A well (10) in a geological structure (11), the well (10) comprising: a first casing string (12a) and a second casing string (12b) inside the first casing string (12a) and defining a first inter-casing annulus (14a) therebetween. A wirelessly controllable valve (16a), in the second casing string (12b) provides fluid communication between the first inter-casing annulus (14a) and a second casing bore (14b). The first or second casing string are less than 250 meters longer in length than the second or first casing string respectively and may be the same length. The distal ends of the first and second casing strings may be in a substantially impermeable formation (11). A number of benefits can be realised from such an arrangement. For example, in the event of a “blow-out”, kill fluid may be introduced into the well bore without the need to drill a relief well.