The effect of well inclination on shale strength relative to bedding. The circles represent failure.
StatoilHydro has several HPHT reservoirs, with the Kristin gas and condensate field as a prime example. Drilling development wells on such fields is particularly challenging owing to:
- the small range of mud weights which can be applied to accommodate safe drilling
- the rapid reduction in reservoir pressure encountered during production, which limits opportunities to drill more wells when the reservoir is significantly depleted.
StatoilHydro researchers have carried out a comprehensive rock mechanics study of HPHT shale intervals to gain further insight into these problems. The results have been used in well planning and design.
An important issue related to the mud weight window is how well stability will be affected by its inclination relative to sediment bedding and laminae, both of which can act as planes of weakness.
This is virgin territory in scientific terms, since most HPHT fields have been developed using vertical or sub-vertical wells with inclinations of less than 45 degrees. However, some of the wells drilled on Kristin could approach the horizontal.
To test this, shale cores cut from within and just above the reservoir were first subjected to thorough geological, petrophysical and rock mechanical characterisation by the Norwegian Geotechnical Institute (NGI).
The results were then used to develop advanced numerical models of the shale’s stability.
Next, the models were experimentally verified by Sintef Petroleum using “hollow cylinder” tests – with the cylinders representing boreholes.
Having altered the direction of the pseudo-boreholes relative to the bedding for three cases (perpendicular, 30 degrees, and parallel), it was shown that shale strength – and therefore borehole stability – is radically reduced when wells are drilled parallel or sub-parallel to it.
Another issue concerns the underbalanced drilling of shale intervals once a reservoir has been depleted – in other words, drilling when the pore pressure exerted by the shales is higher than that exerted by the drilling mud.
In this case, it was found that the low permeability of the shales (0.1 nD perpendicular to the laminations) is sufficient for metre-thick intervals to retain high pore pressures, even when sandwiched between heavily depleted reservoir sandstones with significantly lower pressure.
Hollow cylinder tests were carried out to simulate this, and an underbalance of 37 MPa was attained before shale failure (collapse) in a pseudo-borehole perpendicular to the bedding.
This looks good for underbalanced shale drilling, as long as the mechanical properties and in-situ stresses of the shale intervals are known and evaluated in advance.
