Dupal, K. (Shell International Exploration and Production Inc.) | Curtiss, J. P. (Shell International Exploration and Production Inc.) | van Noort, R. H. (Shell International Exploration and Production Inc.) | Mack, C. (Shell International Exploration and Production Inc.) | Greer, S. (Stena Drilling)
Operations were being conducted with a drill ship in deepwater, harsh environment conditions offshore Nova Scotia. After securing the well, the rig disconnected the Lower Marine Riser Package (LMRP) from the lower Blow Out Preventer (BOP). After disconnecting, dynamic loads caused an uplift of the marine riser, ultimately resulting in a failure of the tensioner ring support and loss of the riser/LMRP to the seabed. No personnel were injured in this incident and no spilling of synthetic base mud to the environment occurred. This paper provides a summary of the root causes and contributing factors for the incident.
The Tripod beta method was used to conduct the review of the incident. The scope of the review included the following:
Measured data (rig heave, tensioner stroke, tensioner pressures)
Moonpool video camera recording of riser and tensioners during and after disconnect
Analytical models for vessel & marine riser dynamics, including the riser tensioner anti-recoil system
Rig/moonpool geometry, riser tensioner ring design, and space-out
Based on initial findings, further studies and analyses were conducted to better understand the dynamic behavior during the transition phase from initial disconnect to the hang-off position.
Forecasted Metocean conditions from a late winter storm indicated the potential to exceed the threshold for rig heave, with the marine riser connected to the well.
In preparation for disconnecting the LMRP, the well was secured with two barriers, a storm packer and closed blind shear rams. Once the rig heave limit was reached, the LMRP was disconnected from the lower BOP stack. Seven minutes after unlatching the LMRP, the riser tensioner profile on the slip joint outer barrel lifted off the riser tensioner ring and landed back onto the tensioner ring off-center. This uneven loading caused the tensioner ring halves to separate, dropping the LMRP and riser to the sea floor.
Analysis showed that one of the most critical phases of disconnecting the LMRP from the BOP occurs immediately after disconnecting and prior to moving the rig a safe distance from well center. The investigation indicated that the root causes of the event included human factors, such as adding additional air to tensioner system and re-setting of the Riser Anti Recoil System (RARS) prior to final hang-off condition. Contributing factors included the dynamic behavior of the riser and a lack of specific procedures for addressing the dynamic system conditions during the critical transition phase.
The paper provides additional information for riser/tensioner configuration and riser dynamics analyses during harsh environment conditions. In particular, additional analyses are presented for the transition phase from disconnect to hang-off position. Initial data is provided for further development of a smart disconnect algorithm, based on machine learning techniques of hind cast data.
Dooply, Mohammed (Schlumberger) | Sianipar, Sakti (Schlumberger) | Rodriguez, Faiber (Schlumberger) | Poole, David (Chevron) | Fuenmayor, Cesar (Chevron) | Carrasquilla, Juan (Schlumberger) | Rosero, Ivan (Schlumberger)
Achieving successful cement placement in tieback casings and liners on deepwater wells is very critical. One of the design challenges is to displace compressible drilling fluid in the tight annulus within the mechanical limitations of downhole tubulars. Accounting for the compressible nature of drilling fluids with changing pressure and temperature, combined with fluid contamination level, will provide better understanding of cementing dynamic pressure during placement.
Cementing tight annulus normally requires managing high placement pressures within the tubulars mechanical limits. Field measurements from case studies in Gulf of Mexico, were analyzed comparing with simulated cementing dynamic pressure accounting for effect of synthetic based mud compressibility as it is displaced by viscous spacer and cement slurry. The rheology of contaminated mixture also provided an input for better interpretation of cementing surface pressure response. These analyses, including estimating hook load variations while cementing, allow selection of appropriate fluid placement rate without exceeding the mechanical limits while also achieving effective fluid displacement.
Comparison analysis of measured and simulated data shows that use of complete fluid rheology profile at various temperature and pressure provides a more accurate prediction of cementing dynamic pressure in tight annulus cementing with synthetic based mud. This approach also allows a better estimation of the minimum rate required for efficient mud displacement enabling an optimal design of the cement slurry thickening time, when coupled with a representative mud circulation schedule.
Precise annular clearance of tieback strings provides better understanding of fluid positions inside the tieback strings and annulus, which ensures achieving planned top of cement to mitigate annular pressure buildup. This is critical to protect the outer casing against any potential collapse loading in a blowout scenario in deepwater drilling environment.
