Erling-P. Bergerod Aase, Tom A. JelmertTo avoid distorted data when analyzing well pressure tests of permeable offshore reservoirs, one needs to account for periodic ocean tidal stress.
Norwegian Institute of Technology
Trondheim, NorwaySven A. Vik
Saga Petroleum A.S
Sandvika, Norway
Quartz-crystal bottom hole pressure recorders provide a high resolution of reservoir pressure but also measure pressure fluctuations from tidal effects during well testing. Periodic oscillations in the reservoir pressure are due to the three mechanisms:
- Solid earth tide
- Barometric tide/effect
- Ocean tide.
PRESSURE
In onshore reservoirs, gravitational forces between the sun, moon, and earth can cause periodic reservoir pressure variation. However, in offshore reservoirs, the observed oscillating reservoir pressure is mainly caused by sea level changes varying the overburden weight. The earth tide effect (a periodic movement of the earth crust caused by the same forces that produce ocean tides) is negligible in comparison to the ocean tide effect.
Barometric changes and weather also can change overburden pressure by significantly influencing sea level. These effects are impossible to predict; hence, measurement of seafloor pressure is required to obtain overburden changes affecting reservoir pressure.
Fig. 1 illustrates the influence of ocean tide on bottom hole pressure. The figure plots the corrected data set and the measured seafloor pressure variations during the test. The time-lag between the seafloor pressure change and the consequent reservoir pressure change is very short, between 15 and 30 min according to other authors.1
Our study confirmed the observations that no visible difference in the corrections was apparent when varying the time-lag between 15 and 30 min. Thus, a time-lag of 15 min was selected.
Based on seafloor pressure measurements and an equation defining ocean-tide efficiency compared to fluid and pore compressibilities, a number of tests from the North Sea were analyzed after eliminating the ocean tide effects.
In some cases, the corrected pressure data enhanced the understanding of the reservoir. In a typical case, the boundary interpretation changed from pressure maintenance to a sealing fault.
TEST EXAMPLE
Fig. 2 plots the pressure derivative from the main buildup of an offshore reservoir. After about 1 hr, the pressure derivative indicates radial flow. Further analysis had been based on this, and an analytical model with a radial composite reservoir matched the data. However, uncertainties appear at late time.
It is difficult to match a simpler model through all of the data. In fact, a homogeneous model with pressure maintenance rules out the last 12 hr of data from the 24 hr buildup.
The possibility of sea tide influencing the interpretation was investigated because tidal effects had been observed frequently in nearby reservoirs during previous well tests.
Seafloor pressures measured during the test were used to correct the bottom hole pressure data. Fig. 3 is a plot of the pressure derivative of the corrected buildup. The new interpretation exhibits a pressure maintenance behavior and the earlier observed late-time effects were ruled out. This change in interpretation dramatically revises the understanding of the reservoir.
IDENTIFICATION
Ocean-tide effects normally are observed in reservoirs with a relatively high kh (permeability times reservoir height). Also a high productivity index indicates that tidal effects are likely.
A linear plot of the pressure buildup curve often hides the ocean-tide effect. However, the tidal effect is more evident on the pressure derivative plot in log-log or linear co-ordinates. The pressure derivative has been shown to be very sensitive to ocean-tide effects. If these effects are not considered, interpretations can be highly inaccurate.
If the data show full tidal effect sequences, simple corrections can be made, and the interpretation normally will have only a limited amount of error. However, if the full tidal effect sequence is not identified, large errors can be made.
OCEAN-TIDE EFFICIENCY
The relationship between changing overburden and subsequent reservoir pressure change is defined as tidal efficiency, TE. In an offshore reservoir, only ocean tide efficiency needs to be considered. This efficiency can be defined as:2
[SEE FORMULA]
The a is a proportionality constant, assumed equal to one in uncemented sand. The ct, cf, and cfl are the total, formation, and fluid compressibilities.
More complex relationships, which include porosity, permeability, Poisson's ratio, etc., have been introduced.1 3 But the simple TE equation proved sufficiently accurate to obtain the dampening factor for correcting the bottom hole pressure data.
To validate the dampening factor (TE) found from the equation, a number of corrections were made by varying the dampening factor to observe its influence on bottom hole pressures. The result on the main buildup and on the pressure derivative are evident in Figs. 4 and 5.
