Improving Hole Quality and Casing-Running Performance in Riserless Top Holes: Deepwater Angola

The operator and co-venturers are developing several Angola Block 15 fields in water depths of up to 1400m. To date, 83 development wells and 33 exploration / appraisal wells have been drilled in the Block. Most have experienced hole quality issues in the riserless interval that manifests themselves as tight hole while tripping the BHA and abnormal/excessive drag running casing. In a few cases, casing could not be run to total depth (TD) and in one unfortunate case, the casing buckled in open water.There has been uncertainty as to the root cause of the hole quality issues which have been attributable to various factors since drilling was initiated. A review of available literature revealed very little published information on riserless drilling and casing running practices. The operator undertook a study of its riserless drilling and casing running practices in order to look for trends that may suggest certain sources as the root cause and to allow a re-examination of long-accepted practices. The study, combined with some new perspectives on directional drilling mechanics, has identified potential root causes not previously considered. Changes to drilling practices have resulted in improved riserless hole quality and smoother casing running operations.

With the current market's high prices for drilling units and sophisticated directional and formation assessment services, low systems reliability or poor directional performance in the execution of complex wells (ERD, HPHT, Deep Water applications) can spell costly Non Productive Time (NPT) for operators. Over the past two decades, many Bottom Hole Assembly (BHA) models have been developed to address the directional performance issue, but their application in an operational environment has generally failed for lack of strong associated BHA analysis methodologies. This paper presents a unique automated methodology for Post Analysis of BHA directional behaviour. It is based on modern 3D analytical models, combining BHA, BIT and formation effects with highly flexible data management. Implemented in user-friendly BHA management software, it can be used to define a BHA run segmentation and to determine the influence of the BHA settings in particular geological formation blocks. The Post Analysis clearly displays the final results of the BHA (objectives and performances versus prediction window), highlighting any problematic intervals drilled. Already, the methodology has solved challenging directional performances issues, generating significant savings and optimising the learning curve process. The field applications referred to in the paper have demonstrated that, without a proper methodology, a model offers little added value. The examples discussed here from Total E&P affiliates deal respectively with Rotary, Rotary Steerable System (RSS) and Steerable Motor (SM) directional BHAs. Case studies have led to BHA and bit design modifications. The results were well beyond expectations, ensuring good well positioning through the reservoir by improving BHA stability and manoeuvrability. Efficient drilling (reduced sliding, improved hole quality) was also achieved with a single SM BHA in 17 1/2" applications during kick-off, and successive build-up and slant drilling intervals. The methodology and its associated BHA Management® software have proved an undeniably valuable tool for the drilling community. The model is also run at the well Pre Engineering stage to check BHA design proposals by making multiple sensitivity analyses and by fine-tuning solutions to the operational context. Finally, in an industry with high personnel turnover, the software's global data management system ensures good capitalization of know-how and ongoing learning curve benefits.

Sidetracking a cased wellbore presents numerous challenges because the operators have to plan ahead to select the sidetracking depth and then ensure that all the objectives are met from a well authority for expenditure and geological target perspective. The quality of the geological target window is of great concern to operators because it ensures that the subsequent bottomhole assembly (BHA) will pass through the window without any problems. Sidetracking the wellbore when the milling assembly has to cut two strings of casing is an additional unique challenge. The centralizer and casing collar locator along the length of the wellbore at and in the region of the kickoff point are significant because they can be additional risks, which can lead to costly multiple trips to ensure that the window is in good-quality form. An operator was faced with potential geological losses at the kickoff point in a wellbore while attempting to sidetrack an existing wellbore containing 9.625-in., 43.5-lbm/ft L80 buttress thread casing, and 13.375-in., 68-lbm/ft K55 buttress thread casing, dual strings at the kickoff point. The BHA for this challenging application was modeled using a finite-element analysis (FEA)-based modeling system to select the optimum BHA to sidetrack the wellbore with the least amount of vibrations. The feasibility of using a bi-mill vs. tri-mill BHA was evaluated. As a result, a parameter road map, taking into consideration the dimensions of the whipstock slide and mill position during the milling operations, was finalized. The placement of the centralizers and casing collars along the length of the casing at the kickoff point was considered. The exact locations where the lead mill would initiate the cut at each casing string were analyzed to determine the whipstock setting depths. A corrosion and collar locator tool was used to identify the collar location along the 13.375-in. casing because standard casing collar locator logging tools were not be able to identify the location of the casing collars along the length of the 13.375-in. casing string. Whipstock simulation software was used to check the bending moments and stresses for the pass-through BHA because the dogleg through the window can create additional issues and challenges. The program calculates the dogleg severity for a liner or BHA pass-through in addition to forces and stresses on the liner or BHA. The total quantity of cuttings that would be generated from the milling operation while cutting two strings of casing was also analyzed. This methodical planning resulted in a successful dual-casing exit operation. The success is the result of a proactive planned initiative to mitigate BHA shock loading, which included real-time monitoring using a predictive compressive-strength analysis system. The proactive plan also increased confidence in the FEA-based modeling system's ability to accurately identify the root cause of damaging vibrations while sidetracking through carbonates in a dual-casing Kuwait well.

