Casing leaks are a major problem in the entire oil, gas and geothermal energy industry all over the world. The reasons for casing leaks are multiple, but the majority of the failures is due to corrosion and mechanical wear, because of reciprocating or vibrating artificial lift systems. Additionally, wellbore treatments such as acidizing or fracturing are well known to be another major source for casing failures.
The impact of casing failures is usually phenomenal and in the worst case scenario the wellbore has to be plugged and abandoned. In this Master Thesis several options are discussed in order to repair the casing failures. The most common methods in the industry in order to overcome this problem are mechanical or chemical solutions. QHSE aspects are considered for every application. Mechanical solutions are most reliable, however the inside diameter of the borehole is reduced and therefore another barrier for the installation of artificial lift systems is created. In such a case the operator must be aware that after such an installation the original productivity of the wellbore cannot be achieved anymore. Repair methods like casing patches, expandable tubulars, and swaging operations are discussed in detail. Chemical solutions like the injection of specially designed cement slurries or resins or a combination of both of them have been investigated. Several methods like low pressure squeeze, high pressure squeeze and running squeeze have been discussed. The design of the cement slurry and all the relevant rheological and chemical parameters has been investigated.
The Master Thesis comprises a comprehensive literature review and analysis of 244 real case scenarios from the Vienna basin. The data sets were supplied by a business partner who operates the wellbores in the Vienna basin for more than 80 years. Therefore, it is obvious after such a long period of production the data material submitted is ideal for investigating in the subject of the Master Thesis.
Furthermore, numerous lab tests on various cement slurry designs have been carried out in accordance to the API standard “API recommended practice 10B-2”, and an optimized slurry design has been sorted out. The optimization criteria have considered Conventional-Cements, Microfine-Cements and Ultrafine-Cements. The type of cement to be chosen predominantly depends on the boundary conditions given by a particular wellbore problem and design. During the execution of the lab test it has turned out that especially the particle size of the cement grains, the density and the rheology of the cement slurry are the main criterion for choosing the right cement for the individual application. Additionally, quality checks on cement slurry samples have been carried out and crosschecked with the data sheets delivered by the cement manufacturer. It has turned out that the data sheets from the manufacturer are most reliable and of crucial relevance for the operators.
The specific test setup for the laboratory test are discussed in detail and documented and calibrated. The existing infrastructure is therefore ideally tailored for future lab tests and master theses.