Publications

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, Malaysia, wells such as Bergading Deep and Sepat Deep-1 encountered high pressure conditions, causing severe mud losses, well kicks and other operational difficulties such as stuck pipes, borehole stability and hole cavings (Mohamad et al., 2006). In addition to elevated risk during drilling 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 low permeability seals, allowing hydrocarbons to escape. Wells such as Guling Deep-1 and Inas Deep-1 in the Northern Malay Basin also have small seal capacity, depending on the definition of a fracture envelope. In Indonesia, the Porong-1 well encountered high pore pressures sufficient to induce fracturing of the overburden. This type of failure i.e. mechanical is in contrast to membrane leakage which is routinely analysed using Mercury Injection Processes (MCIP) and often reported in the literature. In this paper we present a workflow that discusses the method to predict the seal capacity more accurately, based on a combination of regional mapping, reservoir and fluid behavior and rock properties. Some conclusions include (a) trap failure occurs at crestal depths, rather than down-dip stratigraphic tops, (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 breached traps.