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Summary Slimhole well design can reduce well-construction cost through a reduction in steel, fluids, and disposal costs. In the industry, there has been a misconception that slimhole size involves the tradeoff of slower rate of penetration (ROP) and less-efficient fracture treatment. Improvements in downhole tools, drillstrings, rig capability, and drilling-fluid design have been implemented to improve ROP in slimholes. Completion designs were refined for slimmer holes to avoid any significant loss in stimulation effectiveness and maintain well value. Through systematic replication of learnings and designs across basins, slimhole well design has advanced. Introduction In the highly competitive business of unconventional drilling, well UDC (USD/bbl, USD/Mcf) must be continuously reduced. By reducing the well design to the minimum technical scope able to satisfy well functional requirements, significant well-construction cost savings can be achieved in the form of capital-equipment cost--volume of steel, mud, and cement, along with reduced disposal costs of drill cuttings, mud lost because of retention on cuttings, and less overall energy (fuel) expended to physically drill and construct a well of smaller size. Typically, slimhole drilling has been viewed as a suboptimal design because of slower ROP, downhole-tool-reliability issues, unacceptably high equivalent-circulating density (ECD), and increased completion cost. Improvements have been made in tightclearance drillpipe, new higher-torque motors and improved bit designs, rigs with high-pressure pumps, minimum mud weights (clear fluids where possible), and managed-pressure drilling to allow slimhole well-delivery performance to equal and, in some cases, exceed traditional big-hole drilling performance. Slimhole Theoretical Advantages Typical slimhole design approach in unconventionals is to reduce the size of every hole section and subsequent casing string to the minimum functional specification required. Risk-based design might be used to design aspects of the well in a fit-for-purpose fashion.
Bouska, R. (National Oilwell Varco) | Jeffery, C. (National Oilwell Varco) | Ramnarace, D. (National Oilwell Varco) | Brinkman, C. (Shell Oil) | Jorgensen, J. (Shell Oil) | Vasylyev, S. (Shell Exploration & Production Company)
The increasing complexity of deepwater Bottom Hole Assemblies (BHA) requires a complete systematic approach to each phase of the drilling process; pre-job planning, execution and evaluation. This approach was successfully applied to a recent multi well project in the Gulf of Mexico, whose vertical 12¼ in. x 13½ in. sections were historically plagued by shocks and vibrations while drilling. The root cause of inefficient drilling performance could not be accurately determined based on drilling data, but severe vibrations resulted in multiple downhole tool and BHA component failures.
However, with the implementation of a complete, holistic approach to the drilling system, the resultant effects culminated with no downhole tool failures and a significant reduction in drilling time and costs. The key to these successes were the multistage preparation process that was utilized, which included:
• A detailed analysis of all the offset data available for the same field and for fields with similar lithologies and formation properties.
• A rock strength analysis based on Logging While Drilling (LWD) data and Compressive Confined Strength (CCS) measurements.
• A bit and reamer matching process taking into account CCS, lithology, directional profile, cutters layout and at the same time, matching the performance criteria for both drill bit and underreamer.
Advanced tools and technology, including critical speed analysis and asymmetric vibration tool placement, were applied and supported by high-frequency downhole dynamics memory data with corresponding post-run drilling performance and vibration analysis. Some of the contributors to the successful application were the designed-for-purpose engineered BHA and optimized operational environment.
This approach can be applied to borehole enlargement wells, vertical, directional and horizontal wells. The analyses of the measurements gathered during all phases of the project, including a description of the obtained results, are discussed in detail. The results highlight a consistent improvement in drilling performance for the 12¼-in. x 13½-in. well section, where vibrations were the main limiting factor.
Efficient drilling performance is achieved by implementing this complete approach to the deepwater drilling system and these engineering solutions for vibration mitigation, increasing ROP, and improving BHA integrity and wellbore quality.
Copyright 2013, Unconventional Resources Technology Conference (URTeC) This paper was prepared for presentation at the Unconventional Resources Technology Conference held in Denver, Colorado, USA, 12-14 August 2013. The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s). The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein. All information is the responsibility of, and, is subject to corrections by the author(s). Any person or entity that relies on any information obtained from this paper does so at their own risk. The information herein does not necessarily reflect any position of URTeC. Any reproduction, distribution, or storage of any part of this paper without the written consent of URTeC is prohibited.
