The success of a pilot milling operation is dependent on the mill design, adherence to correct milling parameters and precise location of stabilizing members in the bottomhole assembly (BHA), especially while milling through large casings such as 20 inch inside 30 inch conductor. This paper discusses the importance of correct mill design and stabilization of the BHA, along with field results from milling with under-gauged mill and stabilizers.
Pilot milling interventions to facilitate open-hole side-tracking can be very effective and cost-efficient, especially in cases where retaining the original borehole size is necessary for further workover operations, for example, when liner is milled for this purpose. Pilot milling is a suitable option where sidetracking with a whipstock is not viable, as when casing has collapsed, with internal diameter restrictions, or situations where irreparable surface damage to conductor pipe and casing have occurred due to corrosion. Such situations might result in losing an offshore platform slot, which is a huge cost to operators.
One such situation was encountered where 30 inch conductor pipe parted at the water line due to corrosion. Prolonged exposure to corrosion further led to 20 inch casing parting at the water line as well. Surface repairs were attempted but were unable to arrest annulus leakage. In order to recover the slot, an improved and specially designed pilot mill was used. A stabilized milling bottom-hole assembly with precise sizes and locations of stabilizers was incorporated. This new mill design resulted in milling 585.6 feet of 20 inch casing with an average rate of penetration (ROP) of 2.6 ft/hr. The new mill design resulted in good mill life and only two mill runs were made in the entire milling operation. Results of torque and drag simulations to study the bending stresses and torsional stresses on mill string components while milling are discussed. Catastrophic effects of using under-gauged mill and stabilizers were also examined.
This improved mill and stabilized bottom-hole assembly design offers optimum ROP, improved mill life, reduced surface vibrations and a fine metal cutting structure that eases metal debris handling at surface.
For the first time in ADCO, and the UAE, a window was opened in Super 13CR 7 inch casing in well QW-XY. Extensive pre-job planning, two way communication between ADCO and Baker Hughes, and flawless job execution were the key factors for the success which allowed ADCO to immediately reach the reservoir, saving significant time and costs versus the typical window in the 9 5/8 inch casing. The main objective was to avoid cutting the window through the 9 5/8 inch casing to leave space for the ESP completion to be installed deeper for enhanced oil recovery and extending the well life. This also saved resources to drill 8-12/ section including OBM and 7 inch liner and its related accessories that were going to be required in case of the 9 5/8 inch casing window. The team decided to take the challenge presented in the stiffness of the Super 13CR material and the deviation at the window depth to save Rig days and resources. This operation utilized Path-Master whip stock and Silverback milling technology to successfully open the window and drill the required rathole in one trip. Another reaming trip was carried out to ensure seamless reentry with the drilling BHA later on. Teamwork and cooperation are the keywords that made this operation possible; with Baker Hughes technology and ADCO's innovation.
Many good well-construction-technology papers came out last year, as well as a couple on older technologies that either had not been published earlier or had been released and were underappreciated. Two papers on cement technologies offer improved, relatively easy cementing success. The paper on the new, larger-diameter, high-pressure/ high-temperature (HP/HT) casing valve offers new capabilities for reducing risk in some managed-pressure-drilling applications.
Stinger or Tailpipe Placement of Cement Plugs. Everyone knows how to balance a cement plug, right? Well, if you are using the same approach to balance plugs with a stinger as that used without a stinger, this paper does an excellent job of demonstrating why you are performing your stinger applications incorrectly. While this technology has been available in a couple of forms for several years, I think it is still underused. This paper does a very good job of illustrating and explaining in simple terms how we may be contaminating 40% or so of our cement plug from unintended overdisplacement.
Newtonian Fluids in Cementing Operations in Deepwater Wells. The first field applications of this technology were as early as 2000, but this is its first publication. This technology is simple and relatively easy to apply and is most beneficial in wells where one is trying to gain increased cement coverage farther up the annulus when fighting a narrow pore-/fracture-pressure window. Improved mud-displacement efficiency can be an ancillary benefit. The authors have done a good job of detailing some of the more recent deepwater Gulf of Mexico case histories.
New Casing Valve in a Deep-Well Sour Environment. In HP/HT managed-pressure drilling, many bottomhole-assembly (BHA) components are too large to be tripped through the rotating head. Opening the rotating head to pull large BHA components requires a delicate fluid balance between losing and gaining while completely out of the hole, a high-risk operation. The equipment and techniques described in this detailed case history could add a significant safety barrier for similar HP/HT managed-pressure operations.
Thanks to John Lofton, Chevron senior drilling advisor, for his help with selection of these papers. JPT
Recommended additional reading at OnePetro: www.onepetro.org.
SPE/IADC 168056 Experimental Assessment of Casing Expansion as a Solution to Microannular Gas Migration by Darko Kupresan, Louisiana State University, et al.
SPE/IADC 167956 Constructing Difficult Colony Wash Lateral With Innovative Rolling-Cutter Technology Improves Drilling Performance by Greg Bruton, Chesapeake Energy, et al.