Hydraulic Fracture Testing Of Cohesive Soil
Authors: Overy R.F. , Dean A.R. 
Published: October 2008

Hydraulic fracture tests were performed in cohesive soil as part of a geotechnical investigation at three southern North Sea sites. The information was required to guide conductor settings and grouting pressures. Theories available to interpret the data are reviewed, the apparatus is described and its use as part of a normal offshore site investigation explained. The test results are appraised with regard to the soil conditions encountered and comparisons are made with the predictive methods available. It was found that the hydraulic fracture resistance of the strong, medium plasticity clays encountered can be assessed by assuming the soil fails in undrained shear. A highly plastic smectite clay investigated failed at a relatively high pressure consistent with cavity expansion. The equilibrium pressure after a test provided a measure of the minor principle stress in the ground.

INTRODUCTION

If the hydrostatic pressure in a borehole is increased until it exceeds the soil's ability to withstand it, then failure occurs. Hydraulic fracturing is essentially a tensile failure of the borehole wall that causes cracks to open.

The oil industry uses hydraulic failure in reservoir rocks to increase the well-rock interface, thereby providing a greater yield from the well. Because of this use of fracturing most early analysis concentrated on the behaviour of rock. However, as hydraulic fracture has now been recognised as the cause of earth dam failures, theories have been proposed to analyse soil response. This has resulted in the measurement of the ultimate fracture pressure, the horizontal in situ stress and permeability of clays. Based on this work, allowable grouting pressures for dams, conductors and piles have been estimated.

Inadvertent fracture of a soil or rock can have serious consequences for drilling operations and safety of an offshore structure. If it occurs during grouting of a pile or casing then grout will be lost into the soil raising considerations of pile or casing failure during subsequent platform operations. When drilling, fracture is most likely during drilling for the first casing. Then, instead of the drilling fluid being returned to the drill floor it is pumped into the soil. At deeper penetrations hydraulic fracture may cause loss of heavier drilling muds such as used to overcome pressure surges in the well from unexpected gas. Mud loss can have an adverse effect on the platform foundations and can delay or stop drilling progress. Remedial measures will be expensive in time and materials, in addition to the cost of the lost drilling mud.

This paper reports the results of hydraulic fracture tests carried out at three proposed platform locations in the two southern North Sea blocks shown on Figure 1. The tests were performed from a dynamically positioned drillship in about 50 m of water during the geotechnical site investigation. The tool used was fully compatible with the McClelland Stingray system described by Semple and Johnston (1979) that has been widely used in offshore site investigations. The tests were performed at nominal 15-m intervals below 40-m penetration to between 77 and 130-m penetrations, well below possible platform foundation depth.