Measurement and Prediction of Hydrate Formation Conditions for Dry Gas, Gas Condensate, and Black Oil Reservoirs
Authors: Notz P.K. , Burke N.E. , Hawker P.C. 
Published: October 2008

ABSTRACT
Hydrate formation conditions were measured over a range of temperatures and pressures for a Dry Gas, a Gas-Condensate and a Black Oil that were in contact with their native brines. Measurements were repeated on the same fluids, but with hydrate formation inhibitors. A temperature/ pressure cycling technique was used to precisely determine the hydrate formation point for all but the dry gas system. Hydrate formation in the Dry Gas was measured by visual observation. Hydrate formation conditions were rigorously predicted using the hydrate packages available in four commercial simulation tools HYSIM, PROCESS, PIPEPHASE and EQUIPHASE-Hydrates. Good agreement was obtained between the experimental and predicted results.

INTRODUCTION
Gas hydrates are solid clathrate structures consisting of a small molecule encased within a shell of water molecules. Free water must be present for hydrates to form. Hydrates cause serious problems for the petroleum industry, since these compounds form at pressures and temperatures above the freezing point of water. Hydrates can form in production tubing, surface facilities, and transportation pipelines. Hydrates are of special concern to offshore production operations where cooler temperatures prevail.
The purpose of this study is to determine the conditions under which hydrates can form in reservoir fluids and to evaluate the performance of commercially available hydrate prediction programs. Recent research has shown that the formation of hydrates can be determined by monitoring the change in pressure as the temperature is varied for a constant volume and composition system.l The advantage of the above method is that recorded data is obtained instead of only visual observations. The present study uses this technique. Experimental hydrate formation data was measured on a Dry Gas, a Gas-Condensate, and a live Black Oil, both with and without the presence of a hydrate formation inhibitor in the aqueous phase. Hydrate prediction modules from the HYSIM, PROCESS, PIPEPHASE, and EQUIPILASE-Hydrates simulation programs were used to predict hydrate formation for the same fluid systems. The packages were evaluated for accuracy in predictions, applicability to production operations and ease of use.

THEORY OF HYDRATE FORMATION
Gas hydrates are solid clathrate structures involving a small "guest" molecule within a cage of water (ice) molecules. The guest molecules can be light hydrocarbons or compounds such as carbon dioxide and hydrogen sulfide. Free water or ice must be present for hydrates to form from an oil or gas system. Structurally, there are two types of hydrates, Class I and Class II. Class I hydrates encase nitrogen, carbon dioxide, and the small hydrocarbon molecules methane and ethane, while Class II hydrates contain propane and butane. The formation of gas hydrates is dependent upon the temperature, pressure, and composition of the aqueous and hydrocarbon phases. The literature indicates that butane is the largest alkane molecule which will form a clathrate with water.2-4 Solid clathrates can format temperatures much above the freezing point of water when the gas pressure is high, as is the case with petroleum production.