Abstract

Stress tests in two fields in Central Australia revealed insitu horizontal stresses which are significantly higher than what would otherwise be expected assuming linear-elastic fracture mechanics. The state of insitu stresses in the two fields falls into a strike-slip fault regime. The stress regime in the region probably has a tectonic origin, similar to the Chimney Butte field in the Green River Basin, Wyoming where high insitu stresses arise from neo-tectonic activity. The high horizontal insitu stresses and stress anisotropy in the region may cause drilling induced shear fractures and therefore stress heterogeneity around the wellbore. Stress modelling shows that an existing natural or induced fracture prior to hydraulic fracturing, if permeable and aligned at a small angle with respect to the maximum horizontal stress, allows a hydraulic fracture to develop from its tip at a lower breakdown pressure. Such a hydraulic fracture, once initiated, would experience turning in order to align itself with the far-field minimum stress. It is postulated that pre-existing fractures and shear failures of the rock around wellbore during the breakdown stage increases the probability of near-wellbore tortuosity in the form of multiple fractures and fracture turning.

Introduction

Hydraulic fracturing is increasingly used to stimulate low permeability gas reservoirs in Australia, particularly in Central Australia. However, as in other regions of the world, hydraulic fracture treatments in Australia generally experience higher treating pressures than are otherwise predicted by models, particularly during proppant addition stages. Such abnormally high treating pressures often precede premature, near-wellbore screen-outs, generally reduce maximum allowable proppant concentrations, severely reducing the effectiveness of treatments. Previous studies have suggested that fracture width restrictions, in the near-wellbore region, is the major cause of these high treatment pressures. This phenomenon, more commonly called Near-Wellbore Tortuosity, is generally characterised by any complicated, tortuous pathways connecting wellbores with main fractures. The resulting 'bottle-necks' increases pressure drops, and restricts the passage of proppant into main fractures. Cleary et al. grouped Near-Wellbore Tortuosity into two categories:

  1. multiple fracturing and;

  2. fracture reorientation.

Multiple fracturing refers to the initiation and propagation of more than one hydraulic fracture from a single wellbore. Multiple fractures compete for opening space within the same region, such that the individual fracture branches have widths reduced by a factor of (where N is the number of fracture branches) as compared with an idealised single fracture. While multiple fracturing occurs to varying degrees in all hydraulically fractured wells, it is more common in deviated and horizontal wells. Davidson et al. and Stadulis et al. describe case studies in which multiple fracturing were clearly the origin of the type of fracture treatment problems described above.

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