ABSTRACT
As offshore operators explore deeper waters and develop more mature oil and gas fields with complex well structural designs, new and different challenges are encountered that can impact production. Continuous downhole injection of chemicals is increasingly required to manage challenges, such as scale formation and corrosion within the wellbore completion components. Many challenges are met when designing and qualifying chemicals for continuous downhole injection via capillary line for such systems. Discussed herein are the physical properties production chemicals injected via capillaries should possess, including temperature stability, gunking, compatibility with process fluids, and efficiency. However, the main challenge and focus addresses the compatibility of neat chemicals with downhole jewelry materials. Two chemical qualification processes are discussed - a corrosion inhibitor and a combined corrosion- and scale inhibitor. The corrosion inhibitor is a synergistic blend of imidazoline salt and other surfactants, while the combined inhibitor is a synergistic blend of amine, terpolymer, and various additives. Both chemicals are tailored to reduce corrosion, while the combined inhibitor also inhibits mineral crystal growth. The products had to be tailored and qualified for capillary line injection with downhole parts made of UNS S41000, UNS N04405, and cemented tungsten carbide (91.6 HRA, 6% - 9% Co) as chemically wetted components at elevated temperatures. The chemicals have shown good performance and have satisfied defined requirements for safe deployment downhole.
INTRODUCTION
Downhole continuous injection systems (DCIS) via capillary line have been used since 1980s.1 However, until the last decade, they were not commonly employed due to the associated high capital expenditure (CAPEX) costs. The DCIS is a powerful tool within flow assurance that allows delivery of a mitigation strategy as close as possible to the origin of the challenge.2 Application of chemicals via capillary line enables mitigation of downhole challenges, such as scale formation and corrosion, within the wellbore completion components.
A typical DCIS via capillary line comprises facilities for storage, distribution and injection of chemicals into the process systems. Storage and part of distribution system (e.g., chemical skid, injection pump and injection tubing) is located topside, while the remaining distribution and injection elements are located subsea and subsurface.
A capillary line with a ¼? or ?? outside diameter is hooked up to a surface manifold (christmas tree) and fed through and connected to the tubing hanger (part of wellhead) on the annular side of the tubing. The capillary line is attached to the exterior of the production tubing and runs all the way down to the downhole chemical injection mandrel and valve. The downhole chemical injection valve (DCIV) is located at the entrance of the capillary line to the production tubing (Figure 1). The DCIV is usually located upstream of the downhole safety valve (DHSV).
The elements of the DCIS via capillary line are built of different construction materials, depending on the expected operational conditions. UNS S31600 or UNS S32205/S31803 are frequently used for chemical storage tanks and lines of the topside process facility. The main components in these alloys are Fe, Cr and Ni. These alloys are corrosion resistant to most chemicals at ambient temperatures. The capillary line is generally manufactured in UNS N08825. The UNS N08825 is a Fe-Cr-Ni-Mo alloy and is characterized with a very high corrosion resistance under most conditions. Other high-alloyed construction materials, such as UNS N06625 and UNS N07718, have similar corrosion resistance to UNS N08825. These are frequently used for the manufacturing of the capillary line or the downhole jewelry components (e.g., chemical injection mandrel and valve).
