Development of shallow reservoirs using CAN-ductor
Introduction
The CAN well foundation concept was developed to improve the conventional top hole well construction process. It has the basic form of a suction anchor with an additional guide pipe. The installation is performed by vessel prior to rig arrival which provides the operator with more flexibility in planning and saves rig time. Furthermore, it enables pre-installation of the conductor, or can act as a guide to ensure vertical installation of the conductor by jetting or driving (top or toe). The CAN also acts as the primary support for the BOP and subsequent loads induced during the wellbore construction (axial, lateral as well as bending loads), in addition the risk of unstable conductors due to poor primary cement jobs or lack of verticality are mitigated.
The first installations were undertaken by Eni Norway in 2006 in order to enable safe jetting of conductors for their deep water wells Gemini in 1100 m and Cygnus in 860 m water depth. As the well loads were to be taken by the CAN, the use of short conductors (3 and 2 joints only respectively) plus drill-ahead features of the jetted conductor running tool enabled hitherto unmatched conductor installation efficiency at these water depths; both were installed within 24 hours each. This compares favourably with the average conductor setting time in deep-water (> 800 m) at that time of 7.4 days for comparable wells (this average included all 18 previously drilled deep water wells, of which several had to be respudded, resulting in significant cost overruns. Arguably, the average time has dropped since, to typically 2.17 days as reported on the NCS for medium water depths). Further, risks related to conductor stability, verticality and load carrying capacity were also fully mitigated.
Based on this novel and successful method of spudding a well, the concept was further developed into a stand-alone, proven technology as of today, with 15 installations to date in water depths ranging from 125 to 1444 m.
The purpose of the third CAN application was to ensure best possible conductor to soil seal for the very shallow gas reservoir Peon (Statoil), with top of reservoir only 170 m below seabed. Just relying on cementing the conductor in place was evaluated as being a significant well integrity risk. This was resolved by first installing a CAN, through which a conductor was driven with a subsea hammer, using the same installation vessel for both tasks. This solution offered a pre-installed conductor and a perfect soil-to-conductor seal which ensured well integrity.
These projects established the CAN as both a problem solver and cost saver for soft seabed and conductor fatigue related problems (Sivertsen 2011). However, as a result of further technology development/extension, new areas of CAN applications evolved, which has resulted in 15 installations, with the most recent application for Wisting Central II as an example of the CAN technology potential. The background objectives, preparations as well as operations execution of this unique well are discussed in further detail below.
CAN Design
To ensure a successful deployment of a CAN or suction anchor, location specific soil property information is needed. This geotechnical information has to be analysed and used as a basis for calculating the achievable CAN penetration as well as the achievable load bearing capacities of the CAN.
As the well to be drilled was an appraisal well and would be removed after logging and production testing, the well foundation was to be rented for the duration of the well. For this project CAN units of 6 m diameter were on stock in different heights ranging from 7.5 m to 11 m.
Conductor Integration
The integration of the conductor was done in the workshop prior to shipment to the mobilization port. Fig. 1 shows a sketch of the CAN with integrated conductor and 36 in. LPWHH (low pressure wellhead housing) as installed on the seabed. The integration of the conductor is based on the following construction steps (in reference to Fig. 1):
· Basis is a standard CAN design with guide pipe (44.5 in. ID).
· The conductor anchor is welded to the guide pipe at the lower end. This element is designed to transfer 350 ton from the conductor into the guide pipe.
· A centralizer is mounted near the top of the conductor. This centralizer fixs the position of the conductor until the cement is in place.
· The conductor is lifted into the guide pipe and welded to the conductor anchor at the bottom.
· The annulus between the guide pipe and conductor is filled with cement. The cement provides an axial load capacity of 70 ton/m, resulting in about 700 ton over the cemented height of the conductor. In combination with the conductor anchor the total axial load capacity is therefore 1050 ton.
This integration method was chosen for simplicity and has the following advantages:
· No hot work on or close to the conductor housing, therefore no introduction of new hot spots that would compromise the fatigue life of the wellhead system.
· The conductor is welded at the bottom where only a few percent of the loads are remaining.
