Abstrakt

Swelling clay phenomenon is frequently observed during oil and gas drilling operations and has a significant impact on the quality of cementing procedure. Certain types of clayey minerals increase their volume in contact with water-based drilling fluids. After drilling is completed, borehole remains unsupported and filled with waterbased drilling fluids for several hours, before a casing string is inserted and secured with cement. In the period of time between completing the drilling and inserting the casing string the clay can expand hindering proper cementing or blocking the casing string in a wellbore. Filling the annular space between a casing pipe and wellbore walls with cement is crucial for further exploitation of a well. An improper performance of displacement work (primary cementing) may cause both financial losses and environmental damage. The aim of this study is to describe the impact of distorted annular space geometry on cement sheath quality and to examine the possibility of improving the distorted geometry with a prototype wellbore tool. The tool was designed to be mounted as a first pipe section on the casing string (cementing shoe/reamer shoe). Two test stands were designed and constructed. The first one simulates the well cementing process, while the second one simulates the downward movement of the casing pipe in the well (run in hole process) drilled in expansive clay. Six distorted annular space sections were cemented using the first test stand. The sections were scanned with μXCT (computed micro-tomography) to locate discontinuities in the cement sheath. This research has confirmed an adverse influence of annular space obstructions on the cement sheath quality, thus the necessity of removing them before cementing. The obstructions can be removed by means of newly designed clay cutting wellbore tool. Therefore, the prototype of such a tool was tested on the second test stand. The experiment allowed to evaluate an influence of a swollen clay obstruction on the force needed to push the prototype tool through the obstruction. The same experiment was conducted with a standard cementing shoe in order to obtain comparative data. Hole geometry improvement, ability to fragment and remove clay cuttings have been observed. The research has confirmed that the prototype tool efficiently improves the borehole geometry and, consequently, improves the cement sheath quality.

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