Sealing Capacity of API Connections—Theoretical and Experimental Results

Abstract

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.

Design Optimization of a VX Gasket Structure for a Subsea Connector Based on the Kriging Surrogate Model-NSGA-II Algorithm Considering the Load Randomness

“…During the working process, the high-pressure medium with random pressure value fluctuations acts on the gasket to produce a radial force. The corresponding compacting force exerted onto the sealing surface is further increased, the plastic deformation appears in the gasket, and a type of self-tightening seal structure is ultimately formed [14].…”

“…In order to improve the optimal design solution, we regarded the working load P as a random variable, obeying the Gaussian distribution function [14], where the mean value is µ and the coefficient of variation is cv [22]. The corresponding upper and lower bounds were µ − 3 × cv and µ + 3 × cv based on the 3σ-rule [23].…”

A Quantitative Model to Calculate Gas Sealing Capacity and Design Sealing Parameters for Premium Connection

“…Based on those sealing criteria, sealability evaluation and sealing parameter design for premium connections have also been mainly focused on the sealing contact stress analysis by the theoretical model, field testing, and finite element method (FEM). Teodoriu and Badicioiu analyzed theoretically the sealing contact stress for the American Petroleum Institute (API) connection [8], and Xu et al conducted the theoretical analysis of sphere to cone sealing performance for tubing and casing premium connection [9]. Hamilton et al also adopted ultrasonic techniques to examine accurately the seal surface contact stress in premium connections [10].…”

Evaluation of the Elevated-Temperature Performance and Degradation Mechanisms of Thread Compounds

“…What is special about them is the high solids content of up to 50wt% to 60wt% compared to regular greases [1]. The function of the grease and the added particles is threefold; it provides a repeatable friction factor for a predictable assembly torque response [2], provides surface protection [3][4][5][6], and aids in the seal ability of the system by blocking or bridging potential leak channels [7][8][9][10][11].…”

“…Zinc and copper are said to be added to control the friction coefficient. In addition, the copper particles are the largest and probably aid in thread sealing [9,14]. Finally, the graphite is not of lubrication grade and therefore acts mainly as a filler [6].…”