The role of phase trapping on permeability reduction in an ultra-deep tight sandstone gas reservoirs

Abstract

As the first external liquids that are introduced into a reservoir rock, suitable drill-in fluids not only prevent phase trapping-induced permeability reduction (also referred to as phase trapping damage) caused by the drill-in fluid itself but also minimize any phase trapping damage caused by the invasion of subsequent working fluids. An ultra-deep tight sandstone reservoir in the Tarim Basin, NW China is considered as a case study, and this paper presents the results of an extensive series of phase trapping damage investigations that were carried out to determine the permeability reduction in the reservoir rock that comes into contact with different working fluids during the processes of drilling, drill stem tests, completion and well tests. Both the pore microstructure and surface properties were also investigated. The experimental results showed that the integrated phase trapping damage ratio (PDR) of water-based drill-in fluids (WBDF)-organic salt completion fluids (PDR = 0.99) was greater than that of oil-based drill-in fluids (OBDF)-organic salt completion fluids (PDR = 0.88). The analysis suggests that the OBDF is more effectively in inhibiting the absorption of the filtrate of the completion fluids compared with WBDF. A procedure for reducing phase trapping damage by OBDF is presented, and a numerical simulation model is developed to validate the procedure. The results could be useful in understanding and selecting the best drill-in fluids and completion fluids to minimize phase trapping damage in ultra-deep tight gas reservoirs.