Browsing by Subject "Well Integrity"
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Item Cement fatigue and HPHT well integrity with application to life of well prediction(2009-05-15) Ugwu, Ignatius ObinnaIn order to keep up with the world?s energy demands, oil and gas producing companies have taken the initiative to explore offshore reserves or drill deeper into previously existing wells. The consequence of this, however, has to deal with the high temperatures and pressures encountered at increasing depths. For an oil well to maintain its integrity and be produced effectively and economically, it is pertinent that a complete zonal isolation is achieved during well completion. This complete zonal isolation can be compromised due to factors that come into play when oil well cement experiences cyclic loading conditions which can lead to fatigue failure as a consequence of extensive degradation of the microstructure of the cement material depending on stress levels and number of cycles. There have been a lot of research and experimental investigations on the mechanism of fatigue failure of concrete structures but the fatigue behavior of oil well cement is still relatively unknown to engineers. Research in the area of oil well cement design has led to improved cement designs and cementing practices but yet many cement integrity problems persist and this further strengthens the need to understand the mechanism of cement fatigue. This research seeks to develop a better understanding of the performance of the casing cement bond under HPHT well conditions that can lead to best practices and a model to predict well life. An analytical model, which can be used to evaluate stresses in the cement sheath based on actual wellbore parameters, was developed and combined effectively with finite element models to evaluate the fatigue and static loading behavior of a well. Based on the findings of this investigation, the mechanical properties of the casing, cement and formation as well loading conditions play a very big role in the static and fatigue failure of well cement. Finally, recommendations for future work on this subject were also presented in order to understand all tenets of cement fatigue and to develop governing equations.Item The Effect of Cement Mechanical Properties and Reservoir Compaction on HPHT Well Integrity(2012-11-15) Yuan, ZhaoguangIn the life of a well, the cement sheath not only provides zonal isolation but also supports casing and increases casing-collapse resistance. Due to the high-pressure, high-temperature (HPHT) conditions, the cement sheath plays an important role in maintaining wellbore integrity. During the production process in HPHT wells, the pressure differential inside the casing and the surrounding formation is larger than the conventional wells. The stress induced by fluid withdrawal in highly compact reservoirs can cause the cement and the casing failure in these wells. These present a greater challenge to the wellbore integrity than the conventional wells. To have reliable data, extensive experimental work on Class G cement was carried out to measure the principal parameters for mechanical structural calculations. The experiment was also set up to simulate conditions under which cement low-cycle fatigue failure could occur. Zero-based cyclic pressure was applied to the casing in the cement low-cycle fatigue test. Three types of cement (72-lbm/ft3, 101-lbm/ft3 and 118-lbm/ft3) were cured and tested at 300?F to study the cement mechanical properties under high-temperature conditions over the long term. The tests included a 1-year mechanical properties measurement such as compressive strength development; i.e., Young?s modulus and Poisson?s ratio. Finite element methods (FEM) were used to study the casing buckling deformation characteristics of reservoir compaction in some south Texas wells. The 2D and 3D FEM models were built to study the effects of mechanical properties and reservoir compaction on HPHT well integrity. As the confining pressure increases, the cement shows more plasticity and can withstand more pressure cycles. The cement with a higher Poisson?s ratio and lower Young?s modulus showed better low-cycle fatigue behavior. Casing collapse resistance is very sensitive to void location, cement Poisson?s ratio, cement Young?s modulus, and pore pressure. Casing eccentricity and voids shape have minor effect on the casing-collapse resistance. Casing shear failure, tension failure, and buckling failure are the most likely failure modes in reservoir compaction. For different casing wall thickness, the critical buckling strain is almost identical. This study presents a better understanding of casing failure and cement failure in HPHT wells. The results of the study will help improve cement and casing design to maintain wellbore integrity that can in turn be expected to extend throughout the life of the well.Item Well Integrity Diagnostics for Sustained Casing Pressure and Faulty Gas-Lift Valve Based on Pressure Transient Modeling(2014-10-20) Rocha-Valadez, TonyA problem frequently present in the oil and gas industry is the difficulty of measuring well integrity parameters; particularly, for high-pressure high-temperature wells. For this reason, many relevant parameters, indicators of the integrity of the well, are not directly measured but rather qualitatively estimated by testing response variables, sometimes, unfortunately, without understanding the correlation between the key parameter and the response variable. This research presents methodologies to quantitatively evaluate well integrity in wells with sustained casing pressure (SCP) and gas-lifted wells with faulty gas-lift valves (GLV). The phenomenon occurring during these well integrity issues were modeled using the thermodynamic properties and transport phenomena occurring inside the wellbore. Well integrity denotes the ability to maintain intentional isolation between the formation and the well. The consequences of not detecting and managing well integrity issues can go from the activation of rupture discs to a release of oil/gas, fire and/or explosion during a blowout. For the SCP problem, the developed analytic model has been validated against field data and compared to other numerical models showing similar performance. The SCP model allows for early time data to be used to accurately predict the leak?s severity by estimating a seepage factor, which is akin to permeability, to account for leakage occurring through the imperfect cement sheath. In comparison to current practices, the model shortens the testing time and reduces the risk from gas accumulation and pressure buildup, making it an inherently safer testing procedure. The methodology developed to assess wellbore annular integrity, during gas-lift operations, has been compared to acoustic well sounding (AWS) data from different wells. The model divides the well into small elements and estimates average properties which are used to quantify the total amount of mass and hydrostatic pressure in the annulus at any given time. This methodology accurately tracks casinghead pressure and liquid level increase. When fluid intrusion occurs mostly through the gas-lift valve, the model allows estimating the damage coefficient of the faulty GLV. This coefficient serves as a quantitative parameter for GLV replacement; being independent of acoustic well sounding devices. This methodology has the advantages of easy and quick implementation, being accurate, not requiring any specialized equipment, and providing a quantitative damage parameter for the GLV.