An aggressive design and testing program addressing a wide range of technical and environmental issues has been implemented specifically to optimize frac-pack processes in the Italian Adriatic sea region. Three formulations of viscoelastic surfactants have been developed to improve productivity and reduce environmental issues in these mineralogically immature (dirty), unconsolidated sandstone reservoirs. The first-applied viscoelastic surfactant (VES) carrier fluid used a cationic surfactant; the newly developed fluid systems use anionic and zwitterionic surfactants. Laboratory testing has focused on improving fracture conductivity, reducing formation damage, and complying with Italian environmental regulations. This paper describes the laboratory development and field application of an environmentally friendly VES system that complies with current offshore Italian regulations.
In formations with high clay content, filtrate from cationic VES fluids exhibited low retained permeability owing to surfactant adsorption on the clays. Methanol was added to these fluids to solve this problem; however, environmental constraints require the elimination of methanol. Recent laboratory tests have proven that the newly developed anionic and zwitterionc VES fluids achieve excellent permeability recovery after backflooding, without requiring methanol.
The laboratory study was also performed to optimize the VES concentration as a function of the desired rheological and suspension properties. The zwitterionic VES could also be formulated with heavy brines such as calcium chloride (CaCl) and seawater, thus providing a significant cost savings.
Results of this process confirm that environmentally friendly systems provide dramatically improved, cost effective fluids, while reducing the environmental impact of VES applications that use alcohols.
The paper also presents a field case incorporating details of execution and production results. The treatment process has been continuously reviewed to provide a more technical and environmentally acceptable service.
The gas-producing sandstones of the Adriatic sea region contain important hydrocarbon reserves but they also present a sand-control challenge with respect to the fracturing fluids interacting with the reservoir complex mineralogy.
The formations are turbidite sediments composed of interbedded sand, silt, and clay lithologies. The reservoirs are typically poorly consolidated sandstone characterized by a large shale/sandstone ratio in the pay zones. The commonly occurring shale streaks contribute to a high clay content (up to 50%), creating the sensitivity to certain injected fluids.
The production of dry gas in these silty sandstones is dependent on the relative permeability to gas and water and on the irreducible water saturation, both related to the percentage of silt in the sandstone.
The depth of these reservoirs varies between 800 to 2500 m. Production comes generally from a weak lateral water drive with good recovery. The average initial production of each layer is between 30 000 to 100 000 std m/d with a bottomhole pressure drawdown of 10 to 15% of the static value. The producing layers are also characterized by a large range in reservoir pressure (pressure gradient varying from 1.2 to 0.4 kg/cm/10 m) and low temperatures (35° to 55°C), which represent a challenge to the achievement of fluid cleanup after fracturing.