Connectivity of channelized reservoirs: a modelling approach

Abstract

Connectivity represents one of the fundamental properties of a reservoir that directly affects recovery. If a portion of the reservoir is not connected to a well, it cannot be drained. Geobody or sandbody connectivity is defined as the percentage of the reservoir that is connected, and reservoir connectivity is defined as the percentage of the reservoir that is connected to wells. Previous studies have mostly considered mathematical, physical and engineering aspects of connectivity. In the current study, the stratigraphy of connectivity is characterized using simple, 3D geostatistical models. Based on these modelling studies, stratigraphic connectivity is good, usually greater than 90%, if the net: gross ratio, or sand fraction, is greater than about 30%. At net: gross values less than 30%, there is a rapid diminishment of connectivity as a function of net: gross. This behaviour between net: gross and connectivity defines a characteristic ‘S-curve’, in which the connectivity is high for net: gross values above 30%, then diminishes rapidly and approaches 0. Well configuration factors that can influence reservoir connectivity are well density, well orientation (vertical or horizontal; horizontal parallel to channels or perpendicular) and length of completion zones. Reservoir connectivity as a function of net: gross can be improved by several factors: presence of overbank sandy facies, deposition of channels in a channel belt, deposition of channels with high width/thickness ratios, and deposition of channels during variable floodplain aggradation rates. Connectivity can be reduced substantially in two-dimensional reservoirs, in map view or in cross-section, by volume support effects and by stratigraphic heterogeneities. It is well known that in two dimensions, the cascade zone for the ‘S-curve’ of net: gross plotted against connectivity occurs at about 60% net: gross. Generalizing this knowledge, any time that a reservoir can be regarded as ‘two-dimensional’, connectivity should follow the 2D ‘S-curve’. For channelized reservoirs in map view, this occurs with straight, parallel channels. This 2D effect can also occur in layered reservoirs, where thin channelized sheets are separated vertically by sealing mudstone horizons. Evidence of transitional 2D to 3D behaviour is presented in this study. As the gross rock volume of a reservoir is reduced (for example, by fault compartmentalization) relative to the size of the depositional element (for example, the channel body), there are fewer potential connecting pathways. Lack of support volume creates additional uncertainty in connectivity and may substantially reduce connectivity. Connectivity can also be reduced by continuous mudstone drapes along the base of channel surfaces, by mudstone beds that are continuous within channel deposits, or muddy inclined heterolithic stratification. Finally, connectivity can be reduced by ‘compensational’ stacking of channel deposits, in which channels avoid amalgamating with other channel deposits. Other factors have been studied to address impact on connectivity, including modelling program type, presence of shale-filled channels and nested hierarchical modelling. Most of the stratigraphic factors that affect reservoir connectivity can be addressed by careful geological studies of available core, well log and seismic data. Remaining uncertainty can be addressed by constructing 3D geological models.

The application of outcrop analogues in geological modelling: a review, present status and future outlook

“…These typically use models of specific outcrops or related generic models. Such models have been used to investigate the influence of various aspects of reservoir architecture on fluid flow across a range of different reservoir types, such as fluvial connectivity (Georgsen et al 1994), understanding shallow-marine reservoirs (Kjønsvik et al 1994;Bransaeter et al 2005), the connectivity of channelized reservoirs (Larue & Hovadik 2006, 2008, the behaviour of the Ainsa deep-water reservoir analogue (Larue 2004;Labourdette 2008) and the role of clinoforms draping shales as barriers (Forster et al 2004;Howell et al 2008b;.…”

“…For this purpose, an accurate characterization of the connectivity of adjacent sandbodies is required that incorporates the effects of baffles or barriers between the major sandstone elements formed by fine-grained facies. Modelling studies (Larue & Hovadik 2006;Alpak et al 2013) suggest that such fine-grained barriers, if sufficiently common and laterally extensive, can significantly influence fluid flow. Such results have encouraged close examination and quantification of connectivity relationships within deep-water reservoir analogues.…”

Object-based modelling of avulsion-generated sandbody distributions and connectivity in a fluvial reservoir analogue of low to moderate net-to-gross ratio

“…Larue & Hovadik 2006): (1) the largest connected geobody; and (2) the fractional volume of sandstone that is connected between the northern and southern faces of the models (i.e. the connected sand fraction sensu King 1990).…”

“…Sandbody connectivity is a key control on recovery from channelized fluvial reservoirs of low net-to-gross (NTG) ratio (c. <30%) (Jones et al 1995;Larue & Hovadik 2006;Hovadik & Larue 2007). Factors that influence the connectivity of channelized fluvial sandbodies include: (1) the NTG ratio; (2) the width and thickness of the sandbodies; (3) plan-view geometry of the sandbodies, which is typically considered in terms of sinuosity and range of sandbody orientations; (4) the organization of sandbody stacking; and (5) the sandstone content of crevasse-splay and other non-channelized floodplain deposits (Allen 1978;Leeder 1978;Bridge & Leeder 1979;Allard & HERESIM Group 1993;Jones et al 1995;North 1996;Ainsworth 2005;Larue & Hovadik 2006;Donselaar & Overeem 2008;Pranter & Sommer 2011;Pranter et al 2014).…”

“…using object-based modelling algorithms conditioned to vertical sandstone-proportion curves: Clemetsen et al 1989;Hirst et al 1993;Georgsen et al 1994;Holden et al 1998;Deutsch & Tran 2002). In its simplest form, the assumption of randomly distributed channelized sandbodies allows a percolation threshold for sandbody connectivity to be identified in each stratigraphic layer based solely on its NTG ratio (King 1990;Larue & Friedmann 2005;Larue & Hovadik 2006;Hovadik & Larue 2007). This percolation threshold for sandbody connectivity in three dimensions occurs at a NTG ratio of 28% (King 1990).…”