Anti-Agglomerant Hydrate Inhibitors for Prevention of Hydrate Plugs in Deepwater Systems

Abstract

Abstract Anti-agglomerant low dosage hydrate inhibitors (LDHI's) give operators an additional tool for controlling hydrates in their systems. In contrast to kinetic and thermodynamic hydrate inhibitors, anti-agglomerant LDHI's inhibit hydrate plugging, rather than hydrate formation. Thus anti-agglomerant LDHI's allow hydrates to form but keep the particles small and well dispersed. Fluid viscosity remains low, allowing the hydrates to be transported along with the produced fluids. Emulsification of the produced fluids is not necessary; in fact, anti-agglomerant LDHI's have aided in demulsifying several black oil emulsions. In the laboratory, anti-agglomerant LDHI's have been shown to be effective up to 40°F below the hydrate formation temperature, at pressures up to 7000 psi, and for shut-ins up to two weeks in duration. In addition, extensive laboratory testing and formulation work have been performed to ensure that the anti-agglomerant LDHI's are compatible with system metals, elastomers, and treatment chemicals, and that the products will not plug umbilicals nor cause upsets in surface facilities. After a comprehensive laboratory evaluation, an anti-agglomerant LDHI was selected for field testing in a deepwater, Gulf of Mexico oil well. The product protected the system throughout the 1.5 month trial, including two brief shut-ins. This paper presents the results of the laboratory and field tests. Introduction The co-production of water with petroleum fluids leads to numerous problems that diminish the profitability of oil and gas production. Historically, the problems were primarily associated with corrosion, scale formation, and demulsification, but recently much emphasis has been placed on control of natural gas hydrates. Natural gas hydrates are ice-like materials consisting of individual gas molecules surrounded by "cages" of water in a crystalline structure. The thermodynamic stability of these structures increases as pressure increases and temperature decreases.1 While natural gas hydrates have been a nuisance to gas and oil producers for decades, it has only been in recent years with the ever-expanding emphasis on offshore production that the true extent of hydrate problems have come to light. Control of hydrates is now considered by many to be the number one priority for flow assurance projects for one simple reason: natural gas hydrates can completely block production in nearly every system which produces some water and experiences cold temperatures / high pressures. Traditionally, natural gas hydrates that can form in oil and gas production systems have been controlled using either thermodynamic hydrate inhibitors, such as methanol and ethylene glycol, or by insulating the system to remain outside of the hydrate region. Thermodynamic hydrate inhibitors, which also include salt, shift the conditions at which hydrates are stable to higher pressures and lower temperatures. Sufficient quantities of thermodynamic hydrate inhibitors are injected such that hydrates remain unstable throughout the system. Unfortunately, injection rates can be high (0.25–1 bbl methanol per bbl water produced for deepwater systems), leading to high capex and opex costs. Insulation leads to even higher capex costs, and while opex costs are significantly lower, thermodynamic hydrate inhibitors are still needed for extended shut-ins and cold well startups. Producers and service companies have thus devoted significant resources to looking for alternatives to traditional hydrate control techniques. Recently, kinetic hydrate inhibitors have been applied in place of or in conjunction with thermodynamic hydrate inhibitors.2,3 Kinetic hydrate inhibitors are typically water-soluble polymers that delay hydrate nucleation and / or growth. While kinetic hydrate inhibitors are cost effective in some fields, there is a concern that hydrates can form during extended shut-ins. In many cases, a secondary hydrate control mechanism (e.g., blowing down the flowline) is needed for extended shut-ins.