Two North Sea extended reach wells have been analyzed in detail by comparing downhole pressure and temperature measurements with results from an advanced pressure and temperature simulator. Drilling of 12¼ in. and 8½ in. sections are studied.
Early indications of hole cleaning problems were observed both on temperature and pressure. Calculations predict that downhole temperature is very flow regime dependent, and flow regime depends on eccentricity of the inner string, which therefore is important to take into account. It is also demonstrated that the presence of tool joints may influence temperature dramatically.
Improvements in the planning and modeling of the drilling of extended reach wells are suggested.
Extended reach wells (ERW) are interesting from a modeling point of view, because some effects that are masked by cancellations in other wells appear more clearly. Hence, more accurate modeling of fluid properties is necessary.
In vertical wells at moderate or shallow water depths it is frequently observed that pressure and temperature dependence of drilling fluid properties is not very important due to cancellations. Pressure and temperature effects cancel each other partially, with temperature variations going in different directions in different parts of the well. For example when starting pumps after a long static period, temperature normally decreases near the bottom of the annulus because cold mud is flowing downwards. And it increases at the top of the annulus because warm mud is moving upwards.
The situation is often very different in an extended reach well. The drilling fluid flowing down the drillstring is heated by friction and may therefore get the same or even higher temperature than the surrounding formation, which normally has a temperature that is not changing very much along near horizontal sections. Accordingly the annulus temperature in the ERW may increase in most of the annulus when circulation starts after a static period. The result is that the temperature dependence of fluid properties is displayed clearly in the ERW's because the cancellation effects are small. And fluid properties influence significantly calculations of hydrostatic pressure, frictional pressure loss, and heat transfer.
Another important phenomenon is the formation of a cuttings bed below the drillstring. This influences not only bottom hole pressure, but also heat transfer from formation to annulus and from annulus to drillstring. Fluid velocity above the bed increases with bed height, and, as long as flow is laminar, heat is conducted better through the bed than through the fluid above the bed. Increased heat transfer may show up as an unexpected increase in bottom hole temperature, which in principle can be an aid in detecting hole cleaning problems.
The presence of tool joints turns out to have several interesting effects. Flow inside a 5½ in. drill pipe may not be turbulent during drilling, but still turbulence can be triggered at tool joints. This may have a large impact on heat transfer, which gradually becomes poorer along a section with pure laminar flow. The same may be true in parts of the annulus. In some cases it has been observed that far too low temperature has been predicted when ignoring tool joints.
Pressure and temperature model
A transient pressure and temperature model that allows pressure and temperature dependent fluid properties, has been developed1,2,3,4,5. It consists of an accurate dynamic pressure model, which is coupled to a 2D dynamic temperature model. The two models exchange information at each time step.