ABSTRACT:

Shale barriers are formed by shear creep closing an open annulus around a well. Such barriers are attractive options to prevent unintended flow along parts of the well where there is no cement between the casing and the formation. However, in many cases the strength and creep resistance of the shale prevent such closure. Electro-osmosis has traditionally been applied to clay to either increase or reduce the shear strength. Here, we conducted laboratory tests to demonstrate the weakening effect of electro-osmosis to enhance barrier formation. Outcropping Eocene clay was sampled in Denmark, equivalent to Eocene shales of the North Sea, within which shale barriers are observed. Samples were pressurized under isotropic stresses within a triaxial apparatus and both the hydraulic and electro-osmotic permeabilities were measured. Results showed a significantly higher electro-osmotic flow potential compared to the hydraulic flow potential that might occur under realistic pressure gradients. Electro-osmotic treatment with 3 volts across the sample resulted in an increase in pore pressure at the sample top (cathode) from 10 to 15 MPa, reducing the effective stress locally from 11 to 6 MPa after 4-5 days. Following treatment, the sample was dismounted and cut into nine pieces to investigate the effect, and we observed a clear trend of increasing water content towards the cathode. These experiments demonstrate a significant potential of electro-osmosis for barrier formation within a North Sea analogue clay. This method has a significant cost saving potential since direct current may be applied to the well with simple means. Preliminary estimates indicate that a sufficient electric gradient is achievable using the electrical power supply normally available at an offshore rig.

1. Idea and Objective

Shale acts as an excellent seal for hydrocarbon reservoirs and is even used for safe storage of radioactive waste. As most of the overburden above hydrocarbon fields consists of shale, it is no surprise that utilizing the in-situ shale as a natural barrier against flow along the outside of well casing has become popular (Williams, 2009). The use of shale, as opposed to the more conventional concrete, is primarily to save costs but also because shale may be less prone to fracturing.

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