Electrical submersible Pumps (ESPs) have been employed for decades in accelerating oil production and creating pressure head needed to produce sub hydrostatic wells. ESPs are installed downhole and comprise stages of impellers driven by electrical motors that are automatically cooled by the passing fluid.
A major disadvantage of ESPs is blocking access to well re-entry for surveillance and through-tubing intervention. One of such cases is during acid stimulation. In Petroleum Development Oman (PDO), acid jobs have been carried out as part of workovers to retrieve the pumps first. The cost implication can be tremendous ranging from $250,000 to $400,000.
Two deployment techniques were adopted in PDO recently by bullheading acid through ESP and Coiled Tubing (CT) into the reservoir and by so doing, saved 80-93% of comparative cost. The techniques involved the use of abrasive resistant ESP for vertical wells and ESP bypass (Y-Tool) for CT deployment in horizontal wells. The challenges of using these techniques include damaging the ESP with acid or mistakenly circulating acid via the tubing-casing annulus. We have been able to apply these techniques successfully in acidizing a sub hydrostatic carbonate reservoir. In one example, new vertical well was stimulated to enhance matrix withdrawal from 20m3/d (120bbl/d) to 128/d (790bbl/d) dry oil and in another example, a new horizontal well which was severely damaged by high viscous pill was stimulated via CT. These have opened new opportunity for matrix well withdrawal in this reservoir.
This paper presents examples of candidate selection, cost justification, pump selection and acid recipe. It also illustrates the innovative deployment techniques used to prevent pump & tubular damage as well as achieving proper acid placement into the reservoir. These examples can be treated as best practice worth replicating in all ESP applications worldwide.