| Popis: |
Drilling fluids are subjected to cyclic pressure and temperature loads when circulated in the wellbore. However, little information exists about quantitative changes of fluid viscosities during long-term circulation. Therefore the effects of temperature/ pressure cycles on viscosity reduction/ regaining for different Xanthan products - dissolved in pure solutions and in drilling mud - and for Bentonite fluids are evaluated in a laboratory study. To model these cyclic temperature/pressure loads a new HPHT cyclic load test has been developed and the impact of temperature and time dependent viscosity on hydraulics calculations is evaluated quantitatively. First, the ability of Xanthan and Bentonite fluids to break and reestablish viscosity under cyclic temperature and pressure loads was evaluated. Furthermore it was investigated whether fluids are able to build up original viscosity after applying cyclic temperature and pressure loads. This effect is defined as viscosity degradation per load cycle which could be quantified as part of this work. Bentonite, in contrast to Xanthan fluids, however has a different, more complex and stronger time and temperature dependent viscosity behavior when cyclic loads are applied. For the investigated fluids a significant impact of time dependent behavior under cyclic temperature and pressure loads on hydraulics modeling is shown. By performing the new test procedures it is possible to quantitatively determine cyclic viscosity degradation. When using these findings, the accuracy of hydraulic modeling and nozzle optimization can be improved. Better predictions of critical viscosity breakdowns and additive requirements to maintain fluid viscosity can be done. |
