Abstract

The injection of seawater into oilfield reservoirs to maintain reservoir pressure and improve secondary recovery is a well-established, mature, operation. Moreover, the degree of risk posed by deposition of mineral scales to the injection and production wells during such operations has been much studied. However, the current drive within the North Sea to reduce the environmental burden of production chemicals and to reduce oil discharge to the environment has focused attention on the challenge of produced water management and has introduced new challenges for scale management involving produced water re-injection.

This paper will outline the risk assessment process required prior to undertaking produced water re-injection. The factors that will be considered are the location of scale deposition around fractured and unfractured injection wells, formation damage potential and impact, and retardation effects on injected scale inhibitors.

The paper will draw upon computer modelling techniques, laboratory generated coreflood data, and field results that will demonstrate the impact of the following factors on long term water injectivity: viz, scaling tendency, suspended solids content, suspended oil content, injection temperature, reservoir type, and completion type. Furthermore, scale control measures currently being employed (e.g., scale inhibition, hydraulic fracturing, drag reduction, and solvent cleaning) will be assessed and reviewed against the risks identified. Finally, this paper will outline in detail the particular scaling issues associated with produced water re-injection for both platform and subsea facilities.

Introduction

Increased environmental concern for the effects of produced water discharges is increasingly encouraging operators to dispose of produced water by re-injection either into the oil-bearing formation or into a specially selected aquifer. In addition to the environmental benefits of produced water re-injection (PWRI), there are other potential benefits including making cost, space and weight savings through the optimisation of water treatment facilities and produced water re-injection system throughout the life of a field.

Re-injection of produced water is performed in several locations around the world. BP, for example, was an early adopter of the technology with re-injection schemes in Prudhoe Bay and the Forties Field in the early 1990's and the Ula Field in the Norwegian sector of the North Sea from the mid-1990's1–4. Today, BP has a corporate goal of eliminating all discharges to the sea by 2004.

Early experience of PWRI was focused on individual wells and did not include the co-mingling of produced water with seawater. With the move to full-field PWRI and the requirement to maintain voidage replacement, there is an increasing requirement to either co-mingle the fluids prior to injection or to inject both seawater and produced water into the same reservoir but via separate wells. Such practices introduce scale formation risks. These can be both calcium carbonate formation arising from the produced water itself and sulphate scales arising from the co-mingling of barium, strontium, and calcium containing produced waters with seawater. Clearly, the formation of such scales poses a risk to the topside injection system, the injection well itself, and finally the near-wellbore. Managing these risks is critically important to effective field management (in terms of being able to maintain adequate water injection) and to being able to maintain a zero water discharge commitment. This paper addresses methods of assessing the risk that scale poses to PWRI schemes and outlines the various management options that are available. The overall process and methodology is illustrated by field examples from the North Sea Basin.

Risk Assessment
Field Experience

Under favourable operating conditions, the risk of scale damage to produced water re-injection wells should not be significant. However, it is possible that under certain injection conditions rapid build-up of scale could lead to complete loss of injectivity. Avoiding such a catastrophic scenario is the objective of the risk assessment process. The preliminary step is to study field case histories. This is then followed by the procedure of calculating potential scaling scenarios using scale prediction codes and fluid flow simulations to evaluate the risk.

Field Experience

Under favourable operating conditions, the risk of scale damage to produced water re-injection wells should not be significant. However, it is possible that under certain injection conditions rapid build-up of scale could lead to complete loss of injectivity. Avoiding such a catastrophic scenario is the objective of the risk assessment process. The preliminary step is to study field case histories. This is then followed by the procedure of calculating potential scaling scenarios using scale prediction codes and fluid flow simulations to evaluate the risk.

This content is only available via PDF.
You can access this article if you purchase or spend a download.