Types of 3-d Seismic Analyses

The interpretations that a geophysicist might per­form with 3-D seismic data can be grouped conve­niently into those that examine the geometric frame­work of the hydrocarbon accumulation, those that analyze rock properties, and those that try to monitor fluid flow and pressure in the reservoir.

These analyses affect and significantly improve decisions that must be made about volume of reserves, well or platform locations, and recovery strat­egy.

The first general grouping is geometric framework. It’s a collective term for such spatial elements as the attitudes of the beds that form the trap, the fault and fracture patterns that guide or block fluid flow, the shapes of the depositional bodies that make up a field's stratigraphy, and the orientations of any unconformity surfaces that might cut through the reservoir.

By mapping travel times to selected events, displaying seismic amplitude variations across selected horizons, isochroning between events, noting event termina­tions, slicing through the volume at arbitrary angles, compositing horizontal and vertical sections, optimiz­ing the use of color in displays, and using the wide variety of other interpretive techniques available on a computer workstation, a geophysicist can synthesize a coherent and quite detailed 3-D picture of a field's geometry.

The second general grouping of 3-D seismic analy­ses involves the qualitative and quantitative definition of rock properties. Amplitudes, phase changes, interval travel times between events, fre­quency variations, and other characteristics of the seismic data are correlated with porosity, fluid type, lithology, net pay thickness, and other reservoir prop­erties. The correlations usually require borehole con­trol (well logs, cuttings, cores, etc.) both to suggest initial hypotheses and to refine, revise, and test pro­posed relationships. An interpreter develops a hypoth­esis by comparing a seismic parameter in the 3-D volume at the location of a well to the well's informa­tion, often through the intermediary of a synthetic seismogram or 2-D or 3-D seismic model. The hypoth­esis is then used to predict rock properties between wells, and subsequent drilling validates (or invalidates) the concept. Gas saturation in sandstone reservoirs is probably the rock property that has been most suc­cessfully mapped by 3-D seismic surveys. The pres­ence of free gas typically lowers sharply the seismic velocity of relatively unconsolidated sandstones and creates a strong acoustic impedance contrast with surrounding rock. The contrast produces a seismic amplitude anomaly. Since the early 1970s, this "bright spot" effect has been widely exploited to detect gas saturation with standard 2-D seismic sections. When the effect occurs in 3-D volumes, gas-saturated sand­stones can be accurately mapped laterally across fields at multiple producing horizons.

The third general grouping of 3-D seismic analyses consists of those designed to monitor the actual flow of the fluids in a reservoir. Such flow surveillance is possible if one (1) acquires a baseline 3-D data volume at a point in calendar time, (2) allows fluid flow to occur through production and/or injection with attendant pressure/temperature changes, (3) ac­quires a second 3-D data volume a few weeks or months after the baseline, (4) observes differences between the seismic character of the two volumes at the reservoir horizon, and (5) demonstrates that the differences are the result of fluid flow and pressure/ temperature changes.

The standard 3-D seismic data volume is acquired with source and receivers at the Earth’s surface. It is logistically possible to put sources and/or receivers in boreholes and to record part or all of the 3-D data volume with this downhole hardware. This approach is an active area of research. Depending on the acquisi­tion configuration, one records various kinds and amounts of reflected and transmitted seismic energy, which can then be sorted to provide information on geometric framework, rock properties, and flow sur­veillance, just like surface surveys. Advantages of downhole placement are that higher seismic frequen­cies generally can be recorded, thereby improving resolution, and that surface-associated seismic noise and statics problems are lessened or avoided. The main disadvantages are that source and receiver plants are constrained by the physical locations of available boreholes; borehole seismology can be affected by tube waves and the like, so downhole placement is not noise-free; a borehole source cannot be so strong as to damage the well; and the logistics and economics of operating in boreholes are complex, though not nec­essarily always worse than operating on the surface. One can imagine a time when borehole seismic sources and receivers might be standard components of the hardware run into wells and accepted as routine and valuable devices for reservoir characterization and flow surveillance.

The petroleum industry's twenty-year experience with 3-D seismic surveying is an example of a techno­logical and economic success. Today, the investment in a 3-D survey typically results in fewer development dry holes, improved placement of drilling locations to maximize recovery, recognition of new drilling oppor­tunities, and more accurate estimates of hydrocarbon volume and recovery rate. These outcomes improve the economics of development and production plans and make the surveys cost effective.

Notes:

* to take advantage of - воспользоваться

* "bright spot" effect – эффект «яркого» пятна

2. Find the sentences in the text with the word “drilling” and determine its grammar form.

3. What are ing – forms in the sentence below:

By mapping travel times to selected events, displaying seismic amplitude variations across selected horizons, isochroning between events, noting event termina­tions, slicing through the volume at arbitrary angles, compositing horizontal and vertical sections, optimiz­ing the use of color in displays, and using the wide variety of other interpretive techniques available on a computer workstation, a geophysicist can synthesize a coherent and quite detailed 3-D picture of a field's geometry.

4. Answer the questions:

1. When is 3-D seismic survey used? 2. What interpretations can the geophysicist get with 3-D seismic method? 3. What does geometric framework comprise in? 4. Can you name qualitative and quantitative definitions of the rock structure? 5. Where are the standard 3-D seismic data receivers located? 6. Why 3-D surveying method is more appreciated nowadays?