Chemical Compromise: A Thermodynamic and Low-Dose Hydrate-Inhibitor Solution for Hydrate Control in the Gulf of Mexico
Authors: Szymczak S. , Sanders K. , Pakulski M. , Higgins T. 
Published: December 2007

Gas hydrate inhibition is a serious concern for operators who produce at conditions in the hydrate region. As production goes into deeper water, hydrate control grows in importance. Ideally, operators want a total hydrate control without the problems associated with Thermodynamic Inhibitors (THI) and/or Low Dose Hydrate Inhibitors (LDHI). A Gulf of Mexico operator experienced hydrate control problems in an 18,000 ft. umbilical line in spite of the addition of methanol as a thermodynamic hydrate inhibitor. In addition to erratic pressure, the high rates of methanol created a logistical problem as well as an HS&E concern because of the handling issues associated with methanol. This presented operational, logistical and safety issues.

Laboratory studies and previous onshore field experience had indicated that hydrate inhibition synergy is gained through the combination of thermodynamic inhibitors and LDHI.1 This is termed a Hybrid Hydrate Inhibitor (HHI). Because of the performance, logistical and cost drivers presented by the use of methanol, any alternative approach would have to consider those three factors. The performance had to do with hydrate dissolution in the event a hydrate formed during operations. A Kinetic Inhibitor (KHI) can prevent hydrates, but cannot dissolve already formed hydrates. The logistical had to do with pump sizing; i.e. conventional LDHI applications require new pumps and configuration. The cost of methanol is far less than specialty LDHI chemistry. Thus the objective of the study was to provide all of the benefits of the existing technology with improved performance, improved logistics and at a cost not to exceed hydrate prevention with methanol.

After a presentation of lab and field studies to the operator the method of application was approved for use. The differential pressure (Î”p) between the wellhead pressure chemical injection line) and the flow line pressure would serve as the key performance metric. There was a significant decrease of Î”p value after HHI product was introduced into the system. Initially, the HHI was applied at the same rate (and a much higher equivalent cost) as methanol. Rates were decreased in a stepwise fashion until the rates and daily costs were below the daily cost of methanol. On a cost performance basis the new product out-performed the methanol. Whereas the methanol rate had been 120 gal/day, the new product controlled line pressure at a rate as low as 12 gal/day. The HHI dosage was eventually set at 22 gal/day in order to compensate for potential flow and pressure/temperature fluctuations. From the logistical standpoint methanol shipments to the platform were decreased five-fold. This meant less cost and handling as well as a reduction in the footprint for product storage. From an HS&E position the potential for an incident was decreased in line with the reduction in boat trips and crane lifts.

The technology described here created a synergy that assuaged the concerns of operations, technical, logistics, HS&E and personel. After observing that hydrate dissolution is still possible at a lower dosage with less handling and at a comparable cost, the HHI treatment became a permanent hydrate prevention method. The project was a success with possible future expansion.