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PSPrediction of Sub-seismic Sealing Faults Using Simple Numerical Simulation Models*
By
R.C. Bain1, K.H. MacIvor1, B.E. Holt1, and D.S. Beaty1
Search and Discovery Article #40242
Posted May 28, 2007
*Adapted from poster presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007
1Chevron North America Upstream, Houston, Texas ([email protected])
In order to justify development drilling in a partly-depleted, highly faulted gas reservoir in which untapped higher-pressure compartments may exist, convincing evidence for fault separation from existing producing wells must be provided, either by obvious fault breaks on 3-D seismic or by missing section due to a fault encountered in a well. Lacking such evidence, it is difficult to state with certainty that prospect reserves will be incremental, as opposed to acceleration, even when volumetric analysis suggests that existing wells will not capture all of the producible reserves in a reservoir.
The Mid-Continent Business Unit of Chevron North America Exploration and Production has had success in the Lobo Trend of Webb and Zapata Counties, South Texas, using simple, "fit-for-purpose" 3D-earth models and numerical simulation models that provide a level of confidence sufficient to predict the location and expected reservoir conditions of remaining incremental reserves in a partly-developed reservoir. These models have proven to be very useful in their ability to provide quick results with limited geologic and reservoir data. The key factor in their success is the proper integration of flowing pressure data with observed production decline curves. Static reservoir pressure measurements are typically unavailable and also give misleading results when used for P/Z volumetric analysis in compartmentalized reservoirs.
In the first example, a simple simulation model predicted the presence of
sub-seismic faulting that provided a seal for the objective reservoir. The
proposed location was in a syncline between two wells that had already produced
large volumes of gas and were producing at very low bottom-hole pressures. An
iterative approach involving the seismic interpreter and the reservoir engineer
resulted in a geologic model that was supported by the seismic data and agreed
with the 
history
matching efforts. The well, which would not have been approved
without the model to support it, encountered near-virgin reservoir conditions.
The second example provides a lesson learned, demonstrating a reservoir in which
the reservoir simulation and history match correctly predicted the presence of a
sealing fault, but incorrectly predicted which of several faults was the sealing
one. The sealing fault was penetrated by the wellbore and the seal was ruptured
when the well was fracture stimulated.
Quickly demonstrating the accuracy and applicability of simple numerical models in an environment where rig moves are rapid and reservoir data is sparse has generated a new interest in a tool that was heretofore thought too complex and too time consuming to apply. Asset Team Earth Scientists are now working more closely with the reservoir simulation engineers and are using the results from these simple models to help in their interpretation of subsurface geology, especially in highly faulted environments. In some cases, successful wells are being drilled where they otherwise would not have been.
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Poster 1: The Problem, Geologic Setting, and Reservoir Simulation Basics Volumetric calculations indicate that two wells producing from the same gas reservoir have not drained all of the producible reserves in a 200-acre fault block. The challenge is to identify economic drill locations despite the fact that the existing wells appear to be depleted.
Posters 2 and 3: A 3-D
seismic interpretation of the fault block described above was converted
to a GOCAD model and assigned reservoir properties. A 3-D cellular model
was then constructed. By
interpreting tiny offsets of flexures in seismic events as possible
faults, the seismic interpreter was able to segment the objective
reservoir into four blocks (see Line A-A’). The 3-D cellular model was
revised to incorporate the new barriers, and the simulation was run
again. This time the pressure and production
In a
different part of the model created in
Flowing tubing pressure (converted to bottom hole flowing pressure) can be used for the pressure match when other reservoir pressure measurements are unavailable |
