The effect of CO 2 -fluid-rock interactions on the porosity and permeability of calcite-bearing sandstone

Transcription

1 The effect of CO 2 -fluid-rock interactions on the porosity and permeability of calcite-bearing sandstone Benoit Lamy-Chappuis, Bruce Yardley, Carlos Grattoni School of Earth and Environment, University of Leeds, U.K.

2 Key points: Show experimentally that small amounts of calcite contained in sandstone reservoirs dissolve quickly when exposed to CO 2 -saturated brine. Small amount of calcite dissolution induces significant changes in rock properties (permeability, sonic velocity, yield strength, peak strength). mct images of the pore space and mineralogy can be used to predict changes in rock permeability due to calcite dissolution. This method works better than classic semi-empirical relationships to predict the effect of porosity change on permeability.

3 The experimental set-up

4 The experimental set-up

5 Cayton Bay Sandstone

6 Cayton Bay Sandstone

7 SEM observations unreacted after reaction (above) The original rock in simplified form. (right) pairs of images contrasting pore structures before and after the experiment

8 unreacted after reaction

9 Shell fragment dissolution

10 Shell fragment dissolution

11 quartz and clay particles in space between pores

12 Silicates were unaffected by acidified brine injection.

13 Monitoring porosity during the experiments Absolute porosity Relative porosity

14 How does Porosity change? Main change apparent from NMR data is an increase in large pores consistent with the dissolution of shell fragments

15 What are the consequences for Permeability? Flow modelling codes use the Kozeny-Carman equation to predict permeability from porosity. In these experiments the increase in permeability is much greater than K-C equation would predict. We have therefore used pore-scale flow modelling (FLUENT), to see if this can predict the experimental results. Kozeny-Carman

16 Simulating Pore-Scale Flow: a) building a model from micro CT scans Single scan of core with shell fragments

17 Simulating Pore-Scale Flow: a) building a model from micro CT scans Initial model block

18 Simulating Pore-Scale Flow: a) building a model from micro CT scans Model showing pores and minerals of different greyscale intensity.

19 Simulating Pore-Scale Flow: a) building a model from micro CT scans Pore space

20 Simulating Pore-Scale Flow: a) building a model from micro CT scans Pore space +Calcite

21 Simulating Pore-Scale Flow: b) flow paths modelled at constant DP Modelled flow paths in the actual rock core, calcite present. Modelled flow paths in the core after calcite grains have been removed.

22 Simulating Pore-Scale Flow: b) flow paths modelled at constant DP Modelled flow paths in the actual rock core, calcite present. Modelled flow paths in the core after calcite grains have been removed.

23 Simulating Pore-Scale Flow: c) pressure distribution modelled at constant inlet velocity Modelled pressure distribution in the actual rock core, calcite present. Inlet to left. Modelled pressure distribution in the core after calcite grains have been removed Lower pressure gradient reflects increased permeability.

24 Simulating Pore-Scale Flow: c) pressure distribution modelled at constant inlet velocity Modelled pressure distribution in the actual rock core, calcite present. Inlet to left. Modelled pressure distribution in the core after calcite grains have been removed: Lower pressure gradient reflects increased permeability.

25 Simulating Pore-Scale Flow: f) how successful is it? FLUENT simulations are very successful at modelling the rapid increase in permeability as calcite dissolves in the experiments...

26 Simulating Pore-Scale Flow: f) how successful is it? FLUENT simulations are very successful at modelling the rapid increase in permeability as calcite dissolves in the experiments but do not predict the absolute permeability. This may be in part because of the complex pore structure at a fine scale.

27 Simulating Pore-Scale Flow: g) Model success and limitations Real Rock Model Low permeability Dissolution of calcite High permeability

28 Simulating Pore-Scale Flow: g) Real rock vs model

29 Conclusions Acidified brine injection leads to very rapid dissolution of isolated shell fragments. Where large shell fragments dissolve, there is a very large impact on permeability that is not predicted by conventional modelling approaches. Pore-scale modelling using FLUENT and based on a rock model generated by micro-ct scanning IS able to predict the changes in permeability when calcite dissolves. Thanks to Shuisheng He and Yingqing Qu from Sheffield University for useful discussions concerning the Fluent simulations.