Abstract

It is common knowledge that the identification of hydraulic units is vital for the construction of reservoir simulation models that preserve dynamically relevant geological features from the underlying geological model.

To date, both static-based and flow-based techniques were suggested for assessing hydraulic units. The static-based methods are used more commonly and are based on the minimization of a statistical measure of permeability heterogeneity or on establishing a relationship between log and core derived reservoir properties. The dynamic methods are based on single phase numerical simulation.

This paper presents a new flow-based method to determine hydraulic units considering multiphase flow, gravitational phase separation and capillary forces. The underlying concept assumes that, for every geological block, it is possible to estimate the time that is necessary for a fluid particle to traverse it laterally. The best way for that is a three dimensional streamline simulation. A stack of blocks can be lumped vertically to a hydraulic unit, if the time, which is necessary to achieve the vertical saturation equilibrium, is within a defined fraction of the traverse time. A multiphase simulation will be performed considering the reservoir as isolated columns. Water will be placed on top and then it will be calculated how far the vertical equilibrium can be achieved within 10 to 20 % of the local traverse time. The blocks, for which the required equilibrium is achieved, can be members of the same hydraulic unit. This method takes both the lateral flux and the vertical heterogeneity into consideration.

The paper presents a full field example to demonstrate the applicability of the suggested method.

Introduction
Definition of Hydraulic Units

Nowadays, geological models of oil and gas reservoirs are routinely built at a vertical resolution close to the sample rate of well logs. These models attempt to accurately represent reservoir geology and the variation of reservoir parameters on a very small scale. Geological models frequently contain several hundred layers and millions of grid blocks.

The geological model commonly contains too many grid blocks for numerical simulation. Upgridding and upscaling is therefore applied to the geological model in order to obtain a coarse grid that is suitable for dynamic reservoir simulation.

The upscaling procedure contains two major steps:

  1. the generation of the simulation grid (upgridding) and

  2. the sampling of grid properties into the coarse grid (upscaling).

During upgridding, the geological grid is coarsened areally and vertically. Based on sequence-stratigraphic principles, the spatial variability of reservoir parameters is typically expected to be high in the vertical direction, whereas the reservoir parameters are relatively continous in the lateral dimension.

It is therefore of paramount importance that (1) hydraulic units that express the vertical heterogeneity on a largers scale are identified and (2) the selection of simulation layers reflects these hydraulic units. Only when both requirements are fulfilled, the simulation model can preserve the heterogeneity of the geological model on a larger scale.

The concept of hydraulic units, or flow units, has been known to the oil and gas industry for a long time, but has gained increased attention in the last years. Hearn 1 published the first definition of hydraulic units in 1984. He proposed a subdivision of the reservoir interval into hydraulic units to determine the distribution of rock types that most strongly control fluid flow. A hydraulic unit is a reservoir zone that is laterally and vertically continuous and has similar permeability, porosity and bedding characteristics. Certain ranges of porosity and permeability that occur in a particular part of the sedimentary sequence are used to subdivide the reservoir along lines that represent gradations in reservoir quality, the ability of the rock to transmit fluids, both laterally and vertically.1 The identification of hydraulic units is usually constrained to a facies model. Facies models reflect the large scale architecture of the reservoir; hydraulic units are then used to express the vertical heterogeneity within the individual facies types in the large scale geological framework.

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