Gas hydrates are a growing concern in oil or gas production because gas hydrates can present flow assurance problems in onshore wells, in offshore wells and in pipelines. Almost any producing system may be vulnerable to disruption from uncontrolled hydrate formation. A new hydrate control called ICE-CHEK addresses the problem through chemical application, i.e. chemical solvents to dissolve hydrates and chemical inhibitors to mitigate hydrate initiation. Recent field examples testify to the performance of the product under a variety of applications.

Case 1. The new hybrid gas hydrate inhibitor allowed a Gulf of Mexico operator to reduce daily chemical use in a 3.5-mile (5.6-km) long subsea flowline from 120 gal of methanol to 20 gal of the hybrid inhibitor. The new inhibitor also reduced differential pressure from intermittent hydrate formation between the wellhead and the platform. Hybrid chemistry offered the thermodynamic inhibition properties of methanol while adding kinetic and anti-agglomerate functionalities. A cost-benefit analysis of the new process showed significantly lower lifting costs. Benefits included OPEX savings in reduced logistics and handling expenses, plus increased revenue from gas production due to the differential pressure improvement. The operator also improved safety by reducing crane lifts and other safety issues related to handling large quantities of methanol on the platform.

Case 2. A Canadian operator experienced gas hydrate problems in its wells and gas flow lines. Hydrates formed due to ambient thermal conditions as well as system temperatures and pressures. Historically, the operator pumped methanol downhole and into flow lines to inhibit and dissolve hydrate plugs. Although methanol injection allowed the operator to produce gas, significant pressure spikes indicated hydrate formation and dissolution cycles in the system. To address these problems and economic concerns resulting from the resulting inconsistencies, the operator switched to ICE-CHEK chemistry for gas hydrate inhibition. Although starting with the same feed rate as methanol, the operator eventually reduced injection rates until the cost for methanol versus the new product was comparable. The operator noted (and recorded) significant improvement; historical spikes were abated and the charts now reflect consistent well and flow line pressure.

Case 3. A deepwater operator in the Gulf of Mexico faced a problem concerning packer fluid selection. Due to density requirements and the need for hydrate inhibition at a certain interval in the completion string, commonly used packer fluid formulations were inadequate. Completion engineers consulted in-house chemistry experts and conceived a novel solution. By using hybrid, low-dose hydrate inhibition (LDHI) chemistry in the packer fluid formulation, the engineers addressed the density needs and the hydrate inhibition concerns with a new packer fluid formulation. Addition of 0.5% LDHI can reduce hydrate equilibrium curve by 4.5°F (2.5°C) and 2,000 psi in packer fluid. The completion was successful and well fluids were inhibited when the packer fluid was exposed to the production string.

Case 4. Wells in Canada with extraordinarily high levels of H2S (i.e., 35% vs. a few ppm) presented special problems for operators. Gas hydrate formation was just one of these problems. The conventional approach for hydrate control was methanol injection, but transporting and handling methanol presents unique logistical problems in remote areas of Canada. Having seen success with new hybrid low-dose hydrate inhibitors (LDHI) in other Canadian wells, the operator decided to apply the technology in its high concentration H2S wells. The hybrid LDHI inhibited hydrate formation while also exhibiting anti-corrosion properties. Whereas methanol can induce oxygen corrosion (due to the dissolved oxygen in methanol), the new hydrate inhibition chemistry inhibited and significantly reduced corrosion rates.

A production problem

Gas hydrates normally are found in cold climates, in deepwater environments, or at any point in a gas system where the gas experiences rapid expansion. Hydrates are frozen bodies in which a gas molecule, usually methane, is trapped within a water molecule lattice or "cage." As this lattice expands and gains mass, it can block the tubing, the flow line, the pipeline or any conduit through which produced gas flows.

As deepwater drilling and production increases, the problems associated with hydrate formation will increase. By the same token, as operators search for hydrocarbons in colder regions such as Siberia, Alaska and Canada, hydrates increasingly will become a cause of significant production problems.

