Increasingly difficult wells have led to a significant number of new drilling challenges, particularly in the offshore market. Operators that are actively searching for new ways to combat the persistent decline in oil prices are becoming more aware that technical innovation must be the catalyst for industry change. To deliver innovation that drives impactful performance increases in complex offshore well environments, work must be done outside the confines of what has historically been considered “ordinary.”
Drilling the presalt
A conversation on difficult offshore wells would not be complete without discussing how the presalt is drilled. The presalt has become a major area of interest for operators working offshore Brazil and Angola, with important discoveries in the past decade revealing vast reserves of oil and natural gas and a potential to change the landscape of the offshore drilling market. Drilling the presalt, however, presents distinct challenges. The deepwater or ultradeepwater location of typical presalt layers and the large areas over which such layers are spread often present issues of economic feasibility. In addition, the expertise necessary for extraction necessitates significant investment in researching, both onsite and offsite, looking for the best methods and technologies to drill such difficult formations. Despite these challenges, the rise of these new technologies and access to such reserves will provide powerful opportunities for the industry to move forward.
National Oilwell Varco (NOV) has developed the eVolve Optimization Service, a tiered approach ranging from surface data-based optimization to full closed-loop downhole drilling automation, to address the changing economic and technological landscape of the oil and gas industry. The eVolve service brings together technologies from several NOV product families and proven drilling expertise to enable improved operational efficiency and enhanced decision-making. BlackBox memory-mode logging tools placed in the drillbit, bottomhole assembly (BHA) and/ or drillstring capture data on pressure, torque, weight transfer and vibration and provide insight into how drilling practices affect performance and reliability. The driller is alerted to potential dysfunctions such as stick/slip and torsional/lateral vibration, and dynamic behaviors downhole are assessed to provide a deeper understanding of the drilling environment. In addition, combining surface and downhole data to analyze drilling dynamics creates an optimization foundation, enabling better design and component selection for the BHA in subsequent wells.
NOV worked with a South American client to develop a comprehensive solution in a unique presalt application using the eVolve service. The client was experiencing a number of drilling dysfunctions in the project, including high torque, which was stalling the motor and topdrive with the original bit, and deviation issues, which were compromising ROP due to low weight on bit. It was also necessary to reduce and then maintain inclination to optimize ROP. Previous offset wells had used a positive displacement motor and were usually drilled in either two runs with roller-cone bits or a single run with a polycrystalline diamond compact (PDC) bit. The complex lithology of the challenging formations was also a concern.
The first step in this project was to use the BlackBox tools to obtain high-frequency downhole data. These data were then analyzed to determine optimal drilling practices and parameters moving forward, with the client setting the objectives of maintaining low inclination, improving ROP and drilling the entire section in one run through the surface hole formation, stopping in the salt. Hole verticality and tortuosity, limited torque at the topdrive, high reactive torque and potential stick/slip when drilling the anhydrite, and potential equipment damage were the main challenges determined from the analysis. In addition, high circumferential contact needed to be maintained while drilling to achieve improved lateral stability, and caution needed to be taken so that the cutters did not break when drilling the salt.
After redesigning the BHA and implementing NOV’s suggested parameter changes, the client saw a significant reduction in lateral vibration in the optimized well both at the bit and in the drillstring. Mitigating vibration was one of the factors leading to increased performance and reduced equipment damage in this application. (Source: NOV)
NOV worked with the client to design a completely new and innovative BHA, using a 17½-in. PDC bit matched with a 26-in. concentric reamer and impact-resistant Helios cutters. This redesign enabled a proven 17½-in. rotary steerable system to be used to aggressively drill the challenging 26-in. section and reduced the time required to change the BHA for the next hole section, allowing both the bit and the BHA to be reused. Vibration was mitigated at the end of the run in the anhydrite, allowing the client to reach total depth (TD) without damage to the equipment, and the offset and spiral of reamer and midreamer blades provided 360-degree circumferential contact to maximize lateral stability. Drilling practices and parameters were optimized through use of computational fluid dynamics, torque and drag analysis, and finite element analysis, ensuring optimal performance.
ROP in the offset well, using the original 26-in. PDC bit, was about 17.6 m/hr (57.74 ft/hr), and the field average of ROP for the project was about 14.4 m/hr (47.24 ft/hr). Using the redesigned, optimized BHA, improved bit selection and the drilling parameters recommended by the eVolve team, the client was able to achieve a field record ROP of 28 m/hr (91.9 ft/hr) when drilling the presalt. This improvement represented an ROP increase of more than 57% over the best offset well and almost double that of the field average. The client also achieved the objective of reaching TD in a single run without incident, and low inclination was maintained as per the well plan. The significant increase in ROP combined with the various other performance improvements enabled a reduction in total drilling time of more than 1.5 days in comparison to the offset wells, yielding savings in excess of $700,000 for the project.
Improvements in ROP, including an increase from 17.6 m/hr in the offset well to a field record of 28 m/hr with NOV’s recommendations, were one of several benefits on this project. Inclination continued to decrease after the offset well, reaching as low as .35 degrees, and the client was able to drill the entire section in one run on each well vs. four runs on the offset well. Optimization yielded drilling time savings of 1.5 days vs. the offset well. (Source: NOV)
The analysis of the large amounts of data collected by the BlackBox tools and the actions taken as a result of this analysis were not only beneficial to the client on this project, improvements on additional wells, in fact, were enabled through implementation of this project’s lessons learned. On the subsequent well of this drilling campaign, for example, another customized BHA was designed, and the client was able to avoid areas with high-resonance vibration potential in the BHA due to NOV’s parameter recommendations. Implementation of these recommendations combined with continuous, real-time feedback while drilling from the client’s operations center enhanced overall efficiency and yielded ROP gains that were equal to or greater than those achieved on the previous well. Torsional and lateral vibration also were reduced, and mechanical-specific energy use was optimized in the system.