WELL CONTROL / INTERVENTION

Relief well drilling operations allow re-entry and control of a blowout well

Rather than performing dynamic kill operations on a blowout, a relief well was used to re-enter and isolate the problem well, thus averting the high risks associated with a surface intervention

David Barnett, Wild Well Control, Inc., Spring, Texas

fter an offshore well blew out during re-completion operations with a snubbing unit, plans were launched for drilling a relief well. Considerations that figured into the planning, and which are discussed below, include intervention options; structure and responsibilities of relief well team; hazard evaluation and contingency development; project planning, coordination and control; simultaneous relief well and platform operations; drilling operations; and proximity ranging and intercept methods.

Blowout Situation

A well in the South Marsh Island area of the U.S. Gulf of Mexico suffered a blowout during re-completion operations with a snubbing unit. The well flowed dry gas and large amounts of sand for about 24 hours prior to bridging. Well control specialists removed the damaged snubbing equipment and installed additional blowout preventers (BOP) to secure the well at the surface. Kill operations were implemented, but were largely unsuccessful due to damage sustained to the 7-in. production casing during the abrasive flow.

Following the attempted kill operations, the well continued to build pressure in the 7-in. production casing. A similar pressure build up was observed on the 10-3/4-in. intermediate casing. The pressure build up on the 10-3/4-in. casing had to be closely monitored and bled off in regular intervals to avoid the risk of a surface casing failure.

Relief Well Decision

After reviewing the intervention options, it was decided that a relief well would be used to re-enter the 7-in. production casing immediately above the perforations. After intersecting the 7-in. casing at approximately 10,000 ft, a window would be milled that would allow a tubing string to be inserted in the blowout well casing. The tubing string would be used to place a cement plug to isolate the perforated interval.

The factors that lead to the utilization of a relief well, as opposed to direct surface intervention, included:

• Sand bridges were present in the wellbore, but their depth and extent were unknown.
• If a shallow bridge was disrupted during direct re-entry, the well would have to be placed on low-pressure diversion to avoid over-pressuring the 10-3/4-in. casing.
• Significant amounts of sand production could be expected if the well was placed on diverter.
• Bridging above the damaged portion of the 7-in. production casing would almost certainly result in over-pressuring and subsequent failure of the 10-3/4-in. casing.
• The well was located on a platform with numerous other producing wells. Loss of surface control would pose a significant risk to other assets.

This article discusses the planning and operational aspects of the relief well project. In particular, the following topics are examined:

• Intervention options and decision making process
• Structure and responsibilities of the relief well team
• Hazard evaluation and contingency development
• Project planning, coordination and control
• Simultaneous operations (relief well and platform)
• Drilling operations, proximity ranging and intercept method.

A window was milled in the 7-in. casing at a depth of 9,300 ft (TVD) as planned. Almost 600 ft of 2-7/8-in. tubing was inserted into the casing and a 500-ft cement plug was placed above the perforated interval. This allowed plugging and abandonment operations to proceed without the risk of uncontrolled surface flow. Direct circulation was established between the two wellbores after removal of the sand bridges.

The relief well project provided a means to reduce the risks associated with direct surface intervention. The project required stringent directional planning and control in order to intercept the 7-in. target at the required incidence angle that would allow the insertion of the tubing string. A special mill was designed and built expressly for the purpose of milling the required window in the 7-in. casing. Geo steering MWD tools were used to provide the required near-bit directional control. Specialized proximity ranging tools were used to provide target well identification, direction and distance.

This type of operation provides a means to secure wells that have unacceptable risks associated with direct surface intervention.

Original Condition (Post Blowout)

The blowout occurred during completion operations and a gravel pack assembly (4-in. OD blank pipe and screen) was left across the workover BOPs after the well bridged. The well was secured, after bridging, with additional BOPs installed on top of the workover BOP stack, Fig. 1.

Fig. 1. A blowout that occurred during completion operations later bridged, allowing the well to be secured with additional BOPs installed on top of the workover BOP stack. However, as snubbing operations were implemented to re-enter the well, surface pressures became erratic, prompting these efforts to be abandoned.

