Abstract

A large number of North Sea fields have been in production for many years, have matured and reached a plateau in total liquids production. The plateau is often a function of water processing constraints that limit the ultimate production rate. This total production rate often remains constant while the oil production rate declines, resulting in production of water far exceeding that of the oil. When this is applied over a number a fields it can quickly be determined that total discharges of water to the environment are increasing rapidly (Refer to Fig.1). The environmental impact of handling, treating and disposing of this water is a major headache and is testing the industry's ingenuity to come up with viable, sustainable alternatives to discharge. Pressure is also being applied by legislative bodies to limit the residual hydrocarbon content. As signatories to the Oslo Paris Convention, the UK Government agreed (on a trial and voluntary basis) with industry to a maximum oil residual of 30 mg/l for discharges on the UKCS. In the light of ever increasing discharge levels (tonnes/yr), it would not be surprising if additional qualities become legislated (e.g. COD and certain transition metal ion content etc.). Indeed, the potential banning of produced water discharges to the environment cannot be dismissed.

Wellbore separation and same-well disposal is fast becoming accepted by operators as an environmentally friendly tool that provides a unique opportunity to reduce operating costs and enhance the economic viability of higher water-cut wells (>65%), while simultaneously reducing the risk of pollution. The technology is inherently a low-risk approach as there is no adverse affect on well productivity, and tried and tested componentry is employed. It is expected that with time this technology will be considered as just another viable water management tool and will compete directly with shutoff technologies in the strategic handling of produced water.

The Downhole Oil-Water Separator (DOWS) system is able to produce oil to the surface while simultaneously disposing of water into a zone accessible from the same wellbore. This substantially reduces the environmental risk associated with

  1. spills and leaks at the surface,

  2. subsurface contamination of aquifers during casing failure in production and re-injection wells,

  3. chemicals used to treat the produced water and

  4. disposal levels.

The DOWS system has been developed using proven technology. Hydrocyclones, Electric Submersible Pumps (ESPs) and conventional completion equipment are all individually available off the shelf. This greatly reduces the technical risk thereby enabling operators to reach their financial goals via reduced operating costs, increased oil production and even increased recoverable reserves while simultaneously meeting their commitment to the environmental. Long term targets to reduce overall disposal and emission levels can be met.

This paper will focus on the Health, Safety and Environment issues associated with DOWS. It is intended to show that operators, through use of DOWS technology, will be able to handle the ever increasing water levels, while still hitting economic targets and simultaneously making tremendous strides towards the ultimate environmental goal of zero discharge.

Introduction

As water production rises and fields begin to produce more water than oil (WOR>1:1), field economics begin to deteriorate. The water ultimately restricts the level of oil production and can result in significant capital and operational expenditures. Capital expenditure normally means the installation of artificial lift facilities, though it can also include the later installation (retrofit) of additional water treating equipment and/or injection wells.

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