This paper describes how an expandable drilling liner was used to isolate an overpressured gas zone in a well at a depth of over 13700ft. The liner was deployed for a Venezuelan operator in an onshore field. The liner was run below 9-5/8" casing and expanded from 6-5/8" to 9-5/8" thereby retaining hole size and allowing the well to be drilled to TD without a change in the original casing design.
The paper describes the planning involved in preparing the programme and how several new techniques were developed to ensure success in the field. Described in this paper is the selection of under-reaming parameters, development of cement placement techniques for expandable liners, expandable liner burst and collapse calculations and new milling & dressing techniques.
The application of expandable drilling liners to operations in the Asia-Pacific region could be considerable. Several applications have been considered, including offshore Vietnam, Sarawak and offshore Australia. The aim of this paper is to educate the region on the recent improvements that have taken place in other areas.
Geological uncertainty whilst drilling a well can lead to un-planned changes in the original well design. Often, the well will be designed to take account of such uncertainties by including the ability to set a contingency casing string, avoiding a reduction in the desired final hole size. This ‘contingency’ approach to well design leads to increased well costs as a result of drilling a larger well than is necessary. In modern, optimized well designs where costs have been trimmed to a minimum, contingency casings are often omitted from the well design. In deep wells, often traversing several pressure regimes, it is impossible to include contingency casings and still penetrate the objective with the desired hole size.
Traditionally, in circumstances where it is not favourable to set a contingency casing, a cement plug is placed across the problem zone in an attempt to mitigate the problem long enough to be able to drill to section TD and run the planned casing. However, this technique suffers from a number of drawbacks:-
The BHA is uncontrolled and tends to ‘kick-off’ the cement plug causing unwanted sidetracks. This is more likely to occur where the formation is softer than the cement.
The cement is drilled away by the bit and therefore the technique can only work if the hole is considerably larger than the bit size, or if the cement can be squeezed into the formation to a depth greater than the diameter of the bit.
The technique is only suitable for straight hole sections, as the drill out assembly will never follow the same trajectory as the original hole.
The technique becomes very unreliable if the length of the plug is greater than approximately 20ft.
The cement can collapse into the wellbore, re-initiating the problem.
Many have attempted to improve the basic concept of a cement plug by using techniques such as:-
Soft centre cement plugs, whereby the bit should preferentially drill the softer, inner cement, thus maintaining bit control. This technique requires very delicate control of cement placement and even if successful, the cement can still collapse over time.