Abstract

Mechanical property data will be important for the exploration and production of gas hydrate reservoirs, and for the safe exploitation of conventional oil and gas reservoirs beneath hydrate deposits. Two parameters controlling fracture initiation are friction angle and cohesion. The friction angle can be estimated from porosity, mineralogy, and texture of the sediment. In the context of Mohr-Coulomb theory, we relate measurements of the speed of sound, which can be made with logging tools, to the cohesion of the formation. We propose that the cohesion depends on the details of how hydrate grows in the pore space of the sediment.

Introduction

There is a growing realization that geomechanics will play a central role in exploiting gas hydrate reservoirs. Fracture and fault systems play a major role in transporting gas into the gas hydrate stability zone, where it can combine with water to form hydrate1. During production, the low hydraulic permeability of hydrate-affected formations2 means that natural or artificial fractures will be required to produce gas at economic rates3. Fracturing also plays a central role in geohazards such as wellbore instability4 and subsea landslides5,6, which must be avoided if hydrocarbon production from or through hydrate deposits is to be carried out safely7.

Because gas hydrate is not stable at laboratory temperatures and pressures, and can decompose during core retrieval, mechanical properties measurements of recovered core are rare, expensive, and often of questionable validity. Moreover, it is very difficult or impossible to create laboratory samples of hydrate in sediment under conditions that mimic natural processes. Borehole logging measurements most accurately represent the in situ properties of hydrate reservoirs.

In this paper, we present correlations by which well log data can be used to estimate the material properties that determine the stability of marine sediments against mechanical failure. We propose that the strength of sediments depends on the details of how hydrate grows within the pore space of sediments.

Review: Mohr's Circle and Coulomb Failure Criterion

The three principal stresses in the earth are ?1 ??2 ??3. Fracture generation is controlled by competition between the normal stress ?? and shear stress ?? acting on a plane of failure. Given the simplifying assumption that two of the stresses are equal, ?2 = ?3, the normal and shear stresses on the failure plane are

  • (Mathematical equations) (1) and (2) (Available in full paper)

where the failure plane makes an angle ? with the plane containing the two smaller principal stresses, ?2 and ?3. These equations define the Mohr's circle on the (?,?) plane, which is centered at ((?1+?3)/2,0) and has a radius (?1-?3)/2.

According to the Coulomb criterion8 a fracture will occur when the shear stress equals or exceeds cohesion and friction

  • (Mathematical equations) (3) (Available in full paper.)

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