· Short lead time as all elements are standard off the shelf components and will fit all subsea wellhead supplier wellheads.
· The conductor integration is done within a couple of days in a workshop. This ensures both, an efficient work process and a controlled environment.
o Note: This design is slightly heavier than a standard CAN due to the added steel and cement weight, however the total weight of the structure is easily handled by supply bases as well as installation vessels. The total weight of the structure as shown in Fig. 1 is 93.5 ton versus 80 ton for a standard CAN of the same dimensions.
Figure 1 – Section through CAN "as installed".
Fig. 2 shows the detail with the top of the conductor cemented into the CAN, Fig. 3 shows the assembly while deployed through the splash zone.
Figure 2 – Conductor integrated in to CAN
Figure 3 – CAN deployed through splash zone
Guide Post Receptacles
The CAN has guide post receptacles integrated into the top. This further contributes to cost savings and simplifications of the rig operations as no guide base is required to be run.
Geotechnical Assessment
In preparation of the installation of a CAN (as well as for a suction anchor) a geotechnical assessment of the seabed in which it is to be installed has to be performed. The main results of this assessment are to determine the minimum dimensions of the CAN to achieve the required load capacity for the well and also to verify that the required suction pressure is within the structural capacity limits of the CAN. The most critical parameters for these calculations are the undrained shear strength and the clay sensitivity. The first parameter is determined by the cone penetration test (CPT), the latter is only available by analysing core samples, however these are rarely available for exploration or appraisal wells. The literature gives a range of values for this parameter, which is selected conservatively and define the lower and upper bound cases.
Preparation of Input Parameters
The available CPT locations for the well Wisting Central II were unfortunately quite far from the spud location and showed a great deal of variations. Fig. 4 shows an overview map over the Wisting area with wells (drilled as well as planned) and CPT tests taken. With reference to Fig. 4 and Fig. 5 the input data can be evaluated as followed:
· CPT 10 is the closest test set located about 2.8 km east-north-east from the spud location and shows favourable soil conditions with an Su around 50 kPa at 10 m depth.
· CPT 03 is located about 3.5 km north from the spud location and shows an increased strength formation coming in at 2 to 3 m below mudline. This would stop the CAN penetration quite quickly if present at the installation location. The height of the structure would have to be adjusted to cope with this special soil condition
· CPT 09 is located about 4.6 km east of the spud location and shows favourable soil conditions with an Su slightly below 50 kPa at 10 m depth similar to CPT 10
· CPT 06 is located about 5.3 km north of the spud location and shows an increased strength formation coming on at around 12 m below mudline
· CPT 04 is located about 6.5 km north-north-west of the spud location shows a moderately increasing strength formation coming in at around 2 m below mudline
Evaluating all the data above results in the uncertainty about the depth of the increased strength formation in the north-western area (later addressed as Unit II). Therefore a geotechnical correlation based on shallow seismic was undertaken to assess whether this layer is relevant for the geotechnical assessment. In addition two scenarios where such a consolidated layer comes in were investigated:
· Unit II coming in at 8 m below mudline
· Unit II coming in at 6 m below mudline
Figure 4 – Overview map Wisting area (well and CPT locations).
Figure 5 – CPT results for Wisting area.
Geotechnical Correlation
This correlation was done by a 3rd party provider, so only the results are mentioned in this work. Fig. 6 indicates the presence of a horizon at a depth of 27 m, which represents the border between Unit I and unit II. Based on this information and a CAN height of 7.5 to 11 m it could be concluded that this hard layer would not be a challenge for the installation.
The second part of the correlation was to derive the input parameters at the spud location based on the reference wells which concluded as shown in Table 1.
Unit Depth Sensitivity Unit Weight Undrained
Shear Strength
[m] [] [kN/m3] [kPa]
Unit I 0.0 1.75 8.0 6.0
27.0 1.50 8.0 90.0
Table 1 – Shear strength profile.
Figure 6 – 2DHR seismic line section - Wisting Central II.
