Abstract:

The Mount Messenger Formation in Taranaki New Zealand provides both reservoir and seal units for hydrocarbon accumulations. This paper presents the results of geomechanics testing of samples from the formation at the GNS Science Rock and Soil Mechanics Laboratory, in combination with mercury injection testing. Strength testing, which was on blocks from outcrop exposures or shallow (<60 m) depth cores, included unconfined and triaxial compression at effective confining pressures up to 20 MPa that gave Mohr Coulomb strength parameters of cohesion (c') between 1.5 and 2.5 MPa and friction angles (F') between 15° and 26°. Other testing, which included consolidation, ultrasonic (p and s wave) velocities, air permeability gave results typical of New Zealand sedimentary soft rocks with unconfined compressive strength <5 MPa. The mercury injection testing gave a mercury-air threshold entry pressure of ~80 psia. The low to moderate strengths and a low threshold entry pressure together indicate a borderline hydrocarbon reservoir seal quality for the samples.

Introduction

The Taranaki region of New Zealand is known for both onshore and offshore hydrocarbon production. The Mount Messenger Formation [1], which has a widespread onshore distribution in North Taranaki (Fig. 1), is associated with both reservoir and seal units for hydrocarbon accumulations (e.g., [2]).

This paper presents the results of a range of laboratory geomechanics testing, in particular for strength and compressibility properties. The main intention of the paper is to demonstrate the capability of the GNS Science Rock and Soil Mechanics Laboratory in Lower Hutt to obtain mechanical properties at pressures associated with New Zealand hydrocarbon reservoirs.

Background

GNS Science has well-established capabilities in engineering geology and petroleum geoscience. In engineering geology these are largely associated with infrastructure foundations, landslides and geological hazards and for hydrocarbon geology the assessment of hydrocarbon systems, including the development of reservoir traps and associated seals.

The Rock and Soil Mechanics Laboratory was originally established to complement engineering geology programmes (e.g., [3]), while a recent expansion of capabilities has targeted an ability to perform testing at higher loads and pressures including use of a stiff loading frame and associated triaxial cells.

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