The Use of Saturated Fracturing Fluid to Cope With Hydrate in Gas Wells Located in Ultradeep Waters

Abstract

A potential problem when fracturing gas wells in deepwaters is the risk of hydrate formation during the flow for cleaning the well. Some alternatives have been used, such as the pumping of alcohol (methanol or ethanol) in the WCT (wet christmas tree) down the chemical lines or the postponement of the cleaning for some months, counting on the gravitational segregation of the fluids in the reservoir. In some cases, these solutions are not enough. In these cases, a possible solution is the combination of a saturated fracturing fluid together with a saturated brine, what reduces the hydrate formation envelope. This solution can be used in conjunction with other providences, such as the use of alcohol or glycol, sub-surface chokes, etc The present work describes the steps for the development of a fracturing fluid based on saturated brines and the planning, execution and evaluation of fracturing jobs performed in gas wells located in water depths that ranges from 840m to 1950m. Introduction Deep water operations are extremely expensive. Normally rig costs, together with the production unity construction, are responsible for the major part of expenditures in the development of an oilfield. Because of this, minimize non productive time is always an important part of the oilfield developing strategy. Coping with hydrates problems can be extremely time consuming and, in most situations, extremely risky. Normally, melting hydrates in pipelines, risers or wet christmas trees involves the use of heat (through the use of steam, hot water or thermochemical reactions - Marques, 2003), pressure bleed off or the use of chemicals. When pressure bleed off is applied to just one side of the hydrate plug, it can be extremely dangerous, because it can create a huge differential pressure across the hydrate plug making it behave as a missile. Besides that, melting the hydrate can generate a volume of gas up to 170 times the original volume, making this procedure still more risky. The use of chemicals can be ineffective or even impossible, due to the need of contact between the inhibitor and the hydrate plug. All of these solutions are time consuming, what means expensive. In view of these complications, the cliché "To prevent is better than to remedy" is tailor made when dealing with hydrates. In order to prevent hydrate formation a great number of procedures and inhibitors have been successfully developed. Very recently, inhibitors for fracturing fluids had been introduced in oil industry, since cleaning a gas well after a fracturing job has a great potential to hydrate formation. Since the production of gas from fractured wells deepwaters is quite new in Campos Basin, these products were not available when 2 wells were fractured. Because of this, an in house solution had to be implemented, by using a fracturing gel prepared with saturated brines. The steps of this development will be detailed below. Concepts Review Hydrates Hydrates are crystalline solids, with external aspect very similar to ice, which are formed when a specific number of molecules of water create cavities around gas molecules, at specific conditions of pressure and temperature. An example of hydrates can be seen in Figure 1 (a-d), which shows a hydrate deposition over a tree-cap located at 863 m of water depth.

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“…Hydrates are crystalline solids like ice, constituted by a mixture of water and gas (light hydrocarbons). They are formed in conditions of low temperature and high pressure [ 21 ], under normal production conditions or, preferably, when there is an interruption in the well production or when production resumes [ 16 ].…”