Abstract

Down-hole scale control in Statoil's Åsgard field presents a particularly severe challenge. The field is producing from low permeability formations with a high draw down, through sub-sea templates. System pressure and temperature changes experienced during production promote the formation of carbonate scales, which have been identified in the near well bore area and perforated intervals. The high formation temperature and pressure, 165°C and 500 bars respectively, high chlorite content coupled to the water sensitive nature of some of the formations creates a challenging environment for downhole scale treatments.

This paper will describe possible formation damage mechanisms that were identified for this water sensitive formation and the treatment strategy adopted to minimize them. It also presents the extensive laboratory studies that were undertaken to develop a non-damaging, non-aqueous inhibitor/squeeze enhancer formulation suitable for deployment under these harsh conditions. An understanding of the mechanisms by which the squeeze treatment package prevented water block formation and clay mobilization under such severe temperature and reservoir constraints, coupled to the mechanisms of squeeze life extension are discussed. The paper will also highlight the manufacturing challenges that were overcome to improve the environmental profiles of the products.

The paper will finally discuss the two treatments, which have been performed to date with the non-aqueous products. Initial field data demonstrates that the application of this non-aqueous product package significantly reduced the potential for formation damage and allowed the successful treatment of these wells. Residual inhibitor returns remain significantly above MIC and a long squeeze life is expected for both wells.

Introduction

Oilfield scale is a water related problem and it is often associated with the production of produced water in the field once wells start cutting water. In the North Sea, typical scale problems are often related to the formation of BaSO4, SrSO4, CaSO4 and CaCO3. The severity of the problem and the most cost effective solution are highly dependent on the field operating conditions, which vary from mild to very severe scaling tendencies and from relatively simple low pressure, low temperature vertical platform wells to high temperature and pressure, complex subsea, multilateral and horizontal long reach completions. A number of remedial and proactive scale management tools are available including chemical and non-chemical based options. It is considered commonplace to prevent scale from forming in the system by injecting a chemical inhibitor continuously and/or by periodic squeeze treatments into the reservoir depending on the location of the scale deposition.

Åsgard is within the Haltenbanken group consisting of Heidrun, Åsgard and Norne. The Haltenbanken area is environmentally sensitive due to its plentiful fish reserves and licensees are faced with stringent guidelines for the use of chemicals. The field lies on the Halten Bank, about 200 kilometres off mid Norway and is made up of the Smørbukk, Smørbukk Sør and Midgard Fields. Åsgard is considered to be among the world's largest sub-sea developments producing oil, condensate and gas through two production facilities, Åsgard A and B from ten reservoirs spreading over an area of 60Km by 20Km. The production facilities include 50 wells in 16 subsea templates tied together by 300Km of 10 and 20-inch flowlines. Carbonate scale deposition has been identified in the near well bore area and tubing for some of the wells in Åsgard.

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