A method for optimal well design, the method comprising: providing a computational model of a well, the computational model comprising a section of wellbore, surrounding formation, and a well completion arrangement (100, 100A, 100B) positioned between the wellbore and surrounding formation, the well completion arrangement (100, 100A, 100B) defining fluid paths between the surrounding formation and the wellbore, wherein the fluid paths provide an effective inflow area from the surrounding formation to the wellbore; and then a. calculating fluid production from the surrounding formation, through the well completion arrangement (100, 100A, 100B, 150A, 150B), to the wellbore; and b. varying the effective inflow area of the well completion arrangement (100, 100A, 100B, 150A, 150B); and repeating steps a. and b. so as to determine a profile of well performance for that computational model of the well based on a range of effective inflow areas of the well completion arrangement (100, 100A, 100B, 150A, 150B), and determining an optimal well design having a particular effective inflow area based on the determined profile; and wherein the computational model comprises a plurality of well completion arrangements (100, 100A, 100B, 150A, 150B), axially aligned and connected lengthwise; the plurality of well completion arrangements (100, 100A, 100B, 150A, 150B) comprising at least a first well completion arrangement having a first effective inflow area and a second well completion arrangement having a second effective inflow area, the second effective inflow area being different from the first effective inflow area; and the effective inflow area of each well completion (100, 100A, 100B, 150A, 150B) arrangement is individually varied in order to determine the optimal well design. providing a computational model of a well, the computational model comprising a section of wellbore, surrounding formation, and a well completion arrangement (100, 100A, 100B) positioned between the wellbore and surrounding formation, the well completion arrangement (100, 100A, 100B) defining fluid paths between the surrounding formation and the wellbore, wherein the fluid paths provide an effective inflow area from the surrounding formation to the wellbore; and then a. calculating fluid production from the surrounding formation, through the well completion arrangement (100, 100A, 100B, 150A, 150B), to the wellbore; and b. varying the effective inflow area of the well completion arrangement (100, 100A, 100B, 150A, 150B); and a. calculating fluid production from the surrounding formation, through the well completion arrangement (100, 100A, 100B, 150A, 150B), to the wellbore; and b. varying the effective inflow area of the well completion arrangement (100, 100A, 100B, 150A, 150B); and repeating steps a. and b. so as to determine a profile of well performance for that computational model of the well based on a range of effective inflow areas of the well completion arrangement (100, 100A, 100B, 150A, 150B), and determining an optimal well design having a particular effective inflow area based on the determined profile; and wherein the computational model comprises a plurality of well completion arrangements (100, 100A, 100B, 150A, 150B), axially aligned and connected lengthwise; the plurality of well completion arrangements (100, 100A, 100B, 150A, 150B) comprising at least a first well completion arrangement having a first effective inflow area and a second well completion arrangement having a second effective inflow area, the second effective inflow area being different from the first effective inflow area; and the effective inflow area of each well completion (100, 100A, 100B, 150A, 150B) arrangement is individually varied in order to determine the optimal well design.