Abstract
The Middle to Late Cretaceous Natih formation in Oman can be highly compressible and undergo large compaction during depletion. Significant reservoir compaction and surface subsidence has potential risks for fault reactivation, integrity of wells and surface facilities. Petroleum Development Oman produces oil and gas from the Natih formation in a number of fields within its concession area. There are existing experiences in one of the analogue field in Oman, where the compaction of Natih formation has resulted in issues of well damage, well integrity, subsidence damage to facilities and experiences of surface tremors due to fault reactivation.
The focus of this work was for Fahud West oil field producing oil and gas from a Natih reservoir, where analysis of an analogue field had indicated a high potential impact of compaction. A Geomechanical assessment of the formation within the field was therefore undertaken to mitigate operational risks, and to assess the permeability impact with increased depletion.
The Integrated Geomechanical data acquisition and modeling minimized uncertainty and provided clarity on whether the reservoir can continue with increased depletion – without increased geomechanical risks of loss of integrity for wells and facilities, cap rock integrity or reduced productivity. Properly planned and rock mechanics measurements were conducted in the laboratory on core samples.
The measurements revealed that the expected compaction of Natih reservoir in Fahud West field is less severe compared to the analogue field. The maximum predicted surface subsidence expected at depleted reservoir pressure of 10 bars, is within the tolerable subsidence limit for surface facilities. In addition, permeability measurements showed that the permeability at reservoir pressure of 22 bars (the previous base case for end of production), will not change significantly with further depletion of reservoir pressure to as low as 10 bars.
The outcome of this integrated geomechanical assessment demonstrates that the field can be produced down to 10 bars from the previously estimated 22 bars (base case) limit, adding significant risked volume of oil production, allowing further drilling of wells to raise the final field recovery while ensuring safe well and facility integrity
