Time-lapse seismic now integral part of reservoir management
A measure of how important Time-lapse (or 4D) seismic has become is that in the order of 75percent of current seismic survey work on the North-west Europe Continental Shelf, of which the North Sea is the largest component, relates to 4D assessment of existing fields, primarily in the Norwegian and UK sectors.
An excellent example of this is work carried out in the Forties field for Apache. In 2005, the company contracted CGG to acquire a fresh 3D data-set – the fourth such campaign since the surveys of 1988, 1996 and 2000 when Forties was a BP asset.
The purpose of the new survey was to supply the information necessary for Apache to execute a 4D time-lapse assessment of the remaining potential of the first multi-billion barrel oilfield ever discovered in UK waters.
Another good example is the 4D time-lapse monitoring of the Chirag field offshore Azerbaijan through 3D seismic surveys acquired in 1995 (before the 1997 startup of production) and 2002.
4D is evolving, technologically and in breadth of application and, though 3D surveys are currently used to set the baseline, CSEM also being promoted as a complementary/alternative approach to setting the baseline and monitoring thereafter.
While the idea of 4D had been kicked around since the mid 1980s and applied to a handful or so of fields in order to demonstrate that it could provide a valuable means of learning about hydrocarbons migration, it was arguably BP’s UK Continental Shelf Foinaven field development of the mid 1990s that demonstrated the technique’s real potential, indeed established its credentials.
Foinaven, discovered in 1992, is a heavy crude development located on the UK Atlantic Frontier, 190km west of Shetland in some 500m of water. Though not the first find in that sector of the UKCS, it was the first to be developed and was brought onstream in 1997, based on subsea well clusters tied back to a production ship.
A couple or so years later, an analogue discovery located just a few miles south of Foinaven and named Schiehallion was also successfully brought into production and, later, a number of associated satellite.
Where both fields differed from the traditional North Sea fare is that they were found to lie within Palaeocene stratigraphic plays. Discoveries appear to be complex combination traps, which, in most cases, have related deeper structuring. This is the case for Foinaven, Schiehallion, Suilven, Laggan and Torridon.
The simplest way of describing the Foinaven and Schiehallion reservoirs in cross section is to liken them to streaky bacon – with multiple thin horizons. The only way to access the hydrocarbons is by drilling horizontal production wells, plus water injectors.
In order to better understand what Foinaven seemed to be about, and to help manage the reservoir, BP took the bold decision to carry out a baseline 3D seismic survey as a dedicated 4D reference before production started in 1997. A couple of years later, the company acquired the worlds largest 4D survey across the by then three producing fields: Foinaven, Schiehallion and Loyal.
Indeed, by the end of 2000, BP had carried out four full-field 3D/4D surveys over Foinaven alone. The technique rapidly became a standard tool for the company and has been extensively applied across the North Sea and in other locations like deepwater Gulf of Mexico, also the Caspian as mentioned already.
Not only that, BP is credited as being the first to apply ‘life of field’ seismic (LoFS), which is based on a permanent seafloor deployment that monitors the long-term performance and development of subsea producing reservoirs.
The Norwegian sector Valhall field was selected as the test-bed, with deployment taking place in 2003, some 21 years after first commercial production started in 1082.
In addition to conducting repeated 3D sourced surveys at approximately three-month intervals, the nearly 10000 channel seismic system monitors all seismic activity in the vicinity of the reservoir continuously and in real-time. The system was developed and supplied by US company Geospace Engineering Resources International of Houston.
But BP was not the only company interested in the value proposition to the North Sea offered by 4D as, unquestionably, Statoil is another in the vanguard of pioneers.
Indeed Statoil appears to have paved the way in Europe, with time-lapse work carried out over the northern part of the Gullfaks oilfield in the North Sea in 1985, ahead of first commercial oil in 1986, again in 1995, followed by a third in 1996.
Statoil is also noted for the ground-breaking 4D Norne field basleine survey contract issued to Schlumberger unit Western Geco in 2001, followed by a much larger package of work in 2003 on the same field.
For this, Western Geco applied its proprietory suite of seismic technologies known as Q-Reservoir. It was said by Western at the time that: ‘Time is a critical factor in the effectiveness of 4D’ and that, ‘While 4D has been proven technically, the time between surveys had limited its business value’.
What the company was getting at was the fact that less than two years had lapsed since the first Norne 4D campaign and that this would enable Statoil to better maximise hydrocarbons recovery … the objective being to get this above 50percent (oil).
At least 80percent of Statoil’s European assets portfolio is covered by 4D.
More or less in parallel with BP’s UK work and that of Statoil in Norwegian waters, Total broke new ground offshore Angola in the mid 1990s by introducing 4D. The French group was then and continues to make spectacular deepwater progress with a train of discoveries reported and a number developed, starting with Girassol and Jasmin.
In Brazil the story has been similar, though with early application onshore rather than offshore. The target fields were Fazenda Alvorada and Alto do Rodrigues, onshore Reconcavo and Potiguar, with pilot 4D surveys conducted in 1994 and 1998 respectively.
The first offshore application by Petrobras was in the Marlim field. A major challenge was the need to acquire seismic data over 1520km2 in a heavily obstructed oilfield production area … platforms, subsea wellhead arrays, intra-field pipelines and so-forth.
Also onshore, but in California, Chevron applied 4D to the Coalinga ultra-heavy crude field located in the Central San Joaquin Valley approximately 160km north of Bakersfield, California. There, heavy oil is produced from moderately consolidated sand-shale reservoirs.
Chevron started steam injection of the study portion of Coalinga in 1995, with the objective of increasing production in heavy-oil-bearing reservoir, with time-lapse 4D applied initially in 1996 in order to monitor the lateral distribution of injected steam in the producing oil reservoirs.
A second survey was carried out in 1997, since when there have been several campaigns.
Whether surveys are conducted at intervals of several years offshore by vessels, or every few months where Valhall-style subsea arrays are permanently installed, each sweep is a snapshot in time. String them together and it becomes possible to observe dynamic changes in a reservoir and for subsurface teams to interpret fluid saturation and pressure changes.
Used in conjunction with a simulator and designer-4D offers the ability to achieve improved understanding of reservoir by identifying the presence of swept zones, major pressure changes, intra-reservoir barriers and by inference enable effective targeting of infill wells.
In the case of Russian company Yukos, prior to the company being dismembered by the Putin administration in Moscow, extensive work was carried out using a mix of seismic and time-lapse modelling to determine the best means of waterflooding mature fields in Western Siberia and placing infill wells to best advantage. This modelling was immensely detailed, tracking waterflows and hydrocarbon migrations around individual wells.
Early results appeared highly promising, even dramatic, with both Yukos and Schlumberger working closely at the latter’s imaging centre in Moscow which was and, as far as can be ascertained remains, the only ‘Eye’ centre in all of Russia.
