Flow experiments were performed on a ten-inch diameter by fifteen-inch long thick-walled cylindrical sample of poorly consolidated sandstone, with a 1.25-inch diameter borehole. The purpose was to evaluate the effect of changing stress regimes on near-wellbore permeability and liner loading. A one-inch diameter screened liner was installed in the wellbore to preclude sand production. The liner was instrumented with strain gages, in order todetermine stresses resulting from borehole deformation during production. The cylindrical sample was instrumented with pore pressure probes, placed at different distances from the wellbore, in order to assess variation in formation permeability with evolving effective confining stresses and production regimes. The major conclusions of this experiment are as follows:
If sand ablation does not occur and if an adequate annular space exists between the liner and the wellbore, load transfer to the liner is mitigated. If sand ablates and fills the annular gap, the complexity of the load transfer mechanism increases dramatically. This has been addressed elsewhere(Abou-Sayed et al., 1995; Willson and Abou-Sayed, 1998).
Apparent dilatant behavior in the near-wellbore region leads to permeability increase.
An intermediate interval experiences a reduction in permeability, due to stress transfer, following yielding of the inner zone, adjacent to the wellbore.
In unconsolidated formations, it has been speculated that wellbore failure may significantly increase permeability in the failed zone. This could also partially reduce stress transfer to a liner, decreasing the potential for collapse. However, under these conditions, near-wellbore stresses are transferred to an intermediate region away from the wellbore wall. This results in reduced permeability in that intermediate region. Controlled wellbore deformation/failure is required to establish an optimum condition that would lead to overall skin reduction and reduced liner stresses. It may be feasible to manipulate wellbore deformation by "regulating" the annular space between the wellbore wall and the liner (or by using a deformable liner).