Abstract
A multiwell injectivity model is necessary to assess CO2 injection potential in subsurface formations. This necessity becomes almost ineluctable when CO2 is injected into low-permeability formations, which requires multiple injection wells in order to compensate a minimum injection rate for commercial projects. The pressure interference between the injection wells located in the same reservoir causes significant injection losses.
This paper examines well interference when injecting fluids with more than a single injector in a porous medium. Two cases of multiwell injection are studied; water injection and gas injection in a water filled reservoir. In both cases the aim is to examine the degree of well interference caused by the number of wells, formation permeability, well spacing, and total injection rate. A steady-state analytical model obtained using the superposition principle is used for the flow of a slightly compressible fluid of constant viscosity and compressibility in a rectangular homogeneous reservoir of uniform thickness, porosity, and permeability. The wells are represented by fully penetrating vertical line sources with a constant well spacing in a regular pattern. The model is used to calculate the required number of wells for given reservoir parameters and total injection rate. The results are compared with those obtained using single-well and multiwell numerical models.
Comparisons show that there is a good match between the multiwell analytical and numerical models for permeabilities higher than 10 mD. It is found that the single-well analytical model underestimates the well numbers compared to the multiwell analytical and numerical models for this range of permeabilities whereas for permeabilities lower than 1 mD it has a good agreement with the numerical model. It is argued that the reason for this should be well interference because the flow in low permeability media is relatively slow, causing less interference between the wells.
