Treating Oil-Contaminated Seawater and Minimizing Backwash Effluent Discharge
Authors: Lynch Kevin , Evans Wayne 
Published: October 2008

The injection of oil contaminated seawater into a sensitive injection formation requires careful selection of process equipment. The treatment of backwash effluents from such plant pose many problems. This paper looks at how one such project deals with the problems using non-standard oilfield equipment to arrive at a robust solution.

Introduction

A gravity base platform allows storage of stabilised crude oil within the platform subsea tanks. Periodically when the subsea storage tanks are full a tanker is moored alongside and evacuates the crude oil. To maintain platform stability the discharged oil phase is replaced with raw seawater via a subsea valve. When crude oil from the separation train is sent to the subsea storage tanks the ballast seawater is displaced. This displaced seawater is re-injected as part of the water injection system.

The displaced seawater contains some oil originating from the subsea ballast tanks and it is the treatment and disposal of this ballast stream that presents the problem. Due to legislation in force the design basis specified use of the displaced water phase to provide water injection. Displaced water, or any of its consistent components, is not allowed to be returned into the sea even after treatment. The injection reservoir formation is relatively ‘tight’ and requires that injection water is treated to a high specification.

This creates three problems for the design of the water injection system:highly filtered seawater is required for injection into a tight reservoirthe injection process creates backwash effluent which is oil contaminatedthe volume of effluent water must be minimised because of the necessity for onshore disposal as hazardous waste.

Description of Water Injection System

Pressure maintenance is required to maintain the hydrocarbon reservoir pressure at the necessary levels to fully exploit the field development. This is achieved by seawater injection. The reservoirs are relatively tight and require a very high quality of water to prevent permeability decline in the injectors. The source of water for the water injection system is primarily the displaced water from the platform subsea tanks, with sea water supplementing the displaced water as necessary to fulfil the total water injection requirements.

Process Systems

There are 2 principal unit operations associated with a standard water injection system;media filters to achieve the necessary filtration requirementvacuum deaeration to reduce the oxygen concentration to levels required to minimise corrosion

The primary complicating factor was the handling of the displaced water stream and it's integration into the overall scheme. The treatment of this water for high quality water injection is difficult and presents particular challenges offshore.

Figure 1 shows a process schematic for the water injection system. The displaced and sea water streams are filtered separately and combined upstream of the vacuum deaeration unit. The water injection system delivers water at a rate sufficient both to maintain injection pressure and to dispose of any displaced water.