Tidal effects

Tidal Effects

The relative motion of the Earth, Moon and Sun results in some changes in gravitational forces that act on the sea and lithosphere. This creates some tidal changes in water depth and can lead to some changes in hydrostatic head and reservoir pressure for some offshore wells.
These tidal effects can be observed with some oscillations in the pressure data at the seabed:
reference tidal signal on the seabed gauge
As SPE 84376 explains, these periodic oscillations are transmitted through overburden to the reservoir but the tidal signal has then a smaller amplitude (function of the total compressibility) and is shifted in time (function of the reservoir permeability).

These tidal effects may be visible in the downhole gauge pressure data during shut-in periods, as shown below. 
tides on PBU test
Tidal pressure signals may also be detected in onshore wells, due to the expansion and contraction of the earth’s lithosphere. However, the tides are typically an order of magnitude smaller than the sea tides and may not be detected.

Why is this a problem ?

The derivative of the above PBU test is shown below.
derivative with tidal effects
The oscillations on the derivative are due to the tidal effects and mask the reservoir features from few hours onwards, from about 6 to 40 hours of test.
While the well testing team may acquire some nice and long PBU tests looking for large radius of investigation and connected volume, the low amplitude tidal effects may become dominant at late times, as the ΔP changes are getting smaller and smaller. The reservoir features away from the well will be masked and the information on radius of investigation, heterogeneities and volume will be limited to a certain distance from the well (usually hundreds of feet).
The problem in well testing softwares like Saphir is the default “smoothing” in the derivative plot. This could “hide” the tidal effects, among other issues, and could compromise the PBU test and its interpretation.
The petroleum engineer cannot get rid of the tidal signal simply by “smoothing” the derivative. This could create some wrong features that could be mistakenly taken as flow regimes and result in erroneous information on the well and reservoir. In addition, the pressure data will still be affected by the tidal signal, leading to a wrong deconvolution and overall interpretation.

The tidal signal has to be identified and removed from the pressure data. The identification of this signal can also provide some information on total compressibility and subsequent changes with long term production and depletion.

Compressibility

The tidal effects are not simply some noise in the data, but a composite of eight or more signals with different periods and amplitudes (SPE 84376).
As a result, this problem cannot be removed by “smoothing” the data, but using an algorithm and a reference tidal signal (usually from a seabed gauge). The figure below shows the original bottom-hole pressure (green) and the data with the tidal signal removed (purple).
removing the tides from the pressure gauge data
With the tidal signal identified in the well test data:
extracting the tidal signal from the pressure gauge data
By extracting the tidal signal from the bottom-hole pressure data, some critical information on compressibility can be recovered. Over time, the changes in compressibility could be used for reservoir surveillance and be complementary to 4D seismic.
The pore compressibility can be defined in terms of the fluid compressibility:
tidal transmission efficiency and compressibility

Hence the pore and total compressibility values can be calculated from the fluid compressibility and saturation values, and the tidal transmission efficiency T (obtained when removing the tidal signal from the downhole gauge pressure data). These values are relevant to well test analysis, as small pressure disturbances are applied.

Recommendations: 2 pressure gauges at the seabed for offshore appraisal wells

The tidal effects mask the pressure transient response and need to be removed using a reference signal (seabed pressure). The best practice is to run 2 pressure gauges at the seabed (one for redundancy) so as to acquire the reference tidal signal. It is sometimes possible to generate an approximate seabed pressure response using tidal data from simulation models or databases, but it is difficult to account for the topography and the weather. Practices show that generating a tidal reference is not very successful compared to using seabed gauges.
More information on how to detect and remove the tidal effects from the data is available in the Well Test Analysis Workflow session. Some other data issues are illustrated in the full training video: Factors that complicate well test analysis.

Tidal Effects on Wireline Pressure Measurements (RFT/MDT)

The tidal effects impact all pressure data, including the wireline pressure measurements. The pressure data will be under or over-estimated due to the timing relative to the tides.
This could result in some gradient errors and affect the interpretation, for example with the detection of barriers or fluid contacts. Like for well test analysis, corrections can be applied using the time of the measurement and a seabed gauge.

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