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 digitized workflow from pre-drill pore pressure modelling with Monte-Carlo approach, till update of pressure prognosis while drilling from e.g. sonic and/or resistivity data is described. The innovative approach will reduce the uncertainty in the mud-weight window ahead of the bit.
For the 3D pressure modelling, a basin modelling software method is used, where the pressure compartments in the study area are defined by faults interpreted from seismic. Key input parameters like minimum horizontal stress, vertical stress and frictional coefficients for failure criteria are varied. The output is pressure profiles along the planned well path, with uncertainties.
The work presented in this paper was carried out on a North Sea dataset. The results show that the uncertainty in the pore pressures will highly influence the uncertainty span in both the fracture gradient and the collapse gradient. Representing the mud-window in terms of a most likely collapse and fracturing curve, with on each side of both, limits the minimum and maximum pore pressure derived limits, makes for a more realistic prediction, stating the uncertainty in the result.
While drilling, log-data will be automatically used to update the pressure and mud-weight prognosis ahead of bit. The digital updated prognosis can help the drilling crew in the decision-making during drilling campaigns.
A hybrid model based on Physics of failure and Data-driven algorithms is developed that can estimate remaining useful life of production casing (well barrier). The state of integrity of the well barrier is assessed by updating the reliability under operational loads.
The interactions between the casing and surrounding formation, and effects of tribocorrosion on the casing are considered. Tribocorrosion is the process of degradation of a material resulting from a sequential process of (i) mechanical wear (due to sliding, friction, or impact) followed by (ii) a corrosive action of the surrounding environment. The model includes simulating casing wear due to drilling, and enhanced degradation due to conditions in the well.
The main capability of the model is to help well integrity analyst with insight of future health states of a monitored well. This is achieved in two main steps; the first being the offline module comprised of degradation models. The second is the pattern recognition based on well log and features mapping, and estimation of remaining useful life of well barrier. The production casing grade P-110 undergo reduction in strength due to wear during drilling, induced stress and hydrogen induced cracking. The remaining useful life is calculated for the depths of interest and time.
A comparative analysis is carried out using the industry standard soft-string model versus a more comprehensive stiff-string model to estimate wear. The paper presents a unique approach to predict the remaining useful life of a well barrier and the dynamic state of the well's operational integrity. The prediction is not solely based on statistical modeling but also incorporates barrier engineering and physics of failure in the model.
Because wells drilled by the oil and gas industry are becoming more complex, the drillstring is subjected to additional severe loading conditions, resulting in increased failure risk. It is universally known that fatigue is one of the primary causes of drillstring failure, accounting for more than 70% of the failures. Drillstring failure generally results in unexpected catastrophic twistoff of bottomhole assembly (BHA) components and costly fishing operations. For these reasons, it is important to develop a drillstring fatigue life prediction model.
Fatigue is driven by cyclic stresses and accumulates over time. These cyclic stresses can occur in a wide range of conditions, such as rotating the drillstring through a high-dogleg severity well section, and severe bending due to whirling or buckling of the drillstring. The procedure relies on the powerful and accurate drilling dynamics computation engine, which is based on a 3D finite element model and which predicts transient dynamics response and stresses along the drillstring under drilling operations loading conditions. First, the section being drilled is subdivided into multiple small intervals. For each interval, simulation is used to predict the drillstring deformation and contact force. Stresses can then be evaluated for each component in the drillstring.
Due to the cumulative nature of fatigue failure, it is necessary to track the cycle of alternative stresses of the drillstring while drilling an entire section. For stable rotary drilling, the number of stress cycles can be calculated from the number of rotation revolutions within the interval. In the real-time fatigue monitor application, the actual drillstring revolutions measured at the surface can be used directly as the stress cycle. When severe downhole vibration exists, the rain-flow counting method is used to count the stress cycles for the complex dynamics stress history. To consider mean stress effect, the Goodman law is used to compute the equivalent alternative stress. With the pre-collected fatigue S-N curves (stress level versus the cycle to failure) for different connection types and drilling tools, the fatigue life consumption in one interval is calculated. Finally, using Miner’s rule, the fatigue life results in all intervals are summed to obtain the cumulative fatigue damage to the drillstring. The fatigue model, which has been implemented as one of the key components in the drilling analysis workflow, provides engineers with the analysis capability to identify the potential factors influencing the integrity of BHA components. Two case studies validated the drillstring fatigue prediction model.
This paper presents a practical and effective procedure for calculating drillstring and BHA component life due to fatigue accumulation. This modeling tool enables engineers to employ a systematic approach for quantifying the fatigue risk during the well planning and real-time execution phase.
Lyons, N. (Baker Hughes, a GE company) | Izbinski, K. (Baker Hughes, a GE company) | Pauli, A. (Baker Hughes, a GE company) | Gavia, D. (Baker Hughes, a GE company) | Hoffman, M. (Cimarex Energy Co.) | Cantrell, B. (Cimarex Energy Co.) | Bryant, S. (Cimarex Energy Co.)
The development of improved synthesis techniques for polycrystalline diamond compacts (PDC) positively impacted fixed cutter drill bit performance. Coupled with these advances, recent developments in cutter geometry show improved cutter performance in many applications. Laboratory and field testing has demonstrated that modifying the face geometry of the PDC cutter used in a fixed cutter bit is one of the most direct ways to affect the efficiency and longevity of the bit's cutting structure. This paper describes a new non-planar cutter face geometry that has increased footage drilled, rate of penetration (ROP), and improved the bit dull condition in the Meramec formation in western Oklahoma's STACK play.
A drilling mechanics focused team created a finite element analysis (FEA) model of the rock cutting process to optimize cutter face geometry for improved cutting efficiency. The new non-planar geometry enabled better cutting efficiency and improved cutter cooling. Multiple lab tests were then used to verify the model's predictions.
