Until now, the oil and gas industry has drilled a majority of the easy wells, with many complex wells left to drill. These largely hydraulically challenged wells have narrow pressure profiles that require more precision than conventional circulating systems can provide when managing the wellbore profile. Deploying conventional drilling methods to solve unconventional problems can result in exceeding authorizations for expenditure and even loss of the well. One operator had to plan for such a problem prior to drilling a new well in the Dutch North Sea. Before penetrating the target reservoir, Oranje-Nassau Energie B.V. (ONE) faced a daunting challenge that required unconventional means to mitigate any “unconventional kicks” at a shallow depth in the Zechstein Formation.
When drilling a sidetrack from the offset well, a severe kick was encountered in the Zechstein Z3 Carbonate Member. Conventional well kill methods proved unsuccessful in statically stabilizing the well. After circulating the well with the expected kill weight fluid, the bottomhole pressure (BHP) soon increased. Applying a higher-weight kill fluid again raised the BHP. The well stabilized only after finding a sweet spot circulating rate. Returns from the bottom of the well after shut-in showed a clear decrease in mud weight.
The encounter was interpreted into two important points. First, an accumulator effect in the overpressured body increased the pressure in the body with a higher hydrostatic column. Secondly, drilling fluid changed out with formation fluid in a highly permeable system at the bottom of the well. After conventional well kill options were applied, the bottomhole assembly (BHA) could not be pulled out of the hole of the unstable well. Consequently, running and cementing casing to shut off the flowing section was not an option, and the lower part of the drilled interval was abandoned.
Applying lessons learned
Applying these lessons learned from the offset well N05-01 helped to prepare for success in the new well. ONE carried out a thorough review of the potential overpressure hazards in the Zechstein Formation and then sought input from Weatherford on how to best drill its next exploratory well. The data from well N05-01 served as the main input for well N07-04. The collaborative effort consisted of well engineering analysis and risk assessment sessions to meet the objectives in a safe, efficient manner.
A large and relatively undisturbed platform served as the penetration point to decrease the chance of drilling into a fractured and thereby permeable body with potential overpressure. The recommended drilling fluid system included a salt-saturated water-based mud (WBM) rather than an oil-based mud (OBM). In an influx situation, OBM can rapidly lose its stability and carrying capacity. By comparison, a WBM can accommodate a much larger brine influx and allows treatment with chemicals to bring it back into its required specifications. Because of the potential of encountering the overpressured Zechstein Z3 Carbonate Member at an estimated measured depth of 2,630 m (8,629 ft), alternative drilling means were required to avoid tripping pipe out of the well while still managing well pressures in the event of a kick.
The review concluded that a combination of drilling-with-liner (DwL), managed pressure drilling (MPD) and continuous circulating system technologies would best mitigate and manage a potential kick scenario.
DwL involves the use of a liner equipped with a drillable casing bit and a liner hanger as a drillpipe extension, so that a single trip into the well can achieve drilling the hole, cementing the liner and setting the liner top packer. DwL eliminates tripping pipe because it does away with the conventional pilot hole, and the liner can be drilled and cemented in a single trip.
MPD enables precise control of the annular pressure throughout the wellbore. Operators are increasingly considering an MPD approach to maintain wellbore integrity in constricted envelopes. Time-saving, safety-enhancing MPD techniques are especially useful when drilling hazardous intervals like the Zechstein Group. Figure 1 illustrates the pressure profile of the N07-04 well.
In regard to this particular operation, MPD is a perfect complement to DwL because the DwL BHA is deployed on drillpipe and its rotating control head seals around the drillpipe. In this way, the problematic high-pressure interval can be penetrated in a single trip all while maintaining precise control of the annular pressure profile. If the overpressured Zechstein interval was not encountered, the plan entailed retrieving the DwL BHA and drilling the remaining 12¼-in. hole with conventional methods to the Silverpit Formation. At that point, the 9 5⁄8-in. casing could be cemented in place to facilitate drilling an 8½-in. hole to the planned well total depth. Figure 2 shows the steps taken to ensure a successful passage through the potential kick zone.
ONE and Weatherford established a partnership early in the new well planning stages to meet the objectives of drilling, running, drilling and cementing a 9 5⁄8-in. by 13 3⁄8-in. liner in a 12¼-in. by 12-in. hole through a hazardous interval in the Zechstein Group. The team blended engineering and evaluation disciplines to create and execute an effective DwL and MPD solution that included contingencies for unexpected events. The solution, integrated into the ONE drilling plan, eliminated uncertainties and provided risk mitigation to meet the intended well objectives.
The engineering analysis included formation strength evaluation to determine the optimum cutting structure for the 9 5⁄8-in. by 12-in. drillable casing bit, torque and drag modeling to verify the integrity of the liner assembly, and MPD hydraulics and influx simulation modeling while running, drilling and cementing the 9 5⁄8-in. by 13 3⁄8-in. liner. The planning phase included further contingencies to address various scenarios. For example, one contingency involved installing a continuous circulating system to account for a significant changeout of mud and formation fluid during connections. Stands with continuous circulation subs installed at the top would enable continued circulation while making connections. Other thorough assessments of adverse scenarios were related to the potential failure of the cementing operation, which could happen in several different ways. Examples of these include the failure of kick zone isolation by cement, failure of liner or cementing equipment, and loss of backpressure. All of these “what if” scenarios had to be solved because failing to isolate the kick zone could rapidly result in the loss of the well. To improve standoff for cementing and maintain the tangent trajectory, 9 5⁄8-in. by 11¾-in. rigid, heavy-duty centralizers were run.
The combined technologies were integrated into the operation in a seamless manner. The collaboration identified potential concerns and reduced them to a manageable level. Ultimately, the team used the 9 5⁄8-in. DwL system with MPD without encountering the overpressured interval. Therefore, they retrieved the DwL BHA, and the remaining 12¼-in. hole interval was conventionally drilled to the planned depth without incident.
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