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Abstract

For the history matching of large reservoirs it is necessary to change local parameters after successful global matching. These modifications have to be made only in a relatively small part of the reservoir, but the influence of the whole reservoir has to be taken into account, too. The new technique "gathering" enables to combine blocks outside the focused area. This not only reduces costs but also increases the chances of success. Comparative simulations with full and gathered block systems show extensive agreement of the results for the focused area.

Introduction

Conventional block division in the Cartesian system of coordinates does not ensure that each part of the reservoir is represented with the required, sufficiently high resolution. By the new technique of local grid refinement a fundamental block system can be compacted locally. Grid refinement can also be made dynamic in order to improve the description of front-type displacements. It was proved in an earlier paper that in a refined grid the consistency is not worse than in a conventional grid. Numerical comparative examples have proved the practical applicability of this method to a large extent. Practical applications have shown that the reverse procedure, i.e. constructing a block system by block gathering - may be very useful. It not only makes it easier to establish a reasonable grid, but also opens up new ways in history matching. The combination of gathering and block refinement offers a better solution for every problem. This paper deals with gathering.

PRINCIPLE OF GATHERING PRINCIPLE OF GATHERING Fig. 1 shows one part of a conventional grid. 8 blocks are gathered once, and 2 blocks are gathered twice. The following rules have to be followed:

1. No shifted overlappings may appear (Fig. 2).

2. In the vertical direction (I1-coordinate) each block may have only one neighbour on each side.

3. A gathered block may have 4 neighbours at the most in the 12 or T3 coordinate directions (horizontal).

4. The total number of neighbours is limited to 12.

Only point 1) is a matter of principle. Points 2) to 4) prevent the algorithm from getting too complicated. prevent the algorithm from getting too complicated. PARAMETERS OF GATHERED BLOCKS PARAMETERS OF GATHERED BLOCKS The pore volume is the sum of pore volumes of all gathered blocks. The top depths are weighted with the pore volumes, taking into consideration only the uppermost blocks. At vertical gathering the lower blocks have no impact on the calculation of the top depth.

The thickness of a block column formed by gathering is the difference between the depth of the bottom edge of the lowest block and the top depth of the uppermost original block. If a gathered block is derived not only from blocks lying vertically one below the other, but also from horizontally neighbouring blocks, their thicknesses are weighted with the respective pore volumes. The pressure is calculated as a weighted mean value from the pressures of all original elements. Weighting is again carried out pressures of all original elements. Weighting is again carried out by means of the pore volumes. The oil and gas saturations are calculated in such a way that the oil and gas contents do not change.

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