10 - Single-shot titanium/carbon fiber reinforced composites/aluminum stacks holes drilling

This paper describes a new Rotary Steerable System (RSS) and results from its field testing program. The RSS has been designed to handle the technical requirements of the most challenging wells, but also to be economical for use in low cost wells. This important feature was achieved by a unique hydraulics and electronic design and through the use of durable but low cost materials. The tool itself consists of three active and independent steering blades placed close to the bit, and a near-bit stabilizer which acts as a fulcrum. The steering blades apply a constant force to the formation. The displacement of the blades is measured to offset the Bottom Hole Assembly (BHA) in the desired direction. The clamping force of the tool in the hole acts to stabilize the lower part of the BHA and remove unwanted vibration and instability from the drill bit. This significantly improves BHA response, borehole quality, ROP and bit life. Since the RSS has a full 3D capability, it can drill constant curvature boreholes in any direction. The tool has been tested in wells in Texas, Louisiana, Oklahoma and the Gulf of Mexico. The results of these tests will be presented in detail. It is shown that BHA optimization can make a significant difference to the quality of directional holes drilled with a RSS. The data obtained during testing have, in fact, lead to an optimized BHA configuration which is unique compared with any other existing RSS. This optimized BHA configuration was determined from test runs in both point-the-bit and push-the-bit modes. Through the course of this exercise, it was shown that point-the-bit technology can provide superior borehole quality, improved Rate Of Penetration (ROP) and consistent Dog Leg Severity (DLS). Finally, the improved performance and time saving obtained with this new RSS will be compared with offset data from directional wells that were drilled with conventional steerable motor assemblies.

When drilling operations commenced in Sao Francisco basin, the operator knew the exploratory well campaign would present a major investment. However, as operations progressed the well costs and time exceeded assessments mainly drilling the 8½″ production hole section through two hard and abrasive formations. Due to the unconventional nature of the reservoir, well costs must be held to a minimum in the exploration phase to ensure each project is economically viable. To accomplish the objective, several tool technologies were analyzed and proposed with the aim of reducing down-time and tool failures. One of the main issues identified was the high level of shock and vibration (S&V) observed while drilling the Macaubas and Paranoa formations. The environment had a negative impact on drilling efficiency and contributed to tool failures and excessive wear on BHA components leading to additional trips and preventing the operator from achieving higher ROP and run lengths. After an engineering analysis, the drilling team determined the root cause was BHA/bit whirl while along with several other contributing factors related to the challenging hard and abrasive formations. Based on this information, the provider recommended using a roller reamer in the bottom hole assembly (BHA), to mitigate shock and vibration and improve drilling efficiency. Results were positive and the roller reamer reduced NPT and overall well costs in this economically sensitive play. Roller reamers are currently part of standard packed BHA and are being used as a reference for operators experiencing the same type drilling dysfunctions.

