Patent EA-003214-B1 (EA)
Title: Method for simulating a characteristic of a physical system
Inventor: WATTS JAMES W III
IPC: G01V1/30
Filed: 2000-03-15
Abstract: 1. A method of simulating at least one characteristic of a physical system, comprising the steps of: (a) discretizing the physical system into a plurality of volumetric cells arranged adjacent to one another so as to have a boundary between each pair of adjacent cells; (b) assigning an initial estimate of state variables for each cell; (c) constructing for each cell linear equations relating its state variables to the state variables of cells adjacent to it; (d) associating with each boundary a transportability value and ranking the boundaries corresponding to descending transportability values; (e) using the boundary rankings to construct topologically one-dimensional strings of cells; (f) developing a matrix equation for each string by assembling the linear equations associated with the cells of each string; (g) obtaining improved estimates of the state variables of the cells by solving the matrix equation for each string, one string at a time, until all strings' matrix equations have been solved; (h) repeating step (g) until a convergence condition is satisfied, thereby obtaining state variables for all cells that simultaneously satisfy the linear equations for all cells; and (i) using the state variables determined in step (h) to simulate at least one characteristic of the physical system. 2. The method of claim 1 wherein the construction of strings uses a rule that promotes construction of strings having high transportability values at cell boundaries between cells in the same string. 3. The method of claim 1 further comprising the steps of: (i) hierarchically ranking the boundaries corresponding to relative sizes of the transportability values, the boundaries being ranked in descending order <DP=2from the boundary having the highest transportability value to the boundary having the lowest transportability value; and (ii) constructing the strings in step (e) according to a first rule that promotes inclusion of cells having as many as possible of the highest ranked transportability values associated with the boundaries between the cells and a second rule requiring that any given cell in a string shall have boundaries with no more than two other cells in the same topological string of cells. 4. The method of claim 1 wherein for any given string used in step (f), no given cell in the given string is connected to a cell in the string other than a cell immediately before or immediately after the given cell in the given string. 5. The method of claim 1 wherein the string constructed in step (e) is circular, the method further comprising the step of cutting the string at the lowest ranking boundary in the circular string of cells. 6. The method of claim 1 wherein a given string constructed in step (e) contains a cell that touches another cell in the given string that is neither immediately before nor immediately after the cell, the method further comprising the steps of starting at one end of the given string and determining cell by cell whether a given cell in the given string touches another cell in the given string that is neither immediately behind nor immediately ahead of the given cell, identifying the last cell in the string that touches another cell in the string, identifying the lowest ranking boundary between said last cell and the closest cell before the last cell that touches another cell in the string, and cutting the string at the lowest ranking boundary between said the last cell and the closest cell before the last cell that that touched another cell in the string, thereby forming two new stings from the given string. 7. The method of claim 1 wherein the matrix equation assembled for each string in step (g) is constructed for line-successive overrelaxation. 8. The method of claim 7 further comprising the step of applying an additive correction to the improved estimated solution of step (f) within each string, with the additive correction being determined such that its application causes the sum of residuals within each string to become zero. 9. The method of claim 1 wherein iterative solution is accelerated by a Krylov acceleration method. 10. The method of claim 9 wherein the acceleration method uses the Orthomin method. 11. The method of claim 9 wherein the acceleration method uses the GMRES method. 12. The method of claim l, wherein the construction of strings in step (e) forms circular strings, an additional step of breaking each circular string at the boundary having the lowest ranked transportability value. 13. The method of claim 1 wherein the physical system comprises a hydrocarbon bearing reservoir, wells that extend from the surface of the earth to the reservoir, hydrocarbon-bearing flowlines at the surface of the earth, and hydrocarbon processing facilities. 14. The method of claim 1 wherein the physical system comprises a subterranean aquifer. 15. The method of claim 1 wherein the characteristic being simulated is heat transfer in the physical system. 16. The method of claim 1 wherein the physical system being simulated is a hydrocarbon-bearing reservoir. 17. The method of claim 1 wherein the matrix equation being solved results from the use of finite difference approximations. 18. The method of claim 1 wherein the matrix equation being solved results from the use of finite element approximations. 19. The method of claim 1 wherein the matrix equation being solved results from the use of finite volume approximations. 20. The method of claim 1 wherein the volumetric cells comprise a plurality of unstructured cells. 21. The method of claim 1 wherein the volumetric cells comprise both structured and unstructured cells. 22. The method a claim 1 further comprising the steps of (i) identifying a string having a cell that has a boundary with more than two cells of the string; and (ii) cutting the string to thereby form two strings. 23. The method of claim 1 wherein the cutting of the string is at the boundary having the lowest transportability value lying between the two cells that are touching each other and the cells and boundaries between such two cells in the one-dimensional string of cells. 24. The method of claim 1 wherein the linear equations of step (c) relate the state variables of a cell at the end of a time interval to the state variables of cells adjacent to it also at the end of time interval. 25. A method for predicting a characteristic of a physical system containing multiple fluids, comprising the steps of: (a) discretizing the physical system into a plurality of volumetric cells arranged adjacent to one another so as to have a boundary between each pair of adjacent cells; (b) assigning an initial estimate of state variables for each cell; (c) for each cell, constructing governing equations representative of the behavior of the fluids in the cell over a time interval, said equations using fluid and transport properties computed at the end of the time interval; (d) constructing linear equations by linearizing the governing equations; (d) associating with each boundary a transportability value and ranking the boundaries corresponding to descending transportability values; (e) using the boundary rankings to construct topologically one-dimensional strings of cells; (f) developing a matrix equation for each string by assembling the linear equations associated with the cells of each string; (g) obtaining improved estimates of the state variables of the cells by solving the matrix equation for each string, one string at a time, until all strings' matrix equations have been solved; (h) repeating step (g) until a convergence condition is satisfied, thereby obtaining state variables for all cells that simultaneously satisfy the linear equations for all cells; (h) using the results of step (g) to predict a characteristic of the physical system and the fluids it contains at the end of the time interval; and (i) performing steps (b) through (h) for a plurality of time intervals and using the results to predict a property of the physical system and the fluids it contains as a function of time. 26. The method of claim 25 wherein the physical system is a subterranean formation. 27. The method of claim 25 wherein the subterranean formation contains hydrocarbon fluids. 28. The method of claim 25 wherein the physical system comprises fluidcontaining facilities associated with production of hydrocarbons from a subterranean hydrocarbon-bearing formation. 29. The process of claim 25 wherein the fluid-containing facilities are surface flow lines and wellbore tubing. 30. The method of claim 25 wherein the results of step (h) are used to predict the pressure and saturation of a fluid in the physical system. 31. The method of claim 25 wherein the cells are finite difference gridcells and the governing equations are finite difference equations. 32. The method of claim 25 wherein the cells are unstructured. 33. The method of claim 25 wherein the cells are structured. 34. The method of claim 25 wherein the cells are finite elements and the governing equations are finite element equations. 35. The method of claim 25 wherein the cells are finite volumes and the governing equations are finite volume equations.