Liu, Chao (Aramco Services Company: Aramco Research Center-Houston) | Han, Yanhui (Aramco Services Company: Aramco Research Center-Houston) | Abousleiman, Younane (PoroMechanics Institute, University of Oklahoma)
The recently formulated theory of dual-porosity dual-permeability porochemoelectroelasticity is applied to derive the analytical solutions for an inclined wellbore drilled through a shale formation, accounting for the effects of natural fractures and shale chemical activity (
The analytical solution is applied to study two field cases. The Hoek-Brown failure criterion is employed to evaluate wellbore collapse and mud densities. The two case studies indicate that the analytical solution explains the wellbore failure and is capable of predicting the used safe drilling mudweight. Back analysis on the field data with a sensitivity study is able to estimate the range of the fracture permeability once the matrix permeability is defined.
Optimized well clean-up planning and procedures are crucial for the effective development of offshore subsea wells and their subsequent production stage to host facilities. The objective of the well cleanup is aimed at ensuring a successful removal of the completion fluids and drill-in fluid out of the wellbore to restore connectivity with the reservoir, maximize well productivity while minimizing tensile sand failure, and properly conditioning the sand face completion (in a standalone screen scenario). To achieve this goal, the well clean-up time, bean-up procedure, rate and fluid volumes to be produced should be appropriately estimated to properly size the surface testing equipment required for the operation.
Due to the highly dynamic and transient nature of the cleanup process, the use of a dynamic simulator was required to effectively capture the physics of the concurrent flow of the various phases present in the system. An extensive modelling and simulation of the unload process has been performed through the use of a dynamic multiphase simulator to assess the transient displacement of the various wellbore fluids according to several unload strategies. Potential clean-up times and volumes were assessed using flowrate ramp-up schedules designed for different completion fluid distributions in the wellbore. The constrained flowrate cases were considered to represent the constraint on the rig (restricted because of surface handling capacity issues).
The well clean-up procedure was developed to minimize clean-up time, avoid formation damage, and minimize volume of formation liquids on flow back during the rig well tests. During the execution, the movement of fluids along the wellbore, surface production rates, the drawdowns and duration of clean-up to predefined targets were monitored and recorded. The acquired field data from the clean-up operation was compared against simulation prediction and validated the reliability of the predictive model.
This study proves the transient multiphase simulation to be effective in capturing the physics of the multiphase flow process involved in the clean-up operation. It also demonstrates that, when appropriately done, it could be an effective tool for the planning and strategy selection for the well cleanup operation.
The oil and gas industry has operated in Denver Julesburg (DJ) basin for many decades. Currently in the basin, increasing population density and wellbore complexity have resulted in a heightened visibility of long-term well integrity. Failure can lead to future liabilities, loss of public trust, and a revoked right to operate. Operators must demonstrate commitment to well integrity to continue operating in the basin, yet many still report sustained casing pressure (SCP) on a significant portion of wells. Because SCP corresponds to the open communication of fluids to surface, it is a direct metric of well integrity failure. Regulations require operators to report and remediate instances of SCP on all wells. On average, clients experience one well with SCP for every five drilled.
As a primary well barrier element, the cement sheath is vital to well integrity improvement. Enhanced placement techniques of conventional cements failed to prevent SCP, confirming that failure is derived from post-placement dynamic conditions. The solution must account for pressure and temperature stresses, preventing and mitigating mechanical failures throughout the well life cycle. A flexible and self-healing cement design provides a twofold response that is ideal for wells in areas, such as the DJ basin, with SCP risk.
Mechanical properties are optimized based on the results of a mathematical stress model. Although Portland-based cement systems can be optimized to sustain higher levels of dynamic stresses, it is impossible to avoid a mechanical failure entirely. Therefore, a self-healing function is a critical secondary feature. The self-healing mechanism is designed to activate upon contact with an invading hydrocarbon and can be formulated for any type of hydrocarbon, from high gravity oil to dry gas.
Flexible and self-healing cement has been successfully designed and implemented on approximately 250 wells in the DJ basin with a reduction to 2% instances of SCP. Elimination of SCP provides confidence in long-term well integrity, which is essential to continued operation in the basin.
A qualification methodology for advanced rotary shouldered threaded connections is presented. It covers physical tests of connection prototypes and virtual tests with modeling and simulation techniques.
The methodology had been applied to qualify a fatigue-resistant threaded connection design that has recently been released for field testing. The qualification process consists of in-lab makeup and breakout tests, on-rig makeup and breakout tests, sealability tests, fatigue tests, torsional yield limit tests, and tensile capacity tests.
