Summary Hydraulic fracturing has become a state-of-the-art stimulation technique. It has been proved over the years that significant production increase can be obtained by applying the right fracturing technique. Now, the most advanced techniques of geothermal-energy-recovery systems widely use hydraulic fracturing. The following paper presents the experimental results of the tests carried out on four different compounds using the improved "grooved-plate" method. The tests have shown a large variation of the tested-thread-compounds sealing capacity. Starting from the experimental results and the theoretical analysis of the American Petroleum Institute (API) connection, a useful chart was built to determine the real connection resistance, on the basis of its initial makeup torque. The chart offers to engineers involved in the design of a fracturing process the possibility to estimate the maximum pressure that may lead to a connection leak. Introduction Most of the published data show that a long fracture is the key to optimum well stimulation. The desired length of the fracture can be achieved by use of equipment capable to deliver the right pressure and fluid volume. Because the hydraulic-fracturing technique can also be applied to old wells, equipped with standard API connections, the high pressures that are achieved during the pumping phase require the understanding of leak resistance of API connections. It has also been proved that during the injection phase, the high pump rate may lead to additional pressure increase into the well tubulars. The time and pressure values are two key parameters that may affect the sealing capacity of the API connection. Testing the sealing capacity of a casing connection is not an easy task because it depends on many factors: thread type and form, thread compound, aging of the thread compound, and makeup-induced stresses. Actually, there are no standards to evaluate the seal capacity of a thread compound. To date, three approaches have been found in the literature:The fixture designed during the project PRAC 88-51 that consists of two circular-steel plates having a spiral grove from the center to the exterior (Wood et al. 1990).Full-scale testing of threaded assembly using a high load press (ISO 13678:2000 2000) in which not only the thread compound but the entire sealing capacity of the assembly is tested.Small-sized connections as described by Hoenig and Oberndorfer (2006). There are many advantages and disadvantages for each one of the methods, but testing thread compounds separately requires removing all inconsistent parameters that may affect the evaluation process. The main parameters that may affect the thread-compound evaluation are the stress/strain state induced because of makeup and thread tolerances. The fixture proposed by project PRAC 88-51 offers the advantage of comparing only the threaded compounds, by neglecting the makeup- and tolerance-induced errors. This is why it has been considered the use of the same experimental setup as the one described by Wood et al. (1990). The experimental setup will be presented in detail in this paper.

Local stress/strain concept analysis using material properties for low-cycle fatigue can accurately predict fatigue life of casing strings exposed to variable loads. Standard approaches that assume only static loads ignore the variations in temperature and pressure that affect loads on casing strings during steam injection or in geothermal wells and lead to inaccurate estimates of fatigue life. Full-scale experimental results show that our method properly accounts for these variations, reducing time and cost in fatigue life prediction. Our method can be extended to any type of fatigue in which large deformations occur. Introduction The casing string is generally considered to be exposed to static or quasistatic loads. Mostly, these external loads act over long times, and therefore the assumption of static loads is correct. Current industry design standards consider the casing string to be statically loaded, ignoring changes in temperature or internal pressure in geothermal operations that can subject the string to variable loads and thus leading to fatigue failure. Because casing movement is restricted by the presence of a cement sheath, temperature variations induce thermal stresses in the casing string; these stresses may become greater than the material's yield strength. In such cases, the fatigue behavior of the casing material can be considered as low-cycle fatigue (LCF). Existing geometrical changes in the casing body (such as the thread) will amplify the local stress distribution and reduce the low-cycle fatigue resistance. Wöhler diagram or stress-number of cycles curve (S-N curve) is a way to represent the cyclic behavior of materials, see Figure 1. As the plot indicates, the higher the magnitude of the stress, the smaller the number of cycles to failure is. The horizontal curve at higher N values is characteristic to ferrous materials (steels) and it is called fatigue limit or endurance limit. On high stress values there is no endurance limit, and this is also characteristic to casing loads in which the induced stresses are higher then the material yield strength.

Hydraulic fracturing has become a state of the art stimulation technique. It has been proved over the years that significant production increase can be obtained by applying the right fracturing technique. Nowadays, the most advanced techniques of geothermal energy recovery systems widely use hydraulic fracturing. The following paper presents the experimental results of the tests carried out on four different compounds using the improved "grooved plate" method. The tests have showed a large variation of the tested thread compounds sealing capacity. Starting from the experimental results and the theoretical analysis of the API connection a useful chart was built to determine the real connection resistance, based on its initial makeup torque. The chart offers to engineers involved in the design of a fracturing process the possibility to estimate the maximum pressure that may lead to a connection leak. Introduction Most of the published data show that a long fracture is the key to well optimum stimulation. The desired length of the fracture can be achieved using equipment capable to deliver the right pressure and fluid volume. Since the hydraulic fracturing technique can be also applied to old wells, equipped with standard API connections, the high pressures that are achieved during the pumping phase require the understanding of leak resistance of API connections. It has been also proven that during the injection phase the high pump rate may lead to additional pressure increase into the well tubulars. The time and pressure values are two key parameters that may affect the sealing capacity of the API connection. Testing the sealing capacity of a casing connection is not an easy task since it depends on many factors like: thread type and form, thread compound, ageing of the thread compound, make-up induced stresses, etc. Actually, there are no standards to evaluate the seal capacity of a thread compound. To date, three approaches have been found in the literature: There are many pros and cons for each one of the methods, but testing thread compounds separately require getting off all inconsistent parameters that may affect the evaluation process. The main parameters that may affect the thread compound evaluation are the stress-strain state induced due to make-up and thread tolerances. The fixture proposed by the project PRAC 88–51 offers the advantage of comparing the threaded compounds only, by neglecting the make-up and tolerances induced errors. This is why it has been considered the use of the same experimental setup as the one described in paper [1]. The experimental setup will be presented in detail later in this paper. Thread Compounds for Oil Country Tubular Goods (OCTG) Typical threaded compounds for OCTG are formed using base grease in which solid particles are dispersed. The grease is standard lubricating grease made of mineral oil having a metal soap as thickener (i.e. aluminum stearate). In very low amount, additives are added to the compound to improve the following properties: high pressure resistance, wear protection, corrosion protection, etc. The role of solid particles is to provide anti-galling resistance and sealing properties of the compound. Powdered metals and non-metallic particles like graphite or ceramic spheres are used as solid ingredients. Typical metals used for threaded compounds manufacturing are: lead, copper, zinc. The common non-metallic solids used for compounds are graphite, PTFE, ceramics. The so called "green dope" or environmental friendly compounds have a totally metal-free composition. Figure 1 shows a classification scheme of thread compounds after [3]. Table 1 shows the composition of some common threaded compounds used in the oil industry, including the tested thread compounds described in this paper.

No abstract

The heavy weight drill pipe, one of the most expensive components of the drill string, is exposed, beside fatigue and corrosion, to an intensive wear process as a result of the friction with the inner wall of the casing or the borehole wall, that lead to a drastically decrease of it durability. In order to improve the durability, the worldwide specialists apply the hardbanding technology to increase the heavy weight drill pipe wear resistance. The major problem raised by the hardbanding process is the selection of the most suitable wear resistant alloy and the optimum hardbanding technology. The present research work investigate the possibility of hardbanding the heavy weight drill pipes by using the gas metal arc welding process, taking into considerations three different wear resistant materials with the trade name ARNCO 100XT, ARNCO 300XT and FLUXOFIL M58.