Production of oil and gas—as well as the injection of water, gas, or steam—generally causes a change in pore pressure, total stresses, and temperature in the reservoir and its surrounding formations. The resulting stress changes can cause compaction or lead to shear failure or tensile fracturing. That, in turn, can lead to fluid containment issues and compromise the integrity of the wellbore and surface facilities.
Forty-eight active fields in the Niger Delta, some of which have been producing since the 1970s, were subjected to fit-for-purpose screening to ensure geomechanical risks associated with operations were identified and adequately handled. Some of the fields are highly faulted and compartmentalized. Oil accumulations are fault controlled, and several of the fields have gas caps.
The screening identified risks such as sand control, compaction and subsidence, top seal integrity loss, and wellbore instability in the fields. The risks were assessed using a risk-assessment matrix to check the potential effect on safety and production and the probability of occurrence, which ranged from low to medium.
In compliance with process safety requirements, the risk of reservoir compaction was evaluated using 1D analytical models. The screening resulted in proposals of integrated solutions to address and mitigate the risks. The recommendations included (1) acquisition of core measurements to reduce uncertainty of uniaxial strain compressibility required to estimate reservoir compaction and (2) mechanism-based evaluation of seal integrity to analyze the potential of out-of-zone-injection and cross flow between reservoirs. Some recommendations have been implemented to reduce the risks to as low as reasonably practicable.