CAN Dimensions and Load Capacity Assessment
As the well to be drilled was an appraisal well, the CAN well foundation was to be rented for the duration of the well. The available rental units are of 6 m diameter and were in stock in different heights ranging from 7.5 m to 11 m.
As a first step the load capacities of the shortest and the longest available unit were determined, these are given in Table 2 and can be summarized as followed:
· Base case short (7.5 m)
Unfactored load capacities of this scenario are close to the design load, therefore no sufficient load capacity was reached.
· Base case long (11m)
Unfactored load capacities of this scenario are in excess of the double compared to the design load, allowing for a safety factor > 2.
Based on the above results the CAN unit of 11 m height was selected. Fig. 7 shows the penetration resistance and underpressure calculation. At full penetration the resulting under pressure was predicted to be 2.9 bar. Fig. 8 shows the overturning capacity. It has to be noted that the design load (red line) is depending on the penetration depth (resulting moment arm) of the well foundation. The deeper the unit is penetrated the less moment arm which results in a decreased moment
Table 2 – Load capacities for CAN (unfactored).
Figure 7 – Installation assessment (base case), penetration resistance and critical suction.
Figure 8 – Installation assessment (base case), overturning moment capacity (H applied at CAN top).
Backup Scenarios
As mentioned above two additional scenarios were investigated to mitigate any risk connected to an unidentified increase in shear strength similar to Unit II. Fig. 9 shows the shear strength profiles that were assumed for the two scenarios.
Of particular interest was the overturning capacity for the case that the CAN penetration would meet premature refusal due to excessive suction pressure, as caused by an unforeseen higher soil shear strength. Therefore the allowable suction pressure was limited to 3.5 bar although the structure is rated for 10 bar under pressure. The resulting maximum penetration as well as overturning moment for lower and upper bound assumptions are as followed:
· 6 m (Lower Bound)
o 3.5 bar under pressure is reached at 7.2 m penetration
o Resulting minimum overturning moment is 13 950 kNm
· 6 m (Upper Bound)
o 3.5 bar under pressure is reached at 6.6 m penetration
o Resulting minimum overturning moment is 13 322 kNm
With a design load of maximum 6 000 kNm and early refusal at 6.6 m the CAN would still be able to deliver the required capacity with a safety factor of 2.22. The detailed load capacities for both cases are given in Table 2.
Figure 9 – Undrained shear strength profiles for 6 m and 8 m layer.
CAN Operations
Installation
The CAN was installed at the well location in December 2014 (Fig. 3 shows the deployment through the splash zone). The ROV video survey of the seabed showed a significantly sloping seabed. After initial penetration to fix and check the location and orientation of the CAN, it was identified that the slope of the seabed caused a significant CAN inclination from vertical (3.3°). In the further process the CAN was partially lifted out of the seabed to allow gravity to correct the inclination. This reduced the tilt, but active measures had to be taken to bring the CAN back to vertical. This was achieved by moving the installation vessel surface locaton a certain distance into the opposite direction of the CAN tilt, reducing the inclination to 0.6° from vertical, which is within the acceptance criteria of 1.0°. This number improved further to a final inclination of 0.18°. Fig. 10 and Fig. 11 show the CAN fully penetrated and visualizes the slope in the seabed. Fig. 10 represents the low side with a 0.9 m distance from the top of the structure to the seabed level (each depth indication represents 10 cm). Fig. 11 (right hand side beyond the lifting pads) shows the high side. In total the installation parameters can be summarized as followed:
· Verticality:
o Target <1.00°
o As installed 0.18°
§ Pitch 0.11°
§ Roll 0.14°
· Penetration:
o Target 10.70 m
o As installed (high side) 11.00 m (full penetration)
o Comment: Significant seabed slope encountered, 90 cm over the diameter of the CAN, equivalent to 8.53° or 15 %.
· Position:
o Target North (± 50 m) 8 152 829.42 m
o Target East (± 50 m) 603 693.34 m
o As installed North 8 152 831.00 m (+ 1.38 m)
o As installed East 603 695.00 m (+ 1.65 m)
· Orientation:
o Target 180.00°
o As installed 171.30°
Figure 10 – Final penetration (low side).
