Changqing Oilfield has become one of the largest oilfields in China, with equivalent Oil and Gas output over 50 million tons per year since 2013. As a typical block with low permeability of Changqing Oilfield, Jiyuan oilfield has produced 6.7 million tons of crude oil per year. The main formations of Jiyuan oilfield are Yan'an formation of Jurassic and Yan'chang formation of Trias.

The injection water (IW) with sulfate is incompatible with formation water (FW) containing Ba2+/ Sr2+ and Ca2+. The mixing of incompatible injected and formation waters primarily leads to scale formation in the perforations, production casing, tubing, and downhole equipment of production wells and anywhere in surface equipment during oil production. Recently, the scale problem has been worsening as the watercut increases to 50%.

The typical scale and water samples during reservoir core flooding tests have been analyzed by inductively coupled plasma (ICP) spectroscopy, X-ray diffraction (XRD) and Scanning electron microscope (SEM) to make a better understanding of the scale formation mechanism. The scaling tendency of the mixing injected and formation water from several stratums has been predicted by using OLI scale software. To simulate the mixing of IW and FW within the reservoir, the IW and FW were injected into the reservoir core during the experiment, the results showed that the core permeability impairment is over 35%. The occurrence of scale affected core permeability and flow ability of the surface system.

To solve the scale problems of C8 block, nanofiltration sulfate reduction (NFSR) system has been carried out and scale inhibitors were screened. 5 sets of NFSR systems for injection water have been applied to remove high content SO42-. The results showed that this system decreased the pressure of injection wells at the beginning term. However, the long term effect of these systems are full of uncertainty and difficult to evaluate at current stage. The inhibition efficiencies of six chemical scale inhibitors(SI) have been tested, and the maximum inhibition efficiency reached 45% at the same time, the scaling time period was doubled. The new type efficient solidification scale inhibitor was recommended under the consideration of the cost of two scale control methods .

Subject
Scanning electron microscopy, Water, Permeability, Sulfates, Scale inhibitors, Reservoirs, Scale formation, Corrosion inhibitors, Oilfields, Equipment, Scale, Scaling, Oil
Keywords:
scale formation, mechanism, sulfate, injection water, scale inhibitor
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2020
Association for Materials Protection and Performance (AMPP)
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