The intervention of sophisticated downhole tools, such as inflow control devices (ICDs) and inflow control valves (ICVs), has led to increased shear stresses being experienced in the wellbores near these devices. This results in an increased risk of scale formation and adherence even under mild levels of oversaturation - i.e. scale is likely to form as a result of the increased shear conditions. This can lead to situations where mildly oversaturated downhole carbonate scaling regimes result in scaling when they would otherwise remain “meta-stable” and not precipitate until further up the production system where they can be easily managed. Materials of construction and surface conditions can potentially have a critical impact on whether downhole scaling and productivity impairment is observed.
In this work, we have focussed on downhole carbonate scaling under new deepwater field conditions where the drawdown in the near wellbore area results in oversaturated brine systems. Downhole conditions have been modelled and closely simulated in the laboratory to ensure that test conditions match the mild level of oversaturation expected.
The work initially examines the relationship between increased shear and increased scaling examining bulk scaling (homogeneous precipitation) and the relationship between shear and surface scaling, showing that increased scaling can be expected at higher shear. More significantly the work progresses to examine several different materials commonly used in ICVs including different metallurgies, and surface finishes as well as selected coatings. Work at elevated shear conditions clearly shows the potential to minimise surface scaling by careful material selection and/or by the use of appropriately selected coatings.
In addition to tests designed to examine the affect of shear under conventional conditions (e.g. bulk tests and high shear rotating coupon tests), a pilot rig incorporating a mock-up valve has been designed and commissioned, allowing testing of the potential for scaling under realistic downhole flow conditions at representative temperatures and pressures and at test flow rates of up to 8 l/minute. Complementary results from these tests will also be presented in the paper to further illustrate the importance of surface conditioning in critical locations.