Barite sag and poor hole cleaning are not problems; they are symptoms of well control and stuck pipe problems. Issues with barite sag and hole cleaning are routinely encountered while drilling high pressure high temperature (HPHT) wells. Maintaining optimal mud rheology in HTHP conditions (350°F+) can be very difficult. Adding organo-clays or low gravity solids (LGS) to boost rheology can lead to high equivalent circulating densities (ECD) and low rates of penetration (ROP).
The new HPHT organic rheology modifier (ORM) imparts optimal rheological properties to low, medium and high density clay-free invert emulsion fluids (IEF). Clay-free systems have previously demonstrated superior gel strength and rheological profiles over conventional organo-clay and lignite treated fluids. Although significant improvement in these systems seemed unlikely, this was accomplished with the new ORM chemistry.
These IEFs exhibit enhanced low shear rheology (even at 9.0 ppg) with lower or similar plastic viscosity (PV) values when compared to IEFs formulated without the new HPHT rheology modifier. When tested at up to 400°F and 18,000 psi, the IEFs formulated with ORM show similar or higher low shear rheology with low PV than under ambient condition. A good low shear rheology implies better hole cleaning and sag control. A low PV improves ECD control.
Treatment with ORM imparts fragile gel characteristics to 9.0 to 18.0 ppg clay-free IEFs. The rapid gel-to-flow transition helps to minimize surge and swab pressures and reduce mud losses. The new HPHT rheology modifier with a biodegradability of 67% in 28 days also stabilizes the IEF and provides comparatively low fluid loss values. It also eliminates the need to add LGS to boost rheology. The paper presents experimental data demonstrating both the environmental and rheological performance of the HTHP rheology modifier as well as comparative data from the conventional clay-free fluids without the ORM.