Effect of Small/Medium Scale Reservoir Heterogeneity on the Effectiveness of Water, Gas and Water Alternating Gas WAG Injection

Water-alternating-gas (WAG) injection is a technique employed in EOR (Enhanced Oil recovery). WAG injection can be immiscible or immiscible with water and gas being injected into the hydrocarbon liquids reservoir to promote greater recovery. WAG injection is effective as gas typically has greater microscopic sweep efficiency whilst water has better macroscopic sweep efficiency. It is important to be able to characterise and quantify how much the degree and type of small/medium scale heterogeneity during WAG flooding could affect the recovery factor from a reservoir, such that during project evaluation teams are able to properly evaluate the ranges on uncertainty on recovery factors and the economic benefit of the project as well as risks associated with WAG implementation. The Hutton field is located in the North Viking Graben area of the North Sea and the lithology of the reservoir section is made up of Brent group sandstones which are highly heterogeneous in the horizontal and vertical directions at a small scale (i.e. pore scale and plug scale) and at a medium scale (the vertical layering of different formations). The effect of reservoir heterogeneity on WAG efficiency has been evaluated using dynamic reservoir simulation models of the Hutton field. Input parameters were based on an available model of the Hutton Field. A fine grid geological model (grid size 5ft × 5ft × ~2ft) has been created of a small section of the Hutton reservoir. A variety of field development schemes were evaluated including depletion, water injection, gas injection and immiscible WAG production scenarios. Geological models were created for three scales of heterogeneity (small scale and medium scale heterogeneity models, and a homogeneous model) based on interpretation of log data from a set of three control wells. Compositional simulation models were used to model the dynamic behaviour. Two phase relative permeability (oil / water and gas / oil) data was used, as three phase relative permeability data for Hutton was not available. There is no hysteresis data available for the Hutton field, therefore separate test runs were carried out to evaluate how hysteresis might affect recovery factor during WAG injection using two and three phase relative permeability data and parameters for use in the Killough correlation for hysteresis. Immiscible WAG injection is beneficial in reservoirs with small and medium scale heterogeneity and gives ~5% improvement in recovery factor when compared to water injection. However, when hysteresis is included, the recovery factor may be higher than this by another ~10%. WAG injection may provide inferior recovery factors to water injection in homogeneous reservoirs. However, simulations indicated that some limited gas injection into a homogeneous reservoir may prove beneficial for accessing attic oil. It is recommended that laboratory testing of core samples (core flood experiments) be carried out prior to a WAG injection specifically with the aim of identifying the most appropriate hysteresis model and to give good relative permeability data across all three phases.