Pattern and control of gravity displacement between asphaltic heavy oil and drilling fluid

Asphaltic heavy oil developed extensively in Formation K of Y Oilfield. Drilling operation in the formation frequently encounters co-existence of kick and lost circulation. Generally, a higher volume of lost circulation may be caused by the killing fluid with higher density. Conventional rigid borehole theories fail to explain such conditions. To verify existence of gravity displacement between asphaltic heavy oil and drilling fluid and identify factors that may affect such gravity displacement, tests and theoretical researches were conducted through combination of lab tests, numerical simulation and theoretical studies. Eventually, effective control methods and techniques over gravity displacement between asphaltic heavy oil and drilling fluid can be developed. Research results show that drilling fluid density, fracture width and viscosity of asphaltic heavy oil are major influencing factors over displacement of asphaltic heavy oil. These factors display linear, logarithmic and exponential relationship with the volume of displacement, respectively. In addition, researches have been conducted for asphalt solidification, pressure-controlled drilling and while-drilling detachable packing techniques, and comprehensive control over super-thick asphalt layer in accordance with specific conditions has been developed. Field application results show asphalt solidification and pressure-controlled drilling are effective ways to ensure safety of drilling operation in asphalt formations.