1. A method of operating a subterranean region model, the method comprising: identifying sub-matrices and entries outside the sub-matrices in a first band matrix, the first band matrix including flow variable coefficients based on governing flow equations of a model representing well system fluid flow in a subterranean region; for each of the sub-matrices, generating, by operation of one or more computers, a reduced-size matrix based on the sub-matrix and a subset of the entries that are associated with the sub-matrix; generating, by operation of the one or more computers, a second band matrix based on the reduced-size matrices; and operating, by operation of the one or more computers, the model based on the second band matrix.