Res Sim CH4

Question 1

What causes errors and how to improve numerical dispersion?

Answer 1

• Error caused by grid block approximations
  + More block gets gives more accurate solution and reduces numerical dispersion
  + Use pseudo functions (pseudo relative k curves) to control numerical despersion.
  Balance is between accuracy and computational cost.

Question 2

What is Numerical Dispersion?

Answer 2

Numerical dispersion is the artificial spreading of saturation fronts due to the numerical grid block structure in the simulation.
It arises because we take large grids to represent moving fronts.
It can be improved by refining the grid (globally or locally) or by using improved numerical methods.

Question 3

Issues when choosing a res simulation grid

Answer 3

1. Dimension, 1D, 2D, 3D
2. Geometry/Structure, cartesian (x,y,z) or r/z, LGR, distorted
3. Fineness/Coarseness, therefore how many blocks do we need? 10’s or 1000’s?

Question 4

What are 2D cross sectional (x/z) grids used to study?

Answer 4

(a) vertical sweep efficiency in a layered system;
(b) water/oil displacements in a geostatistically generated cross-section;
(c) generate pseudo-relative permeabilities (3D to 2D system);
(d) the mechanism of a gas displacement process e.g. the importance of gravity etc.

Question 5

Streamline method

Answer 5

1. Permeability map with INJ and PROD well
2. Solve pressure field.
3. Compute the velocity field and trace streamlines.
4. Move saturation along streamlines and compute saturation values on the grid.

Question 6

Dynamic methods of pseudo-isation (upscaling) describe how numerical dispersion is taken into account.

Answer 6

1. setup fine grid simulation representing 2 or more cells
2. Run waterflood simulation and calculate flow from 1 coarse cell to another.
3. Record correct relationship between flow and saturation which converts to relative K.

Question 7

What is the process of Upscaling?

Answer 7

Calculating the correct effective parameters (i.e. relative K) to Reproduce a Fine Grid Model for Larger Scale Grid Blocks.

Question 8

What is meant by “Upscaling” with reference to the modelling of say a water flood.

Answer 8

Upscaling in a water flood essentially means getting the correct (effective) parameters (-e.g. rel. K) for the larger scale grid blocks which will reproduce a “correct” fine grid model.

Question 9

What is the difference between “rock” relative permeabilities and pseudo-relative permeabilities?

Answer 9

“Rock” relative permeabilities are intrinsic properties of a piece of reservoir rock at the “small” (i.e. core plug) scale.

Pseudo rel. Perms are effective properties at the “larger” (usually gridblock) scale that includes other effects (e.g. numerical dispersion, heterogeneity..) in addition to the “rock rel. perms”.

Question 10

What is gas coning?

Answer 10

Gas coning is the drawdown of highly mobile (low viscosity) gas into perforations. Causes high GOR production at a level above the solution gas value.

Question 11

In which two ways would the grid used to investigate gas coning be different from that which was used in the full field waterflooding simulation

Answer 11

1. Different geometry: R/Z for coning and cartesiian or corner point for full field.
2. Different fineness: very fine for near well and coarser for full field.
   Also 2D versus 3D for grid geometry.