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In the past few years, operators have been increasing the lateral lengths of horizontal wells to maximize the reservoir contact and production rates. However, the frictional forces between the coiled tubing (CT) and casing in those long lateral wells also increase, limiting the ability of conventional CT sizes to reach the end prior to lock-up occurring. Technologies such as lubricants, vibratory tools and tractors are usually used to extend the CT reach. However, the downhole performance of some of these friction-reducing technologies is sometimes unpredictable and inconsistent. In addition, with the current industry's trends to lower the overall intervention costs, lubricants may be considered too expensive in long laterals. This paper reports on the laboratory evaluation of the friction-reduction performance of a novel CT surface treatment. This surface treatment has been proven to be effective at reducing the frictional forces by altering the CT surface finish. After the treatment, the CT surface is smoother and has micron-size dimples that work as small reservoirs, preventing a lubricant from being easily washed off the CT surface. The new metal surface treatment was applied to several CT samples. The friction between the treated CT samples and various actual casing samples was studied in a laboratory on a linear friction apparatus. This instrument is specifically designed to measure the coefficients of friction between CT and casing at downhole conditions, such as with or without fluids relevant to coiled tubing operations and at temperatures as high as 100°C. Additionally, laboratory tests were performed to determine the ability of the treated and un-treated CT samples to retain lubricants when sliding on the casing surfaces. Currently, there are two main operational challenges of using lubricants for reducing the CT friction. First, to reduce the lubricant volume in long laterals, and therefore the intervention costs, many operators choose to pump lubricant slugs instead of pumping the lubricant continuously. However, most of the lubricant is consumed inside the CT, and only a small lubricant amount adheres to the outside CT and casing surfaces where the friction needs to be reduced. Secondly, even if the lubricant coats the outside CT surface, there is a risk of being quickly washed off, unless new lubricant is pumped continuously. The laboratory testing results obtained from this study have shown a reduction of the coefficients of friction after the CT metal surface treatment. These results prove the friction-reduction potential of manufacturing a CT with the new treated surface for extending the CT reach with or without friction-reducing technologies such as lubricants, vibratory tools and tractors. The advantage of utilizing the new CT metal surface treatment is that a lubricant remains longer in the micron-size pores on the CT surface and reduces the CT friction more consistently. The novel idea in this paper encompasses the fact that the CT metal surface treatment has the potential to reduce the CT friction by itself and further in combination with friction-reducing technologies such as lubricants, vibratory tools or tractors. The new CT surface is smoother and has micro-pores that can prevent a lubricant from being easily washed off the CT surface. The laboratory tests with the new CT samples have shown reduced coefficients of friction when comparing to conventional CT coupons with un-treated surfaces.
In the past few years, operators have been increasing the lateral lengths of horizontal wells to maximize the reservoir contact and production rates. However, the frictional forces between the coiled tubing (CT) and casing in those long lateral wells also increase, limiting the ability of conventional CT sizes to reach the end prior to lock-up occurring. Technologies such as lubricants, vibratory tools and tractors are usually used to extend the CT reach. However, the downhole performance of some of these friction-reducing technologies is sometimes unpredictable and inconsistent. In addition, with the current industry's trends to lower the overall intervention costs, lubricants may be considered too expensive in long laterals. This paper reports on the laboratory evaluation of the friction-reduction performance of a novel CT surface treatment. This surface treatment has been proven to be effective at reducing the frictional forces by altering the CT surface finish. After the treatment, the CT surface is smoother and has micron-size dimples that work as small reservoirs, preventing a lubricant from being easily washed off the CT surface. The new metal surface treatment was applied to several CT samples. The friction between the treated CT samples and various actual casing samples was studied in a laboratory on a linear friction apparatus. This instrument is specifically designed to measure the coefficients of friction between CT and casing at downhole conditions, such as with or without fluids relevant to coiled tubing operations and at temperatures as high as 100°C. Additionally, laboratory tests were performed to determine the ability of the treated and un-treated CT samples to retain lubricants when sliding on the casing surfaces. Currently, there are two main operational challenges of using lubricants for reducing the CT friction. First, to reduce the lubricant volume in long laterals, and therefore the intervention costs, many operators choose to pump lubricant slugs instead of pumping the lubricant continuously. However, most of the lubricant is consumed inside the CT, and only a small lubricant amount adheres to the outside CT and casing surfaces where the friction needs to be reduced. Secondly, even if the lubricant coats the outside CT surface, there is a risk of being quickly washed off, unless new lubricant is pumped continuously. The laboratory testing results obtained from this study have shown a reduction of the coefficients of friction after the CT metal surface treatment. These results prove the friction-reduction potential of manufacturing a CT with the new treated surface for extending the CT reach with or without friction-reducing technologies such as lubricants, vibratory tools and tractors. The advantage of utilizing the new CT metal surface treatment is that a lubricant remains longer in the micron-size pores on the CT surface and reduces the CT friction more consistently. The novel idea in this paper encompasses the fact that the CT metal surface treatment has the potential to reduce the CT friction by itself and further in combination with friction-reducing technologies such as lubricants, vibratory tools or tractors. The new CT surface is smoother and has micro-pores that can prevent a lubricant from being easily washed off the CT surface. The laboratory tests with the new CT samples have shown reduced coefficients of friction when comparing to conventional CT coupons with un-treated surfaces.
