An extensive study has been executed to understand the mechanism of calcite scale deposition occurring in the main heat exchanger at the MPP (main production platform) facilities at SVSA. The deposit necessitated a change out of the internals of the heat exchanger, which caused production deferment and capital expenditures.
The conclusion of this investigation was that the mechanism for accelerated calcite scale build-up in the HX heat exchanger is driven by the considerably high skin temperature (above 120°C at normal heating conditions) in a high pH environment. This latter is caused by the triazine, which is part of the H2S scavenger package (used to remove H2S from some of the sour gas production), and by-products originating from the reaction of triazine (an amine) with H2S. The CaCO3 scaling tendencies and indices as calculated by scaling software (ScaleChem, StreamAnalyzer) predict the adverse impact of the high temperature and alkalinity conditions encountered in the heat exchanger.
Several technical options to mitigate the effects of the scavenger were considered from an economic and operational perspective. The most effective of the options was the injection of an appropriate scale inhibitor at a suitable dosage upstream of the heat exchanger.
A multidisciplinary technical group composed of a development engineer, process engineer, chemist specialist, operational engineer and researchers came together, during an extended period of time, to understand the mechanism of calcite scale deposition occurring in the main heat exchanger at the MPP (main production platform) facilities at SVSA.
The schematic of the MPP facilities, showing the location of the heat exchanger, is shown in Figure 1. Oil from a large number of wells is collected at manifolds from where it is transported to the MPP. Two flow lines board the MPP and production is passed through two G/L separators.
The relevant operating conditions for normal production are summarized in Table 1.
Occasionally, spent acid from a well frac job will be flowing through the production system. Because the liquid stream on the MPP is collected from many wells, the returning spent acid is diluted with the normal production from the other wells. The spent acid flow rate is typically 1,000 blpd and a treated well takes a few days to clean out.
The liquid streams from the separators are combined and heated in a heat exchanger (HX) for handling. The flow trough the heat exchanger is 40,000 blpd with a water cut of 3%. Outlet temperature of the heat exchanger is 65°C. The skin temperature in the heat exchanger is considerably higher and can easily reach 120°C.
The heat exchanger HX, plugged progressively over a period of several months starting in early 2003 to a stage that the pressure drop over the heat exchanger caused operational problems at the MPP (unstable process). Due to the increased back pressure on the system, considerable production was lost as well (5% of total production per day).
To get back to the original production levels, it was decided to clean/replace the heat-exchanger tube-bundle at a cost of around US$250k and considerable production deferment (85 kbbl) in early June 2003. When the tube bundle was removed and examined, large accumulations of calcium carbonate deposits of various thicknesses were discovered on all the tubes.
Replacement of the tubes did not prevent the increase in pressure drop across the heat exchanger from reoccurring within the next few months. This lead to another shut-down in Jan 2004, with the obvious consequence in costs for SVSA.
To address this recurring problem of calcite scaling in the heat exchanger, an intensive study was performed to determine the root cause of the problem, to come up with different possible technical solutions. As a result of our investigations, we have been able to mitigate the scale build-up process by putting in place a chemical program which includes the application of scale inhibitor, use of monitoring tools, etc.
Looking further ahead, different long term solutions to avoid conditions that caused the scaling problems have been considered as well.