Figure 11 – Final penetration (high side).
Geotechnical Back-Calculation
After completing the installation, the recorded data were used to update the geotechnical model to determine the actual "as installed" load capacities of the CAN. It is important to note that this is a unique feature of this type of well foundation. No other conductor installation method allows such an exact load capacity verification.
The back-calculation uses the actually measured underpressure versus depth curve to match the soil parameters at the installation location. This was especially important for this case as the input parameters for the spud location had to be extrapolated from CPT data with several km distance. Fig. 12 shows the match between measured (onsite data) and corrected (back-calculated suction) after the model update. The remaining deviations can be explained as followed:
· Depth range 2 – 5 m:
There was an operations delay in the beginning of the installation to correct the inclination. This possibly caused short term set-up effects which resulted in slightly higher values.
· Depth range > 10 m:
Due to the sloping seabed the CAN lid came in contact with the seabed at around 10 m penetration, causing an increase in penetration resistance.
The resulting load capacities are summarized in Table 2. The differences in under pressure and load capacity is explained by the different bearing mechanisms:
· Prediction: skirt friction inside and outside (ceiling of structure not in contact with seabed)
· Actual: skirt friction outside and end bearing (ceiling of structure in contact with seabed)
Figure 12 – Updated suction pressure based on measured data.
Drilling Phase
One of the two main objectives of the appraisal well Wisting Central II was to prove that it is possible to penetrate such a shallow reservoir horizontally. This is one of the requirements to economically develop these resources. The well was drilled as planned, and already in the 20 in. surface casing section it proved possible to build slightly more angle than planned for. The initial goal with the 26 in. BHA was to kick off from vertical and if possible build 1 to 2° of inclination. This was successfully executed and resulted in an inclination of 2.5° at 50 m below mudline. Fig. 13 shows the whole trajectory for the well including the 1.4 km horizontal section.
Besides proving the feasibility of the very shallow kick off, this installation also demonstrated the fact that shallow set conductors (e.g., 10 m penetration) will suffice for continued drilling below the CAN. There were no signs of borehole instability or washouts when drilling the 26 in. hole for the 20 in. surface casing or flow broaching on the outside of the CAN structure.
Another crucial benefit for the operator was to have the conductor capacity and inclination verified prior to rig arrival. Out of five available reference wells in the area, three had a conductor inclination (before cementing) of more than 1.5° (specified limit of the drilling contractor). Even though the inclination could be reduced by keeping the conductor in tension until the cement was set (up to 15 hours), this causes additional rig time. As referenced above, the CAN integrated conductor could be installed at an inclination of 0.18° only. Even though the load capacity of the well foundation was undisputed, daily checks of the bulls-eye mounted on CAN were performed for documentation (see Fig. 14). No movements of the CAN were detected during the whole project, which included the drilling and production test program of more than 70 BOP days.
The installation of the conductor by vessel also has other beneficial effects with respect to cost and safety:
· The well positioning is done on vessel rather than on rig day rate and less personnel needs to be mobilized to the rig.
· There is no handling of heavy 36 in. conductor and handling equipment on the supply vessel or on board the rig. This reduces HSE risks for personnel and frees deck space on the vessel and rig.
· The maximum BHA size is reduced from 42 in. to 26 in. Again this reduces HSE risks for personnel and frees deck space on the rig.
· The cement job for the conductor is avoided, saving rig time and cost for materials and chemicals. The conductor cement job is usually the second largest cement job in terms of consumables (cement, chemicals, etc.).
· No guide base is necessary, as guide posts are directly integrated into the CAN. This saves cost and reduces HSE risks for personnel by reducing the handing of large and heavy structures through the rig's moonpool.
· Casing cutting risk mitigation: The integrated conductor allows to simplify the cutting of the casing strings (see Fig. 15). Conventionally the 20 in. casing, cement and the 36 in. casing have to be cut. As shown in Fig. 15 the volume to be cut for the conventional method (left hand side) is substantial larger compared to cutting the 20 in. casing only (right hand side). When the cutting operation goes as planned the tie saving would be less significant, possibly about one hour. However, often circumstances like cutter arms braking, premature cutter wear and/or eccentrically casing strings may require a second cutting run, which would easily cause additional 8-12 hours rig time.
