Locating the zones where fluids enter the wellbore in a producing or injecting well is an important aspect of production logging. It is relatively straightforward to establish flow profiles in single-phase situations using conventional methods based on flow and density, but multiphase systems present a greater challenge to quantifying gas and liquid holdups.
Obtaining representative samples of reservoir fluids is a major challenge in reservoir engineering, particularly in the complex carbonate reservoirs of the Middle East. Reservoir studies, crucial to production planning and process design, determine and utilize reservoir fluid properties, pressure-volume-temperature behavior, viscosity, and gas/oil ratio, on the basis of a reservoir at its original conditions. A sample free of contamination and contained at its downhole PVT conditions when it reaches the surface gives confidence in the analyses that follow.
Multilaterals are effective in complex carbonate reservoirs but have been slow to develop because of general skepticism about the risks and more practical deterrents. One solution is a prefabricated subsurface wellhead assembly that can be unwrapped and installed downhole, splitting the main bore into two equal-size lateral bores and providing a high-pressure seal at the junction.
Knowing whether a boundary is being entered from above or below takes away a lot of the guesswork for drillers and geologists, and the ability to react immediately is of great value to the geosteering engineer. Adding an azimuthal capability to resistivity and gamma ray LWD tools enables real-time measurement in a number of equally spaced sectors around the borehole. Increased sensitivity allows detection of features that would have remained unseen using conventional methods.
The complexity and unpredictability of carbonates have frequently frustrated geologists, geophysicists, petrophysicists, reservoir engineers and drillers alike. Advances in areas such as wireless telemetry and real-time electronic imaging have contributed to the range of formation evaluation-while-drilling tools.
In some areas of the Middle East zonal isolation is particularly difficult to achieve in tophole sections. The optimization of cement slurry to take account of the difficulties presented by formations, borehole conditions, and wellbore fluids, and to produce a set cement with the necessary mechanical properties is a complex business.