Stabilization Of In-Situ Hydrates Enhances Drilling Performance And Rig Safety
Authors: Schofield T.R. , Judzis A. , Yousif M. 
Published: December 2007

In the Alaskan Arctic, hydrates can occur below the thick layer of permafrost. Some of the biggest hydrate accumulations have been found during the recent development of Cascade, a small field on the south eastern edge of the Milne Point Unit. They are found in two zones at around 2300' TVD and 3200' TVD. The hydrates sublimate into gas when drilled through, presenting a safety hazard. The need to isolate the hydrates has caused surface casing to be set almost immediately below them at around 3500' TVD.

Hydrates have been drilled on the North Slope for many years and a number of techniques have evolved to counter them by controlling fluid temperatures and pressures. These proved to be inadequate at Cascade and so BP Exploration looked at the possibility of chemically stabilizing the hydrates. After a series of experiments, the most promising candidate turned out to be Lecithin, a widely used clarifying agent in the food and beer industries. Lecithin is also used in a liquid form as an oil wetting agent in oil based drilling fluids.

This paper discusses test results and field data showing that Lecithin can help stabilize hydrates even when using mud weights considerably lower than before. The ability to cut back mud weights has saved rig time and reduced the risk of lost circulation. It has also allowed surface casing to be pushed deeper, saving the need for an intermediate string.

Introduction

Gas hydrates are crystalline solids formed by low molecular weight hydrocarbon gas molecules combined with water. The interaction between the water and gas is physical in nature and is not a chemical bond. They are formed and remain stable over a limited range of temperatures and pressures and exist in a number of structure types depending upon gas composition.

Conditions suitable for hydrate formation occur in three types of environments:Arctic areas where thick permafrost exists,deep ocean sediments where there are sufficiently high pressures to favor hydrate formation andshallow offshore sediments in Arctic areas.

As far as onshore hydrates in Alaska, most are known to be present in the Prudhoe Bay-Kuparuk Unit areas. They are in a series of sandstone and gravels interbedded with multiple thick siltstone units and contain predominantly methane (92–95%). In 1971, Katz predicted that gas hydrates could be stable at Prudhoe Bay up to depths of 3,940 ft. This was confirmed by ARCO and Exxon in 1972 when hydrate core samples were obtained at several depths between 1,893 and 2,546 feet from the Northwest Eileen #2 well. Gas hydrates in offshore areas are found in sediments in the Beaufort Sea, Bering Sea, and Gulf of Alaska. It is likely that the extent of areal hydrate occurrences in Alaska exceed 6,500 square miles.

The Challenge for Drilling Surface Hole Hydrates are a safety hazard to drilling operations because they become unstable under typical wellbore conditions and sublimate into gas. This is a problem because there are approximately 175 SCF of natural gas per cubic foot of hydrates. The gas influxes are neither rapid nor at high pressure and would not usually present a serious risk. Unfortunately, the hydrates are found in shallow formations above the surface casing point and before the BOP equipment is installed. This means that when gas from the hydrates enters the wellbore, it rises to surface with the circulating drilling fluid and cannot be isolated from the rig floor. Under these conditions, the presence of gas provides the potential for explosions and fires.

Since the earliest experience with hydrates on the North Slope, the need to minimize the amount of gas generated by maintaining relatively low temperatures and high pressures was recognized. Not only is the gas a safety hazard but when left uncontrolled the severity of the problem increases. P. 43^