Abstract

Petrophysical evaluation of the Moki Formation in the Toka-1 exploration well, required rapid quantification of fracture porosities, enabling likely hydrocarbon volumes to be estimated. A resistivity imaging tool had been run over the approximately 400m of fractured interval. This information was being processed for fracture apertures by the acquisition contractor, but the rate of quantification was too slow to meet the operators objectives. A technique was developed to rapidly estimate fracture porosities using the difference between the density and the compressional sonic porosities and the difference between the shear and compressional sonic porosities. The fracture porosity estimates were cross-checked against the image log analysis completed. Owing to washouts across the most heavily fractured intervals, it was expected that fracture porosities from the density and compressional sonic logs were maximum values. The most likely fracture porosities were thought to be those derived using the shear and compressional velocities. The fracture porosities estimated by hand-picking fractures from the image data were considered the minimum values, since the identification process was incomplete. The work described represents a new technique for fracture porosity quantification. The comparison with image log interpretation over a short interval gives confidence in the methodology. The results are significant in that they show rapid quantification of fracture porosity is possible by using conventionally acquired logs, although it should be recognised that the uncertainty in this approach is significant. Application of this technique to other fields with limited image data may prove helpful in reservoir modelling and understanding of production behaviour.

Introduction

During 1995, the Toka-1 Exploration well was drilled in Taranaki. The well was drilled from onshore in New Plymouth, deviating to the north and into an offshore exploration permit.

The Moki Formation was recognised as being highly fractured and possibly hydrocarbon-bearing during drilling. Unfortunately, the deeper primary target reservoirs were found with low permeability. Hence rapid quantification of the possible hydrocarbon volumes in the Moki Formation was required so that a decision whether to complete the well or abandon could be made. It was at this stage that additional expertise was sought.

Data Available
Mudlog Data

The Moki section was drilled with significant gas shows from 2364–2665 m AHBKB. These shows corresponded to increased blue-white to dull orange fluorescence in the cuttings. Losses were encountered at 2709 m AHBKB. Gas detector readings approaching this depth and thereafter are likely to be erroneously low owing to excess mud filtrate invasion into both the fracture and matrix systems.

Wireline Log Data

Standard resistivity, sonic, density, neutron and gamma ray logs have been acquired across the Moki interval. More specialised logs acquired were a resistivity imaging log and an array sonic. The latter tool has allowed the measurement of shear and Stoneley wave velocities in addition to compressional velocity.

Fractured intervals are evident on the caliper, density and neutron logs at 2365–2420, 2476, 2600–2765 m AHBKB. These sections have been confirmed by analysis of the image log data.

Mudlog Data

The Moki section was drilled with significant gas shows from 2364–2665 m AHBKB. These shows corresponded to increased blue-white to dull orange fluorescence in the cuttings. Losses were encountered at 2709 m AHBKB. Gas detector readings approaching this depth and thereafter are likely to be erroneously low owing to excess mud filtrate invasion into both the fracture and matrix systems.

Wireline Log Data

Standard resistivity, sonic, density, neutron and gamma ray logs have been acquired across the Moki interval. More specialised logs acquired were a resistivity imaging log and an array sonic. The latter tool has allowed the measurement of shear and Stoneley wave velocities in addition to compressional velocity.

Fractured intervals are evident on the caliper, density and neutron logs at 2365–2420, 2476, 2600–2765 m AHBKB. These sections have been confirmed by analysis of the image log data.

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