In discussing reservoir containment and subsurface integrity, several themes emerge. Understanding each of these themes allows engineers, drillers and operators to mitigate risks associated with subsurface integrity.
Exploration and production traditionally focus on the reservoir interval, but the overburden, underburden (basement) and side-burdens all provide opportunities for data collection. Characterizing overburden generally requires interpretation of geologic units, relative timing and three-dimensional heterogeneities including faults, folds and salt bodies; identification of rock properties; and the determination of the stress state and pore pressure in each layer. The benefits of combining these inputs into a single geomechanical model of the overburden/reservoir system include stabilizing wellbores, refining drilling trajectories, identifying potential fluid migration pathways and mitigating the potential for induced seismicity.
In-Situ Stress States
Faults influence the containment geomechanics of the overburden and the reservoir as they can serve as a mechanism by which subsurface fluids can migrate through a caprock or top seal. Several factors influence a fault’s capacity to serve as a conduit for fluids including host-rock lithology, clay content of fault zone, displacement distribution and magnitude, relationship to the geometry of the producing zone, geologic history, and in situ and dynamic stress states. Determining the overall stress state by interpreting models of pore pressure, principal stresses and stress azimuth enables an analysis of fault stability once fault attitude (strike and dip) and strength (friction and cohesion) are known. Further, developing a full understanding of subsurface pressure dynamics allows operators to predict the effects of depressurization of a reservoir as hydrocarbons are produced.
Caprock and Top Seal Integrity
The stratigraphic sequence that overlies a producing reservoir must contain hydrocarbons through eons of geologic time as well as through the time-frame of production. The top seal influences the initial retention of hydrocarbons over millions of years of geologic time, and a full top seal analysis, may include fault seals, side seals and salt seals. Similarly, the same caprock sequence must mechanically and hydraulically seal against pressures and fluids associated with production. Sequences with better propensity for containment have low permeability and high tensile and shear strengths.
Subsurface injection of wastewater introduces risks to subsurface containment loss along several fronts. First, fluid injection at high pressure and high volume for an extended period of time can significantly impact the stress states of the disposal formation. Sealing sequences should be carefully selected to prevent out-of-zone migration of fluids. Second, an increased risk of induced seismicity may arise from fluid injection; nearby faults may either slip or act as conduits that may compromise the containment integrity of the injection zone as well as of other producing zones.
Reservoir Monitoring and Surveillance
Exploration, production and disposal introduce dynamic changes to the reservoir and overburden that can be monitored through the lifecycle of a well or reservoir. Movements in the subsurface including compaction and subsidence can induce slip on faults, introduce deformation of well casing, or create complex leakage paths from reservoirs directly to the undesired locations such as groundwater aquifers or the surface.
Images: “Graphic” by Top Energy Training