Results from single cutter lab tests showed an 11% increase in cutting distance as measured in a vertical turret lathe test, a 30% decrease in cutting edge temperature from a pressurized cutting test, and a 10% increase in load capacity compared to a previous non-planar geometry in a face load test. Full-scale pressurized drilling tests in the lab showed that a PDC bit with the new geometry was 15% less aggressive with equivalent-to-lower mechanical specific energy (MSE) when compared to the same PDC bit with a previous generation non-planar cutter.
Field tests were conducted with the new non-planar geometry applied to a commercial 0.529 inch [13mm] cutter on a standard 8-1/2 in. drill bit design used in the Meramec Lateral application. The paper reviews in detail three test cases in this multiple bit lateral section using the same bit design with and without the new non-planar cutters. In two test wells, we saw direct improvement of 205% distance drilled on average and a 33.5% boost in ROP. At least 17 bit runs have been completed in this application using the new non-planar feature, proving it to be a beneficial enhancement. Similar performance improvement has been observed in other applications as well.
The optimized cutter geometry has led to further and faster runs, resulting in significant time savings and improved consistency. The use of advanced cutter geometries provides a significant boost in drilling performance beyond that normally achieved through fixed cutter bit design optimization and materials improvements.
The Barlow equation for tubular internal yield pressure is widely used in American Petroleum Institute (API) and International Organisation for Standardisation (ISO) standards, but its provenance and accuracy have never been established: indeed, until very recently, the original reference had been lost to the industry. This has led to doubt and confusion about its use. This paper presents the work done by ISO TC67 SC5 workgroup 2 to remedy this, and explains the background and technical basis for the upcoming revisions to ISO TR 10400.
It is shown that Barlow's 1836 derivation violates the 3D constitutive law, and the result is therefore incorrect as originally purposed (a thick wall hoop stress). Moreover, hoop stress is a uniaxial (1D) check: the modern approach is 2D or 3D checking, based on a material failure condition such as the Von Mises yield criterion.
However, the result also happens to represent the thin wall approximation to the VME failure pressure for plane stress (i.e., zero axial load), which gives an accurate measure of the yield pressure. Remarkably, this does not seem to have been recognised in previous work. The derivation is given, and the assumptions and limitations explained.
Present design practice is over-conservative for thick wall pipe, and potentially unconservative for thin wall. This is not the fault of the Barlow equation
The industry should therefore consider revising OCTG burst ratings and accompanying design practice to achieve a more uniform safety level over the full D/t range of casing and tubing.
This paper presents a new model that integrates relevant aspects of geomechanics and drilling fluids to provide practical solutions for wellbore strengthening. This integrated approach accounts for the influence of stress state, rock properties, wellbore design parameters, and the impact of particle-laden fluids on the creation, growth, and arrest of an induced fracture. A dimensionless parameter called the Wellbore Strengthening Index (WSI) is introduced as a measure of desired strengthening normalised to the rock stiffness. WSI is used to estimate the concentration of wellbore strengthening material (WSM) required for a given application. A semi-empirical correlation then relates solids loading to the length of an induced fracture at time of sealing. A closed-form equation is proposed to estimate fracture width using this calculated fracture length. Once induced fracture dimensions are determined, appropriately sized WSM is selected for efficient sealing. The model is relatively straightforward to code into a practical and user-friendly design tool for wellbore strengthening. Two case studies are presented to show successful implementation of the new model.
Premature failures of rotary-shouldered connections (RSC) resulting from improper re-cut and repair operations performed on used connections can cause immense financial loss to oil and gas operators. One of the reasons for improper re-cut and repair procedures is the desire to limit the loss of tool length during thread repair operations. However, this leads to residual damage which can compromise the connection performance and lead to premature connection failures.
To study the issue, finite element analysis (FEA) modeling was utilized to determine the distribution of the stresses in RSCs and to map the stress profile at critical locations which could be affected by improper repair operations. In addition, two case studies where the connections of the components failed prematurely in their first run after the connections were freshly re-cut were investigated. The thread repair protocols and post-threading nondestructive inspections were reviewed. Visual examination and optical metallography were utilized to characterize the failure mechanism along with the location of the failures. The failures were not located at the last engaged threads of the connection, which is not typical. Both case studies indicated that the residual damage left from improper thread recut operations had led to the thread failures at non-standard locations.
The paper presents the case studies, the details of the metallurgical analysis along with the FEA simulation data. The paper lists the various options available for thread repair along with the commercial benefits associated with these repair options in terms of tool length savings. The paper also presents the failure risks associated with the thread repair techniques. Additionally, the paper discusses the correct re-cut and repair procedures for RSCs along with required NDE inspections that need to be performed after the repair of the connection. By applying and following the recommended repair and inspection protocols, premature connection failures resulting from improper thread repair may be eliminated.
Wellbore stability in shale is often hampered by the detrimental effect of existing weak bedding planes on the strength of the rock surrounding the wellbore against shear failure. This paper presents results from a formal closed-form analytical solution to the wellbore stress problem that incorporates rock failure along weak bedding planes. The solution is used for a case study of a highly inclined well section in a laminated layer of troublesome shale with a strike-slip faulting regime above the target formation in the Latin America region.
Connector joint strength and leak resistance depend on internal and external pressures. Axial tension or compression opens (dilates) the connector, while the pressures close the connector. This paper presents a "toy connector" model that incorporates actual connector elements, but in a simplified form useful for analysis, so equations of joint strength and leak resistance can be determined as functions of axial load and internal/external pressures. The model and example case in this paper confirm that dilatancy occurs and hydrostatic pressure plays a critical role in connector failure (leak, pull-out, or thread shear). Von Mises stress alone cannot model dilatancy and the hydrostatic effect.