Several offshore gas fields are present in Adriatic Sea (Italy), producing since the 60s. The gas is produced from multilayer metric sand reservoirs. The declining production in these mature fields is normally offset by drilling new deviated wells. Recent technology evolution shifted the focus from metric reservoirs to thinly laminated intervals (thin beds), until now not produced due to difficulties in indentifying gas bearing zones. This paper proposes a critical review of existing thin beds evaluation techniques and shows a successful case history using wireline formation testing technology.
While gas identification in metric reservoirs can be achieved with standard petrophysical measurements, thin beds are challenging since lamination thickness is half inch or less. Even the most advanced petrophysical logging tools struggle in providing discrimination of gas from water in this environment. Conventional pressure gradient approach also does not work, since thin beds often overpressurized and pressures are supercharged due to low mobility.
A new wireline formation testing approach for thin beds to discriminate gas from water zones was proposed, using a dual packer string with downhole fluid analysis capabilities, including fluid density measurement. Twelve stations in thin beds were attempted, all resulted representative in terms of fluid identification and pressure measurement. Productivity forecast of the tested intervals was also provided using as input pressures and permeabilities derived from dual packer tests.
The possibility to verify gas presence in zones with high uncertainties saved the cost of multiple well testings, optimized the completion strategy of the different reservoirs and allowed to increase the field production and reserves.
Several gas fields today producing from metric reservoirs will be revisited in the very near future in order to start production from thin beds, until now untouched. The wireline dual packer approach described in this paper, along with enhancement of petrophysical logs applications, will certainly play a key role in optimal exploitation of thin beds gas reserves.