34;- WELL
Chapter 5: Application to a Real Life Problem
5.2 Geology and Geophysics
The field has been mapped by interpreting a seismic data set comprising a series of seismic lines. Data from each line are processed to give a vertical slice of subsurface reflections along the length of the line. The vertical axis of the plot is based on the two-way travel time for the seismic pulses to travel from surface to a reflector and then back again. Such a plot is known as a seismic section. The reservoir interval is characterised by a change in seismic impedance in moving from the overlying shales into the reservoir sandstone, followed by a reversal at the base of the reservoir. This is visible on the seismic section as a peak that is related to the top of the reservoir interval followed by a trough related to the base of the reservoir, which occurs at a slightly later two-way travel time. Mapping out the peak and converting the travel
time to a depth gives a depth map of the top of the reservoir. This, and the various well locations, is depicted in Figure 5.1.
FIGURE 5.1: DEPTH TO THE TOP OF THE RESERVOIR INTERVAL
Geologically the reservoir is believed to be part of a turbidite system triggered by changes in sea-level during the Cretaceous era approximately 100 million years ago.
Typically the turbidite system is initiated by a relative decrease in sea level that exposes much of the continental shelf. The sediment on the shelf is then subject to erosion. Accumulation of the eroded material at the self-edge can be unstable. A turbidity current forms when portions of this material break free and rush down the continental slope as a slurry of sediment and water, moving together. At the base of the continental slope the velocity of the current diminishes and thus its ability to carry solids decreases. Since the coarsest grained particles require the highest velocities to remain in suspension they are deposited first. Finer grained particles are deposited as the velocity of the current decreases. Sediment associated with a single turbidity
current is known as a turbidite. The sandstone reservoir is a stacked succession of many individual turbidites.
As the strength of each current diminishes it is possible for finer grained material to be deposited leaving an impermeable layer above the sandy sediment. The next turbidity current may erode all or part of this layer or may leave it intact forming a barrier to vertical flow. Frequently, remnants of the finer grained material may be found incorporated into the next sandy layer in the form of rip-up clasts even though no intervening layer remains. Vertical permeability within the overall package is therefore largely a function of the extent to which the impermeable (claystone or siltstone) layers have been preserved. Within each of the sandstone layers the vertical permeability is expected to be similar to the horizontal permeability. This has been confirmed by core experiments on wells A and D which compare measurements made using vertically cut plugs with those taken from horizontally cut plugs. Typically the ratio of vertical permeability to horizontal permeability was about 0.8.
Outcrop studies conducted on turbidites near Laingsburg and Tankqwa47 In South Africa have shown that the lateral extent of claystone and siltstone layers within similar sandstone packages can vary enormously. In some cases the clay and siltstone layers are almost entirely absent or only extend for a few metres. The ratio of vertical to horizontal permeability for these cases would be close to unity. In other cases the claystone layers can be followed for several hundred metres and the ratio of vertical to horizontal permeability could be of the order of 0.01 or less.
Close examination of the core for well A has provided evidence that the energy of deposition was high for the lower portion of the reservoir interval. Although claystone layers are preserved in the core it is likely that these are not laterally extensive. Closer to the top of the interval the energy of deposition appears to be less and the claystone layers are believed to be more continuous. This upward fining sequence is typical of channelized turbidites. The overall geological model for the field is that of an amalgamated channel complex within a broad erosional valley.
Towards the edges of the valley the sediment is mud rich, as in wells Band C for example, whereas the sediment in the centre of the valley is predominately sandstone as detected by wells A, D, and E.
The reservoir properties and lithologies for well A, the well to be modelled, are depicted in Figure 5.2.
FIGURE 5.2: PROPERTIES OF THE RESERVOIR INTERVAL, WELL A
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CORE POROSITY
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Data used to create Figure 5.2 were gathered from a variety of sources. A portion of the reservoir was cored and the rock sample brought to the surface for examination by geologists. Rock plugs were subsequently cut from the core and subjected to laboratory measurements that included porosity, permeability, and electrical properties.
Special geophysical logging tools were run into the borehole to measure the natural gamma ray radiation, density, and resistivity of the rock in-situ. The gamma ray readings give an indication of the clay content. On the whole the intervals with a high gamma ray reading consist of claystones or silts whereas the intervals with low gamma ray values are sandstones. Density readings are used to derive a continuous porosity curve that is calibrated against core porosities. Resistivity readings, when used in conjunction with porosity, allow the hydrocarbon saturation to be determined.
Close examination of Figure 5.2 will reveal a prominent interval of high gamma ray readings between the depths of 2391 and 2394 mbKB corresponding to interbedded claystone and siltstone layers. Similar intervals have been noted in wells D and E. It is therefore believed that this feature could be present over a large portion of the reservoir and would represent a regional hiatus in sand deposition. As such, the feature could provide a widespread seal between the upper and lower portions of the reservoir. The three wells in question do, however, fall on a line that is almost parallel to the direction that the channels are believed to follow. A higher degree of continuity can be expected in a direction parallel to the channel axis than at right angles to it. It is therefore possible that the layer is discontinuous in a direction orthogonal to channel axis.