Kees Hornman

Distributed acoustic sensing for reservoir monitoring with vertical seismic profiling

Distributed Acoustic Sensing is a novel technology for seismic data acquisition, particularly suitable for Vertical Seismic Profiling. It is a break-through for low-cost, on-demand, seismic monitoring of reservoirs, both onshore and offshore. In this article we explain how Distributed Acoustic Sensing works and demonstrate its usability for typical Vertical Seismic Profiling applications such as checkshots, imaging, and time-lapse monitoring. We show numerous data examples, and discuss Distributed Acoustic Sensing as an enabler of seismic monitoring with 3D Vertical Seismic Profiling. Key words: Borehole geophysics, Acquisition, Seismics, Time lapse, Monitoring.

Cable development for distributed geophysical sensing with a field trial in surface seismic

Fibre-optic distributed sensing has the potential to revolutionize well and reservoir surveillance in the oil and gas industry. Benefits include the passive nature of optical fibre sensors, the potential for cost-effective installations, combined with the possibility of densely distributed measurements along the entire length of the fibre. Amongst a range of fibre-optic sensing technologies, Distributed Acoustic Sensing has the potential to provide a low cost alternative for conventional seismic technologies. To widen the geophysical application scope further, the fibre-optic sensing cable should be made more sensitive to incoming seismic waves that arrive at the cable perpendicular (“broadside”) to its axial direction. We introduce the development of such cable concepts, and present results of a successful cable deployment in a surface seismic field trial. Efforts continue to realize cost-effective directionally-sensitive cables for geophysical use, for deployment down-hole and on surface.

Microseismic Applications using DAS

Distributed Acoustic Sensing (DAS) is a rapidly maturing fibre optic technology with many applications for wellbore monitoring and geophysical surveillance. DAS transforms a fibre optic cable into a distributed array of acoustic sensors. Shell is developing DAS technology in partnership with OptaSense, a subsidiary of QinetiQ U.K. DAS has been proven to work for VSP applications (Mateeva et al., 2012). The technology has been improved through numerous field trials and it has been tested in a variety of installations, where it has been compared to geophones and sonic logs as a check-shot tool, and as an imaging tool for walk away VSP data. Signal to noise ratio, directionality and repeatability among other aspects have also been studied through these field trials and laboratory experiments. The performance of DAS for micro-seismic monitoring applications is still under evaluation. Although DAS offers the advantage of recording along the whole well at once, significantly increasing the number of receivers, it has several challenges with respect to geophone arrays: the current DAS system records data with a higher noise floor and with a more constrained angular sensitivity since it behaves as a doublet of one-component geophones. In this paper we report the current status of this technology regarding micro-seismic monitoring. A field trial specifically designed to test this application in a micro-seismically active area is described. The trial was jointly carried out with Petroleum Development Oman (PDO) in a field in Oman.

Continuous Land Seismic Reservoir Monitoring of Thermal EOR in the Netherlands

A continuous reservoir monitoring system has been installed for Shell, on a heavy-oil onshore field situated in the Netherlands, to re-develop oil production by Gravity-Assisted Steam Drive. The challenge was to continuously monitor using seismic reflection the expansion of the steam chest injected in the reservoir during production. The main problems for onshore time-lapse seismic are caused by near-surface variations between base and monitor surveys which affect the seismic signal coming from the reservoir. In our system, a set of permanent shallow buried sources and sensors has been installed below the weathering layer to both mitigate the near-surface variations and minimize the environmental footprint. The very high sensitivity of our buried acquisition system allows us to track very small variations of the reservoir physical properties in both the spatial and calendar domains. The 4D reservoir attributes obtained from seismic monitoring fit the measurements made at observation, production, and injector wells. A daily 4D movie of the reservoir property changes allows us to propose a scenario that explains the unexpected behavior of the production and confirms that the steam does not follow the expected path to the producer wells but rather a more complicated 3D path within the reservoir.

Recent Advances in Seismic Monitoring of Thermal EOR

Well-planned and executed reservoir surveillance has proven to add significantly to the production and ultimate recovery of hydrocarbons, notably in areas of Improved and Enhanced Oil Recovery (IOR/EOR). Recent technological advances in the area of data acquisition and integration have led to increased use of well and reservoir surveillance data to optimize such processes. In the case of thermal EOR, one of the most important subsurface uncertainties impacting performance is heat and steam front conformance, both vertically and arealy. This paper illustrates new geophysical technologies used for monitoring various thermal EOR recovery strategies in The Netherlands, Canada, and Oman. We focus on permanently buried seismic sources and receivers, refraction seismic, down-hole seismic, and the newly developed Distributed Acoustic Sensing (DAS) to enable low-cost and non-intrusive seismic surveillance. These technologies are not without challenges, but our field trials indicate they have the potential to broaden the successful application of reservoir monitoring onshore.

Data acquisition over the Lunskoye gas cloud

Obtaining a good seismic image under a gas cloud can be a considerable challenge. The gas cloud can cause strong lateral variations in the P-wave velocities, leading to severe ray bending and scattering, and it can cause high absorption and high transmission losses. Lunskoye Field, in shallow waters offshore Sakhalin (a Russian island north of Japan), is one of those fields suffering from imaging problems due to gas above the reservoir, close to the seafloor. As shown in Figure 1, the field is elongated in the N-S direction. A time migration of a 3D streamer survey, acquired in 1996, did show a large wipeout zone over the crest of the structure. This 3D survey was acquired with inlines along the long axis of the field for operational reasons; on the west side the water depths get very shallow, complicating any vessel turns in the E-W direction. In order to correct for the ray bending effects of the gas, a 3D prestack depth migration (PSDM) was carried out in 1997 and these results were a clear improvement over the time migration (Figure 2). However, Figure 3 shows that, even after 3D PSDM, a wipe-out zone with a width of 1.5 km still covers the crest of the structure. Figure 1. Bathymetry map of the survey area. Sakhalin Island is to the west of this map. Lunskoye Field is indicated by dark blue. The rectangular survey outline measures 13 × 25 km. Figure 2. A crossline (E-W) of the time migration with fixed gain (left) and the prestack depth migration (right, displayed in depth) of the 1996 survey. Figure 3. Wipe-out zone after time migration (top) and after prestack depth migration (bottom). Clearly, the available seismic data are inadequate for field development. Production wells have to be drilled through active fault planes and with these data it …