Operators of Gulf of Mexico (GOM) wells frequently reported overtorque issue of bottomhole assembly (BHA) connections when drilling the 26-in. section through salt. Such overtorque often leads to costly tool damage beyond repair (DBR), additional trips, and high nonproductive time (NPT). The average DBR cost per BHA can be as high as USD 1 million. Combined with a complete BHA roundtrip, it can easily cost more than USD 3 million for operators if such failure happens. This has been a problem for several years and has caused significant damage: In 2014, of 15 26-in. PDC bit runs in salt, 40% had overtorque connections and 20% led to DBR. This paper discusses how an integrated multidisciplinary team identified the root cause of and the solution to the overtorque problem. Torsional vibration was believed to be the cause of such failure. Comprehensive drilling dynamics simulation software that is based on empirical bit design knowledge was used to design a new bit to reduce the vibration. A newly developed high frequency downhole recording tool used in the 26-in. section recorded high-frequency torque, acceleration, and RPM fluctuation downhole. This dataset became the key to understanding the downhole vibration in detail because it provided information that cannot be acquired by a traditional MWD tool. Field-recorded data were fed into drilling dynamics simulations to accurately calibrate the drilling dynamics model. The simulations resembled downhole drilling conditions and clearly identified the root cause. The simulations precisely predicted the torque along the entire drillstring and identified why overtorque is present in only a certain part of the drillstring. The calibrated model was used to compare old and new bit designs. The newly designed bit showed much lower torque amplitude with similar torsional vibration frequencies. The simulation indicated that the newly designed bit can significantly alleviate the overtorque issue. Implementation of the new bit mitigated the overtorque issue immediately. As of May 2016, there have been 18 runs with the new bit. Only one run had a slight overtorque issue whereas the rest showed no sign of overtorque connections. DBR and NPT related to overtorque were eliminated. As a byproduct, the average on-bottom rate of penetration increased by 9%. This case demonstrates the effectiveness of the integrated approach to solving drilling challenges.

Hydraulic fracturing experiences were gathered over the years, from a tight sandstone reservoir of Late Carboniferous to Early Permian origin to find the root causes for the persisting challenges of high formation breakdown pressures and low fluid injectivity. It was soon realized that a single root cause could not account for these challenges. Instead, a multitude of reasons acting at the same time during the injecting sequence of that particular fracturing stage could easily be responsible, and no single re-occurring pattern would explicitly repeat in the consequent stages under similar conditions. It was found that the known complex nature of the depositional environment of the targeted formation played an important role, but the overall hydraulic fracturing mechanics played an equally critical part in the success of the fracturing attempt. Having realized this complexity in deciphering hydraulic fracturing performances it became imminent that not only the fracturing execution had to be refined, but all the preceding activities from drilling operations, to well placement, to wellbore preparation, to the choice of completion equipment and its deployment. These operations critically influenced the near wellbore region. The drilling fluid properties, wellbore hydraulics, well trajectory, the wellbore clean-out procedures, and hole quality were important contributors in the geo-mechanics domain coupled with the impact of the geological setting. Thus, a workflow was developed to analyze and rationalize the root causes for the injectivity challenges with the goal of optimizing every well construction step that would enhance hydraulic fracturing and ultimately deliver optimum well productivity.

Lightweight materials are finding plentiful applications in various engineering sectors due to their high strength-to-weight ratios. Hole-making is an inevitable requirement for their structural applications, which is often marred by thermal damages of the drill causing unacceptable shortening of tool life. Efficient cooling of the tool is a prime requirement for enhancing the process viability. The current work presents a novel technique of cooling only the twist drill between drilling of holes with no effect of the applied cryogenic coolant transferred to the work material. The technique is applied in the drilling of two commonly used high-strength lightweight materials: carbon fibers reinforced polymer (CFRP) and an alloy of titanium (Ti-6Al-4V). The efficacy of the cooling approach is compared with those of conventionally applied continuous cryogenic cooling and no-cooling. The effectiveness is quantified in terms of tool wear, thrust force, hole quality, specific cutting energy, productivity, and consumption of the cryogenic fluid. The experimental work leads to a finding that between-the-holes cryogenic cooling possesses a rich potential in curbing tool wear, reducing thrust force and specific energy consumption, and improving hole quality in drilling of CFRP. Regarding the titanium alloy, it yields a much better surface finish and lesser consumption of specific cutting energy.

