Management of Scale Deposition in Deepwater Subsea Systems: An Offshore West African Case Study

Abstract Deposition of mineral scales in the wellbores and on production flowlines can cause loss of millions of dollars due to production deferment and equipment damages. This financial impact becomes greater in an offshore environment particularly for subsea assets. In a deepwater field located in West Africa, production comes from 11 sandstone reservoirs of varying reservoir properties. To improve ultimate recovery of the field, treated seawater is injected into the reservoirs via subsea injector wells to provide sweep to their paired subsea producer wells. Initial study of the field highlighted the need for scale inhibition at the topside due to the self-scaling nature of formation water and incompatibility of the formation and sea water at low pressures and surface temperatures. Thus, scale inhibition had always been done continuously on the topside to minimise the risk of scale deposition. During a routine pigging exercise of the subsea flowlines, scale was recovered unexpectedly, with large volumes that led to a stuck pig and higher than planned production deferment. Analysis of the recovered samples was carried out with the objective to understand the mechanism causing the subsea scale deposition, identify the exact location of the deposition and develop an effective subsea scale management strategy. The methodology for carrying out this investigation involved a detailed lab analysis on the scale recovered subsea, followed by a compositional analysis of the produced water of all wells on the impacted flowlines. The result of the compositional analysis was then used in a Scale Prediction Software to determine the scaling culprits. This paper presents the multidisciplinary approach used in analysing the problem of subsea scale deposition in an offshore asset using actual scale samples recovered during the pigging exercise. An insight to the deposition mechanism will be discussed as well as what led to the deposition. The paper also discusses the various experiments done to identify the suitable subsea scale inhibition deployed and the results obtained from the subsea scale inhibition. Furthermore, this paper also highlights recommendation to mitigate future subsea scale deposition.