Monday, December 4, 2017

Wellbore Damage chapter 1

Scales, organic deposits and bacteria are three types of damage that can cause havoc anywhere, from the tubing to the gravel pack, to the formation pore space. Scales are mineral deposits that in the lower pressure and temperature of a producing well precipitate out of the formation water, forming a crust on formation rock or tubing.

With age, they become harder to remove. The treatment fluid depends on the mineral type, which may be a carbonate deposit, sulfate, chloride, an iron-based mineral, silicate or hydroxide. The key is knowing which type of scale is blocking flow. 

Reduced pressure and temperature also cause heavy organic molecules to precitate out of oil and block production. The main culprits are asphaltenes and paraffinic waxes. Both are dissolved by aromatic solvents. Far more troublesome are sludges that sometimes occur when inorganic acid reacts with certain heavy crudes. There is no known way of removing this type of damage, so care must be taken to avoid it through use of antisluding agents.

 Bacteria are most commonly a problem in injection wells, and they can exist in an amazing variety of conditions, with and without oxygen, typically doubling their population every 20 minutes or so. The result is a combination of slimes and assorted amorphous mess that blocks production. An additional reason for cleansing the well of these organism is to kill the so-called sulfate-reducing bacteria that live off sulfate ions in water either in the well or formation. Sulfate-reducing bacteria produce hydrogen sulfide that readily corrodes tubulars. Bacterial damage can be cleaned with sodium hypochlorite and it is as important to clean surface equipment, when injection water originates, as it is to clean the well and formation.

Two further types of damage can contribute to blocked flow in gravel pack and formation -silts and clays, and emulsions. Silts and clays, the target of most mud acid jobs and 90% of all matrix treatments, can originate from the mud during drilling and perforating or from the formation when dislodged during production, in which case they are termed fines. When a mud acid system is designed, it is useful to know the silt and clay composition, whatever its origin, since a wrongly composed acid can result in precipitates that block flow even more. Emulsions can develop when water and oil mix, for example when water-base mud invades oil-bearing formation. Emulsions are highly viscous and are usually removed using mutual solvents.


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