The Moyno Tri-Phaze system allows all fluids produced at the well site to be transported simultaneously through one pipeline to a central processing station. (Image courtesy of Moyno)

While drilling in Pluspetrol’s high-profile Pagoreni field in Peru, excessive bottomhole assembly/polycrystalline diamond compact (BHA/PDC) bit vibration was causing unacceptably low ROP and reducing tool life. The problems were most noticeable when using an expandable reamer in a rotary steerable BHA to drill a 105?8-in. pilot hole reamed open to 121?4 in.

Specific issues while holding angle in the tangent section included excessive torque induced stick-slip as well as high axial and lateral vibration problems. Operator and rotary steerable system (RSS) service company engineers experimented with a variety of operating parameters and BHA configurations, with limited success. Although axial and lateral vibrations subsided slightly, stick-slip increased, causing more downhole problems then the original BHA configuration.

Pluspetrol asked Smith Technologies i-DRILL engineering group to optimize the BHA design (including PDC bit selection) for their Pag-1004D well. The study concentrated on data and drilling practices from three previous wells where similar problems were encountered. To optimize the rotary steerable/reamer/PDC BHA, the service provider used proprietary simulation software to model drillstring configuration and behavior.

Dynamic simulations overview

To address ROP-limiting stick-slip and high axial/lateral vibration issues in drilling the first three development wells, offset data were modeled. The model included the PDC bit, RSS, logging while drilling, expandable reamer, and drillstring up to the surface drive system. The model considered the precise positioning of each individual cutter on the bit and physical characteristics of the formation. It also included all dimensions of the drill string and hole caliper from offset wireline data in addition to the construction materials of the BHA components. Other detailed offset data included RPM, weight on bit (WOB), surface torque, pump strokes, hook load, and several other downhole measurements.

The model was run to test the effects of different bit types, BHA designs, drive mechanisms, and operating parameters as a function of hole size and lithology. It allowed engineers to visualize the bit, reamer, and RSS interacting with the borehole wall to help determine the root cause of poor offset drilling performance. The effort had the potential to significantly improve drilling performance regardless of the source of individual tools and components.

Simulations were repeated until the offset conditions were duplicated with an accurate statistical match. Specific components including the PDC bit, underreamer, stabilizers, roller reamers, and drive mechanisms were then systematically substituted for the original BHA’s components in order to identify the damaging behavior and element. The goal was to determine the root cause of bit/BHA vibrations to ultimately eliminate the cause and thereby increase ROP.

Bit analysis

To determine what style PDC bit would be the most stable and deliver the maximum ROP, a series of virtual case studies were run to quantify design changes in the PDC bit profile, blade/cutter count, gauge length, bottomhole patterns, and force balance on the four bits.

Laboratory testing determined the most appropriate PDC cutting structure in terms of aggressiveness when used in combination with the 121?4-in. reamer’s 13 mm cutters.

To determine cutter aggressiveness, the service provider was able to simulate confining pressure (3,000 to 5,000 psi) of the specific formations to be penetrated. ROP potential was then calculated using a finite element model (FEA) that considered:
• Precise dimensional details and properties of the cutting structure;
• Rock hardness (UCS);
• Lithology; and
• Confined pressure based on laboratory tests.

Tests included ROP vs. WOB and imbalance force/WOB to derive percentage of imbalance vs. ROP on the four different bits. After examining the results from laboratory testing, engineers concluded the new-style rotary steerable six-bladed design — certified directional six-bladed bit with 16 mm (52) cutters — had the best chance of offering the proper balance between cutter aggressiveness for maximum ROP and bit body stability for vibration/stick-slip reduction while offering an opportunity for increased ROP.

Expandable reamer

Virtual BHA analysis also included an RSS-compatible expandable reamer that could simultaneously open the 105?8-in. pilot hole to 121?4 in. The reamer would have to drill an outside diameter hole substantially greater than the pass-through of the casing yet be substantial enough to handle the heavy weight of the rotary steerable BHA set below the reaming tool.

To meet the challenges, the service provider recommended their expandable reaming technology that can reliably cut long intervals through a wide range of formations types at fast penetration rates.

Contingency planning

The program allowed engineers to model the individual BHA components to test a comprehensive range of “what-if” scenarios to identify and reduce vibration and stick-slip risks inherent to the drilling operation. This was achieved by simulating different bits and BHA components based on anticipated lithology and using a PDC wear model that allowed engineers to “see” if the particular bit could sustain an acceptable ROP throughout the entire hole section. It was an advantage to have a quantitative measure before making the final bit selections.

Scenarios

Engineers identified four critical scenarios concerning transitional drilling between the Vivián, Chonta Superior, and Chonta Inferior formations with the 121?4-in. reamer and 105?8-in. pilot bit:
1. Bit and reamer in Vivián;
2. Reamer in Vivián, bit in Chonta Superior;
3. Bit and reamer in Chonta Superior; and
4. Reamer in Chonta Superior, bit in Chonta Inferior.