It is interesting to note that a wrong dampening factor induces periodic oscillations on the data. More complex equations will often give wrong dampening factors because of uncertainties in some parameters. The correct dampening factor is found through trial and error by varying dampening of the seafloor pressures and subtracting these from the well test data.
THEORETICAL RESULT
With the fluid and pore compressibilities found in the North Sea Tordis field, the theoretical dampening factor is:
[SEE FORMULA]
From Figs. 4 and 5, it is evident that the theoretical dampening factor corresponds very accurately to one determined by visual inspection through a trial and error approach.
CORRECTION PROCEDURE
Measured seafloor pressures provide good data for correcting bottom hole pressure. The seafloor pressures are used as follows:
- Time shift the seafloor pressure within 15-30 min. For any North Sea reservoir, a time lag of 15 min is sufficiently accurate.
- By using the mean seafloor pressure during the period investigated and varying the dampening factor, the seafloor pressures are set to an oscillation around a horizontal axis at zero.
- The difference between the original pressure data and the transformed seafloor pressures is the correction for the specific dampening factor used.
- Calculation of the dampening factor is not necessary before doing the correction. A visual inspection of the corrected data obtained at different dampening factors will give the correct ocean tide efficiency.
The theoretically calculated dampening factor can then be compared to the actual one, found from the graphical analysis. If the dampening factors are different three possible explanations are as follows:
1. Reservoirs containing water have the largest dampening factor. The smallest factors are for gas reservoirs.
2. A reservoir with high water saturation will have a higher dampening factor than theoretically found using oil compressibility.
3. A reservoir with high gas saturation will have a smaller dampening factor than theoretically found using oil compressibility.
TWO SCENARIOS
With time, the dampening factor will change because of changes in either fluid or rock compressibility. G. Dean3 thoroughly investigated drive mechanisms caused by changing rock compressibility in the chalk reservoir of the Ekofisk area.3 In reservoirs with small changes in rock compressibility, fluid compressibility may change.
Dampening factor changes can lead to some interesting conclusions. An increased dampening factor would over time indicate that the reservoir water saturation has increased possibly because of water injection. A decreased dampening factor would indicate that the reservoir gas saturation has increased possibly because of gas injection.
RECOMMENDATIONS
The use of tide tables to determine the maximum difference between high and low tide is insufficient to decide whether test data are affected by ocean tide effects. A very small difference would lead to the conclusion that the reservoir would not be affected by ocean tides, but any changes in tides because of weather would have a much greater impact. Such events as weather cannot be corrected for unless seafloor pressures are available.
When doing a well test program, the cyclic changes of the ocean tide should be taken into consideration. Fig. 6 shows the effect on an ideal pressure derivative of a buildup on a log-log scale and the same buildup affected by the ocean tide. A 6-hr shift of the effected buildup indicates that the effects are in an opposite direction.
Because of this, it is important to start the next shut-in at a 6 or 18-hr time difference from the first one. Start time of next shut-in must then be at: _t = 6i after the start of the previous shut-in. The i is any odd number.
This procedure allows for the identification of whether the pressure response is an ocean tide effect or a reservoir effect. In the example previously discussed, the two curves could be interpreted as a sealing fault or pressure maintenance.
A high resolution seabed pressure gauge is important for correcting bottom hole pressures. Lower resolution in seabed pressures will induce additional noise in the bottom hole pressures.
ACKNOWLEDGMENTS
The authors would like to thank Saga Petroleum for the data in the example.
REFERENCES
1. Furnes, G.K., Kvamme, O.B., and Nygaard, O., "Tidal response on the reservoir pressure at the Gullfaks oil field," Pageoph, Vol. 135, No. 3, 1991.
2. Hemala, M.L., and Balnaves, C., " Tidal effect in Petroleum well testing," 6th SPE Offshore South East Asia Conference, Singapore, Oct. 28-31, 1986, pp. 139-51.
3. Dean, G.A., Hardy, R., and Eltvik, P., "A new method to monitor compaction and compressibility changes in offshore chalk reservoirs by measuring formation pressure variations caused by the sea tide," SPE 23142, the Offshore Europe Conference, Aberdeen, Sept. 3-6, 1991.
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