In recent years, BP has moved into reservoirs in deep water subsea projects where sea water flooding is required for reserves recovery. The introduction of sulfate rich seawater into a reservoir producing a formation brine rich in barium ions significantly increases the potential for barium sulfate scale deposition. This type of scale is not acid soluble, unlike the carbonate based scales traditionally encountered in many regions. Alkaline based chelants, such as EDTA and DTPA, are only effective at removing small accumulations. Mechanical removal is generally considered to be the only effective removal option for significant sulfate scale deposits in the tubing but is not appropriate for removing scale from within the near well bore area or within a frac-pack or screen. Thus the recommended management strategy is one of prevention rather than remediation.
A combination of the low geomechanical strength of these reservoirs and other rock characteristics has led to the adoption of frac-pack completions for effective sand control. This type of completion presents a new challenge for placing scale inhibitor and managing the scale risks in frac-packed wells. BP has significant experience of preventative squeeze treatments in sea water flooded reservoirs in the UK and elsewhere. However, the majority of this experience lies with cased and perforated completions. Unlike cased and perforated wells, mechanical removal of sulphate scale from behind a frac-pack screen or within proppant filled perforations is not physically possible with mechanical methods. The damage to the rock and the gravel or screen completion elements can be permanent. If the accumulation is significant and chemical dissolution is considered impractical then, ultimately, sidetracking may be the only option to recover productivity. The cost of restoring production to a scale damaged frac-packed deep water subsea well is very significant due to the high cost of intervention and limited rig availability. In addition, as most deep water subsea projects have a relatively low well count the impact of scale formation to a particular well has a greater impact on the overall production loss. Effective prevention is therefore critical. However, industry experience in squeezing deep water, subsea, frac-packed and multi-zoned wells is limited with uncertain treatment longevity. At the same time, subsea scale inhibitor squeezes, while shown to be effective, have a significant treatment cost.
This paper describes a particular aspect of the multi faceted scale management strategy adopted by our deep water subsea projects for managing wells with frac-packed completions. It describes the inclusion of scale inhibitor within the proppant pack at the completion stage to provide protection against scale related damage during the early stages of water breakthrough. The paper highlights the testing necessary to qualify the products selected and describes initial performance data from several of our major deep water fields.