The state-of-the-art approach to downhole shock and vibration mitigation is to insert into the bottom-hole-assembly (BHA) a shock-sub or torque-limiter with response characteristics pre-configured prior to tripping into the borehole and then
continuously monitor downhole vibrations with the driller making adjustments to surface control parameters whenever the downhole shock levels are excessive. Some top-drive control systems are also capable of mitigating the torsional stick-slip
dysfunction. A novel approach is to use downhole shock and vibration measurements to actively adjust the damping characteristics of a downhole shock-absorber, thereby changing the axial (and optionally torsional) stiffness of the BHA
itself, in response to transient downhole dysfunctions and without the inherent delay of transmitting data from downhole to the driller at the surface.
At the heart of this new adaptive shock-absorber is a magneto-rheological damping fluid whose viscosity is modified whenever excessive downhole vibrations are detected by adjusting the electro-magnetic field through which the damping
fluid passes. This is similar technology to that used for protecting buildings during earthquakes and in high performance vehicles that have intelligent shock-absorbers that let the driver select a preferred stiffness for the vehicle's suspension.
This paper describes this innovative downhole self-adapting vibration damper and its autonomous control system that detects drilling dysfunctions and then iteratively adjusts the device's damping characteristics until the destructive downhole forces
and motions are mitigated - potentially before the driller at the surface is even aware that there was a problem. Modeling of the magneto-rheological fluid response is presented together with some initial downhole test results demonstrating the tool's
ability to control the stiffness and vibration characteristics of a bottom-hole-assembly. Comparisons of downhole shock levels and the amplitude of various types of vibrations at different frequencies illustrate how the self-adapting damper can
suppress different modes of BHA dysfunction and thus also demonstrating the device's ability to extend bit life and drilling tool life and to improve overall drilling performance.
Automatically controlling drilling dysfunctions from within the BHA at their source is timelier and potentially more effective than the current approach of telemetering downhole vibration information to the driller at the surface after the bit and drill
string components have already been damaged over some incremental time period. The more instantaneous response of a self-adapting tool has the ability to prevent drilling tool failures, reduce bit wear, sustain higher penetration rates with a
sharper bit and extend the interval drilled during a single bit run. For certain drilling applications this can significantly reduce the number of bit runs per hole section as well as the related drilling time and cost.
Mechanical specific energy (MSE) has been widely used in the industry to monitor drilling efficiency. However, it does not give detailed information about energy flow in the drilling system and lacks the resolution to identify the root cause of energy loss. The drilling operation is a dynamic process. Energy input may be from a surface-drive system (top drive or rotary table) or a mud motor placed downhole. In a perfect world, all of the energy is used to drill the rock. However, some of the input energy may reside in the drillstring as strain and kinetic energy due to the deformation and motion of the drillstring. Drilling energy is dissipated due to shock, vibration, fluid damping, and frictional contact between the drillstring and wellbore. A novel method has been developed to calculate the drilling energy flow in the drillstring and to enable better drilling energy management by maximizing useful energy consumption and reducing energy waste. The method provides a new way to understand and improve drilling efficiency.
The method is based on an advanced transient drilling dynamics model which simulates the full drilling system from surface to bit. The entire drillstring is meshed using 3D beam elements, and its dynamic response history is solved by the finite element method (FEM). The energy input can be calculated from surface drilling parameters, such as torque, rotation speed, flow rate, and motor differential pressure. With the simulated history of forces and dynamics of the drillstring, the corresponding strain energy and kinetic energy of the drillstring can be evaluated. The detailed cutting structure model can provide insight on the energy amount consumed by the rock cutting action of the bit and reamer. Putting all the components together leads to a holistic calculation workflow of drilling energy.
Field case studies were conducted to examine the effectiveness of this method. The studies showed the drillstring strain energy and kinetic energy are good performance indicators for drillstring reliability and stability because these energy variables reflect the severity of loading and vibration in the drillstring. The energy variables possess clear signatures for interpretation of different downhole vibration modes. Currently, the drilling efficiency is normally evaluated by MSE, which represents the amount of energy needed to remove a unit volume of rock using the surface drilling data. In this study, the energy loss is calculated to understand the percentage of input energy dissipated due to the interaction of the drillstring with the environment. In contrast to MSE, the calculation provides a more direct and detailed measurement of drilling efficiency. It gives a methodology for understanding detailed energy flow in the drilling system under different drilling vibration modes. It can be applied to bit selection, bottomhole assembly (BHA) design, and drilling parameter optimization to achieve better drilling energy management and improve drilling efficiency.
The novel approach calculates drilling energy based on the transient dynamics simulation of the full drilling system. It provides a detailed and holistic view of drilling energy input, propagation, and consumption. This method could help identify the inefficient drilling conditions and optimize drilling operation through evaluating and comparing different options.