Drilling extended reach drilling (ERD) wells starting from the planning phase and engaging various disciplines including drilling, cementing, drilling fluids and geoscience teams. The pre-well engagement and integration between multiple disciplines are vital to define the associated drilling/geosteering challenges and accordingly optimize the drilling program to deliver a successful ERD well. These challenges are included and not limited to geological model uncertainties, differential sticking, high torque and drag, ECD limitation, friction factors and expected mud losses. An integrated and optimized plan was constructed to meet the associated challenges. The drilling engineering team optimized the bottom hole assembly (BHA) design in all sections to ensure a smooth profile using optimum drill bits designs. The BHA included LWD technologies to mitigate the geological challenges and helping in determining the casing points and geosteering operations. A new generation of intelligent fully rotating high dogleg pushthe-bit rotary steerable system was selected with matched drilling bits to geosteer the well in the thin target layer while maintaining the planned target trajectory with minimum borehole tortuosity by means of realtime drilling optimization. Effective collaboration led to successful delivery of the first extended reach well, the geosteering objectives were achieved with 100% reservoir contact and delivered 20,000 feet targeting thin carbonate layer and overcoming the complex geology environment. The well was drilled to record depth of 32,300 feet with 29% ROP improvement in same field. ECD was always maintained below the fracture gradient along with optimal hole cleaning without cuttings buildup or tight hole while reducing the wellbore friction to ensure smooth pulling out of hole operation. Cementing operations were successfully achieved and ensured zonal isolation. Furthermore, a customized and innovative drilling fluid with free RDF Non Aqueous Fluid (NAF) and compatible lubricant were deployed along the different hole sections to reduce the expected induced losses and provide proper hole cleaning. The cementing program has been optimized for the 18 5/8", 13 3/8" and 9 5/8" casings using an innovative flexible expandable lead and tail slurries with enhanced mechanical properties to mitigate the expected losses while cementing and ensure proper isolation across all formations. The best practice of the multidisciplinary approach along with the captured lessons learned opens the door to drill more challenging wells. in addition, it proved that proper planning and execution can shift the boundaries further and gave confidence to drill even deeper.

Riserless drilling poses numerous operational challenges that adversely affect the efficiency of the drilling process. These problems include increased torque and drag, buckling, increased vibration, poor hole cleaning, tubular failures, poor cement jobs, and associated problems during tripping operations. Riserless drilling in deep and ultra-deep water, as well as drilling to deeper depths, requires improved models and comprehensive analyses, especially when larger-diameter casings are run and cemented in a deviated or directionally drilled tophole environment. Current methods that lack proper modeling will severely overestimate hookload values when casing strings are run in and underestimate in the pick-up load estimates. A new modeling approach is proposed to calculate appropriate hookload values, as well as torque and drag. This method can model situations in which the drillstring/casing string is in open water or in openhole under various operating conditions. Both soft- and stiff-string models are used in the hookload estimates and in the wellhead side-loading calculations. Hookload calculations and buckling limitations for the scenarios in which casing and inner strings are run with drilling mud inside the inner string, seawater in the outer string, and pad mud in the hole below the mudline, are presented. The study indicates that the results are influenced by various parameters, including depth of the mud line, offset of the wellhead from the rig center, wellbore inclination, curvature, wellbore torsion, and angle of entry into the wellhead, as well as by the complexity arising from wind, wave forces, and ocean currents. This study compares simulated predictions with actual well data from wells from Angola and the North Sea and describes the accuracy and applicability of the model. It also presents several examples of surface casing-string failure.

Improving hole quality is a major factor for accurate formation evaluation and reduced well construction costs in deepwater. Further optimization of the borehole quality using new generation drilling assemblies is now possible. Not only to improve formation evaluation, increase penetration rates, and reduce the risk of drill string failure, but also to reduce the load points of salt formations on completions. Concurrent drilling and reaming techniques have been successfully used to mitigate the effect of non-uniform, transverse loading of salt formations on completions as well as improve hole quality for higher wireline log success rates. During the execution of deepwater wells in offshore Brazil, multiple bottomhole assembly configurations were deployed to drill the various hole sections and the use of logging-while-drilling (LWD) imaging and wireline calipers provided a direct comparison of hole quality performance of each drilling system. A culmination of various drilling systems were evaluated, during a drill-off test, in a single well while performing enlarging-while-drilling (EWD) of a 12 ¼" to 14 ¾" hole. Salt formations were drilled to fully compare each system in a common drilling environment without variations due to formation types.

