Ekofisk Field: fracture permeability evaluation and implementation in the flow model

Abstract

The Ekofisk Field is a naturally fractured chalk reservoir located in the Norwegian sector of the North Sea. The natural fractures clearly control the permeability distribution, as the effective permeability can reach 50 mD whereas the matrix permeability only ranges between 0.1 mD and 10 mD. Permeability mapping in this field has been challenging due to the structural, stratigraphic and mineralogical complexity, tectonic history and non-negligible matrix permeability. A detailed fault interpretation has resulted in a complex fault pattern. A fault intensity (P21) parameter calculated from the fault pattern has proved to be the critical component for permeability mapping. Correlations were found between the fault intensity (P21) values and (1) the fracture distributions from cores and logs at individual wells, and (2) the fracture component of the well test permeability ( K frac – total permeability less matrix permeability). These relationships allowed field-wide fracture permeability maps to be developed based on the P21 results. Total permeabilities were obtained by summing the matrix permeability and the calculated fracture permeability. These permeability maps were introduced in the Ekofisk Flow Model 2002 and refined to match the rate performance of the 50 initial wells run in prediction mode (well head pressure constraint). The runs in prediction mode have proved to be very effective for calibrating the permeability distributions, based on initial well performance above the bubble point. This simulation technique was extended to cover all producers (262 wells) during the entire history of the field to refine the maps further. After calibration with the performance data, a satisfactory history match was obtained by making minor changes to permeability and other dynamic parameters. Additionally, the running of the model of the mature Ekofisk Field in prediction mode for its full field life has provided a robust tool for calibrating field performance.

Characterization of dense zones within the Danian chalks of the Ekofisk Field, Norwegian North Sea

“…In chalk, matrix permeability is relatively low (0.1-10 mD) and production is enhanced through a network of fractures with effective permeability up to 50 mD (Toublanc et al 2005). Fractures within mechanically brittle dense zones tend to remain more open compared to those within more porous chalk (Thomas et al 2010), and dense zones may behave as conduits that facilitate water imbibition into adjacent high-porosity intervals with subsequent efficient hydrocarbon displacement (Fig.…”

Using mechanical models to investigate the controls on fracture geometry and distribution in chalk

“…Fortunately, chalk is often heavily fractured, both in the subsurface (e.g. Ekofisk: Toublanc et al 2005;Kraka: Jørgensen & Andersen 1991;Valhall: Bauer & Trice 2004) and in outcrop. Therefore, understanding the distribution, orientation and connectivity of the fractures is important to produce the fields efficiently.…”

Integrated Research as Key to the Development of a Sustainable Geothermal Energy Technology

“…Examples representI)aphase field model (PFM) of fracture sealing; [20] II) as ynthetic model [21] of atypical self-affine,rough fracture surface (fractal dimension = 2.2 [22] ); III) alocal cubic law visualization of typicalf low channelsbased on aperture distributions; [23] IV) amedical X-ray computed tomography (CT) scan of af racturedc ore sample; [24] V) lineament interpretationsb ased on remote sensing; [25] and VI) arandom stochasticD FN model. [26] Fracture or DFN studies provide improved permeability predictions of single fractures(analytical, [27] numerical [24,28] )and fracturedg eothermalreservoirs (analytical, [29] numerical [24,25,30] availability of outcrops and subsurface data;this is often provided, for example,b yf ield measurements,t errestrial laser scanning, [39] and remote sensing [40] (Figure6V), as well as aa pplied samplingm ethod, such as scanline or window sampling. [38] In conclusion, the evaluation of fluid flow in fractured reservoirs is am atter of scales,r angingf rom single fracture scales (mmt oc ms cale,m echanically and chemically induced geometries)t ot he field scale (cm to km scale,t ransferability of DFN geometries), the interactions of which have to be better understood to provide more reliable geothermal reservoir models.…”