Abstract

The current practice of overboard discharge of produced water from offshore facilities is becoming increasingly less attractive owing to environmental concerns and the impact of more stringent regulations. An alternative to overboard discharge is produced water re-injection (PWRI) into depleted reservoirs or into non-communicating aquifers. However, as offshore developments become more complex, with new fields producing through existing facilities, PWRI must face the problems involved in combining and re-injecting incompatible fluids and/or production chemicals. In this paper we describe a laboratory testing programme which was devised to aid in the development of a PWRI scheme for a North Sea facility. In the proposed re-injection scheme, produced waters from five fields are combined at a central facility prior to re-injection. Initial brine/production chemical compatibility tests identified potentially problematic incompatibilities. A series of formation damage core floods were performed to investigate the effect of these incompatibilities on injectivity into reconditioned reservoir core material from the proposed re-injection well. These tests were followed by further core flooding work in which alternative solvent-based strategies for the amelioration of incompatibility issues were investigated. From this sequence of tests emerged a PWRI protocol suitable for advancement towards field trials.

This paper describes typical problems encountered in the development of multi-well produced water re-injection projects using commingled waters produced from different fields containing potential incompatibilities both from the co-mingled brines themselves and also as a result of the diverse range of treatment chemicals present in the individual production streams. The strategies employed to overcome these problems in this field case are discussed.

Introduction

The Cleeton field forms part of the BP ‘Villages’ complex in the Southern North Sea. Though production from the Cleeton reservoir ceased in early 1999, the Cleeton facilities continue to be used to handle produced fluids from adjacent fields. Currently, all produced waters on Cleeton are processed prior to overboard discharge.

As part of the development of the Easington Catchment Area (ECA) fields, the Cleeton facilities are being converted into a transportation hub. Phase 1 of ECA involves production from the Neptune and Mercury fields. The hub requirements will then be expanded by the JUNO project, which will require services for a further five ECA Phase 2 fields. Figure 1 shows a schematic of the proposed ECA development.

To comply with UK regulatory limits for oil discharge and with BP Federal Goals of eliminating produced water discharge, and to design the facilities for limited access (simple process facilities), it is proposed that the produced waters from the new fields will be processed on the Cleeton platform and then re-injected into a depleted Cleeton production zone, along with produced waters from Phase I fields. This will entail the topside mixing of various produced waters containing a mixture of chemicals (scale inhibitor (SI), corrosion inhibitors (CI1 and CI2), kinetic hydrate inhibitor (KHI) and methanol), followed by injection of the mixture into the Cleeton reservoir. The different production chemicals encompass a wide range of potentially incompatible chemical species including anionic scale inhibitor molecules, cationic corrosion inhibitors (quaternary ammonium salts, imidazolines etc.) and polyvinylcaprolactam-based kinetic hydrate inhibitor. The potential for incompatibilities between the chemicals produced from the different fields and also brine incompatibilities therefore exists in a similar manner to that previously discussed[1,2]. In addition, very mild chemical incompatibilities may exist which would not be expected to be severe enough to result in significant production issues. It is recognized that such minor incompatibilities may have minimal impact on produced water reinjection into thermally fractured wells.[3,4,5,6] However, more significant reduction in injectivity may be recorded when considering re-injection of produced waters into low permeability non-fractured reservoirs, as in this proposed field example.

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