Snubbing operations were implemented to re-enter the well. However, surface pressures became erratic during the re-entry operations and these efforts were quickly abandoned.

Pressure continued to build in the 7-in. casing, as well as the 7-in. x 10-3/4-in. annulus. Annulus pressures had been observed that were well in excess of the highest expected shoe strength. This led to the conclusion that the 7-in. x 10-3/4-in. annulus was closed off (probably by cement) between surface and the casing shoe. Constant monitoring and periodic venting were required to maintain the annulus pressure at a safe level.

Well control specialists were contacted and asked to provide recommendations for further actions.

Intervention Options

A thorough assessment of the intervention options was necessary to determine the most appropriate methods. The assessment required consideration of the various aspects of each intervention option, such as:

• Safety
• Probability of success
• Consequences of failure
• Operational / logistical aspects.

The options that were identified and considered suitable for further evaluation included:

• Snubbing operations to remove the gravel pack assembly
• Snubbing operations to re-enter through the gravel pack assembly
• Top kill (bullhead) operations
• Relief well intervention.

Some aspects of the situation were common to any attempted intervention:

• The depth, extent and competency of the bridging was uncertain.
• The pore pressure of the perforated interval was sufficient to over-pressure the 10-3/4-in. casing at the surface.
• The well could not be shut in at the surface until the hole in the 7-in. casing was isolated.
• There was no means to vent pressure from the 7-in. x 10-3/4-in. annulus, except through the small (11-3/16-in.) casing spool outlets.
• The well was located on a platform with 20 other producing wells.
• The adjacent wells and possibly the stability of the entire platform could be compromised by a shallow failure of the 10-3/4-in. casing.

A brief discussion of each intervention option follows.

Snubbing operations. The most expedient intervention method would be to re-enter the well with a snubbing unit. If a macaroni tubing string could be run to the bottom of the 7-in. casing, kill mud could be circulated and cement plugs could be set to secure the wellbore. The macaroni string would have to be run through the gravel pack assembly. This would require milling through the bull plug on the bottom of the gravel pack screen.

The other snubbing option would be to remove the gravel pack assembly under pressure. Once the gravel pack assembly is removed, a tubing string could be run to bottom and conventional P&A operations could be implemented.

The snubbing operations had significant drawbacks associated with them, most notably:

• Personnel would be placed on top of the well during re-entry operations.
• Disruption of the sand bridge(s) might cause sudden, sustained surface pressure that would have to be diverted through the 7-in. casing.
• Bridging above the hole in the 7-in. casing would cause full surface pressure to be exerted on the 7-in. x 10-3/4-in. annulus.
• The only avenue available to bleed the surface pressure from the casing annulus would be through the casing spool outlets, which could easily plug with sand.

Bullhead operations. An assessment was made of the advisability of bullheading kill fluid down the 7-in. casing. If surface pressure could be controlled for a sufficient amount of time, the gravel pack assembly could be removed. This would allow a snubbing unit to be employed for conventional P&A operations.

The problems identified with the bullheading option included:

• No means to ensure flow path down 7-in. versus down casing annulus
• Possible disruption of sand bridge(s) before adequate kill fluid introduced to keep surface pressure below burst of 10-3/4-in. casing.

Relief Well Intervention

The relief well intervention plan was deemed most suitable based on the assessment criteria. Whereas most relief wells are designed to facilitate a dynamic kill, this relief well would be used to isolate the perforated interval with a cement plug.

The logic used to select the relief well intervention included the following considerations:

• Safety – no personnel required for direct intervention at the wellhead
• Isolation of perforated interval provides protection against surface over-pressure / shallow casing failure
• Direct intercept and re-entry considered feasible with available technology
• Probability of success acceptably high
• Negative consequences only if early / unintentional intercept is made
• Dynamic kill can be instituted immediately if intercept causes surface flow.

The difficulties associated with the relief well option included:

• Precise planning would be required to coordinate the efforts between the relief well and the platform operations.
• Extraordinary directional control would be required to intercept the 7-in. casing at the appropriate depth.
• The angle of intercept would need to be closely controlled. Maximum incidence angle at intercept would be 3°.

