Subsurface Uncertainties. Opportunities or Challenges?

@inproceedings{AITETTAJER_RM2021,
 abstract = { “Three months ago, the drillbit of the 748' long drilling rig positioned at 150 miles south west of Houston, started its subsurface journey at 6000' below sea level. Two hours ago, it successfully reached, at a record time and at the lowest cost, its planned target at 28,000' below sea level. The Gulf of Mexico team, was the first to discover the 1,200' of net pay section, with the highest permeability-thickness among analogs”. This statement is not the beginning of a fiction novel, but the result of the thorough field study including safety, economics, drilling and subsurface. Furthermore, the latest production results show a confirmation of the excellent performance of the well, with very low environmental impact due to the absence of flaring and the prevention of leaks. In this paper, based on previous publications in SPE-ATW, EAGE and OTC conferences, we will focus on the workflow for the assessment of the subsurface uncertainties due to the geological complexity of this lower tertiary reservoir and its overburden. This workflow encapsulates the experimental design process for the selection of the key subsurface uncertainties, the Monte-Carlo stochastic simulation for the computation of different realizations and the use of in-house tool for the optimization of the selection of the main realizations for the drilling planning and the production forecast. A cross-section through this field shows a post-salt section starting at the seabed defined by a sequence of shale, sand and carbonates layers, followed by a salt-section, then a pre-salt section defined by a sequence of limestone and sand with faults that could act as conduits for active tar. The top of the reservoir section lies between 26,000' and 28,000' below sea level. Four groups of subsurface uncertainties were defined based on the drilling section from the sea-bed to the base of the reservoir: Post-salt: The geomechanics properties, of the alternating shale, sand and carbonate layers, were critical for the assessment of the optimal mud-window and rate of penetration of the drillbit. This optimization aimed at reducing the non-producing-time (NPT), due to stuck pipes or unsuccessful leak-off tests related to setting the casing shoes. Salt: The structure of salt body varies from 10' to 11,000' within less than 500', with a local presence of salt rafts. Pre-salt: Two main challenges were addressed in this section, the pore-pressure prediction at the exit of the salt which could lead to fluid loss and/or high NPT, and the presence and the downward extension of the vertical and listric faults. Reservoir: Buckskin reservoir is compartmentalized, therefore the position of the different faults, has an important impact on the size of the drilled compartment, and hence the maximum drainage area. The expected productivity index, which is related to the petrophysics characteristics of the reservoir, was another key parameter to asses during this study. The assessment of the key subsurface uncertainties allowed the execution of a safe and optimal drilling program, and the landing of the well in the main compartment of the reservoir at high inclination angle.   },
 author = { Ait Ettajer, Taoufik },
 booktitle = { 2021 RING Meeting },
 publisher = { ASGA },
 title = { Subsurface Uncertainties. Opportunities or Challenges? },
 year = { 2021 }
}