EAGE Workshop on Permanent Reservoir Monitoring (PRM) - Using Seismic Data 2011

The Clair LoFS array was installed with the objective of confirming whether a useable 4D response could be obtained from the moderate to low porosity reservoir. The LoFS array covers just over 11km2 which is only a part of the Phase 1 Development area. Seismic surveys have been acquired in 2006, 2007, 2008 and 2010.

Petrobras is installing its first offshore PRM system in Jubarte Field, Campos Basin, Brazil. The water depth at the target area - which has 9 sq km and around 30 km OBC length -is approximately 1,250 meters. 6 years R&D prepared the company to propose a project to the market for the full solution, since project the system, manufacturing the equipment up to acquisition and data processing, should be presented. Petrobras main target is to reduce project/acquisition/processing/interpretation cycle time. The presented solution is is fully optical, using inferometers as sensors and telemetry. Topside preparatory equipments were installed at P-57 FPSO platform, and subsurface system will be installed end/2011-begining 2012.

Although not a very new technique, Land PRM has been successful in projects associated with large acoustic impedance variations such as steam or gas injection. To become applicable to many onshore reservoirs, which produce only small changes, it must increase its sensitivity by one order of magnitude or even more.

During the past decade Shell has actively participated in the full range of permanent reservoir monitoring (PRM) activities including economic justification, acquisition planning, proof-of-concept trials, participation in two actual field deployments in joint-venture assets, and 4D processing and interpretation. Key learnings from the past show that permanent installations offer significant technical benefits over streamer-based systems such as: 1) improved survey repeatability (NRMS), 2) cost-effective frequent repeats for filtering noise, 3) use of sparse acquisition methods, 4) use of other wave types such as refracted p-waves, Scholte waves, and mode-converted waves, 5) improved processing through addition of true WAZ and long offsets, 6) ability to detect rapid changes in the subsurface, and 7) additional capability for monitoring production-induced hazards around our producing and injecting wells. The past has also presented us with a number of challenges that need to be overcome. Examples are 1) the high upfront costs in the presence of uncertainty on both signal strength and differential value as compared to streamer 4D, 2) the complexity and cost of cable deployment in the vicinity of platforms, subsea infrastructure, and other man-made and natural obstacles, and 3) uncertainties about the reliable lifetimes of these systems over field life.

This presentation reviews state-of-the-are for some options of passive geophysical observations above a producing reservoir. These are surface deformations (mostly subsidence or uplift), gravity and microseismic data. A wide range of technologies exist for high-precision measurements of changes in these properties. The data can be used for understanding the reservoir, and can complement each other, 4D seismic measurements, production data and well pressure measurements. Some of the installations are best made permanent in a field-life perspective.

Surface deformation monitoring can provide valuable constraints on the dynamic behaviour of a reservoir, by allowing the evaluation of volume/pressure changes with time, as well as an estimation of reservoir permeability. Levelling campaigns, tiltmeters, GPS and InSAR are all geodetic techniques used to detect and monitor surface deformation phenomena. Among them, InSAR data from satellite radar sensors are gaining increasing attention for their unique technical features and cost-effectiveness. In particular, Permanent Scatterer InSAR (PSInSAR™) is an advanced InSAR technique, developed in the late nineties, capable of providing very precise 1D displacement measurements along the satellite line-of-sight (LOS) and high spatial density (typically exceeding 100 measurement points/sqkm) over large areas, by exploiting point-wise radar targets already available on ground. Recently, some significant advances have been reported in InSAR data processing that can further increase the quality and the effectiveness of this data source for reservoir monitoring: (a) the development of new InSAR algorithms and in particular the so-called SqueeSAR™ approach, which allows a significant increase in the spatial density of measurement points; (b) the availability of an increased number of satellite radar sensors characterized by higher sensitivity to surface deformation, higher spatial resolution, and better temporal frequency of acquisition.

The seismic ACROSS (Accurately Controlled and Routinely Operated Signal System) is a very stable and semi-permanent seismic source developed in Japan. Six units are in nearly continuous operation in Japan and one has been continuously operated since 2004. In order to examine the possibility of continuous imaging of reservoir characteristics change, we carried out simulation using single ACROSS and multi-geophones. We assumed 20% velocity changes in reservoirs. We used rectangular shape reservoirs such as 1) 500 m-wide and 50 m- thick, and 2) 50 m-wide and 10 m-thick located at 1 km-depth. We included the velocity change in shallow sedimentary layer. We can synthesize any directional single forces simulating seismic ACROSS. By use of synthesized full-wave seismograms, the back propagation can generate P, S and P-S phases. If velocity change of the sedimentary layer is < 0.1%, we can clearly obtain the rectangular shape for the reservoirs using before and after change of characteristics. However, the extent of knowledge on velocity structure and large velocity change at the surface strongly affect to the results. Use of a vertical geophone array can reduce the effect of surface velocity change. We will test the method in Japan and Saudi Arabia.

With the increased employment of autonomous planted nodes for seabed seismic acquisition, the 4D capabilities have been demonstrated. Planted nodes are characterized by very high data quality and their flexibility for use in obstructed areas with complex infrastructure, also in ultra deep waters (<3000m). Taking into account that there are no added investments for manufacturing and installation, it is shown by an example that nodes are favorable to permanent systems and, as such, a real alternative for reservoir monitoring.

It is proposed to use the refracted wave associated with the guided waves propagating in the water layer to detect shallow gas or CO2 close to the seabed. At long distances from the source, the refracted wave train in the water layer is clearly observed on field data, and by measuring changes in amplitude and traveltime for this wave, we think this can offer a nice alternative to more conventional methods. Simple formulas for amplitude and traveltime changes based on ray theroy is presented. The optimal configuration is for a situation where the leakage is midway between source and receiver. Major limitations are coupled to the size of the anomaly (a lateral extension of 100 m is probably needed) and the water depth. Deep waters are more challenging for this method. Further investigations related to seismic modeling and eventually a field test is necessary to clearify the practical use of the method.