Utfordringer med pustende brønner under boreoperasjoner

Abstract

Wellbore breathing, also known as ballooning, observed while drilling has often been mistakenly interpreted as an influx of formation fluid or the loss of drilling fluids. This misinterpretation leads to costly well control procedures that may make the situation even worse. Wellbore breathing effects are significant in High Temperature High Pressure (HTHP) wells because of the high mud pressure required to control the well, and the small drilling pressure window gives small room for error.

In order to avoid a dangerous well control situation, it will be crucial to be able to diagnose the breathing event as quickly as possible. There are a number of ways to identify a breathing well, using information gathered on the surface or by downhole logging tools. By reading the flow “signature” of the well during pump stops and interpret its signals, it is possible to make the correct action and thus prevent hazardous situations.

It is important to get a thorough understanding of the mechanisms and factors controlling the breathing phenomenon in order to be able to distinguish it from a real well control situation. The main mechanisms of wellbore breathing are usually quoted:

1. Expansion and reduction of the drilling fluid due to compressibility effects. This is usually the most significant breathing effect.

2. Elastic deformation on the borehole wall. This usually gives minor losses; the effect depends on the elastic properties of the formation.

3. Drilling induced fractures which are most significant in HTHP formations, where the ECD pressure can be difficult to control.

4. Mud flow into natural fractures which can give large breathing effects in natural fractured reservoirs.

A numerical model was built, in order to analyze a mud loss / mud gain event into a natural fracture. The model showed that:

. Increasing the length of the fracture will give a longer lasting mud gain / mud loss event, and the cumulative loss will also increase significantly.

. A higher pressure difference between the wellbore pressure and the formation pressure, will give a shorter mud loss / mud gain event, and increase the cum. loss.