Til hovedinnhold

Undrained pore pressure changes in rocks surrounding a reservoir: From poroelastic theory to fault activation?

Undrained pore pressure changes in rocks surrounding a reservoir: From poroelastic theory to fault activation?

Kategori
Konferansebidrag og faglig presentasjon
Sammendrag
Rocks surrounding a depleting oil or gas reservoir, or an inflating CO2 storage reservoir, are characterized by extremely low permeability in order to provide seals over geological time. It is well established that pore pressure change in a reservoir leads to stress changes around it, however, it is less recognized that this also implies pore pressure change outside the reservoir. Based on soil mechanics concepts, short-term (undrained) pore pressure change is described by two Skempton parameters commonly referred to as A and B. Because of the large scale and the prevalent low permeability of the rocks surrounding the reservoir, short term in the field is likely to be on the scale of several years. In shale, which is the most common overburden rock, Skempton's parameters can be deduced from anisotropic poroelasticity theory. This has been verified by controlled laboratory experiments along different stress paths and with different orientations of the core samples. Given the laboratory data, geomechanical models for stress changes around the reservoir can be used to estimate pore pressure changes in the field. The pore pressure change depends on stress path and Skempton parameters. The in-situ stress path is governed by geometry, by elastic contrasts and by non-elastic behavior. Although pore pressure most likely will increase as a result of injection and decrease because of production, the opposite scenarios are also possible, depending on stress path, the value of Skempton's A and the specific location. The presentation will include a brief review of the theory in addition to laboratory results. Focus is on pore pressure measurements during controlled stress path experiments in a triaxial apparatus, including derivation of the Skempton parameters. In addition, analysis of laboratory derived stress path and pore pressure dependent wave velocities at seismic and ultrasonic frequencies will be briefly described, in order to link to time-lapse ("4D") seismic. Besides the generic outcome of linking laboratory measurements to geomechanical models, a field example will be given to demonstrate estimated pore pressure response as a result of pore pressure change in a reservoir. The possible impact of pore pressure on faulting and fault activation in rocks surrounding the reservoir will be discussed. Elements of this discussion is how Skempton parameters change when moving from an elastic towards a failure state, which is likely to affect the failure process. Also, the orientation of the stress field with respect to the symmetry plane of the shale plays an important role.
Oppdragsgiver
  • Norges forskningsråd / 294369
Språk
Engelsk
Institusjon(er)
  • Norges teknisk-naturvitenskapelige universitet
  • SINTEF Industri / Petroleum
Presentert på
13th EURO-Conference on Rock Physics & Geomechanics
Sted
Potsdam
Dato
01.09.2019 - 05.09.2019
Arrangør
Helmholtz Centre Potsdam GFZ
År
2019