A laboratory test apparatus which enables one to investigate the scale plugging tendency of different sand control completions has been designed and constructed. The functionality of the small scale test set-up has been verified in a test program adjusted to the Tordis field conditions. The influence of the scale sensitive parameters formation water/sea water-ratio, flow velocity, sand grain size distribution and slot width on the performance of a single wire wrapped sand screen has been studied. Mixed scale depositions, with (Ba,Sr)SO4 as main component and in addition some CaCO3, SiO2 and NaCl, have been observed in the sand packs and on the sand screens in all tests, independently of the choice of the scale sensitive parameters. Screen plugging has been logged reliably by differential pressure measurements along the flow line. The inhibition efficiency of 2 commercial scale inhibitors and one novel combination inhibitor has been determined together with the scale dissolution performance of 2 commercial scale dissolvers. No substantial difference has been found in the inhibition efficiency of the scale and combination inhibitors performances. They retarded scale deposition, but could not prevent it over periods of more than 2-3 days. The tested scale dissolvers removed all mixed scale depositions on the screens completely within few hours. A combined scale dissolver and scale inhibitor treatment seems to be an effective method for preventing sand screen plugging caused by scale depositions.
Prepacked screens and gravel packs have become the most commonly used means for sand control in horizontal wells producing oil from unconsolidated sandstone reservoirs. Difficulties with effective gravel placement in extended-reach horizontal wells, flow reductions through the gravel packs caused by depositions of fines 4 or scale and need for recompletion operations may disfavour gravel pack completion within a cost reduction strategy. Sand screen installations are today regarded to be more cost effective in horizontal wells.
But a recent evaluation of 220 offshore wells with horizontal screen completion in the Gulf of Mexico showed a screen failure rate of 25% with partially and completely plugged screens. Another evaluation of completion techniques and production performance of 25 wells on the Statfjord field in the North Sea 4 showed that sand production strongly affects the oil production rate after water breakthrough. Water production is often the start of scale depositions. Even if a screen may originally be properly designed, scale deposition will change the screen slot geometry and can induce screen plugging. Many of the problems which have occurred with the first completion designs could have been avoided, if a realistic onshore test program had been performed before offshore installation.
A potential for scale precipitation is found in most oil wells in the North Sea and scale depositions can develop to a serious problem in open hole screen completions. In order to investigate the consequences of such depositions, a laboratory test program has been initiated with the objective of evaluating the susceptibility for scale depositions in and around single wire wrapped screens, and the severity of such depositions. A test set-up has been designed and constructed. A test program varying the relevant scale sensitive parameters has been carried out, and scale inhibition and scale dissolver treatments have been tested. This will give the basis for the selection of an effective scale prevention strategy.