Thermodynamics

To solve gas hydrate problems, the industry traditionally used thermodynamic chemistry to dissolve and inhibit hydrate formation. Gas hydrates offer two distinct problems for the scientists and engineers who design systems to mitigate the hydrate effect. The first problem concerns dissolution. When a hydrate plug forms, it must be melted to unblock the transmission conduit. For example, if a hydrate plug forms at the mudline in a deepwater completion, the operator must find a way to melt the ice plug in situ before production can proceed. The second problem concerns inhibition. The goal is to prevent hydrate formation in the first place. But to inhibit hydrate formation, the inhibitor must be present before a system reaches hydrate-forming conditions - and these conditions may exist in any low-temperature, high-pressure flow regime.

The traditional chemical approach to hydrate inhibition and dissolution has been to add sufficient quantities of a thermodynamic inhibitor to the production system. "Thermodynamic inhibition" refers to the chemicals' abilities to suppress the point at which hydrates will form. A thermodynamic inhibitor will lower the temperature at which hydrates form (at a constant pressure), but it may also increase the pressure at which hydrates form (at a constant temperature). By shifting the hydrate equilibrium toward higher pressure and lower temperature conditions, inhibitor chemicals make the water/gas system more resistant to hydrate formation. In either case a thermodynamic inhibitor remains effective as long as the produced fluid stays within a defined temperature/pressure regime.

Methanol is the most common thermodynamic inhibitor. Glycols, usually ethylene glycol or triethylene glycol, also are used for hydrate control. Because glycols are significantly more expensive than methanol, their use is usually limited to facilities that include a glycol recovery or regeneration system.

A new approach

In the 1980s and '90s the industry began searching for a new approach to hydrate inhibition. This search primarily was driven by concerns in the North Sea and elsewhere regarding discharge of methanol-containing water into the ocean because methanol is toxic to certain organisms. A hydrate research consortium based at the Colorado School of Mines set out to develop a low-dose hydrate inhibitor (LDHI). The first fruit of this effort was a kinetic hydrate inhibitor (KHI). In this case, "kinetic" refers to a time limitation for inhibition efficacy. Kinetic inhibitors work for a set time period determined by the concentration of KHI and temperature/pressure conditions. Conventional thinking was that hydrates form in a discrete interval of the production system. If a fluid could be inhibited during its time in that critical zone, that is all that is required.

Subsequent to developing the KHI products, scientists discovered a different LDHI mechanism. In the lab they learned certain LDHI products failed the kinetic inhibitor gas uptake test, but performed well in the dynamic loop test. Upon further investigation these products were characterized as anti-agglomerate (AA) inhibitors. The chemicals allowed hydrate seed crystals to form (this is what the gas uptake test measured) but disallowed lattice formation. This chemistry's benefit is best seen in a time vs. temperature graph. Instead of abruptly forming hydrates at a distinct temperature, gas streams inhibited with AA products form a slush and show a gradual increase in back pressure. This allows an operator time to increase product dosage and prevent a hydrate block from forming.

Risk limited early LDHI applications

At first glance the LDHI products appeared to satisfy all the operational considerations. LDHI products eliminated the methanol toxicity issue and inhibited hydrate formation long enough for production fluids to pass through critical hydrate-forming zones. However, the other critical problem in long-term hydrate control, i.e. hydrate dissolution, was not addressed. If the LDHI chemical pump failed, or if system conditions changed to exceed LDHI treatment parameters, then a hydrate block could form, and the operator would have to shut-in production until methanol could be applied to dissolve the blockage. Thus, although LDHI chemistries offered a new methodology for hydrate control, many operators balked at switching due to the risk associated with having no thermodynamic inhibitor in the system.

With the early LDHIs, the industry accomplished half the goal: to replace methanol as the primary product for hydrate inhibition. To address the other half, i.e. hydrate dissolution, scientists looked at formulations that created chemical synergy. At BJ Services Company, that work resulted in the ICE-CHEK product line.

Hybrid approach

BJ Chemical Services has developed new chemistry that combines the thermodynamic characteristics of methanol and glycol with the kinetic and anti-agglomerate inhibition properties of the newer classes of LDHIs. This technology has had beneficial effects on production, logistics, safety and overall lifting costs for several operators who are using it. The line of gas hydrate inhibitors easily handles the primary concerns about treating gas hydrates.

As operators explore and produce from wells in deeper water, and in colder and more remote areas of the earth, the economic and operational benefits of hybrid hydrate control systems will increase. Hydrate formation is a flow assurance problem for operators to manage. The new hybrid LDHI solutions are available. As evidenced by the case histories in this article, the new technology already has replaced and improved upon the performance and cost of methanol.