Relief Well Planning

A project plan was developed to define the organization, scheduling and tasks associated with the relief well program. The project was organized into four separate groups, each with clearly defined tasks and objectives:

• Intercept operations
• Drilling engineering
• Kill (P&A) operations
• Hazard assessment.

A brief discussion of the tasks undertaken by each group follows.

Intercept operations. The Intercept Group was charged with planning the following aspects:

• Surface location
• Directional planning
• Surveying
• Proximity ranging
• Intercept methodology.

The first task was to determine a suitable surface location. Once the proposed surface location was chosen (based on safety and trajectory requirements), preliminary directional planning ensued while the Hazards Assessment Team confirmed the suitability of the location based on the shallow hazards survey. Once the proposed location was deemed safe, directional planning was finalized.

A relatively simple directional plan was developed that involved an initial locating pass at about 7,500 ft TVD. The relief well would then be brought around to follow the target and aligned prior to intercept.

Drilling engineering. The Drilling Engineering Group was placed in charge of developing the drilling program. This included customary casing design, cementing, mud program, etc. A preliminary drilling program was developed and issued for review by the other three operational groups. Modifications were implemented based on the information available from, or specialized requirements identified by, each of the groups. Following review, a final drilling program was issued.

Kill operations. The Kill Operations Group developed the procedures and identified the required personnel, equipment and services needed to implement the contingency dynamic kill, as well as the plugging and abandonment of the blowout well.

Hazard assessment. The Hazard Assessment Group was charged with identifying all surface and subsurface hazards and simultaneous operations procedures during well intercept.

Shallow hazard surveys (sparker) were performed to assess the presence of shallow gas accumulations at the proposed surface location and along the well path. Divers and ROV seabed surveys were performed to determine if any broaching was occurring under the platform.

A program was developed to provide the maximum capability to divert the blowout well in the event the flow resumed (either naturally or as a result of the relief well intercept). Additional BOPs and a 7-in. diverter system were installed on the blowout well. A remotely actuated water deluge system was also installed.

A simultaneous operations program was developed that included platform shut-in and de-manning prior to intercept, remote air and electrical power from a satellite platform and continuous monitoring of the surface pressures at the blowout well before, during and after intercept.

Relief Well Implementation

Drilling began after spotting and surveying the relative surface positions of the rig and wellhead platform, Fig. 2. No shallow gas was encountered during the top-hole sections and 13-3/8-in. casing was set at 4,075 ft.

Fig. 2. Drilling began from a jackup rig (background) after spotting and surveying the relative surface positions of the rig and wellhead platform. An initial target well location was determined from the proximity log at 7,628 ft TVD.

An initial target well location was determined from the proximity log at 7,628 ft TVD. The proximity tool indicated the target well to be 28 ft (±12 ft) at 165° (±15°) TN. Further drilling was required to increase the resolution of the proximity tool and to allow triangulation to resolve the relative distance.

The relief well angle was dropped through vertical and built back to start following the target well. Subsequent proximity logs indicated the target distance and azimuth to be:

• 8,686 ft TVD – 51 ft at 359°
• 9,134 ft TVD – 25 ft at 8°.

Drilling continued with proximity logs run as required to determine the trajectory of the target well and align the relief well with the target. Figs. 3 and 4 show the proximity results at 9,297 ft TVD. The target well is indicated 22-in. away (center-to-center, 14-in. edge-to-edge).

Fig. 3. Relief well drilling was guided by proximity logs run as required to determine the trajectory of the target well and align the relief well with the target.

Fig. 4. Proximity results are shown at 9,297 ft TVD. The target well is indicated 22-in. away (center-to-center, 14-in. edge-to-edge). Original surveys were used with proximity triangulation measurements to determine the target well trajectory, interpolate ahead of the relief well depth and align the two wells in azimuth and inclination.

The original surveys were used in conjunction with proximity triangulation measurements to determine the target well trajectory, interpolate ahead of the relief well depth and align the two wells in azimuth and inclination.

