Study On Mechanism And Anti-collapse Drilling Fluid Technique Of Shallow Formations Borehole Stability In Deep-water Drilling

Deepwater drilling was different from land and shallow water drilling, it was facing many complex technique difficulties. Shallow formations stability in deepwater drilling was one of many complex technical problems which determined the success or failure of a being drilled well. The paper carried out the research of shallow formations stability and anti-collapse drilling fluid technique in deepwater drilling, which combined with one sub-project named“key technique of deepwater drilling and completion”of“863”plan—“research of deepwater drilling fluids”.The main factors which influenced stability of shallow formations in deep water were analyzed initially and shallow formation stability in deep water was simulated by using finite element(FEM) software ANSYS. With the basic parameters of shale formation of deep water oil field in the South China Sea, the results of the security density window of this field got by numerical simulation were basically consistent with the actual drilling data, with the relevant parameters of hydrate formation in Gulf of Mexico got the upper drilling fluid density by numerical simulation.Combined with sedimentary characteristics of deepwater shallow formations of South China Sea, with deep water drilling simulator laboratory equipment and other experimental installations, experimental investigation on instability of shallow formations mechanism was made. The result showed that their high water saturation, greater degree of compaction, inequality of hydration expansion, low intensity of themselves, long soaking and cyclic erosion time by fluid may lead to borehole instability of shallow foemations in deepwater drilling. Decomposition of hydrate,the existence of shallow gas and shallow water could cause borehole instability of shallow formations.The result showed if drilling in hydrate formations by using deepwater drilling fluids with no inhibitors, it would not cause decomposition of hydrate. And if the deepwater drilling fluids had only thermodynamic inhibitors, it would make the decomposition immediately, but the combination of thermodynamic inhibitors and dynamic inhibiters in deepwater drilling fluids could delay the hydrate resolving time. Two water-based deepwater drilling fluid systems were optimized by experiments. At low temperature and high pressure in deepwater drilling, the comprehensive performance evaluation results indicated that two systems had good rheology, lubricity, sealing ability, high inhibition, anti-pollution and strong stability and other advantages. The optimization oil-based drilling fluid formula showed good low temperature rheology property and strong hydrate inhibition.On the basis of analysis of the borehole stability mechanism, drilling fluid technology to prevent borehole instability of shallow formations were initially proposed.