Unconventional shale drilling has developed rapidly during the last decade. The development of substantial prospects, especially in shale gas plays, requires advanced technologies and prudent well monitoring. Many new operational challenges are associated because of simultaneous operations, such as drilling and fracturing nearby wells.
While drilling, the interaction of the fractures from nearby wells results in pressure communication and unexpected well kicks. Because of the low porosity of those tight reservoirs, those uncommon well kicks have so far not been severe, but taking the time to control wellbore pressures adversely affects the efficiency of the drilling process and increases the non-productive time. Additionally, as fracturing becomes more efficient, and conductivity increases, possible future events might have much more dramatic consequences. This paper presents a method that uses the positional uncertainty of the well being drilled, positional uncertainty of the fractured/fracturing wells, and uncertainty of the fracture length and fracture orientation to avoid this problem. The paper discusses the reasons and challenges, techniques, and lessons learned to solve the simultaneous moving boundary conditions between the reference wells and multiple offset wells. The method of explicit solution avoids a trial-and-error procedure and provides excellent maneuverability of planning requirements. The modified model and methods provided the use of exact mathematical solutions and uncertainty ellipses estimates.
This method can also be used to monitor the influence of other variables, such as temperature cycling, oil drainage distance, wellbore storage effect, and reservoir pressure transients. The method presented can be used in the planning stage to ensure optimal well placement; it can also be used for designing super fractures between wells. The study indicates that the use of real-time data from drilling and fracturing wells can predict the need for remedial action.
The benefits of casing while drilling have become apparent to the industry as complex wells are drilled through depleted reservoirs. Casing while drilling operations help to reduce lost circulation, provide wellbore strengthening, mitigate formation damage, and eliminate non-productive time (NPT). The plastering effect mechanism responsible for pulverizing and smearing the cuttings in the formation to increase the fracture gradient is under extensive research to effectively realize the above benefits. An analytical model used to predict the temperature of the drilling fluid downhole while drilling with a casing will provide an improved understanding of this plastering effect. An estimate of the downhole temperature increases attributable to the persistent contact between the casing and the borehole wall, a characteristic of casing while drilling, will add to the ongoing quantitative analytical studies.
This study proposes an analytical model to analyze the heat generated from the contact between the casing and the borehole wall during a casing while drilling operation and predicts the downhole temperatures of the drilling fluid at any depth of the well. The torque acting on the casing as a result of contact forces was used to model the heat generated, and a steady-state heat transfer solution is presented. The model also incorporates the heat dissipated downhole as a result of frictional pressure losses along the drillpipe, casing, and bottomhole assembly (BHA), as well as energy dissipated through pressure losses across the bit.
The paper presents four practical casing while drilling field cases to suggest potential applications for the proposed model. Downhole temperatures of the drilling fluid were calculated along the well profile, and the increase in mud temperature along the target zones was estimated. The effect of the increase in downhole mud temperatures while drilling with a casing is then analyzed in the context of improving the fracture gradient attributable to the plastering effect. In addition, the effect of drilling parameters on the increase in fracture gradient has also been presented. This simple analytical model can be applied to casing while drilling operations to enhance our understanding of the plastering effect and to use it to our advantage.
Wellbore friction has led to an increase in the temperature of the drilling fluid downhole as well as heating of the drillpipe and the bottomhole assembly. Severe downhole heating can lead to potential well-control safety issues and result in costly remedial efforts. An estimation of the increase in temperature of the drilling fluid downhole as a result of this wellbore friction will certainly help in taking measures to prevent such dire circumstances and will lead to an improved drilling plan.
This study uses a simple analytical model to analyze the heat generated from borehole friction and then predict the downhole temperatures at any depth in the well during a drilling operation. The torque acting on the drillstring as a result of contact forces has been used to model the heat generated from friction, and a steady-state heat transfer solution has been presented. The model also incorporates the heat generated downhole owing to frictional pressure losses along the drillstring as well as across the bit.
Practical field cases, including a horizontal well, have been presented in the paper to validate the model. Downhole temperatures of the drilling fluid have been calculated along the well profile using the model and then compared with the field data measured using MWD tools. The zones having the maximum increase in temperatures can be identified based on the temperature profile generated during the drilling operation. The impact of drilling parameters on temperatures has also been analyzed and can be used effectively to maintain a better check on undesired temperatures.
This simple analytical model can be suitably applied to field cases based on the well profile and can be effectively used to predict the maximum temperatures to be encountered downhole while drilling ahead as planned. An accurate estimation of maximum temperatures will help us prevent severe downhole friction heating in the future.
