Topside payloads range from 5 to 50,000 tons, producing oil, gas, or both. A vast array of production systems is available today (see Figure 1). The concepts range from fixed platforms to subsea compliant and floating systems. In 1859, Col. Edwin Drake drilled and completed the first known oil well near a small town in Pennsylvania, U.S.A. This well, which was drilled with cable tools, started the modern petroleum industry.
The majority of offshore fields have been developed with conventional fixed steel platforms. One common feature of fixed steel structures is that it is essentially "fixed" (i.e., it acts as a cantilever fixed at the seabed). This forces the natural period to be less than that of the damaging significant wave energy, which lies in the 8- to 20-second band. As the water depth increases, these structures begin to become more flexible, and the natural period increases and approaches that of the waves. The consequence of this is the structure becomes dynamically responsive, and fatigue becomes a paramount consideration.
Interpreting seismic data requires an understanding of the subsurface formations and how they may affect wave reception. This article discusses some of the key stratal interfaces and their implications for interpreting the data received. All instantaneous seismic attributes (amplitude, phase, frequency) can be used in interpretation. In practice, most interpreters use instantaneous amplitude, or some variation of an amplitude attribute, as their primary diagnostic tool. Amplitude is related to reflectivity, which in turn is related to subsurface impedance contrasts.
Figure 1.6--The Baldpate Compliant Tower is one of the tallest free-standing structures in the world – Empire State Building (right) for comparison (Web Photograph, Amerada Hess Corp., New York City). Figure 1.9a--Worldwide fleet of installed and sanctioned semisubmersible FPS (courtesy of BP). Figure 1.9c--Worldwide fleet of installed and sanctioned spars (courtesy of BP). Figure 1.10--Semisubmersible FPS planned for the Thunder Horse field (courtesy of BP). Figure 1.11--Alternative proven technology field development options (courtesy of BP). Figure 1.12--Subsea production trees used in conjunction with a fixed jacket structure (Intec Engineering, Houston).
Commercial resistivity measurements made while drilling first became available in the late 1970s. Because the drilling environment is much more adverse than the wireline logging environment, a simple short normal tool mounted behind the drill bit was used as the first LWD resistivity tool. However, short normal tools were only able to provide information for basic interpretation, such as correlation of geological markers and estimation of gross water saturation, because of their shallow depth of investigation and relatively poor vertical resolution. Being DC electrode devices, normal tools are also limited to conductive mud environments. To expand the LWD resistivity market to oil-based mud (OBM) environments, induction-type propagation measurements were introduced in the early 1980s.
Historically and currently the oil industry has delivered only about 75% of what was forecasted. There are three paradoxes we struggle with when forecasting oil rates or reserves, these will be discussed in the initial portion of the chapter. Next, we then quote the typical error ranges of forecasts in the literature. However there is a shortage of published data on error ranges. Following this, the chapter summarises a look-back procedure that will improve the next generation of forecasts when dynamic or new data comes in. In addition to technical factors internal to the reservoir or production system, external factors such as project delays play a role so this will be discussed as well.
The numbers come from different sources. Take the red jack example. There are 52 cards in a poker deck (excluding the jokers), two of which are red jacks. We simply take the ratio for the probability. Similarly, when we toss two fair coins, we know that there are four outcomes, and we believe that they are "equally likely," for that is indeed what we mean by a fair coin.