P.D. Howlett, Caledus Ltd.
Slimming down well geometry has been an elusive goal among operators to reduce construction costs for many years. The challenge has been to achieve the optimum size of pipe across the zone of interest while at the same time slimming down the rest of the well geometry, and still maintaining the desired size and configuration of the completion.
Prior attempts have not stacked up due to the additional detailed work required or loss of well productivity/functionality. However, a slender well construction system by Caledus may be a breakthrough. The system involves constructing oil and gas wells from close-clearance, flush-jointed cemented liners from top to bottom to form the entire well.
The new technology has the potential to compete favorably with other methods already being deployed and could be used in a wide variety of well types.
In its full well construction format of four or more close-clearance liners, the cost of constructing the well can be reduced significantly, perhaps more than 50%, while enhancing safety and reducing environmental impact. In its partial format of one to three close-clearance liners during a sidetrack or well extension, the available flow area across the zone of interest can be increased by 100% as a result of an increased tubular size deployed at total depth compared with convention.
Essential considerations
In the development of a slender well construction system it was considered essential that the system provide flexibility in well architecture options while maintaining optimum pipe sizes at the zone of interest. The main difference between a slim-hole design and a slender well design is reduction in the annular clearance between consecutive cased sections. Significant reduction in the telescoping geometry of the well means a wider variety of well architecture options exist.
The new slender well system allows annular clearances between consecutive cased liners to be as small as 1⁄8-in. radially in the lower reaches of the well and as much as 1⁄4-in. in the upper reaches. This annular clearance can be planned within the tolerances already allowable with API casing and the radial clearance will be a minimum radial clearance within these tolerances.
The benefits of reducing the casing sizes from top to bottom while maintaining the optimum size across the zone of interest are significant:
- Economic: Fewer consumables such as casing, drilling fluids, and cement; faster overall drilling; lower logistics; and reduced rig costs
- Environmental: Fewer cuttings and less drilling fluid disposal
- Reduced risk: Fewer big casing operations; reduced risk during transportation and handling
- Contingency: Additional casing strings may be run without affecting the final hole size. Additional liners may be spaced over trouble zones
- Bottom-up design: Allows the well to be designed for the required production/tubular without excessively large top-hole sizes
- Abandonment: Simplifies the well abandonment process due to the lack of overlapping casing strings and potential leak paths at the top of the well
- Well integrity: The ability to use API casing and normal cementing techniques means integrity is straightforward to engineer and plan
- Technical: Constructing wells from close-clearance liners reduces the telescoping effect in wells compared to conventional techniques.
There are technical challenges to overcome to deploy, cement, anchor, and seal each close-clearance liner in this slender well system. However, the equipment developed makes the system both safe and practical to deploy as already demonstrated by a full system test and successful field trial.
The most significant challenge when deploying and moving the close-clearance liners in the well is the potential for swabbing and surging due to the lack of annular flow area between the casings, especially when the new liner is inside the previous one. The development of a flow diversion shoe, artificial inner annular space created by an inner tubing string and deployment tool with internal by-pass, create an inner flow area for fluid to travel inside the liner, over the top of the liner, and around the outer diameter of the drill pipe deployment string.
The artificial inner annular space means that a path of least resistance is present to allow passage, by pipe displacement or circulation, of mud and any cuttings or debris that may remain in the wellbore. The outer annular space around the outside of the liner has not been sealed or shut-off, so fluid movement and flow still can occur dependent on the fluid properties and relative actual flow areas between inside and outside. As the liners are deployed on standard drill pipe connected to the inner tubing string via the deployment tool and connected to the flow diversion shoe via a retrievable seal stem and drillable seal bore, the ability to circulate to the end of the string for well control purposes is available always. There is at least one non-return valve in the flow diversion shoe to prevent back-flow up the inner tubing string and drill pipe deployment string. The system allows rotation of the complete string, while circulating and running into the well with the necessary fluid passing up the inner annular space. The flow diversion shoe can be equipped with a drillable or millable reaming type nose.
Once the deployment and potential for swab and surge have been overcome, the next significant challenge is to avoid high pressure and associated equivalent circulating density (ECD) changes during the cementing operation. This challenge is overcome by drilling and enlarging the open hole beneath the previous liner sufficiently to create an annular space around the liner equivalent to the standard acceptable space provided during conventional casing cementing operations. The enlargement is unlikely to be more than 20% of the previous liner pass-through diameter and can be achieved with bi-center drill bits or drilling with under reaming tools.
Centralization of the liner in the enlarged open hole can be achieved by deploying in-line centralizers on the liner with integral bi-center, non-rotating bow springs that allow passage through the previous casing but still centralize inside the open hole. A simpler inline centralizer design with straight or spiral blades can be deployed in the overlap between casings. The length of the overlap is determined by cement modeling simulation but overlap lengths between sequential liners of several hundred feet will be possible.
There is an engineered artificial annular space inside the liner during deployment to reduce swab, surge, and ECD, and there is also one on the outside during cementing. As each new string is deployed and cemented, the system calls for dedicated joints of casing to be deployed at planned positions in that casing string. These dedicated joints contain a controlled inner diameter to anchor the hanging system inside and to set a metal-to-metal end cap seal inside but still maintain the API drift diameter or drill through inner diameter of the casing they are deployed on. Below the controlled inner setting diameter is another controlled larger inner diameter that increases the flow area around the hanging system during cementing. This increase in flow area is necessary to reduce the ECD during cementing and to allow passage of any residual cuttings and debris from the open hole below.
The hanging and sealing system at the top of the liner is required to fit within the space of the API drift diameter of the previous casing and the next liner or between the drill-through inner diameters of these same casing strings. This is accomplished with a novel hanger system effectively built into the wall thickness of the casing being deployed. This hanger is anchored and sealed through hydraulic pressure applied at the end of the cementing operation. The anchoring is achieved by a dimpling process, controlled by the deployment tool positioned inside the hanger, radially displaces numerous circular disc slips embedded in the outer diameter of the hanger into the controlled inner diameter of the dedicated joints. Once hydraulically anchored, the deployment tool displaces a metal-to-metal end cap seal into the space created between the inner diameter of the dedicated joint and the outer diameter of the hanger.
To precisely locate the hanger system across the dedicated joints during cementing, then anchoring and sealing requires accurate space-out of the drill pipe deployment string. To avoid inaccuracy, a spring loaded dog locator tool is placed above the hanger in the drill pipe string to locate and cause an increase in tension indicated at surface on the weight indicator when the locator tool is pulled through the dedicated joints. This enables the operator to visually confirm the correct position in the well as well as by pipe tally.
Rotation of the close-clearance liners within system limitations is possible during installation of the pipe and during cementation prior to anchoring the system and setting the seal.
Editor’s note: This article is a condensed version of a paper presented at PennWell’s Deep Offshore Technology (DOT) conference in Perth, Australia, in December 2008.
