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

摘要

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: 1. A single well injection and following production test (push-and-pull) with tracers, confirmed high potential for spontaneous imbibition. 2. 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.