Salt
bodies in particular offer a good opportunity for imaging with TISA 3D.
There are four compelling reasons why the conditions for GPR in salt
are favourable:
very low electrical conductivity and therefore very deep penetration;
homogeneity and thus little scattering;
little depolarisation;
no significant dispersiveness.
In
terms of penetration depth, GPR propagation in pure salt is comparable
or even superior to that in pure ice. For these ideal GPR conditions,
the composition of the salt has to meet certain criteria. Specifically,
the salt should consist of 90-100% halite, contain little impurities
such as anhydrite and have hardly any pockets of unbound water, i.e.
brine. Publications by Holser et al. (1972), Robert and Stewart (1976),
Gorham et al. (2008) and Connoly et al. (2008) demonstrate through the
observation of unparallelled GPR reflection distances that if these
pure halite conditions are met, kilometer-scale penetration distances
can be achieved in salt mines. Translating this situation to salt-dome
imaging in a hydrocarbon or engineering setting, we can expect similarly
good results and excellent imaging opportunities.
Salt flank monitoring while/after drilling Salt
domes constitute common traps for hydrocarbons, especially their
flanks. Consequently, these flanks are desired targets for drilling.
Imaging the flanks with geophysical methods however is still a
difficult process in spite of large advances in particularly seismic
imaging. Because of this persistent uncertainty in flank geometry, many
measure-while-drilling (MWD) techniques exist aiming to monitor the
surroundings and conditions for drilling. No borehole tools currently
exist that image structure tens of meters away from the wellbore. A
tool which could guide the drilling to the very near vicinity of a
hydrocarbon trap in a salt dome flank would be very useful. Radar is a
method that can do just this and the TISA 3D is fit for the job. In
an open hole with electrically resistive hard-rock environment, the
TISA 3D tool can perform reflective imaging up to 15 m of the borehole.
If
the flank is approached from within the saltbody and the salt
conditions are consistent as specified above, imaging can be done from
at least from hundreds of meters out.
Internal salt dome delineation and volumetrics Robert
and Stewart (1976) managed to map the flanks of the Cote Blanch salt
dome from within a salt mine making use of the large GPR penetration
distances. Holser (1972) achieved the same by downhole logging with a
GPR tool in the Pine Prairie salt dome. Having disposal over appraisal
boreholes, a downhole GPR tool with directive capabilities such as the
3D BHR can make a reliable volumetric estimate of large salt bodies.
This can either be a stand-alone estimation or be used as a constraint
together with seismic or gravity data. Conceptually, this would be like
a flashlight illuminating a dark body. Many such salt domes exist in
Northern Germany and the Gulf of Mexico.
Floating carbonate reef detection in salt layers. During
drilling through salt layers, certain drilling hazards are present due
to rapidly changing viscosity and pressure conditions. A typical
hazard is the occurence of so-called ‘floaters’. These are bodies of
hard-rock, usually detached from host-rock, that have been encapsulated
by flowing salt. A well-known example of such floaters are carbonate
reef units. They can be broken away from their sequences and dragged
away within the salt. Quite often they are severely overpressured,
leading to blow-out and breaching potential. Sometimes they are even a
promising target as hydrocarbon reservoirs. Being able to detect and
locate such hard-rock bodies within salt layers is of great help to safe
drilling practises. Again, the TISA 3D is a suitable tool for this, as
it can scan the salt over large distances and detect reflections from
the contrasting floaters. TISA 3D’s ability to transmit and receive
reflections directionally is crucial in locating the floaters. In the
Northeast of the Netherlands, the Zechstein salt layer is
stratigraphically well-developed and subject to much exploration by the
likes of Akzo Nobel and NAM. Floaters are known to exist in the
Zechstein salt layer and its many salt domes and lenses. This would
make for an excellent oppurtunity to employ the TISA 3D in a well. References
W. T. Holser, R. J. S. Brown, F. A. Roberts, O. A. Fredriksson and R. R. Unterberger, 1972, Radarlogging of a salt dome, Geophysics, vol. 37, no. 5 (October 1972), p. 889-906.
Robert
D. Stewart and Robert R. Unterberger, 1976, Seeing through rock salt
with radar, Geophysics. vol. 41. no. 1 (February 1976), p. 123-132.
A.
Connolly, A. Goodhue, C. Miki, R. Nichol, D. Saltzberg, Measurements
of radio propagation in rock salt for the detection of high-energy
neutrinos, 2008, doi:10.1016/j.nima.2008.11.008, arXiv:0806.2042v1P.
Gorham,
D. Saltzberg, A. Odian, D. Williams, D. Besson, G. Frichter & S.
Tantawi, Measurements of the Suitability of Large Rock Salt Formations
for Radio Detection of High Energy Neutrinos, 2008,
doi:10.1016/S0168-9002(02)01077-X, arXiv:hep-ex/0108027v2
