Multilayer carbonate reservoirs are often characterised by hydrodynamically isolated units that develop different formation pressures during production. These differences can result in wellbore and behind-casing crossflows. Conventional sublayer formation pressure measurements involve using wireline formation testing tool before production begins. Once a well is completed and onstream, it is difficult to determine the sublayer formation pressure.
An infill well recompleted as a deviated dual-string oil producer was producing from two strings, Unit A into the short string and Unit C into the long string, the well had an initial formation pressure of 4,200 psi. The aim of the diagnostics survey was to determine the current formation pressure in Unit C and to compare the results with wireline formation testing data which indicated that the formation pressure in Unit C was 800 psi below the initial reservoir pressure value.