Impact of Heterogeneity on Flow in Shallow-Marine Reservoirs: Application to a Thin Oil Column Produced via Horizontal Wells
Dilib, Fahad A.; Jackson, Matthew D.; Graham, Gavin H.; Hampson, Gary J.
Numerous studies have demonstrated that heterogeneity in shallow-marine reservoirs can have a significant impact on flow. However, most of these focus on oil recovery by water injection via vertical producer and injector wells. Yet the impact of heterogeneity on flow depends also on the fluid properties, flow rates and principal flow direction. The aim of this study is to quantify the impact of heterogeneity on flow in a shallow marine reservoir that hosts a thin oil rim, overlain by a large gas cap and underlain by an active aquifer, with oil produced via long horizontal wells.
In the sector of interest, the reservoir comprises a series of stacked parasequences that each coarsens upwards from micaceous, silty, fine-grained sandstone at the base (10's-100's mD, termed M-sands) to clean, coarse-grained sandstone at the top (100's-1000's mD, termed C-sands). A sector model, measuring 3200m x 750m x 150m and containing a single horizontal well, was taken from a full field geological model, and downscaled onto a highly refined grid to capture flow. Parasequence-bounding flooding surfaces and intra-parasequence clinoforms are associated with zones of calcite cement; these are not included in the full field model and were added to the sector model using an in-house algorithm.
The most significant heterogeneity that impacts on oil production is the permeability contrast between C- and M-sands. A higher contrast reduces the pressure drawdown into the C-sands, delaying gas breakthrough and yielding higher recovery. The next most significant heterogeneity is the presence of stratabound calcite cements along clinoforms; similar cements along flooding surfaces have a much lower ranking. Cements along clinoforms reduce cumulative oil production, because they cause oil to be bypassed and channel gas into the well completions; they do not ‘hold back' the encroaching gas. Uncertainty in the kv/kh ratio of the sands, which reflects bed-scale heterogeneity, has only a small impact on oil recovery because flow is primarily through the high permeability C-sands.
The results presented here are often counter-intuitive, and contrast with those obtained from waterflood simulations with vertical wells. They provide insight that can be used to guide model construction and history matching. Moreover, they demonstrate that clinoforms can have a large impact on production, even though these features are typically neglected in geocellular models of shallow-marine reservoirs.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013