Uncertainties in the Pore Pressure Evaluation in Deepwater: a Statistical Approach (2002)

Uncertainties in the Pore Pressure Evaluation in Deepwater: a Statistical Approach Jose L. Falcao Jose L. Falcao Petrobras S.A. Search for other works by this author on: This Site Google Scholar Paper presented at the SPE/ISRM Rock Mechanics Conference, Irving, Texas, October 2002. Paper Number: SPE-78247-MS https://doi.org/10.2118/78247-MS Published: October 20 2002 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Get Permissions Search Site Citation Falcao, Jose L. "Uncertainties in the Pore Pressure Evaluation in Deepwater: a Statistical Approach." Paper presented at the SPE/ISRM Rock Mechanics Conference, Irving, Texas, October 2002. doi: https://doi.org/10.2118/78247-MS Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex Search nav search search input Search input auto suggest search filter All ContentAll ProceedingsSociety of Petroleum Engineers (SPE)SPE/ISRM Rock Mechanics Conference Search Advanced Search AbstractThe Pore Pressure is a critical point of any drilling project, once it influences the definition of the mud weight, casing setting depths, casing design, riser safety margin (RSM), etc. During the design stage of offshore and shallow water wells, safety factors (ranging normally from 0.5 to 1.0 ppg) are introduced to compensate the lack of accuracy in pore pressure evaluation. However, this may impose a limit in deepwater drilling operations. In such environment, as the water depth increases, the formations tend to show lower fracture gradients, which makes the difference between the pore pressure and the fracture gradient curves quite small, complicating the project.In this work the methods employed by Petrobras to evaluate the formation pressure are reviewed. Next, a procedure, employing statistical analysis, to evaluate the uncertainty in the pore pressure determination in the offshore portion of Campos Basin - Brazil, is presented. For that, a DST (drill stem test) and RFT (repeat formation test) database collected along the years in this area is employed. As a result, the safety factor applied over the Pore Pressure could be reduced, simplifying deepwater well projects.IntroductionThe Campos basin is currently the most productive of the Brazilian petroleum areas. This basin extends roughly from 15 km (9.3 mi) in land up to 3,400 m (11,155 ft) of water depth, coverings an area of about 100,000 km2 (2.47 106 acres) along the southeastern Brazil passive margin. To date, more than 30 oil fields have been discovered, including the giant fields of Marlin, Albacora and Roncador in about 2,000 m (6,562 ft) of water. As a consequence there is increasing interest in exploration in still greater water depths in this area (1).The offshore exploratory drilling is usually a high-cost and high-risk activity. One of the most significant sources of risk during drilling is associated with the unforeseen occurrence of formation pressure. Such situations can result in: stuck pipes, formation damage, well instability, kicks and eventually a blowout. Therefore, one of the most important objectives of the formation pressure evaluation is to drill a well safely and economically, without causing formations instabilities (collapse or fracture), without allowing inflow of formation fluids (water, oil or gas) and without causing damage to the reservoirs. In other words, the optimization of the drilling project depends on the correct evaluation of the geopressures.This evaluation is more critical in well design for deepwater wells. In this scenario, with the increase of the water depth, the formation tends to show lower fracture gradient (FG) due to having been compacted under low overburden gradient. Uncertainty in the PP evaluation is usually compensated by introducing a safety factor. Such factors range normally from 0.5 to 1.0 ppg, according with the uncertainty associated with the determination. That, however, imposes limits during the well design phase. The available margin for the mud weight, which must stay between FG and PP, can be very narrow complicating deepwater well projects, as it is schematically represented in Figure 1.In the next section, some important definitions related to the formation pressure evaluation will be given. Keywords: reservoir geomechanics, upstream oil & gas, log analysis, annular pressure drilling, pressure evaluation, exponent, database, reservoir characterization, information, spe isrm 78247 Subjects: Pressure Management, Reservoir Characterization, Formation Evaluation & Management, Well control, Reservoir geomechanics, Open hole/cased hole log analysis This content is only available via PDF. 2002. Society of Petroleum Engineers You can access this article if you purchase or spend a download.