Technical and Economic Analysis of the Downhole Scale Risks and Uncertainties for Subsea and Deepwater Field Developments

Abstract

Scale control is conventionally achieved by the use of chemical inhibitors, introduced to the production system either by squeeze treatment into the reservoir or continual injection into the well.However, as new field developments encounter more challenging production environments or when the associated economic impact of chemical intervention is large (e.g. subsea completed wells with poor chemical placement by bullhead treatments, or limited access to wet subsea wellheads), other methods of scale control such as sulfate removal must be considered during the front end engineering design (FEED) stage for the development. However, the final selection of a fields scale management strategy relies on various technical and economic factors which themselves are controlled not just by the severity of the anticipated scaling regimes but also on the proposed field development plan.For subsea completed wells, especially those located in deepwater, the initial field development concept and drainage strategy can have a significant bearing on the costs ultimately associated with various scale management strategies and can, in extreme cases, significantly reduce the Net Present Value (NPV) of the project.The decisions taken at the concept selection stage for scale management in such environments are therefore crucial to the success not only of the chosen scale management strategy but also to the fields economic viability. This paper reviews the various challenges associated with scale formation and control in subsea and deepwater environments and the impact that different field design scenarios may have on the potential success of various scale management options. In particular, the paper will focus on technical, economic and risk-based analysis conducted for a number of new and proposed subsea field developments with different scaling challenges.The options for scale management in selected subsea and deepwater developments including examples from the North Sea (Shallow water), offshore Angola, and Gulf of Mexico cases will be reviewed.The impact of the various risks and uncertainties associated with determining the severity of the scale challenge (including the quality and source of initial water compositions, the accuracy of simulation tools used for the prediction of future scaling challenges prior to field production, the ability to effectively squeeze treat and place scale inhibitors into particular wells and the potential for changes in the field drainage strategy) are assessed in terms of the total costs associated with particular cases.In summary, the paper will describe the decision process used at the pre-FEED stage to determine a field scale management strategy for different development concepts.Detailed economic analysis of different options considered in recent field cases will be presented to support the different strategies adopted.

Successful Scale Mitigation Strategies in Saudi Arabian Oil Fields

“…When long horizontal or highly deviated wells produce from areas of high permeability contrast or with wellbore cross flow, coil tubing operations can often be the only manner in which effective chemical placement can be guaranteed. Associated costs are extremely prohibitive compared to conventional bullhead operations 6,7 . Therefore, it is very important to use proper squeeze models such as Place iT to optimize field treatment designs to improve and maximize field treatment lifetimes.…”

“…Therefore, the effect of the chemical placement on the squeeze life effectively becomes a moot point. Therefore, a simple and less expensive bullhead treatment was preferred to expensive spot treatment by coiled tubing 2,7,16 .…”

“…In most of the regions throughout the world (North Sea, West Africa, Offshore Brazil, deepwater Gulf of Mexico, etc.) sulfate scales, particularly BaSO 4 , is a significant challenge in the production environments 6,7 . CaSO 4 was a concern for SW flooding for many fields in Saudi Arabia 8 .…”

“…The scale inhibitor is squeezed or injected into the near wellbore region where the chemical reacts with formation rock and is retained by adsorption and/or precipitation with the chemical being returned in the PW by elution from the rock surface [1][2][3][4]7 . 2) Encapsulated scale inhibitor treatment if the producers have a "rathole".…”

Maximising Chemical Placement in Complex Wells

“…low permeability) zones. 8 In recent papers, we have described some of the results obtained in an ongoing Joint Industrial Project investigating the design and use of modified scale inhibitor squeeze treatments to effect self-diversion for scale inhibitor squeeze treatments (SISQ) in complex wells in the presence of heterogeneity, and other factors impacting placement such as crossflow and pressure gradients etc. 9,10 This work concentrates on the use of lightly viscosified shear thinning fluids in various stages (pre-flush, main treatment and overflush) of conventional bullhead SISQ treatments.…”