FIG. 1 is a cross-section of a known formation testing/sampling probe assembly 100 viewed in a plane containing the wellbore axis. The assembly 100 includes a packer 101 configured to be pressed against a formation of interest via hydraulically actuated pistons schematically shown as 103 and 104. Deformation of the packer 101 around an extendable probe barrel 102 and against the formation creates a seal that isolates the formation fluids and pressure from the wellbore environment. The probe barrel 102 extends forward, due to high pressure hydraulic oil entering chamber 105 at the same time pistons 103 and 104 are actuated to apply an extension force. The area of investigation by the probe assembly 100 is limited to the area that is in direct contact with the extendable probe barrel 102, usually called the orifice. This area is limited by the diameter of the probe barrel 102, which is about the same dimension as the diameter of the hole 106 in the packer 101 through which the probe barrel 102 passes.
Other prior art has sought to improve upon the design of the probe assembly 100 by increasing the diameter of the probe barrel. Nonetheless, the area of investigation is still limited to the largest diameter of the probe barrel, which is currently between two and three inches. This could be problematic for formations with thin laminations where the formation may have a small (e.g., ˜0.5 inches thick) production zone sandwiched between thicker zones of impermeable formation. With prior art designs, finding a non-producing zone is much more likely due to the small lateral extent (the probe barrel diameter) of the area of investigation.
One attempt at addressing this thin lamination problem involved elongating the packer, such as with the known probe assembly 200 shown in FIG. 2. The elongated packer 201 of the probe assembly 200 is pressed against the formation by backing plate 202 to create a seal. However, in this case, an annular shaped metal spacer 207a fills the upper hydraulic chamber 207, preventing the probe barrel 208 from extending forward. Since the packer 201 and backing plate 202 are elongated along the central axis of the wellbore, a number of formation laminations could be investigated. The axial length of investigation using the probe assembly 200 could approach several inches, including one known embodiment in which the length was about seven inches.
However, problems with the probe assembly 200 have been discovered. For example, the packer 201 is not as constrained at its inner boundary as it is near the raised rim 203 of the backing plate 202 that defines the orifice of the probe. It has been found that the packer material 201 migrates into the probe's orifice during formation testing operations. The large difference in pressures between the wellbore, pushing on surfaces 204, and the probe orifice, pushing on the packer surface near the raised rim 203, forces the packer material to move into the orifice. This movement decreases the ability of the packer 201 to seal against the formation and prevents the tool's pressure gauges from equilibrating to that of the pressure of the formation fluid. As the elastomeric material of the packer 201 is sucked into the probe orifice, the hydraulic pressure declines in the pistons (schematically shown in FIG. 2 as 205 and 206) applying the extension force (a consequence of the tool design). This, in turn, results in a decline in the force pushing the packer against the formation, further exacerbating the situation, and resulting in both packer damage and loss of seal. This design has lead to a high number of lost seals.
Another problem that exists with the design shown in FIG. 2 is that the upper surface of the raised rim 203 of the backing plate 202 is located well below the surface of the elastomer 201 that seals against the wellbore wall. As a consequence, elastomer must be compressed before the raised rim 203 makes contact with the wellbore wall. This is of considerable disadvantage because it requires performing large volume pretests. On the other hand, if this difference in heights between the surface of the raised rim 203 and the sealing surface of the elastomer 201 didn't exist, then it would not be possible for the elastomer 201 to form a seal against the wellbore wall, because the metal spacer 207a makes the probe barrel 208 and the backing plate 202 move in unison.