Hydrocarbon in place volumes are often inaccurate as a result of poor representation of the reservoir structure (by means of a 3D grid), that in combination with the use of traditional saturation calculation methods, lead to erroneous hydrocarbon volumes and poor investment decisions.
Traditionally a reservoir model is represented with a 3D grid, in a complex setting such as fault intersections and stacked reservoirs. A corner point grid is often used, which has limitations to represent this complexity. Further, the hydrocarbon saturations are then derived on a cell by cell basis on that 3D grid using simple averaging techniques of saturation height functions. The poor structure representation on the pillar grid in addition to the simplistic averaging methods lead to inaccuracies of the in place volumes especially where a prominent transition zone is present.
This paper presents new advanced saturation averaging methods (volume and height weighted) using saturation height functions on 3D grids. The new advanced saturation averaging methods are used on different reservoir models to compare the saturation distribution and volumetric differences against the traditional saturation calculation methods. A 4-way dip closure reservoir model with a tilted free water level (typical example of a carbonate reservoir in the Middle East), and a faulted S-grid model of the F3-FA field (North Sea) are used.
For the 4-way dip closure reservoir model, when comparing the advanced ‘volume weighted’ and traditional ‘by center of the part of the cell’ saturation averaging methods, a significant difference in the water saturations is observed which leads to about 5% difference in the calculation of in place hydrocarbon volumes. Further, it is observed that changing the thickness and orientation of the 3D grid cells can result in even larger differences of 5-10%.
The faulted F3 model shows that the difference between the hydrocarbon saturation values is largest where it matters most, that is, around the fluid contacts and in the transition zone. The new advanced saturation averaging methods give accurate hydrocarbon saturations irrespective of the size or complexity of the 3D grid and without any discretization effects.
The F3-FA field is a small, offshore gas field in the Dutch sector of the southern North Sea. The discovery well drilled there in 1971 located rich gas in the Jurassic Scruff Greensand (SGSS) formation; however, its constrained size meant that field appraisal could not take place for another decade, with two wells drilled in 1982. Following appraisal, the field struggled to compete for investment capital and pipeline access against other development opportunities. Ultimately, the original operator exited the field in 2008, and Centrica Energy purchased its equity and field operatorship. A different opportunity set allowed Centrica and its partner EBN to advance the field development. Uncertainty regarding the dynamic performance of the field and a recognition of its limited volumes, and, therefore, limited producing field life, led to the adoption of a self-installing platform (SIP). The platform is currently the largest of its kind in the world at 133 m high, being slightly heavier than the Eiffel Tower and designed for future reuse at another location for the purpose of producing some other small accumulation. The platform was installed in September 2010, with full processing and compression facilities in place, and tied into the northern offshore-gas-transport (NOGAT) pipeline system. Production is from a single, near-horizontal production well. The reliance on a single development well reflected the marginal nature of the development, with a range of reserves that reflected uncertainty about gas distribution and the dynamic behavior of a reservoir that did not have any analogs to draw on. First production was achieved in January 2011, and, to date, the field has performed strongly, with no indication of the aquifer influx that was an uncertainty going into development. The condensate yield has also proved to be much higher than anticipated. The prolonged time between exploration and first production is a familiar trait for many small developments in recent years in the southern North Sea, often executed by a new breed of operator. This length of time is also typical of the choices needed to bring such fields to production, given modest reserves and unresolvable subsurface uncertainties.
F3-FA field is a small offshore gas field in the Dutch sector of the Southern North Sea. The discovery well drilled in 1971 located rich gas in the Jurassic Scruff Greensand Formation. Its constrained size meant field appraisal did not take place for a further decade with two wells drilled in 1982.
Following appraisal, the field struggled to compete for investment capital against other development opportunities and also for pipeline access. Ultimately the original operator exited the field in 2008 - Centrica Energy purchased its equity and field operatorship.
A different opportunity set allowed Centrica and its partner EBN to advance the field development. Uncertainty regarding the dynamic performance of the field and a recognition of limited volumes and therefore producing field life led to the adoption of a Self-Installing Platform (SIP). The platform is a first of its kind at this scale, being the largest mobile production gas platform, 133 meters high and slightly heavier than the Eiffel Tower and designed for re-use on another location to produce some other small accumulation.
The platform was installed in 2010 with full processing and compression facilities in place, tied into the NOGAT pipeline system. Production is from a single, near-horizontal production well. The reliance on a single development well reflected the marginal nature of the development with a range of reserves that reflected uncertainty about the dynamic behaviour of the reservoir that did not have the luxury of any analogues to draw on.
First production was achieved in 2011 and to date the field has performed strongly with no indication of aquifer influx which was an uncertainty going into development. Condensate yield proved much higher than anticipated.
The prolonged time between exploration and first production is a familiar trait for many small developments of recent years in the Southern North Sea, often executed by a new breed of operator. It is also typical of the choices needed to bring such fields to production given modest reserves and unresolvable subsurface uncertainties.
In recent years, gas production in the Netherlands has peaked and appears to have gone into a slow but continuous decline. In an attempt to arrest this decline the Dutch government is investigating possibilities to tap into a previously unattractive class of reservoirs: the stranded fields.
Since the fifties, some 440 gas fields and 45 oilfields have been discovered in the on- and off-shore sectors of the Netherlands. Total gas reserves amount to over 4500 BCM of which some 71% has been produced. Oil reserves amount to 1.15 billion barrels of which 75% has been produced. From the 485 proven fields around 120 accumulations have not been developed and are considered stranded fields. Total volumes in these "contingent resources?? amount to over 200 BCM GIIP and over 60 million m3 STOIIP. Tight reservoirs, distant infrastructure, small volumes, and anomalous gas qualities are amongst the main reasons why these resources have not yet been developed.
In this paper, a screening methodology will be presented showing which of these fields might still qualify for development given recent technological and economic changes. By applying a hybrid methodology of GIS tools, data-mining applications and a quick screening economic evaluation the most attractive candidates for development are highlighted. In addition, the impact of new/improved technology, CAPEX changes or tax changes on the development economics are being modeled.
In order to renew interest in these stranded fields it is the intention of the Dutch Government to share results both on the data used for the analysis and any notional development options resulting from this exercise.