The predictions of hookload, tension, and stresses are important for the successful completion of any well operation, as well as for the selection of optimum operating parameters. The terms effective axial force and true force or effective tension and true tension are used for various tubular designs, including riser designs. To further complicate matters, they are also defined as buoyancy and pressure area methods of calculating forces. Discussions of these two forces occasionally raise doubts regarding how to treat the internal and/or external pressures. The main problem stems from the perception that effective force as a fictitious force, and the inclusion of the internal and external pressures as counter intuitive source of misunderstandings and wrong designs. Although several authors have used the calculation and concepts correctly, other previously published papers failed to address the issue clearly and in a simple way; as a result, confusion still exists. These two forces further result in two neutral points, which raises the questions of "when you apply pressure, does the neutral point change??? and "which neutral point should be used for stuck pipe calculations??? The working envelop using yield limit with combined loading of uniaxial force and pressure forces are important. Using the wrong force/tension will result in an incorrect interpretation of the burst and collapse limits. This paper provides the mathematical treatise and discusses the theoretical basis of these two forces, including how they are used in torque, drag, buckling, stress, and limit calculations. This paper describes the details, apparent definition, pitfalls, and context in which these forces can be used. Simple guidelines are presented, as well as several example calculations to help explain various tubular modeling aspects for future generations of drilling engineers.
Riserless drilling poses numerous operational challenges that adversely affect the efficiency of the drilling process. These challenges include increased torque and drag, buckling, increased vibration, poor hole cleaning, tubular failures, poor cement jobs, and associated problems during tripping operations. These challenges are closely associated with complex bottomhole assemblies (BHAs) and the vibration of the drillstring when the topholes are drilled directionally. Current methods lack proper modeling to predict drillstring vibration.
This paper presents and validates a modified model to predict severe damaging vibrations, analysis techniques, and guidelines to avoid the vibration damage to BHAs and their associated downhole tools in the riserless highly-deviated wells. The dynamic analysis model is based on forced frequency response (FFR) to solve for resonant frequencies. In addition, a mathematical formulation includes viscous, axial, torsional, and structural damping mechanisms. With careful consideration of input parameters and judicious analysis of the results, the author demonstrates that drillstring vibration can be avoided by determining the 3-D vibrational response at selected excitations that are likely to cause them. The analysis also provides an estimate of relative bending stresses, shear forces, and lateral displacements for the assembly used. Based on the study, severe vibrations causing potentially damaging operating conditions were avoided, which posed a major problem in the nearby wells.
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. This study compares simulated predictions with actual well data and describes the applicability of the model. Simple guidelines are provided to estimate the operating range of the drilling parameter to mitigate and avoid downhole tool failures.
Methodology and Calculation
The dynamic analysis model is a finite-element-based forced vibration-frequency response (FFR) analysis, which is used to determine the steady-state dynamic response of a drillstring section owing to specified harmonic excitation. The model computes the stiffness matrices, including the linear, geometric, contact, and friction stiffness matrices. The structural mass is based on the material of the drillstring components; the non-structural mass is also included. Structural damping accounts for the energy losses resulting from mechanical means, such as shock subs, motors, jars, etc.; whereas drilling fluid accounts for the energy loss in the fluid medium in the axial and torsion directions.
The added mass owing to the drilling mud results in fluid inertia as the string moves laterally, resulting in displacement of fluid. It has been found that added mass decreases the natural frequency of the drillstring compared to that measured in air. For lateral fluid damping, the closed-form solution derived by Chen is used, which actually uses the flow equations for fluid moving around a cylinder in a confined space. Hence, the formulation is viscosity dependent. Structural damping is proportional to the linear structural stiffness, with the proportional factor as a function of the angular frequency of the system and a constant damping ratio. The internal damping ratio used varies from 0 to 1 and is controlled internally. The damping matrix is dependent on the beam element lengths, which can be adjusted by the user. If the damping is not included, the solution becomes singular, resulting in infinite amplitude. Caution is to be exercised when overdamping the system, as it will skew and shift the critical speeds. In addition, the mathematical formulation includes fluid, viscous, axial, torsional, and structural damping mechanisms.
The advent of extended-reach drilling (ERD) has led to increased effects of friction on the drillstring and the drilling fluid. Wellbore mechanical friction attributable to pipe rotation or to torque and drag plays a significant role in drilling operations and is considered to influence the temperatures downhole. An analytical mathematical model will help to provide physical insight to understand the borehole conditions.
This study aims to develop a simple mathematical model to analyze the heat generated from borehole friction and to predict the temperature of the drilling fluid during a drilling operation at any depth in the well. The model presents a steady-state solution for heat transfer between the drillstring and the fluids in the drillpipe and annulus, as well as heat transfer between the annular fluid and the formation. Heat generated from friction has been modeled using the torque acting on the drillstring as a result of contact forces. A linear temperature gradient for the formation and a constant borehole wall temperature has been assumed to simplify the model. Frictional pressure losses in the drill pipe, in the annulus, and across the bit have been incorporated in the model because they contribute to the heat generated in the borehole.
This paper will present the derivations of the generalized heat transfer model and the application of the model in a field case. The temperature of the drilling fluid has been calculated both in the annulus as well as inside the drillpipe at various depths for the considered well profile. The drillstring temperature profile has been estimated using the calculated drilling fluid temperatures. Wellbore mechanical friction and downhole temperatures were thoroughly studied by varying the underlying drilling parameters in the model.