During the development of the design, modeling and simulation techniques were extensively used to optimize the design prior to physical prototyping and testing. Some of the qualification tests, such as torsional yield limit tests and tensile capacity tests, were carried out by using advanced modeling and simulation techniques. Because fatigue tests under nominal loads take a long time to complete, accelerated fatigue tests under calculated overloads were conducted to assess whether the design would meet the fatigue life requirements with low risk. After positive results were obtained from the accelerated fatigue tests, nominal loads required by the product specifications were then applied to complete the qualification fatigue tests. It was found from the experimental results that the predicted performance of the connections, such as fatigue life, critical sealing pressure, and breakout torque, matched with the physical test results well.
Despite the fact that numerous rotary shouldered threaded connection designs have been developed in the oil and gas industry, there has been no formally defined qualification procedure for such designs. The objective of this paper is to introduce, for the first time, a systematic and complete qualification procedure for rotary shouldered threaded connections.
Baldino, S. (The University of Tulsa Drilling Research Projects) | Miska, S. Z. (The University of Tulsa Drilling Research Projects) | Ozbayoglu, E. (The University of Tulsa Drilling Research Projects)
Occurrence of reversible mud losses and gains while drilling in naturally fractured formations is of primary concern. Borehole breathing can greatly complicate the already difficult practice of fingerprinting the changes in the return flow profile, hence undermining the reliability of kick detection. Issues can also derive from misdiagnosing a kick and attempting to kill a breathing well. The objective of this work is to correctly address the phenomenon and increase insights of its physical characterization. The fluid progressively flows in and out of the fractures as a consequence of three mechanisms: (1) bulk volume deformation, (2) fluid compressibility, and (3) fracture aperture variation. To represent this complex scenario, a model involving a continuously distributed fracture network is developed. A time dependent, 1-dimensional dual-poroelastic approach is coupled with a variable fracture aperture and a passive porous phase. Finite fracture length is considered and no limitation on the number of fractures is posed. The latter permits us to analyze long open-hole sections intersecting several fissures, which is a more realistic approach than the available single fracture models. The proposed model is able to quantify the pressure distribution in the fractures and the pores, together with the flow rate entering or exiting the fractures. When the fissured space is reduced to zero and incompressible bulk volume is considered, the solution reduces to that of classical reservoir engineering. A sensitivity analysis is performed on the physical properties of the formation and the drilling fluid. The latter provides a deeper insight on the factors that significantly influence breathing phenomena (i.e. drilling fluid weight, rheology and formation mechanical properties). Furthermore, a very useful application of the model is proposed by suggesting its application as breathing discriminator during kick diagnosis. The shut-in drill pipe pressure, recorded from a real kick, has been compared to the one caused by a simulated breathing case. Although the two SIDPPs show great similarities, the correct modelling of breathing can significantly help the identification of the major differences between kick and breathing. Altogether, a comprehensive in-depth characterization of borehole breathing can help with kick diagnosis and can be used to effectively design unconventional drilling techniques such as Managed Pressure Drilling.
Water-based drilling fluids are an economical and environmentally appealing option for wellbore construction. Both conventional and high-performance varieties of water-based systems typically use biopolymers to provide viscosity, suspend solids, and control fluid loss in the wellbore. Some examples include both naturally occurring biomaterials produced by plants or bacteria (e.g., starch, guar, xanthan) as well as their chemically modified analogues. However, new materials that could help improve efficiency, rate of penetration (ROP), or high-pressure/high-temperature (HP/HT) performance are necessary to expand the use of economical water-based systems in increasingly demanding conditions. Recently identified nanostructured biomaterials, such as nanocellulose, have been observed to have outstanding mechanical, structuring, and thermal properties and are known to be potent viscosifiers at low concentrations. This paper discusses a study that investigates the performance of water-based fluids by either replacing or augmenting their common oilfield biopolymers with cellulose nanofibrils (CNFs).
In this study, CNFs produced from technical-grade kraft pulp were mixed in aqueous dispersions with commercial biopolymer viscosifiers, such as xanthan and guar gum. Measurements were made of rheology, dispersion stability, CNF/biopolymer interaction, and filtration behavior as they relate to desirable fluid properties. Unexpected synergies were discovered when the CNFs were blended with secondary biopolymers. Increases or decreases observed in system viscosity were dependent upon the type of biopolymer mixed with nanocellulose but independent of the mass balance of the ingredients. In some mixtures, lower biopolymer concentrations increased viscosity within mixed systems while other mixtures decreased viscosity with increased concentrations.