There are four key technical issues to be addressed when drilling directional wells with steerable bits and bottom hole assemblies: maximizing hole quality, addressing tool face control, eliminating bit and assembly vibration, and predicting hole curvature or build rates. Hole quality1 and tool face control2-7 have been covered in several previous SPE papers, as has bit and assembly vibration elimination8-9. This paper covers the recent development of a fundamental model for the prediction of hole curvature based on measured bit response and modeled bottom hole assembly behavior. Bit side cutting is a nonlinear function of side force, gauge design, rotary speed, rate of penetration, rock strength, and cutter dull state. To characterize the bit's side cutting response, a series of tests are run in a full scale drilling simulator10 under a variety of drilling parameters and sideloads. Axial and lateral bit trajectories are measured and a response curve is generated. The bit side cutting angle measured in the lab, is equivalent to bit tilt calculated on a bottom hole assembly (BHA). Several hundred tests have been run to measure the response of numerous bits. These response curves are then used to relate bit performance to BHA response. The BHA is modeled with a finite element analysis program over the range of hole curvatures and tool settings which define the operating envelope for the tool. The result of the analysis is a bit side force to bit tilt response surface for the tool over its operating range. The response curve for the bit and the response surface for the BHA are then plotted on the same set of axes. The intersection of the two graphs represents the steady state response of the system and is used to predict the achievable hole curvature of the bit-BHA system. This work shows that bits often selected for rotary steerable systems are more laterally aggressive than required of the BHA to achieve the desired hole curvature. The model has been verified with full scale field tests. While this work was originally developed for PDC bits and rotary steerable systems, the fundamental model can be applied to all types of bits and drilling systems. It has also been applied to motor and vertical drilling systems with PDC and Impregnated bits.

Under this climate of oil price and energy uncertainty, it is mandatory to limit the non-productive time (NPT) and achieve the highest levels of operational excellence. This is a key factor toward overcoming the evolving economic challenges, reducing budget and spending, and optimizing the return on investment. Worldwide, stuck pipe and borehole problems represent one major contributor into the NPT while drilling, reaming, tripping, casing and running completions. This NPT category becomes even more critical when dealing with shaly formations. Saudi Aramco constantly deal with offshore shaly formations in Saudi Arabia where stuck pipe and borehole problems contribute with over 24% to the overall drilling and workover NPT. Establishing best practices to minimize or prevent these problems will enhance the overall drilling performance and result in significant operating time reductions and cost savings. The major offshore re-entry operation challenges are first screened: formation instability, hole size, well trajectory, bottomhole assembly and experience and communication. The shaly formation rock nature is analyzed to understand the stressed shale instability root cause: mechanical and/or chemical. This diagnostic step establishes the formation shale properties and behavior. In relation to this information, three basic stuck pipe mechanisms (pack-off and bridging, differential sticking, and wellbore geometry) are discussed where the major contributing factors for each category are identified. This leads to the establishment of proactive recommendations and best practices. These practices will tackle the problem from different angles to construct an integrated solution. This includes drilling fluid design (rheology, filter cake, filtrate volume and properties, etc.), hole cleaning (rate of penetration, sweep pills, bottoms-up volume, etc.) and drilling parameters (trend analysis, flow rates, string motion, etc.). This paper will highlight the recommended practices and provide actual well examples where stuck pipe tendency was reduced in shaly formations.

The use of motorized rotary steerable system (RSS) and advanced measurement-while-drilling (MWD) tools optimized the open-water work (OWW) scope of deepwater drilling operations in Guyana. The objective was to improve drilling efficiency, reduce slide mode drilling, and enhance directional control. While similar BHAs had been used in vertical wells in the Gulf of Mexico, their application in the challenging OWW scope in Guyana was a novel endeavor. To overcome the limitations of the standard bottomhole assembly (BHA), a novel approach was adopted, incorporating a 26-in. rotary steerable system, straight mud motor, and advanced MWD for dynamic surveying. Detailed planning and tool selection, including RSS and motor tendencies analysis, were crucial for establishing initial motorized RSS BHA design. Ten out of fourteen planned wells used motorized RSS BHA, while the remaining four employed a motor and gyro due to anticollision concerns. The study involved optimizing BHA configuration, drilling parameters, and survey acquisition processes to enhance performance. The implementation of the motorized RSS along with advanced MWD tools led to significant improvements in deepwater drilling operations in Guyana. A 75% improvement in drilling efficiency was achieved compared to previous conventional drilling assemblies. The use of the motorized RSS BHA design resulted in the elimination of slide mode drilling, enabling directional objectives to be met more effectively. The average 26-in. section drilling time was reduced by 18 to 28 hours per well. This method allowed for better control and reduced borehole tortuosity, ultimately improving well performance. The success of this approach has now become the standard for future OWW campaigns in the region. Apart from the enhancement in drilling efficiency, this case study has not only pinpointed solutions to overcome directional challenges but has also paved the way for the industry to attain higher inclinations in top hole. This achievement, in turn, addresses challenges encountered in intermediate and production sections by facilitating the creation of smoother catenary curves and delivering less tortuous boreholes. The implications extend to the realm of production and completion, providing a foundation for more streamlined processes and improved precision in the installation of production and completion components.