During the last three decades a coiled tubing (CT) modeling software package has been continuously developed to assist in the planning and executing of global CT operations. The first models were steady-state. In the past decade these models have been extended to consider transient effects of operations as they are executed. These models will also be used in the not-so-distant future to automate CT operations. In this paper, a review of all these models is presented for the first time. The development of a computer program was initiated in the early 1980s to help understand the downhole flow and pressure conditions during CT operations. Utilizing multi-phase rheological and frictional correlations obtained from laboratory flow loop testing, the CT flow model was extensively validated against global field data. Later, CT force and stress analysis models, considering such effects as the specific well geometry, mechanical friction, CT size, shape and material strength, were developed to predict lateral reach and assist in preventing downhole CT failures. While the most common and simplest approach within the industry is still to use steady-state models, in practice, the downhole conditions during CT operations, such as well cleaning, well unloading, well control, stimulation, cementing, underbalanced drilling with nitrified fluid, etc., are transient. Consequently, the steady-state models have been extended to account for downhole transient effects at the pre-planning and execution stages of CT operations. In addition, with the advent of the state-of-the-art CT telemetry systems, it is possible to acquire the downhole data in real time and use the transient CT software model to automate and optimize CT operations, increasing their safety and efficiency. A review is presented for the first time about the steady-state and transient models included in the CT software model, with details about each model and how they performed during 30 years of operations. Results and discussions regarding the extensive validation of the software against laboratory and field data are also presented. Several field cases from around the world help illustrate the transient nature of CT operations and the benefits of using the transient simulation in the pre-planning and execution stages of these operations. The paper presents the results from 30 years of global experience with the CT modeling software program. The mathematical models, validation against laboratory and field data, verification against other models available in literature, and case histories are described. The current trends within the industry are leading to a shortage of experienced CT field engineers, so the use of CT software models to increase the efficiency, compliance and safety of CT operations has never been as important as now.
Summary Microhole–horizontal–well–drilling technology is a high–efficiency and low–cost technology that has developed rapidly in recent years. However, during microhole–horizontal–well drilling, cuttings deposit easily at the bottom of the wellbore because of gravity and nonrotation of the pipe. The pipe sliding on the cuttings bed will cause extremely serious friction between the pipe and cuttings bed, which is an important limiting factor on the extended length of the microhole horizontal wellbore. Therefore, it is necessary to study the influencing factors and establish a model for evaluating the friction between the pipe and cuttings bed. In this study, laboratory experiments on the sliding friction between pipe and cuttings bed were conducted. By analyzing the comprehensive sliding–friction coefficient (CSFC) between the pipe and cuttings bed, the effects of dimensionless buried depth (0.2 to 1.0) and average cuttings size (0.249 to 2.667 mm) on the CSFC between the pipe and cuttings bed were obtained. CSFC is a function of dimensionless buried depth and relative roughness in the developed model. The results suggest that the sliding–friction resistance between the pipe and cuttings bed increases as the buried depth of pipe increases or the average cuttings size decreases. We propose a model for estimating the CSFC using experimental data and the least–squares method. The predictions show good agreement with the experimental data within suitable ranges of models. This work is expected to provide the basis for predicting the friction resistance between the pipe and cuttings bed.
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