Weatherford discusses how a collaborative approach helped in the planning and execution of a North Sea drilling with liner operation.
Fig. 1: Pore, frac, overburden and mud-pressures in the well. Images from Weatherford.
Historically, it has been standard practice to use conventional drilling techniques as the go-to method of choice for drilling challenging wells. However, in some cases, this can lead to major difficulties, such as catastrophic mud losses with resultant high-expense, non-productive time and even complete loss of the hole. Such a situation was facing an operator when drilling a new well in a mature field in the southern sector of the North Sea.
The target of this new well is the virgin pressured Carboniferous reservoir in 3300mTVDSS with a pore pressure of 14ppg EMW (equivalent mud weight). The Volcanics, a heterogeneous formation consisting of around 15 lava flow sequences, separates the Carboniferous from the by production depleted shallower fan carbonate reservoir in 2500mTVDSS with a pore pressure of only 1.7ppg EMW. Above the fan carbonates are overpressured shale formations requiring mud weights of around 14.5ppg to avoid well bore stability issues. Figure 1 illustrates the pressure regime in the well.
Drilling operations in 2012 were unsuccessful in casing off these highly problematic zones using standard drilling practices, including two unplanned sidetracks. All three conventional attempts resulted in major well bore stability problems and severe fluid losses, while not achieving the objective of the well.
Following is a brief breakdown of the three attempts:
Fig. 2: Casing seats in problematic zones.
The following lessons were learned from these three attempts:
As a result of these unsuccessful operations, the operator carried out a thorough review of the situation and then sought input on how best to approach the problem and achieve objectives, and they in turn provided well engineering and project management services to address the situation. This led to a collaborative approach between the two entities and an extensive evaluation of all the drilling problems associated with the well objectives in the over-pressured Jurassic section of the original holes. As a part of this collaborative environment a front-end engineering design study was performed, which resulted in the decision to drill the shale using drilling with liner (DwL) technology to mitigate the problems encountered. In this way it would be possible to set the liner in the top of the depleted reservoir and subsequently drill it with low weight mud to prevent or at least reduce total losses. This would require that a 11-3/4in. liner be set in the shales, safely above the depleted reservoir and a 9-5/8in.x 11-3/4in. DwL section drilled through the shale and set with the 9-5/8in. casing shoe into the top of the loss zone (Fig. 2).
Fig. 3: 9-5/8in. x 10-5/8in. Defyer DPA drillable casing bit.
This approach involved the use of a liner equipped with a drillable casing bit (Fig. 3) and a liner hanger, as a drill pipe extension, so that drilling the hole and setting the liner and top packer can be achieved with a single trip into the well. This method has proved to be a dependable way of achieving such an objective due to the following:
Early partnering between the operator and service company prior to the planning of the next attempt to achieve the drilling target resulted in a seamless amalgamation of all the engineering disciplines encompassed in the application of DwL technology being integrated into the drilling plan. This resulted in the formation of a detailed well engineering analysis, which took a holistic approach to the well objectives.
This analysis included torque and drag modeling to verify the mechanical integrity of the proposed liner system, special testing, detailed preparation of procedures and simulation modelling for running, drilling and cementing of the 9-5/8in. x 11-3/4in. liner as well as allowing for a contingency plan to run a 7-5/8in. x 9-5/8in. DwL liner in the event that the initial liner could not be drilled deep enough.
Fig. 4: Well schematic with all casings.
As a further contingency, in the event that the 7-5/8in. liner shoe is still in the loss zone or inside the shale section, a 200m length of 6-1/2in. expandable liner was sourced and made available. To prevent differential sticking in the permeable depleted reservoir all liners are equipped with special designed stabilizers even the expandable liner.
During the planning phase regular meetings were held and software to track the project scheduling was created to be sure that equipment deliverables would be completed in the required time frame to meet the various well objectives. Potential problems, drilling hazards and check points were established in order that both entities had a clear understanding of the project and its intricacies. This resulted in additional equipment being sourced and made available, such as surge reduction tools, auto-fill collars, underreamer, centralizers, etc.
The initiation of the above described collaborative endeavor commenced in Febuary 2013, and after a lengthy application engineering process, and some delay in the rig schedule, the operations finally began in early 2014. After re-entry the well was re-drilled from below the 18-5/8in. surface casing shoe. 13-3/8in. casing was set and 11-3/4in. liner had been set and cemented at a depth of 3273m safely above the loss zone. The 9-5/8in. x 11-3/4in. DwL was assembled and run in the well consisting of the following components:
The assembly was run and washed down to the 11-3/4in shoe at 3273m and then to bottom at 3282m and drilling commenced using 14.5ppg OBM to a depth of 3302m where fluid losses were encountered. Drilling continued to a depth of 3312m, at an average rate of 3.55m/hour at which point the liner got eventually stuck. Logs later confirmed that the DwL shoe was indeed inside the loss zone proving that the troublesome shales have been successfully cased-off. Some complications were encountered prior to releasing the hanger setting tool, due to high differential pressure between the work-string (seawater) and the annulus (14.5ppg oil based mud), and therefore the secondary release method was used – running and setting a wireline plug at 3163m and pressuring up to 5600psi to release it. A 9-5/8in. tie back string to surface was run and successfully cemented accomplishing the 9-5/8in. production casing string.
The lost-circulation zone was then drilled with seawater and 7 5/8in. liner was run and cemented 60mMD/30mTVD below bottom of this zone in a DwL configuration in case of any loss of hole diameter after drilling it, but no such problem was encountered. A successful formation integrity test to 15.5ppg was obtained proving the successful isolation of the loss zone.
However, continuous lost circulation issues appeared while drilling the subsequent 6-1/2in. section even after successful cement squeezes. This indicates that the upper part of the Volcanics is connected through fractures to the depleted reservoir formation.
In order to isolate these fractures, a 6in. contingency HydraSkin solid expandable liner was utilized and, due to the unexpected extended length that was required, an additional 260m of HydraSkin solid expandable liner had to be swiftly shipped to the location.
Meanwhile, the 6-1/2in. section was drilled and subsequently enlarged to 7.8in. with a RipTide drilling reamer. During the deployment of the expandable liner it became differentially stuck 230m above bottom, most likely in the additional section which could not be equipped with centralizers on short notice. The liner was expanded and cemented successfully in place, and the drill out of the expandable shoe track was achieved without problem. Because the solid expandable liner was set high there were still some fractures uncovered and therefore a 5in. conventional cemented drilling liner was installed which successfully cased off the remaining fractures. Finally the reservoir could be drilled, although with a reduced ID of 4-1/8in. and as a result a 1200m long 3-1/2in. liner had to be located, assembled and shipped to location all within a very limited time frame (Fig. 4)
The successful conclusion of this operation was an excellent example of an operator and service company working together to solve a major challenge and should be a model for future endeavors in difficult and hazardous conditions.