The wellbore instability control mechanism of fuzzy ball drilling fluids for coal bed methane wells via bonding formation

As a novel drilling fluid, Fuzzy-ball Drilling Fluid has been successfully utilized to control the collapse and lost circulation in approximately 1000 coal bed methane wells by now. However, in terms of current research, its mechanism of controlling wellbore instability has not yet been revealed clearly enough to satisfy the requirement from field applications. In order to achieve further understanding, (1) fuzzy-ball drilling fluid was firstly prepared in the laboratory with 2.0% fuzzy-ball coating, 0.5% fuzzy-ball floss, 0.4% fuzzy-ball core, and 0.4% fuzzy-ball membrane blended by Warring Blender at 7000 rpm, whose density and apparent viscosity are respectively 0.85 g/cm3 and 40mPa·s , close to the in-situ reality.(2) Secondly, both uniaxial and triaxial compressive tests under 5MPa confining pressure were conducted to measure the strengths and obtain the stress-strain curves of different coal samples after the separate injections of 2% potassium chloride solution, low solid-phase polymer drilling fluid and fuzzy-ball drilling fluid, as well as the coal plunger (Φ50) without any treatment serving as the control group respectively for three times. (3) Next, the mechanical parameters which influenced the collapse pressure were figured out with regards to sampling cores acquired from the coal reservoir. Through aforementioned experimental procedures, incorporating with a classical collapse pressure prediction mathematical model that contains these mechanical parameters, the collapse pressures corresponding to coal seams subject to different types of drilling fluids were calculated. (4) Finally, two cases, Hancheng and Qinping were employed to verify the results in the laboratory. That is, collapse and loss simultaneously took place in sites, where both issues were tackled in the same well. The findings are as follows. (1) The uniaxial compressive tests show that the strength of natural coal cores after the injection of fuzzy ball drilling fluid rises from 2.6 to 3.6MPa in average while that after the injection of KCl displays a decline and that after the injection of polymer witnesses a modest alteration. (2) The calculation results based on the stress-strain curves illustrate that the Young's modulus (E), Poisson ratio (υ) and cohesion (Cm) of the cores being injected with fuzzy ball drilling fluid change from 3.0665 to 3.9385GPa, 0.229 to 0.318 and 0.5 to 0.9 MPa respectively, whereas those mechanical parameters being injected with other fluids manifest only slight changes. Similar results can be observed in the experiments with reconstructed artificial coal cores. (3) The in-situ application demonstrates that fuzzy ball drilling fluid is capable of overcoming the dilemma brought by collapse and loss taking place simultaneously in the same well. In conclusion, the wellbore instability control mechanism of CBM fuzzy ball drilling fluid lies in that through bonding formation, CBM fuzzy ball drilling fluid changes the rock mechanical parameters, enhancing the strength of rocks microscopically and thereby magnifying the safety density windows macroscopically so as to finally realize the control of wellbore instability.