A study into the mapping of an induced fracture in a fine grained sediment in biaxial and triaxial stress formations

Penketh, S

Abstract

Unconventional hydrocarbon extraction has increased in the last twenty years due to the post-industrial west needing to increase energy security. This has meant tapping into hydrocarbon resources that are trapped within impermeable sediments. Shale hydrocarbon reservoirs have a permeability of less than 0.01mD, therefore to extract hydrocarbons the shale has to be fractured to release the trapped fluid.
The ability to predict when a fracture will occur and what the resulting shape of that fracture will be has recently become increasingly important in engineering, and, in particular the petroleum industry. This has been done previously using the fracture growth equation for brittle material with additional modifications for ductile materials. A number of pseudo 3D equations have been developed which are used by these software simulations to try and predict the dimensions of a fracture so that reservoir engineers can calculate the volume of fluid expected to be produced by any drilling well. However, the simulations provide only a reasonable prediction of fluid flow and can be under or overestimated by up to 60% of returns. Previous fraccing experiments carried out to determine how the models could be improved have relied on data whereby reservoir rocks have been placed under bi-axial pressure, (two forms of pressure acting perpendicular to each other) rather than tri-axial (σ1σ2σ3) which would more adequately reflect the pressures acting on in-situ shale formations.
The present investigation has focused on building a pressure rig that allows rock samples to be placed under tri-axial pressure. To this end a concrete rig structure has been built to house three flat jacks which place a 100mm3 rock sample under pressure of up to 120 bars (12MPa) in three different directions, to represent rock at in situ depths. In conjunction with the flat jacks, 6 acoustic sensors can be placed around the rock sample to allow 3D mapping of the acoustic emissions occurring during the drilling and fracturing phase.
10 different shale samples were scanned (using the GE Phoenix v|tome|xs CT scanner) and their images reconstructed to show 3-D visualisations of any pre-existing fractures or structures. 9 of those samples were then drilled and fractured and scanned again. Using reconstruction software each drilling fracture was mapped and the dimensions measured at regular intervals in order to provide a more accurate picture of how the fracture developed when the rock is subjected to tri-axial loading.
Two types of shale rock were tested, samples from the Westbury Shale Formation and samples from the Accrington Mudstone Formation. Two Accrington and one Westbury were fractured under bi-axial conditions and two Accrington and four Westbury under tri-axial pressure.
The Accrington Mudstones formed a fracture that was almost elliptical in shape with ultra-thin extended tips along both the top and bottom of the fracture. There was a direct correlation between fracture width and height, those fractures that narrowed along the half-length increased in height, whilst fracture half-lengths that didn’t narrow resulted in a reduced height. The Westbury Shales, which are more organic rich, had a more distorted shape due to fractures opening up along the weaker bedding planes helping to disperse the pressure from the fracturing fluid. The acoustic sensors accurately picked up and located the fracture inducement, proving that they opened instantaneously and also revealing damage that occurred during drilling at a small distance (2mm ±1mm) below the position of the drill tip. The acoustic data also revealed existing fractures and planes of weakness that the induced fracture would follow.

• Thesis Type: Thesis
• Acceptance Date: Sep 28, 2018
• Deposit Date: Oct 3, 2018
• Publicly Available Date: Nov 3, 2018