Drilling in the Qiongdongnan Basin, offshore China's Hainan Island Province, has often resulted in failure to reach desired objectives. Bottomholetemperatures up to 475°F, and pressures requiring mud weights up to 19.5lbm/gal equivalent, place severe limitations on the performance of drillingfluids and often contribute in failure to reach the desired drillingobjectives.
Well Yacheng 21–1–4 was drilled in this basin with the COSDCsemi-submersible rig Nanhai V and was spudded on 27th November 1998. TD of5,250 meters was attained on 20th May 1999 where the bottomhole statictemperature was 414°F and the pore pressure was 18.5 lbm/gal equivalent. Logswere run to bottom without incident with no significant drilling fluid relatedproblems and the primary drilling objectives achieved. The success isattributed to innovative, fit-for-purpose drilling fluids and rigorous pre-wellplanning over a 2-year period prior to the well commencing.
The paper describes the holistic approach to drilling fluid engineering forextreme well conditions. The development of innovative drilling fluids specificto these well conditions, and the rigorous laboratory testing necessary togenerate detailed engineering guidelines, are described. Large-scale abrasionand pressure tests were also conducted. Modifications made to the rig designfacilitated the management of drilling fluid properties at high density withhigh flow line temperatures. A portable drilling fluids laboratory, staffedwith trained technicians, was installed on the rig to continually pilot testdrilling fluid samples and treatments under simulated down-hole conditions. Theimportance of good communications and global technical support networks provedinvaluable during the pre-well planning and for the execution phase of extremehigh temperature and high pressure wells drilling.
When bottomhole temperatures exceed 400°F, the design and formulation ofdrilling fluids can present a host of problems. Drilling fluids destabilizeunder such extreme conditions, possibly causing wellbore instability, wellcontrol problems, and ultimately loss of the well. Operators andservice-company responses to this risk are well documented.1,2,3,4,5The critical issues are:
High-Temperature Gelation: This can occur in both water- and oil-baseddrilling fluids. In water-based drilling fluids (WBM), gelation under prolongedstatic conditions at temperature is caused by clay (bentonite) flocculation andis compounded by the thermal degradation of chemical thinners, a drop in pH andan increase in the filtrate loss. In oil-based drilling fluids (OBM), theinteraction of colloidal particles (clays and fluid-loss additives) andbreakdown of emulsifiers may cause gelation.
High-Temperature Fluid Loss: Regardless of the type of drilling fluid, thestatic and dynamic fluid losses usually increase with temperature and areaffected by gelation and thermal degradation of synthetic polymers.
Rheological Property Control: High-density drilling fluids, by definition, have high volume fractions of weight material formulated to maintainhydrostatic pressure and well control. Properly controlling the rheologicalproperties in the field depends on efficient solids-control equipment andhigh-performance drilling fluid additives. Small increases in colloidal-sizeddrilled solids can rapidly escalate the fluid's rheological properties, leadingto unacceptable pressure losses, drilling fluid gelation, and excessive swaband surge pressures. Conversely, low rheological properties promote poor holecleaning, sag of weight material, and a non-uniform density profile in theannulus that can promote drilling fluid losses to the formation or potentialwell control problems.