Because of the varying formation hardness, engineers concluded the bit was out-drilling the reamer when penetrating the relatively soft Chonta Superior formation. To fully understand the complex interdisciplinary downhole dynamics
critical to increasing PDC/reamer drilling efficiency, the service provider conducted five virtual analyses using the four bits in combination with the reamer in the four critical scenarios including:

Weight distribution (WOB/WOR) vs. ROP. For the scenario 2 where the bit is in soft formation, reamer in hard and the weight is hung on the reamer.

Lateral vibration (bit/reamer) vs. ROP. The scenario where the weight is hung on the reamer (bit in soft, reamer in hard), reamer lateral vibrations levels are higher. For the scenario where the bit is in hard formation and the reamer in soft, the more WOB applied, the more likely the scenario for lateral bit vibration. For the scenarios where both are in the same formation, there are no bit or reamer lateral vibration issues.

Torsional vibration (% torque bit/reamer) vs. ROP. The scenario where weight is hung on the reamer (bit in soft, reamer in hard), reamer and surface experience a higher likelihood of stick-slip. There are no bit stick-slip issues.

Average torque (bit/reamer) vs. ROP. The scenario where the weight is hung on the reamer (bit in soft, reamer in hard), reamer and surface experience higher average torque.

Risk of stick-slip (% RPM) vs. ROP. Based on the thresholds for all scenarios in the Vivián Formation, engineers determined there is a medium level of stick-slips risk based on percent RPM. In general, all values are below 150% (severe stick-slip).

Weighted statistical analysis

In order to weight the different issues by importance as determined by the operator, the service provider performed a statistical analysis of each drilling parameter. The study was based on a normalized results equation where each selected drilling parameter was assigned a specific weight according to the operator importance. In this specific project, an equal weight distribution was made for average ROP, bit, reamer and surface stick-slip, bit and reamer lateral vibration, and the change in downhole RPM.

Based on this analysis, engineers concluded that scenario 2 (bit in soft formation and reamer in hard Vivián) is the most critical of all scenarios. Scenario 2 is also the least efficient for the reamer. The worst scenario for the bit is when the reamer is in soft Chonta Superior and the bit is in hard Chonta Inferior. Based on the resulting scores, the modeling suggested the best bits for each scenario showing the MDi616 as the best in thee of four scenarios.

BHA recommendation/operating parameters

Analysis identified the best BHA system and operating contingencies along with sensitivity plots of the relationships between drilling parameters, ROP, vibration, torque, and critical speed analyses. This plot allows drillers to maintain the correct RPM and WOB to keep the system operating at maximum performance. The sensitivity plot is used to develop “sweet spots” for maximizing ROP through drill-off tests early in the run, while respecting the operating limits established by the analyses and their relationship to reducing stick-slip and axial/lateral vibrations.

It was determined that reducing rotation to 140 RPM or slower would reduce the occurrence of stick-slip, while controlling weight in the critical scenarios where the weight is hung on the reamer would help to mitigate vibration, especially when the reamer or both reamer and bit are in the Vivián formation.

BHA

Based on all the above input, engineers concluded the following BHA would mitigate vibrations/stick-slip and improve ROP to an acceptable level:

• When the weight is hung in the reamer during the three critical scenarios described above, the BHA will experience unacceptable levels of lateral and torsional vibrations. The only way to mitigate the vibration levels is to control the weight, letting the weight drill-off. When the hard interval is completed, parameters can be returned to normal to optimize ROP.
• The recommended BHA offers the best results for the three critical scenarios where the weight is hung in reamer (bit in Upper Chonta [soft] and reamer in Vivián formation [hard]) and both the reamer and the bits’ cutting structures are in the Vivián formation.
• For normal drilling conditions (where the weight is not hung in reamer) the addition of an expandable stabilizer would potentially produce the best results in terms of vibration, thus extending the life of the reamer.
• Engineers also recommended using an expandable stabilizer in those cases where the reamer shows excessive wear after it’s pulled out of hole.
• The vibration sensors must be monitored to avoid the weight hangs in the reamer.

Performance results

The optimized BHA was run on Pag-1004D with outstanding results. Recommendations from the 4-D simulation program successfully eliminated vibrations, allowing the 121?4-in. by 105?8-in. hole section to be drilled and under-reamed to total depth in one run with a new-style, 6-bladed, 16 mm cutter PDC bit. The BHA solution efficiently delivered all directional requirements and increased ROP from the three-well offset average of just 5.92 m/hr to 11.4 m/hr, an increase of 93%. The operator realized a 15% reduction in cost per meter compared to the authorization for expenditure plan for a total savings of US $1.2 million in the 121?4-in. by 105?8-in. hole section alone.