Capturing the Effect of Fracture Heterogeneity on Multiphase Flow During Fluid Injection

SummaryCarbonate fractured reservoirs introduce a tremendous challenge to the upscaling of both single- and multiphase flow. The complexity comes from both heterogeneous matrix and fracture systems in which the separation of scales is very difficult. The mathematical upscaling techniques, derived from representative elementary volume (REV), must therefore be replaced by a more realistic geology-based approach. In the case of multiphase flow, an evaluation of the main forces acting during oil recovery must also be performed.A matrix-sector model from a highly heterogeneous carbonate reservoir is linked to different fracture realizations in dual-continuum simulations. An integrated iterative workflow between the geology-based static modeling and the dynamic simulations is used to investigate the effect of fracture heterogeneity on multiphase fluid flow. Heterogeneities at various scales (i.e., diffuse fractures and subseismic faults) are considered. The diffuse-fracture model is built on the basis of facies and porosity from the matrix model together with core data, image-log data, and data from outcrop-analogs. Because of poor seismic data, the subseismic-fault model is mainly conceptual and is based on the analysis of outcrop-analog data. Fluid-flow simulations are run for both single-phase and multiphase flow and gas and water injections.A better understanding of fractured-reservoirs behavior is achieved by incorporating realistic fracture heterogeneity into the geological model and analyzing the dynamic impact of fractures at various scales. In the case of diffuse fractures, the heterogeneity effect can be captured in the upscaled model. The subseismic faults, however, must be explicitly represented, unless the sigma (shape) factor is included in the upscaling process. A local grid-refinement approach is applied to demonstrate explicit fractures in large-scale simulation grids. This study provides guidelines on how to effectively scale up a heterogeneous fracture model and still capture the heterogeneous flow behavior.</jats:p

Developing Gullfaks Shetland/Lista Fractured Carbonate Reservoir - From Hope and Pray to Trial and Error

Abstract The Gullfaks Shetland Group and Lista Formation (Sh/L) are fractured chalk reservoirs located above the main Gullfaks reservoirs. Fractured carbonate reservoirs are heterogeneous, due to deposition, diagenesis and fracturing, so they are challenging to characterize and model. Shetland/Lista is considered to be a type II reservoir, according to the Nelson classification (Nelson 2001), where most of the oil is stored in the relatively high porosity low permeability (0.1 mD) matrix. The oil is mainly transported through the high permeability (several darcy) fracture system. Production by depletion started late 2012. Historical out of zone injection that occured some time between 1994 and 2010 from the Gullfaks main reservoirs increased the reservoir pressure in Sh/L. Production from Sh/L therefore initially improved the drillability to the underlying Gullfaks main reservoirs. However, presently further reduction of the pressure is not advisable to preserve the drillability. Most of the Shetland producers are consequently shut-in. Water injection to maintain the pressure is therefore considered to be a promising new drainage strategy. In addition to pressure support, fractured, water-wet reservoirs can benefit from water injection through increased oil recovery by spontaneous imbibition. Oil can be mobilized by spontaneous water imbibition from fracture to matrix. Special core analysis experiments indicated water-wet conditions and potential for oil recovery by spontaneous imbibition in the Shetland reservoir. Complementary field tests were conducted to confirm this: A single well injection and following production test (push-and-pull) with tracers, confirmed high potential for spontaneous imbibition. A multi-well pilot also showed clear indications of imbibition taking place between injector and producers, when analysing production and tracer data. The objective of the work described in this paper is to confirm feasibility of a new drainage strategy, and confirm that pressure support by water injection will be beneficial for the oil recovery in the Shetland/Lista reservoirs through imbibition.</jats:p