Once the two wells were within a few feet of each other, a Baker Inteq NaviGator geo-steering directional assembly was used. This provided critical near-bit inclination data to help make the final wellbore alignment.

Once the two wellbores were within 14-in. edge-to-edge and lined up within 3° combined azimuth and inclination, the 9-5/8-in. casing was set and cemented. Immediately after drilling out of the 9-5/8-in. shoe, the bit contacted the 7-in. casing of the target well. Contact was confirmed by marks and broken teeth on the bit when retrieved, as well as a final proximity log to confirm the azimuth orientation.

A specially designed mill was run with a drilling motor and steerable system. The mill was designed to make immediate penetration into the 7-in. casing and then center up on the casing wall due to the deep concave face, Fig. 5.

Fig. 5. A specially designed mill was run with a drilling motor and steerable system. The mill was designed to make immediate penetration into the blowout’s 7-in. casing and then, due to the deep concave face, center up on the casing wall.

Initial contact was made with the 1.5° bent sub oriented downward. This orientation was maintained while drilling approximately 5 ft and establishing good contact with the 7-in. casing. The drilling assembly was then re-oriented so that the bent sub was pointing toward the 7-in. casing and penetration was achieved.

The mill centered up on the 7-in. casing wall as expected and a 20-ft long window was cut out of the casing in about 1 hour. Metal cuttings were monitored with ditch magnets throughout the milling process.

Minimal mud losses were observed during the casing penetration and there were no pressure anomalies reported from the wellhead platform. Approximately 2,000 units of gas were observed on bottoms up circulation.

The drilling assembly was removed and a 600-ft, 2-7/8-in. tubing stinger was run on 5-in. drill pipe. The tubing entered the casing window without difficulty and was run 500 ft into the 7-in. casing. A cement plug was circulated into place and the tubing was pulled into the relief well casing. The top of the cement plug was tagged with the tubing after 10 hours WOC.

The relief well remained on standby, circulating inside the casing, and prepared to implement a dynamic kill immediately while the blowout well was monitored.

Results

Surface pressure at the blowout well began to decrease immediately after the cement plug was set. Surface pressure was 0 psi within 24 hours of setting the cement plug.

The relief well remained on ready status while a snubbing unit was used to remove the gravel pack assembly and perform conventional P&A operations. Direct communication was achieved between the two wellbores once the sand bridges were washed out of the 7-in. casing. The relief well was abandoned per U.S. Minerals Management Service regulations.

Conclusions

Most people associate relief wells with dynamic kill operations on blowouts that cannot be controlled at the surface. This relief well was used to re-enter and isolate a well that involved unusually high risks associated with surface intervention.

The planning methods and decision factors associated with this relief well were similar to many other relief wells, with added importance placed on directional control and simultaneous operations. The relief well intercept involved some risk of causing a resumption of the blowout flow. Thus, the relief well had to be designed so that a dynamic kill could be implemented upon intercept if required.

This project confirmed the feasibility of deep re-entry for plugging purposes. There are many circumstances that arise where this method of cased-hole wellbore isolation might be preferred. These include:

• Severely corroded or otherwise damaged wellheads that pose unacceptable risks to personnel and / or other assets
• Bridged off well with shallow casing failure
• Submerged wellhead (storm damage, ship collision, etc.).

Acknowledgments

This project represented a unique implementation of proven technology that can be used to reduce the risk associated with future wellbore isolations.

Special thanks to Vic Saucier and Carl Alexander (Drilling Consultants), Everet "Babe" Lee and Mickey Colvard (Baker Hughes Inteq) and the crew and management of the mobile offshore drilling unit Sam Noble for their dedication to the success of this project.

The author
	David Barnett is vice president of Engineering Services for Wild Well Control, Inc., a Spring, Texas, based company specializing in worldwide firefighting, well control and related engineering services. He has more than 20 years of drilling, snubbing, coiled tubing and well control experience. Mr. Barnett has been involved with the planning and implementation of numerous relief well and high-pressure snubbing and well recovery operations. He earned a BS degree in mechanical engineering from the University of Houston, and is a member SPE, API, AADE, ASME and IADC