The implications of these unusual findings suggest that performance efficiency can be tailored simply by mixing CNFs with biopolymers that are already used extensively in water-based fluids, allowing an operation to use less material. This discovery can enable a new method to maintain drilling fluid properties during drilling operations with the added benefit of increased temperature stability.
By modifying the surface of CNFs with secondary biopolymers, a wide range of fluid behaviors were achieved through changes in surface chemistry, surface morphology, and gel-network formation. Such nanocellulose fluid systems could serve as a renewable, nontoxic, and potentially cheaper alternative to synthetic polymers in high-performance, water-based fluids with the added benefit of controlling and helping to improve fluid properties using a mixture with common oilfield biopolymers.
Lost circulation is one of the most costly drilling issues and a major contributor of non-productive time. Wellbore strengthening has been successfully applied to reduce the associated cost and increased the wellbore stability in the industry over the past two decades. It is of critical importance to accurately predict the extra drilling mud weight after wellbore strengthening. However, previous research assumed fixed boundary conditions and only considered the stress intensity factor in the calculation of fracture reopening pressure (FROP). The change of pressure boundary on the fracture surfaces was ignored, which may overestimate the FROP. This paper employed a dislocation-based fracture model to determine the FROP in wellbore strengthening. The proposed model is compared with finite element simulation. An excellent match is obtained for the fracture profile and a clear inflection point can be observed between the plugged zone and unplugged zone. We present how wellbore pressure can change the pressure boundary in the model. Thus, the FROP calculation should be modified with the consideration of fracture plug width.
Results show that the fracture plugged zone pressure can affect the fracture profile. Specifically, lower fracture plugged zone pressure results in higher FROP. Thus, better wellbore strengthening can be achieved in the depleted sections during drilling. On the other hand, the fracture plug width plays an important role in determining FROP. With a fixed fracture plug location, larger fracture plug width can lead to higher FROP. However, there exists a critical fracture plug width for the maximum FROP, which is the value predicted by the previous research. The study reveals the importance of fracture plugged zone pressure and fracture plug width for FROP in wellbore strengthening. The model is useful for the design of wellbore strengthening materials (WSM), which are critical to achieve the best wellbore strengthening effects.
A successful cement placement can provide zonal isolation and environmental safety. Effective design of cement placement and mud removal impact all the stages of the wellbore life from drilling ahead to production. Accurate predictions of fluid displacement in the wellboreare vital to design fluid properties and plan the cementing job. In this work, an analytical model is developed to simulate the displacement of fluids in eccentric annuli.
This paper presents a novel method for the solution of cement/mud displacement and evaluation of inter-fluid contamination during displacementfor eccentric annuli. This new approach starts by addressing the problem of single fluid flow in eccentric annuli by solving analytically the governing transport equations for a flow inside an unwrapped annulus. The solution is, then, extended to a system of two fluids in a vertical annulus by adjusting the boundary conditions for displacement. The model is completed by adding the time-dependent calculation of interface between the two fluids, enabling the accurate determination of the amount of mixing and displacement efficiency.
The analytical method proposed is used to simulate single and multi-fluid flows and study the effect of fluid properties of cement, spacer, and drilling mud at different flow rates on displacement efficiency, for both concentric and eccentric, vertical annuli. 3-dimensional Computational Fluid Dynamics (CFD) simulations were also performed and the results were compared to the analytical solution. Moreover, instability of the interface in all the cases was studied and the analysis offers an understanding of the role of fluid properties and proposes applicable optimized design to enhance the displacements. The amount of mixing and contamination that occurs during the displacement was calculated for both methods. The analytical solution and CFD produce results within a 13% difference, which validates the analytical model.Evidence was gathered to support that the improper design of fluid properties and flow rate along with a highly eccentric annulus can lead to substantial cement contamination. This may lead to under-designing the amount of fluids to be pumped to provide a complete mud removal and an efficient cement placement. On the other hand, learnings and models developed allow optimizing fluid properties that may lead to best outcomes even for highly eccentric annulus.
The undeniable importance of a complete cement displacement is addressed by means of an analytical solution of the displacement, which provides a realistic prediction of the amount of inter-fluid mixing and cement contamination. This approach coupled with the interface instability analysis, which offers improvement techniques for the displacement, provide beneficial enhancements for practical industrial applications.