March 2010

The Fracture Completion Log (FCL) is a computerized process which allows the determination of primary and secondary porosity and water saturation in the primary porosity secondary porosity and composite systems. The process uses data from conventional well logs, such as resistivity density neutron and/or sonic. It has the advantage of detecting fractured systems even when the fractures are not intercepted by the borehole.

This paper presents, the theoretical background behind the interpretation techniques, and examples of applications in the recompletion of an Austin Chalk well, and the completion of fractured carbonates in the Williston Basin of Montana and in Canada.

Introduction

Naturally fractured reservoirs are reservoirs which, in addition to primary or sedimentary porosity, contain secondary or induced porosity. Induced porosity is generated mainly by tension or shear stresses in a competent or brittle formation, diastrophism, volume shrinkage, solution, recrystalization, dolomitization and other geological processes.

Figure 1 shows the location of some of the largest fractured reservoirs in the world. The ultimate recovery from currently producing naturally fractured reservoirs has been estimated at more than 40 billion barrels of oil. In spite of this attractive potential, fractured reservoirs had not received the attention they deserved until rather recently.

Naturally fractured reservoirs behave significantly different from unfractured reservoirs due to the relative high capacity of the secondary porosity system. This high capacity allows high initial production rates which have led to extremely optimistic production forecasts and numerous economic failures. In order to understand the behaviour of naturally fractured reservoirs, estimates must be made of hydrocarbons-in-place within the primary and secondary porosity systems.

A process (FCL) is presented which allows reasonable estimates of primary and secondary porosity, and water saturation in the primary, secondary and composite systems. This-process utilizes conventional well logs, such as resistivity, density, neutron and/or sonic. It has the advantage of detecting fractured systems even when the fractures are not intercepted by the borehole.

The FCL cannot be considered as a panacea. However, it

(figure in full paper)

has provided excellent results in the evaluation, completion and recompletion of naturally fractured reservoirs in several places around the world.

Principles

The theoretical principles behind this process have been published previously in the literature by Aguilera(l-7) and have been used by others with reasonable success in Mexico(8), Venezuela(9,10) and the United States(11). In this process, the basic formation evaluation equations(12)

Analysis of Eq. 1 indicates that a crossplot of porosity vs R1 on log-log coordinate paper should result in a straight line with a slope of -m for zones of constant a Rw and constant I. For reservoirs with secondary porosity, the value of m is smaller than the cementation exponent, mb, determined from a primary porosity sample in the laboratory at simulated reservoir overburden conditions or estimated from lithological descriptions. This is reasonable because secondary porosity results in a reduction in tortuosity and cementation.

The value of m is determined by calculating the slope of the line drawn through the lower points in the plot.