Inverting for reservoir pressure change using time-lapse time strain: Application to Genesis Field, Gulf of Mexico

Abstract

There are increasing numbers of published examples from around the world in which significant 4D time shifts have been observed in the overburden above producing reservoirs.Indeed, this topic prompted the TLE special section "Rocks under strain" in December 2005. The significance of these 4D observations is that, if we wish to fully understand the 4D signature of compacting reservoirs, we can no longer think of the reservoir in isolation. The seismic response outside the reservoir changes because the nonreservoir rocks deform in response to reservoir activity. While these nonreservoir 4D seismic changes can obscure or contribute to the reservoir-level signal, making the 4D interpretation uncertain, if utilized appropriately they may also be used to provide information on the actual reservoir pressure changes. This new pressure information can thus be used to complement well measurements or other 4D seismicbased methods (such as the multi-attribute pressure and saturation inversion outlined by Floricich et al., 2006), providing valuable data for reservoir monitoring and management. In this paper, we build on the work of several authors who have presented methods to invert surface deformation measurements for reservoir volume or pressure change. We show how 4D seismic can extend this approach by focusing on the inversion of 3D strain deformation estimates for the overburden derived directly from the repeated seismic data. The method is then applied to Genesis Field in the Gulf of Mexico, in which there are series of compacting unconsolidated stacked turbidite reservoirs. Time-lapse time strain.Rock velocities are sensitive to changes in stress and strain so that if a volume of rock strains, the change in traveltime through it will be made up of a contribution due to the change in distance traveled by the seismic wave and a contribution due to the change in velocity. A perturbation formula relating changes in vertical traveltime t, velocity v, and vertical layer thickness z, assuming small changes in thickness and velocity, is given by Landrø and Stammeijer (2004) (1)Hatchell and Bourne (2006) went on to make the assumption that changes in thickness and velocity can be linearly related by a constant of proportionality, R, which relates the fractional change in velocity and vertical strain so that ∆v/v = -Rε zz , resulting in the following relationship (2) where we have replaced ∆z/z by ε zz , signifying the vertical component of the strain tensor. The left side of Equation 2 is the derivative of the time-shift field, which we term time strain. A method to obtain time-lapse time strains from 4D seismic is described in the companion paper in this special section ("4D time strain and the seismic signature of geomechanical compaction at Genesis"). If we have knowledge of the magnitude of R, we can obtain estimates of vertical strain directly from 4D seismic observations. With estimates of vertical strain for the overburden, a linearized inversion can be employed to obtain reservoir pressure change. Segall (1992) shows ...

Reservoir monitoring and characterization using satellite geodetic data: Interferometric synthetic aperture radar observations from the Krechba field, Algeria

“…1988, Dussealt et al 1993, Bruno and Bilak 1994, Castillo et al 1997, Massonnet et al 1997, Fielding et al 1998, Mossop and Segall 1999, Vasco et al 2000a, Stancliffe and van der Kooij 2001, Schmidt and Burgmann 2003, Vasco and Ferretti 2005 or by timelapse seismic monitoring (Guilbot and Smith 2002, Landro and Stammeijer 2004, Tura et al 2005, Hall 2006, Roste et al 2006, Rickett et al 2007, Staples et al 2007 to constrain volume and fluid pressure changes within a reservoir. Our analysis assumes that a set of deformation measurements have been gathered.…”

“…These studies utilized geodetic data, such as leveling measurements, in order to quantify surface deformation related to fluid production or injection. Currently, time-lapse seismic surveys are providing additional deformation measurements, distributed throughout the overburden (Guilbot and Smith 2002, Landro and Stammeijer 2004, Tura et al 2005, Hatchell and Bourne 2005, Barkved and Kristiansen 2005, Hall 2006, Roste et al 2006, Rickett et al 2007, Staples et al 2007). Recently, the observed deformation has been used to better understand fluid flow and the heterogeneity of flow properties within the reservoir (Vasco et al 2000a, Vasco et al 2001, Du et al 2005, Vasco and Ferretti 2005, Hodgson et al 2007, Vasco et al 2008.…”

Modelling broad-band poroelastic propagation using an asymptotic approach

“…For example, recent studies highlight the role of pressure changes and associated deformation in observed time-lapse seismic anomalies below, within, and above a producing reservoir (Guilbot & Smith 2002;Landro & Stammeijer 2004;Hatchell & Bourne 2005;Tura et al 2005;Hawkins et al 2007;Hodgson et al 2007;Rickett et al 2007;Roste et al 2007;Schutjens et al 2007;Staples et al 2007). These studies document both changes in layer position and thickness as well as seismic velocity changes due to stress variations.…”

A new approach to separate seismic time-lapse time shifts in the reservoir and overburden

“…An example of using time strain for estimation of reservoir pressure changes is shown by Hodgson et al (2007). On the other hand, methods related to seismic interferometry (Snieder et al, 2009;Wapenaar et al, 2011;Galetti and Curtis, 2012) can be used to virtually move surface sources below a complex overburden and therefore enable time-lapse monitoring of reservoir without any velocities of the overburden (Bakulin and Calvert, 2006).…”