The combination of large pressure depletion and high bulk compressibility lead to large deformation in the reservoir horizons.
Compaction, known as the “Beauty and the Beast”, can be both a blessing and a curse.
The natural drive mechanism assures good production.
Yet the associated failure mechanisms of this process are numerous and varied.
ADVANTEK has intimateknowledge of this process.
ADVANTEK provides an accurate field verified mode for compaction modeling.

Well evaluation and descriptive reservoir characterization are particular strengths of ADVANTEK.
The effectiveness of the recovery mechanisms and the integrity of the reserve can be met through
the use of ADVANTEK’s geomechanics knowledge base. Many compaction related failures threaten
the production life of a well. Reservoir Seal Integrity and nearby Fault Activation can be disastrous
from a recovery and operational perspective. Casing failures are quite common and may substantially
decrease the planned production life of a well. Surface subsidence causes serious issues specifically
near residential areas.

Pore volume compressibility depends on the reservoir geometry and the material properties surrounding the reservoir. The affect of reservoir geometry and surrounding material on the pore volume compressibility has been modeled by assuming an elliptical inclusion in an infinite solid (Figure 1). The modeling results show that the amount of stiffness of the inclusion (reservoir) will depend on two factors:

• The ratio of reservoir areal extent, R, to the height of the depleted zone, H (aspect ratio, R/H)).
• The ratio of the bulk compressibility of the surrounding rock (shale) to the reservoir bulk compressibility, Csand/Cshale.

Figure 1. Reservoir geometry and some relevant geometrical parameters

The deformation experienced by many materials depends on the rate at which they are loaded. The reservoir analog is that the amount of compaction at a particular time depends on the rate of depletion (see Figure 2, for example). From an experimental point of view, Rate Type Compaction Measurements (RTCM) are sometimes done to allow scaling between rapid laboratory testing rates and much slower rates for changes in reservoir pressure in the field.

One can also address plastic deformation (grain breakage), the influence of the in-situ stress state, the influence of previous depths of burial, the influence of core disturbance and the influence of loading rate on compressibility. The lower the loading rate, the more the sample will be compacted at a given stress level. This philosophy provides a logical methodology for accounting for the transitional period to the stabilized virgin compaction curve.

Figure 2 is an example of the predicted pore volume compressibility for a Gulf of Mexico deepwater reservoir. The values are uncorrected for reservoir geometry and associated stress transfer. Compressibility starts low and grows to a stabilized value with time.

The impact of the time-dependence of pore volume compressibility on a reservoir's performance can best be illustrated by the pressure decline curve. During early stages of reservoir depletion, the pressure decline rate is quite rapid because the early rock response is stiff. This means that the pore volume does not change substantially and pressure can change a great deal - the analogy is pressure response in a gas reservoir compared with pressure response in a liquid-filled reservoir. As time progresses, the pressure decline is moderated and the reservoir responds with higher compressibility.

Figure 2. Time-dependent pore volume compressibility

Finally it should be noted that including rate-corrected PVC values in reservoir simulators require a two-step procedure. First an initial simulation run is executed using a base case constant PVC value for all reservoir simulation blocks. The initial pressure-time profile from this case is determined and a pressure depletion rate is calculated. This pressure decline rate is then used to calculate the pressure-dependent values of PVC using the RTCM model and the pressure rate to convert time dependent PVC into pressure-dependent values

Experience	Solutions/Tools
Casing Failure Evaluation Evaluation of Pore Volume Compressibility and Compaction Evaluation of Well Integrity and Casing Failure Well Integrity and Casing Stress Analysis	RTCM software solution @Log
Geographical Environments	Contact
Equatorial Guinea Gulf of Mexico Offshore Venezuela	Mr. Karim Zaki Ph. 713.532.7630 Email: Karim@ADVANTEKInternational.com