Hydraulic Top Seal Failure - The Determination of Seal Capacities for Undrilled Prospects

O'Connor, Stephen; Lahann, Richard W.; Swarbrick, Richard E.
Ikon Science, Durham, United Kingdom.

Where the drilling window (pore pressure/mud-weight minus fracture pressure) is narrow, borehole stability problems are commonly encountered. For instance, in the Malay Basin, wells such as Bergading Deep and Sepat Deep-1 encountered High Pressure/High Temperature conditions, causing severe mud losses, well kicks and other operational difficulties such as stuck pipes, hole stability and hole cavings. In addition to this elevated drilling risk in such wells, the close proximity of the pore pressure to fracture strength of the rock (or "seal capacity") increases the risk of hydraulic failure of the top-seal, allowing hydrocarbons to escape. Small seal capacity magnitudes are not limited to strictly HP/HT conditions, however, and shallower wells in the Malay Basin such as Inas Deep-1 have pore pressures very close to Leak-Off Test data, an estimate of rock strength at depths of only 2km TVDss. In other areas of SE Asia, such as Sabah, Miocene carbonate reservoirs are the targets of many drilling programs, and many of these are associated with fluid escape phenomena indicative of seal breach.

Part of the exploration strategy for prospects in SE Asia, in those basins where drilling windows are likely to be narrow, therefore, is an assessment of seal breach risk. In this paper we present a workflow that discusses the methodology to more accurately predict the seal capacity in undrilled prospects, based on a combination of regional mapping, understanding of pore pressure generation mechanisms and rock properties. Some conclusions include (a) trap failure occurs not at Top Reservoir but at the crest of the associated overpressure cell, (b) accurate derivation of a predictive fracture pressure algorithm should include a pore pressure/stress coupling ratio term (which relates pore fluid pressure to horizontal stress magnitude through poro-elastic fluid-stress interaction) and (c) in some basins data show that the overburden can be the least stress, suggesting a near-isotropic stress state at depth. Interestingly, not all basins show a relationship between small seal capacities and hydrocarbon preservation.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012