The focus of this work is to present practical solutions to help stabilizeclay-based (shale) formations extensively encountered in deepwater environmentswhile drilling with water-based muds. Water-based drilling fluids due to theirenhanced economics and environmental acceptability are being increasingly usedto drill through troublesome offshore clay-based formations. However improperapplication of water-based muds while drilling sensitive shale formations canlead to costly wellbore instability related rig down-time. A properunderstanding of the physico-chemical interaction of water-based drillingfluids with shale formations is beneficial when utilizing water-based drillingfluids in challenging deepwater conditions.
The approach presented here is to generate a highly efficient membrane withinthe shale with water-based drilling fluids (similar in performance toinvert-emulsion oil-based muds). Laboratory experiments on shale samples underrealistic downhole conditions exposed to water-based muds are presented toemphasize the time-dependent alteration of shale properties (e.g., strength, stiffness, etc.). The next generation of membrane efficient water-based mudspresented here show significantly high membrane efficiencies (greater than 80%) approaching those of invert-emulsion oil-based muds.
A fundamental understanding of the mechanisms for shale stability and theapplication of experimental data to field conditions have resulted in improvedapplication of waterbased drilling-fluids to successfully achieve keyobjectives. Practical guidelines based on the understanding of these mechanismshave resulted in trouble-free drilling of shales which has significantlyreduced non-productive time related to wellbore instability while drillingdeepwater wells.
The US Minerals Management Service (MMS), the oversight agency for federalwaters has defined deepwater as water depths in excess of 1000 feet (305 m) andultra-deepwater as water depths greater than 5000 ft (1,525 m). With severalwells drilled successfully in water depths greater than 6000 ft drilling indeeper waters is being established as not only feasible but also economicallyjustifiable.1
Wellbore stability problems are common while drilling in deepwater andultra-deepwater. In the shallow (surface) hole sections high pore pressure(related to excessive deepwater hydrostatic head), a lack of a fully developedoverburden (vertical) stress and low formation compressive strength narrows themud weight operating margin between pore pressure and fracture pressure. Majorproblems associated with drilling surface holes are shallow water flowhazards2, lost circulation and sea floor subsidence problems. Oncethe surface conductor casing is in place drilling the intermediate holesections can be problematic due to shale (gumbo-type) instability, time-dependent loss of mud column support, etc. In deepwater environmentsdrilling of shale can result in a variety of problems ranging from washout tocomplete collapse of hole as well as bit balling, sloughing or creep.
There are increasing economic demands on reducing rig downtimes associated withborehole instability problems. A major collaborative effort was undertaken todevelop novel environmentally acceptable water-based drilling fluids with highmembrane efficiency to meet the future requirements of the petroleumindustry.