--> Overpressure in the Abadan Plain, Iran

International Conference & Exhibition

Datapages, Inc.Print this page

Overpressure in the Abadan Plain, Iran

Abstract

Risk of blowouts and well control increases in HPHT environments. Pore pressure prediction is essential in well design and execution, particularly with regards to proper mud weight and casing point selection. Conventional methods to predict pore pressure often can't be applied to carbonate rocks due to their porosity which is not always a function of vertical effective stress. Cementation and dissolution also strongly control inter-grain porosity and permeability of carbonate rocks, and carbonates can have a rigid framework that minimizes the influence of loading on compaction (Lucia, 1999). This study examines overpressures in several wells in the Abadan Plain, SW Iran. The investigated fields are comprised of carbonate reservoirs and occasionally exhibit overpressures. The Abadan Plain sits in the foreland of the Zagros Fold-Thrust Belt, which is a result of Arabia-Eurasia collision and is a highly active tectonic environment (Falcon, 1974, Lacombe et al., 2007). The stratigraphy of the Abadan Plain contains several interbedded shale deposits between the carbonate reservoir units. The porosity of clastic sequences has been reduced due to the high compressive stresses in the basin, exhibiting abnormally higher acoustic velocities, making typical pore pressure prediction difficult. Indeed, pore pressure prediction in regions of high tectonic stress is well known, but remains a largely unresolved challenge (Weakley, 1990, Harrold et al., 1999, Couzens-Schultz and Azbel, 2014) Overpressure origins in the Abadan Plain have been examined using density-velocity cross-plots (Hoesni, 2004, Lahann et al., 2001) as well as vertical effective stress-velocity cross-plots (Bowers, 1995), revealing that disequilibrium compaction is the main origin of abnormal pore pressure in Abadan Plain. There is also a possible signature of minor fluid expansion or clay diagenesis effects in some wells. Pore pressure prediction in the intra-carbonate shale units has then been tested using careful shale discrimination and petrophysical log data. An Eaton (1975) exponent of 0.6 was found to provide the most accurate estimate of observed pore pressures.