· Installation of the CAN can be done independent of the rig schedule, preferably in the good weather period to de-risk well spud operations with the rig in bad weather seasons.
Figure 13 – Well trajectory of well Wisting Central II.
Figure 14 – Bulls eye on CAN.
Figure 15 – Simplification of casing cutting operation.
Results and Conclusions
All goals set for this appraisal well were achieved as planned, which now represents the shallowest horizontal well drilled from a floating drilling unit to date. The enabling factor was to combine existing technologies in a new way as described by Hollinger (2017). One of the key elements creating confidence in achieving the high dog leg trajectory was the integration of the conductor into the CAN well foundation. This allowed to shorten the conductor from minimum four joints to only one joint, increasing the available TVD for building inclination by more than 10%.
Despite the challenges with respect to the missing CPT data at the spud location it was possible to select a fit for purpose CAN design that was installed without any incidents and also delivered the required load and performance capacities as planned.
The CAN was not only one of the technologies that increased the likelihood of success to deliver the planned well trajectory, but also managed to contribute to net cost savings.
Cost Savings
The operating company reported an average conductor setting time of 3.0 days for the Wisting area. This somewhat higher number than the industry standard is related to the circumstance that for 3 out of 5 wells the conductor had an inclination of more than 1.5° before cementing it in place. To correct this the conductor had to be kept in tension until the cement had set completely.
It was stated by OMV that 60 % of the rig cost savings were used on the CAN system (rental of equipment, installation and recovery vessel and logistics, etc.). This results in a net saving of 40 %, or 1.2 rig days. In detail the savings include but are not limited to:
· Equipment and services not required when using the CAN with pre-installed conductor:
o 3-5 conductor joints (depending on soil conditions)
o Conductor running tools and services
o BHA including bits, stabilizers and hole opener
o Cement job for conductor
o Stinger, cement plugs and other equipment
· Guide base
· P&A time savings and risk mitigation (as mentioned above)
· Risk reduction (possible NPT (non-productive time)) linked to above equipment and operations
Further Developments, Optimization Possibilities
Even though the Wisting Central II well was an undisputable planning and operational success, options for further improvement and risk reduction are to be assessed. The applied casing program was the conventional combination of 36 in. conductor, 20 in. surface casing, 13 3/8 in. intermediate casing, etc. Based on the successful Wisting II well it is projected that the well architecture can be simplified by omitting casing sizes, thus allowing a further significant reduction of the cost of tubulars required for the well construction.
Besides the fact that the reduction of casing sizes can be commercially attractive if the completion strategy allows for this, smaller casing sizes might also be beneficial to be run in higher dogleg environments. The CAN technology will allow reducing the casing sizes substantially, as the conductor loads are reduced to a few percent as soon as it is supported by the CAN.
Figure 16Fig. 16 shows the bending moment along riser, BOP and conductor for both cases, for a conventional conductor (red line) and a CAN integrated conductor (green line). It is clearly visible that the bending moment drops dramatically as soon as the CAN supports the conductor (green area). This allows to reduce the casing diameters for the conductor and all subsequent casing strings, which again allows to increase the dogleg to reach even shallower reservoirs.
Furthermore, it is possible to integrate the kick-off point into the conductor (also shown in Fig. 17). To kick-off the well from vertical is a critical operation, it is important to attain the planned azimuth when initiating the deviation. By integrating the kick-off point into the CAN well foundation it is possible to attain immediate control of the kick-off direction.
Figure 16 – Bending moment along riser, BOP and conductor.
Figure 17 – CAN with integrated slender conductor.
Nomenclature
Abbreviations
CAN Conductor Anchor Node
CPT Cone Penetration Test
HPWHH High Pressure Wellhead Housing
LPWHH Low Pressure Wellhead Housing
NPT None-Productive Time
NCS Norwegian Continental Shelf
ROV Remote Operated Vehicle
Symbols
St Clay sensitivity
Su Undrained shear strength