CO2 Storage Geomechanics for Performance and Risk Management

Abstract

Abstract Controlling the trapping of CO2 in the subsurface, i.e. storage containment, is of fundamental importance for a safe geological storage of carbon dioxide. During CO2 injection, increasing fluid pressure, temperature variations, and chemical reactions between fluids and rocks inherently affect the state of stress inside the reservoir and in its surroundings. Besides, the mechanical properties of the rocks exposed to CO2 may be altered. The impact of the resulting deformations on seal integrity must therefore be assessed in order to properly manage containment performance and leakage-incurred risks. The analysis starts with the construction and the calibration of a Mechanical Earth Model of the site, through joint analysis of geologic, seismic, logging, drilling, and laboratory test data. Such a model consistently describes ambient stresses, fluid pressures, and poro-mechanical and strength properties of the formations. It is linked to a reservoir model to achieve initial equilibrium and also to further simulate the coupled transport, chemical and mechanical processes occurring during CO2 injection operations and the subsequent re-equilibration. The predicted stress path allows the evaluation of the mechanical stability of both cap-rock and faults (which may bound the reservoir, penetrate the cap-rock or intercept wells). The stability of wells in formations experiencing strain is also investigated. In addition, an accurate Mechanical Earth Model contributes to optimizing well construction and stimulation operations. Profiles of stresses and mechanical properties along a planned-well trajectory allow designing a drilling operation that will maximize subsequent hydraulic isolation of the well by optimizing the wellbore condition. Similar information along existing wells helps to control hydro-fracture propagation when injectivity enhancement is required. The Mechanical Earth Model can be used to develop operating envelopes for well placement, hydraulic fracturing, and CO2 injection that best ensure containment while achieving injectivity and capacity requirements. Introduction The climate of the Earth is warming, with widespread changes in ocean salinity, wind patterns, precipitation and aspects of extreme weather. This is very likely forced by the increase in anthropogenic greenhouse gas concentration in the atmosphere. It is estimated that over 60% of this increase is due to carbon dioxide (CO2) emissions alone[i]. Carbon Capture and Storage (CCS), that is the capture of CO2 from industrial and energy-related sources, transport, and injection into the subsurface for long term sequestration purposes, is a viable means to keep a significant fraction of emitted CO2 out of the atmosphere. CCS is thus recognized as a promising solution to mitigate climate change.[ii] Along with capacity and injectivity, containment is agreed to be a primary function in geological storage performance. As evidenced by oil, gas, and even CO2 natural accumulations, rock formations can be impervious enough to act as flow barriers over geological periods of time. Delineating such a seal, safeguarding its integrity under operational conditions, and verifying whether isolation is effective or not are key objectives in achieving a successful storage project. In particular, seal integrity must not be impaired by the mechanical effects of storage operations. Indeed, rocks and faults permeability may drastically increase as they undergo stress changes and deformation[iii]. The mechanical response of the sealing components to the loads induced by well drilling and completion, CO2 injection, and the corresponding effects on the risk of leakage must therefore be assessed when evaluating the suitability of candidate sites, designing operations, and planning monitoring schemes. This paper presents a methodology where characterization, modeling, monitoring, and construction technologies are integrated for containment performance and risk management. The first section frames the performance and risk management methodology under whose umbrella the geomechanical analysis takes place. The following sections describe the building of a Mechanical Earth Model (MEM) and how mechanical modeling is coupled with fluid flow simulation so as to forecast the dynamic response of the rock mass to fluid pressure increase, temperature variations, and fluid/rocks chemical interactions caused by massive CO2 injection and subsequent re-equilibration. Implications in terms of risk evaluation and strategy for risk control are discussed in the last sections.Working Group I contribution to the Intergovernmental Panel on Climate Change Fourth Assessment Report "The Physical Science Basis", 2007.Working Group III contribution to the Intergovernmental Panel on Climate Change Fourth Assessment Report "Mitigation of Climate Change - Summary for Policymakers", 2007.Sibson, R.H.:"Brittle-failure controls on maximum sustainable overpressure in different tectonic regimes", AAPG Bull, v. 87(6), p. 901–908, 2003.