A real time electrochemical noise corrosion monitoring field trial was conducted at the Cahn 3 Water Treatment Plant in Lost Hills, California in late 1995. Five corrosion-monitoring probes were installed at different locations in the plant that had a history of weight loss corrosion between 0.5 to 450 mils per year (mpy). The five-element probes simultaneously performed Linear Polarization Resistance (LPR) and Electrochemical Noise (EN) measurements. Instantaneous and time averaged corrosion rates were calculated and recorded along with various other statistically relevant parameters for each probe. While absolute measurements of corrosion rates differed from weight loss measurements made on probe elements by factors of between 101 to 103 , the on-line corrosion monitoring system detected and measured changes in plant operation and process chemistry which impacted corrosion in the plant system. Problems encountered with this field trial of corrosion monitoring probes and systems included, probe fouling due to iron sulfide and sludge, data collection volume and awkward data manipulation routines for post collection processing of data. Modifying the probe electrode design reduced probe fouling, but regular cleaning was still required. The project showed that real-time corrosion monitoring in a production plant environment was feasible and provided valuable plant operating information.
INTRODUCTION AND BACKGROUND
The Lost Hills field is unique in that the oil is deposited in a diatomite formation that has a very high porosity, but little or no natural permeability [1 ]. Because of the low permeability, primary oil recovery was limited and the field was not significantly developed until a program of hydraulically fracturing the reservoir was begun in the late 1980's. Hydraulic fracturing is a process where an aqueous/complex sugar (guar polymer) gel with coarse sand is pumped into the well, fractures the formation, and forms a permeable path for oil to follow to the well. Due to the fracturing operation, the produced fluids contain fracture sands that accumulated 1 to 3 inches deep in the six o'clock position in the main lines coming into the Cahn 3 plant. The sand is conducive to microbiologic growth, especially acid producing and sulfate reducing bacteria that cause microbial influenced corrosion (MIC). Nitrates and some sulfates from the Tulare wellwater and the guar polymer from the fracturing operation provide a food supply on which the bacteria can flourish. In addition, the produced water in the field contains approximately 15 part per million of soluble iron which reacts readily with hydrogen sulfide and oxygen to form particulate iron sulfide (FeS). The main 8", 12" and 16" lines failed in 1993 due to MIC . Replacement costs for the failed piping were substantial. Chemical inhibitor costs in 1993 were substantial and problems persisted despite an aggressive chemical treatment program. In 1994, a pigging facility was added to the front of the plant to remove solids in the system. The net effect was to reduce the cost of the biocide treatment program by approximately two thirds. However, corrosion rates, as measured by weight loss coupons in the system, are highly variable, between 0.5 to 450 mils per year (mpy). Chemical treatment operating costs for Cahn 3 continued to be significant.
OBJECTIVES OF THE PROJECT
1. Implement EN and LPR corrosion monitoring techniques to investigate the process corrosion problems.
2. Determine the effectiveness of the corrosion inhibitors and biocides used.
3. Correlate changes in measured corrosion rate with plant operations or changes in plant conditions.
OVERVIEW OF PLANT OPERATION AND PROCESS FLOWS
The Cahn 3 wate