Development of a Downhole Oilwater Separation and Reinjection System for Offshore Application

Abstract

Abstract Hydrocyclone-based systems for downhole separation of produced oil and water and subsequent disposal of the produced water by reinjection within the same wellbore have been successfully applied in a number of onshore wells. Benefits of these systems include reduced produced water lifting and disposal costs, as well as significant reduction in liquid load on surface processing facilities1. Reduction in surface equipment requirements is of primary interest in offshore applications, where significant reduction in the size and cost of surface separation facilities promises to extend field life and make marginal developments economically viable. Further development of downhole separation and reinjection systems is under way in order to provide a range of hardware systems suitable for a variety of offshore applications. Introduction Initial onshore installations of Downhole Oil/Water Separation (DOWS) systems were designed to prove the concept with off the- shelf pump and hydrocyclone designs, and included only minimal downhole instrumentation. Target wells were often of little value, with low oil production and very high water cut. The successes achieved using this approach have proven the viability of the system, and a significant number of systems have been installed based on their commercial merit.2,3,4 Failures to date have highlighted limitations of the initial hardware design and well selection criteria, indicating areas' where further development is required. Owing to the increased financial risk involved with offshore application, significant development has been undertaken to increase the success rate and commercial benefits of the system. An analysis of applications to date, including both successful and unsuccessful installations, has allowed understanding of the reasons for success or failure. Current and planned development activities include improvements in durability of the pump/separator system, improved hydrocyclone separation performance, improved application screening methods, application of downhole instrumentation and control systems, development of higher capacity systems, and development of completion designs for reinjection of water either uphole or downhole from the producing zone. DOWS Systems - Basic Configurations There are a variety of downhole separation systems in use today, including systems for gas/liquid, liquid/solid, and liquid/ liquid separations. A range of separator types are employed, including in some cases the wellbore itself as a gravity separator. This paper relates specifically to high volume (2,000 to 25,000 bpd) oil/water separation systems using centrifugal separators. Downhole centrifugal separators have also been employed with Rod Pumps, but these are not considered economic for offshore application and will not be discussed in detail. ESP DOWS. Fig. I illustrates the most common configurations where Electric Submergible Pumps (ESPs) are used. For the purposes of this paper, an ESP is defined as a downhole, multi-stage centrifugal pump, driven by a downhole electric motor. Production is drawn into the Injection Pump, which boosts the pressure and feeds the separator. The separator divides the liquid into two streams: a concentrated oil stream to be lifted to surface; and an oil-depleted water stream for injection into a disposal zone. The water stream flows through the injection tubing string, past an isolation packer, and to the injection perforations. The oil stream flows through a channel (normally consisting of 2 or 3 small diameter tubes) which routes the flow around the pump and motor, and to the production tubing.