Jerome Schubert

Detecting and Modeling Cement Failure in High-Pressure/High-Temperature (HP/HT) Wells, Using Finite Element Method (FEM)

Detecting and Modeling Cement Failure in High Pressure/High Temperature Wells Using Finite-Element Method. (December 2005) Mehdi Abbaszadeh Shahri, B.S., Petroleum University of Technology, Iran Chair of Advisory Committee: Dr. Jerome. J. Schubert A successful cement job results in complete zonal isolation while saving time and money. To achieve these goals, various factors such as well security, casing centralization, effective mud removal, and gas migration must be considered in the design. In the event that high-pressure and high-temperature (HPHT) conditions are encountered, we must attempt to achieve permeability in the set cement to prevent gas migration and to prevent any other fluid passing through to collapse the entire structure. Therefore, the design of the cement must be such that it prevents: • Micro-annuli formation • Stress cracking • Corrosive fluid invasion • Fluid migration • Annular gas pressure In HPHT cases, we need more flexible cement than in conventional wells. This cement expands more at least 2 to 3 times more in some special cases. The stress in the cement is strongly connected with temperature and pressure, as well as lithology and in-situ stress. If we can define a method which connects the higher temperature to the lower stress field, we would have the solution for one side of the equation, and then we could model the pressure (stress principles) at the designated depth and lithology. Since the stress is so dependent on temperature, the temperature variation must be accurately predicted to properly design the fluid and eliminate excessive time spent waiting on cement. In addition, a post-job analysis is necessary to ascertain zonal isolation and avoid unnecessary remedial work.

Experimental Measurements of Mechanical Parameters of Class G Cement

The new quest of unconventional resources is the achievement of well integrity which is highlighted by the inadequacy of conventional cementing procedures to provide zonal isolation. High temperatures and pressures or even post-cementing stresses imposed on the cement sheath as a result of casing pressure testing and formation integrity tests set in motion events which could compromise the long term integrity of the cement sheath due to fatigue. Knowledge of the mechanism of fatigue in cement and factors that affect it such as the magnitude of the load, strength and composition of the cement, mechanical properties of the cement and pattern of load cycles are important to achieve a realistic design of a cement system that will be subjected to fatigue loading. Such a design will go a long way to ensure the long term integrity of a well operating under downhole conditions. Finite element investigations help engineers to assess the stress magnitude and evolution for a given well configuration, but when structural calculations for casing-cement system are required, missing input parameters reduce the quality of the results. In order to have reliable data we performed an extensive experimental work using Class G cement in order to measure the principal parameters for mechanical structural calculations: compressive and tensile strength, Young modulus, Poison Ratio. The data was measured under room conditions and elevated temperature and pressure. The results were also extrapolated for a time period for more than 300 days. The paper will provide an excellent data inventory for class G cement that can be used when mechanical studies on cement, like finite element studies, are required.

Casing Failure Mechanism and Characterization under HPHT Conditions in South Texas

Casing failure probability is high in Ann Mag Field, South Texas due to the high pressure high temperature operational environment. The formation sands are over-pressured where the pore pressure ranges from 0.85 to 0.93 psi/ft. Casing damage has been experienced in over 11 wells from 18 that have been drilled in the area, near 61% of total wells were damaged during their production life. Casing failure may be caused by formation shear failure, formation compressive failure, casing tension failure, casing collapse or fault activation. Casing buckling is not considered in the study because cement bond logging shows that the cement sheath is good. Triaxial tests were carried out to measure formation mechanical properties that were used for reservoir compaction, fault activation and formation failure analysis. The Mohr-Coulomb failure criterion was applied to study the formation failure and the minimum pore pressure required to activate the fault. The finite element methods were used to analyze the casing tension failure caused by reservoir compaction. According to the formation shear failure analysis, the minimum allowable pore pressure around the casing is around 2,000 psi. In the study of fault activation, for shale formation, at the internal friction coefficient of 0.51, the probability that the normal fault can be activated is very high. The maximum drawdown and depletion were calculated based on potential casing failure types. In the plot of workability operational limits, shale formation has narrower safe zone than the sand formation. Recommendations for drilling and production were made to increase the well service life and improve the gas recovery. This paper presents the casing failure mechanism and characterization under HPHT conditions in south Texas that can be prevented in the future wells and provides workability operational limits for different formations.

Mathematical Modeling of Turbulent Flows of Newtonian Fluids in a Concentric Annulus with Pipe Rotation

Abstract In this study, a mathematical model is proposed to predict flow characteristics of Newtonian fluids inside a concentric horizontal annulus. A numerical solution, including pipe rotation, is developed for calculating frictional head losses in concentric annuli for turbulent flow. Navier-Stokes equations are numerically solved using the finite differences technique to obtain the velocity field. Experiments with water are performed in a concentric annulus with and without pipe rotation. Average fluid velocities are varied in the range of 1.1–3.3 m/s at various inner pipe rotations (0–120 rpm) in a horizontal concentric annulus. To verify the proposed model, estimated frictional pressure losses are compared with experimental data and the commercial software package ANSYS Workbench 10.0. The numerical model predicts frictional head losses with an error less than ±10% in most of the test cases.

