Sealing Capacity of API Connections - Theoretical and Experimental Results

Abstract

Abstract 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.