Selecting Optimum Completion Strategy – Examples from Saudi Arabia's Unconsolidated Unayzah Reservoir

Abstract

Abstract The Unayzah reservoir in SA-1 field is highly unconsolidated with heterogeneous sequence of Early Permian sandstones saturated with condensate rich gas. Reservoir characterization based on the seismic, geological, and petrophysical analysis has indicated many stratigraphic units within the main Unayzah-A interval, which consists of well developed Eolian sandstones represented by Eolian dunes, sandsheets and closely associated inter-dune and lake deposits. Due to the unconsolidated nature of reservoir rock, conventional drilling and completion pose a very high risk of producing formation sand along with gas production, which can cause immense damage to production string, wellhead, surface flow lines, and gas plant equipment. In the early development stage of SA-1, frac-pack (F&P) was used and had been successful for several years. The method consisted of creating a short fracture, packing it up with proppant, and placing a screen as part of the completion system across the reservoir section. After years of producing from wells completed with this F&P completion, the scale due to fines migration buildups in the proppant within the annulus and causes high positive skin and deterioration of the screen, and consequently, needs a complete workover to regain lost potential. The workover for a F&P well is nothing less than drilling a new sidetrack as the completion cannot be de-completed and pulled out. After detailed study and modeling, it was decided to change this strategy and complete the wells with an expandable sand screen (ESS). The completions using the ESS were effective and successful in terms of trouble-free deployment of the equipment and sustained a long-term sand-free rate. The conventional sand screen (CSS) assembly has also been used in this reservoir. The initial test of this equipment conducted in one of the high producers was successful; however, long-term sand-free production as well as maintaining screen integrity cannot be ascertained. Among the disadvantages of CSS completion is the possibility of wellbore collapse on the screens with the depletion of reservoir pressure (changing near-wellbore stress environment) during production life of the well. In such case, the screen can incur partial or total damage and can no longer resist sand production. The paper describes in detail the reservoir characteristics and presents numerous examples of different completion methods implemented in the field to prevent sand and ensure high, sustainable flow rate. Long-term performance data are analyzed and presented in this paper to show the performance of sand screen technology.