The complexity of oil and gas wells has increased significantly during the past decades. In addition to improved equipment, a better understanding of the subsurface environment is required to efficiently drill these wells.Fracturing, as related to circulation losses, is a contiuous challenge when drilling these wells. For that reason two modeling activities have been pursued in Norway during the past decades: 1) establishing a fracturing model for shallow sediments offshore, and 2) developing a fracture model for deepwater drilling.Data from seabed investigations as well as from conductor and surface casings were collected and normalized for varying water depths. This model works well and has been used for many years. When deepwater drilling started in 1997, the same concept was applied, and the results show that the same type of model also applies for deepwater drilling. Wells from Norway, UK, the Gulf of Mexico, Angola, and Brazil have been analyzed and show remarkably similar behavior when the water depth is considered.This paper brings this work further by presenting a generalized fracture model for shallow sediments in relaxed depositional environments. Data normalization is a key method for making the fracture model applicable for all water depths. Normalization methods are presented so fracture data can be converted to other water depths and serve as a prognosis for new wells (based on rig floor elevation, water depth, differences in overburden stresses, and on mud properties).Several field cases are presented with water depths ranging from 380 m to 1350 m, demonstrating the wide applicability of the new model.It is shown that the generalized fracture model for shallow sediments provides a good correlation with an error within a few percent.
Torque and drag modeling is regarded as an invaluable process to assist in well planning and to predict and prevent drilling problems. It discusses how to use torque and drag calculations and measurements to plan long-reach well profiles, to execute drilling operations that minimize torque and drag effects and to monitor hole cleaning. In this study a general overview on most of the available literature on the subject is presented. Different models that have been developed for torque and drag predictions along with pros and cons of the models will be discussed. The application of our new fully 3-dimensional analytical friction model will be presented. Moreover a new criterion has been implemented into the new model in order to capture the effect of weight in the horizontal section especially when BHA is dragged into/out of the wellbore. The model validity will be checked by applying the model for two field cases of ERD wells in North Sea. Field cases also demonstrate the importance of buoyancy effects, tripping speed, hydraulic piston force, pipe stiffness as well as well path effects.
Borehole stability has played an important role in the evolution of long and deep wells in later years. Borehole collapse and circulation losses are costly problems for the entire drilling industry. It is often assumed that high borehole inclination leads to lower mud weight margins.To investigate this aspect, a study of borehole stability in 3-dimensional space was undertaken. This paper presents results and applications of the findings in this study. The in-situ stress state plays a key role for borehole instability. The stress state is usually classified as normal fault, strike/slip fault and reverse fault stress states.One major conclusion of the study was that optimum stability was obtained for a lithostatic stress state, which is where all 3 normal borehole stresses are equal.For this case the window between fracturing and collapse was maximum.Bounds on the magnitude of the in-situ stresses were also invoked, resulting in a constrained 3-dimensional model that covered both fracturing and collapse pressures. Studying both fracturing and collapse pressures, it was found that maximum hole stability could be obtained at intermediate orientations, and that they are controlled be the magnitude and directions of the in-situ stresses. Field cases are also presented in the paper. It is shown that under optimal conditions combination of wellbore inclinations and azimuths can give the highest wellbore stability. The 3-dimensional fracture and collapse model presented is an efficient tool for well path optimization. Platform placement and planning of difficult wells can be improved by using this method. It is presented as analytical equations, and can easily be implemented in a calculator.
One of the most important design parameters in all wellbore stability analysis is the two horizontal stresses. The third principal stress, the overburden stress is determined by integrating the rock bulk density from cuttings or from logs. There exist several approaches to estimate the minimum horizontal stress, σ h , but it is more difficult to determine the maximum horizontal stress σ H . Inversion methods have been used with success, but usually the maximum horizontal stress is assumed, not measured. A wellbore that collapse during drilling or during production usually assumes an elliptical shape because of anisotropic stress loading. Recently an exact solution for tangential stress of an elliptic wellbore was derived. This model couples the ovality of the wellbore to the stress anisotropy. The rock strength plays an important role, such that a strong consolidated rock will have less ovality than a less consolidated rock. In addition to the solution above, the full 3-dimensional in-situ stress state is investigated to obtain physically relevant solutions. This means the fracture pressure always exceed the collapse pressure for any well orientation. This condition is imposed on the new solution above. Leak-off data are used for calibration. In other words, the solution is based on real caliper data and real fracturing data. This new solution has many practical applications, first of all to develop fracture and collapse curves for deviated wells, but also for sand production, well stimulation and reservoir subsidence. The paper will present several field cases, both from Norway and Brazil, demonstrating improvements in practical wellbore stability analysis.
The wellbore friction, torque and drag, between drill string and the wellbore wall is one of the most critical issues which limits the drilling industry to go beyond a certain measured depth. For this reason many studies on torque & drag modeling have been performed. In this regard different approaches have been used; the difference in these approaches is often on how to include bending stiffness and shearing forces in the T&D calculations. These approaches are (1) the effect of shear forces calculations with the assumption of continuous contact of the wellbore wall and the drillstring as well as constant curvature trajectory, (2) the effect of bending stiffness and shearing forces calculations with assuming of clearance between drillstring and wellbore-wall and (3) the effect of bending moment and shearing forces, assuming continuous contact of the pipe and wellbore-wall and using a non-constant curvature trajectory in which the first and second derivatives of the curve exist (i.e. spline type trajectories). This paper consists of two main parts: first a review on literatures on the subject of stiff-string T&D will be considered. Secondly, an approach will be discussed in which despite of the assumption of constant curvature trajectory and full-contact of pipe and wellbore wall, the effect of bending stiffness on torque and drag calculations are considered. The simple and robust stiff-string model is implemented in two example wells in order to find out the importance of the stiff-string calculations.
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