Abstract
The use of systematic changes in flow rate in a retrograde gas well can reduce the formation damage caused during drilling or completion. When a formation containing retrograde gas is drilled with water based fluids that are incompatible with the native water, a process of permeability reduction occurs. The same process may occur when the completion fluid is incompatible and the formation is perforated overbalanced. When the well is put on production the near borehole region has a pressure below the dew point of the gas. In this scenario the flow rate turbulent effects may lead to the formation of an emulsion of the precipitated oil and the invading water which cannot be reabsorbed into the gas phase even if pressure is increased by a rate reduction. These emulsion droplets can nevertheless be dislodged mechanically by creating a fatigue effect in the capillary forces that bound them to the pore throats.
This paper describes the production behavior of a retrograde gas well in a consolidated sandstone formation in the Dnieper-Donets basin which was subjected to a series of choke changes in an attempt to reduce skin. Choke sizes ranging from 3 mm to 9 mm were systematically used over a period of 7 months. The theory behind the skin reduction and the selection criteria for the choke changes is presented. Results from the well show a dramatic increase of gas rate of more than 100% over the evaluation period, while wellhead pressure also increased by 33%. The prognosis has been validated by buildup tests.
The technology presented here may be used in wells which have suffered the formation damage described above. It requires almost no capital expenditures and if properly implemented may increase the flow rate and hence the net present value of retrograde gas fields.