Modeling of Newtonian Fluids in Annular Geometries With Inner Pipe Rotation

Flow in annular geometries, i.e., flow through the gap between two cylindrical pipes, occurs in many different engineering professions, such as petroleum engineering, chemical engineering, mechanical engineering, food engineering, etc. Analysis of the flow characteristics through annular geometries is more challenging when compared with circular pipes, not only due to the uneven stress distribution on the walls but also due to secondary flows and tangential velocity components, especially when the inner pipe is rotated. In this paper, a mathematical model for predicting flow characteristics of Newtonian fluids in concentric horizontal annulus with drill pipe rotation is proposed. A numerical solution including pipe rotation is developed for calculating frictional pressure loss in concentric annuli for laminar and turbulent regimes. Navier-Stokes equations for turbulent conditions are numerically solved using the finite differences technique to obtain velocity profiles and frictional pressure losses. To verify the proposed model, estimated frictional pressure losses are compared with experimental data which were available in the literature and gathered at Middle East Technical University, Petroleum & Natural Gas Engineering Flow Loop (METU-PETE Flow Loop) as well as Computational Fluid Dynamics (CFD) software. The proposed model predicts frictional pressure losses with an error less than ± 10% in most cases, more accurately than the CFD software models depending on the flow conditions. Also, pipe rotation effects on frictional pressure loss and tangential velocity is investigated using CFD simulations for concentric and fully eccentric annulus. It has been observed that pipe rotation has no noticeable effects on frictional pressure loss for concentric annuli, but it significantly increases frictional pressure losses in an eccentric annulus, especially at low flow rates. For concentric annulus, pipe rotation improves the tangential velocity component, which does not depend on axial velocity. It is also noticed that, as the pipe rotation and axial velocity are increased, tangential velocity drastically increases for an eccentric annulus. The proposed model and the critical analysis conducted on velocity components and stress distributions make it possible to understand the concept of hydro transport and hole cleaning in field applications.Copyright

Casing Point Selection for Drilling Operations at Shallow Depths

In shallow sediments, unlike deep sediments with elastic behavior, the failure mechanism of the casing shoe is strongly affected by the plasticity of the rock. Hence, the common practice in casing design which is based on using the pore pressure and fracture pressure gradients plots is not applicable in shallow sediments. Moreover, because of the plastic behavior of the sediments, the interpretation of Leak-Off Tests (LOT) in Shallow Marine Sediments (SMS) could be inconclusive. Therefore, because of uncertainties in prediction of formation fracture and pore gradients, the conductor and surface casing setting depths have always been subject to debate. Also, incorrect interpretation of LOT would lead to costly problems that might jeopardize well progress such as; well control issues, unnecessary squeeze jobs, premature setting of casing, and lost circulation problems. Two of the most important factors in any design are safety and cost. Since safety is one of the most important concerns during drilling an offshore well, planning a design based on the well control aspects would be an appropriate approach to come up with a safe and better design. A well control simulator was used to plan for well control situations. In this paper, the results were generalized for different design scenarios and a simple design method is presented. Also, a new method, supported by field data from LOT in SMS, is presented to accurately analyze casing shoe leak-off pressure in the SMS. A safe design based on the optimum lengths of conductor and surface casing would enable the operator to handle possible formation kicks. Extension of this method to well design in general suggests the potential for safer drilling operations and cost optimization.Copyright

Dual Gradient Drilling Will Control Shallow Hazards in Deepwater Environments

Currently the “Pump and Dump” method employed by Exploration and Production (EP the kick detection methods are slow and unreliable, which results in a need for visual kick detection; and it does not offer dynamic well control methods of managing shallow hazards such as methane hydrates, shallow gas and shallow water flows. These negative aspects of “Pump and Dump” are in addition to the environmental impact, high drilling fluid (mud) costs and limited mud options. Dual gradient technology offers a closed system, which improves drilling most simply because the mud within the system is recycled. The amount of required mud is reduced, the variety of acceptable mud types is increased and chemical additives to the mud become an option. This closed system also offers more accurate and faster kick detection methods in addition to those that are already used in the “Pump and Dump” method. It has the potential to prevent the formation of hydrates by adding hydrate inhibitors to the drilling mud. And more significantly, this system successfully controls dissociating methane hydrates, over pressured shallow gas zones and shallow water flows. Dual gradient technology improves deepwater drilling operations by removing fluid constraints and offering proactive well control over dissociating hydrates, shallow water flows and over pressured shallow gas zones. There are several clear advantages for dual gradient technology: economic, technical and significantly improved safety, which is achieved through superior well control.Copyright

Evaluation of Real-Time Torque-Turn Charts With Application to Intelligent Make-Up

A wide spectrum of power tongs can accommodate the increasing interest in mechanized operations at the rig floor. The importance of a torque-turn recording has been stated in many papers as a good control method for the make-up process, and the reliability of a drill string can be improved using feed back of make-up recordings. State-of-the-art devices not only allow the precise mechanized make-up of connections, but they also provide sufficient data to analyze the quality of the connection make-up. The motivation behind the research performed is the desire to make use of the data that are already provided to increases the lifetime of the connections and the reduction of drill string failures, especially for drilling under extreme downhole conditions like HPHT or deep water. This paper presents a new algorithm to evaluate the slope of the torque-turn recordings and to extract information that can be used to predict the quality of the process, with application to extreme drilling situations. This work is based on more than 300 make-up and break-out laboratory tests.Copyright