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PSPrediction of Sub-seismic Sealing Faults Using Simple Numerical Previous HitSimulationNext Hit 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])

 

Abstract 

In order to justify development drilling in a partly-depleted, highly faulted gas Previous HitreservoirNext Hit 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 Previous HitreservoirNext Hit.  

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 Previous HitsimulationNext Hit models that provide a level of confidence sufficient to predict the location and expected Previous HitreservoirNext Hit conditions of remaining incremental reserves in a partly-developed Previous HitreservoirNext Hit. These models have proven to be very useful in their ability to provide quick results with limited geologic and Previous HitreservoirNext Hit data. The key factor in their success is the proper integration of flowing pressure data with observed production decline curves. Static Previous HitreservoirNext Hit 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 Previous HitsimulationNext Hit model predicted the presence of sub-seismic faulting that provided a seal for the objective Previous HitreservoirNext Hit. 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 Previous HitreservoirNext Hit 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 Previous HitreservoirNext Hit conditions. 

The second example provides a lesson learned, demonstrating a Previous HitreservoirNext Hit in which the Previous HitreservoirNext Hit Previous HitsimulationNext Hit 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 Previous HitreservoirNext Hit 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 Previous HitreservoirNext Hit Previous HitsimulationNext Hit 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.

 

uAbstract

uPoster 1

  uProblem

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uCase Previous HitStudyNext Hit #1

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uCase Previous HitStudyNext Hit #2

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uLessons Learned

uBest Practices

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uPoster 1

  uProblem

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uCase Previous HitStudyNext Hit #1

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uCase Previous HitStudyNext Hit #2

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uLessons Learned

uBest Practices

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uPoster 1

  uProblem

  uFigures

uCase Previous HitStudyNext Hit #1

  uFigures

uCase Previous HitStudyNext Hit #2

  uFigures

uLessons Learned

uBest Practices

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uPoster 1

  uProblem

  uFigures

uCase Previous HitStudyNext Hit #1

  uFigures

uCase Previous HitStudyNext Hit #2

  uFigures

uLessons Learned

uBest Practices

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uPoster 1

  uProblem

  uFigures

uCase Previous HitStudyNext Hit #1

  uFigures

uCase Previous HitStudyNext Hit #2

  uFigures

uLessons Learned

uBest Practices

Poster 1: The Problem, Geologic Setting, and Previous HitReservoirNext Hit Previous HitSimulationNext Hit Basics 

The Problem 

Volumetric calculations indicate that two wells producing from the same gas Previous HitreservoirNext Hit  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.

 

Selected Figures 

 

Posters 2 and 3: Case Previous HitStudyNext Hit #1 

A 3-D seismic interpretation of the fault block described above was converted to a GOCAD model and assigned Previous HitreservoirNext Hit properties. A 3-D cellular model was then constructed. History matching of the actual well performance to the numerical Previous HitsimulationNext Hit indicated that internal boundaries were required to adequately explain the pressure and rate performance of the wells. This caused the seismic interpreter to revise the 3-D model. 

By interpreting tiny offsets of flexures in seismic events as possible faults, the seismic interpreter was able to segment the objective Previous HitreservoirNext Hit into four blocks (see Line A-A’). The 3-D cellular model was revised to incorporate the new barriers, and the Previous HitsimulationNext Hit was run again. This time the pressure and production history of the wells matched the Previous HitsimulationNext Hit model closely. This provided the confidence necessary for proposing additional wells to fully develop the block. Prior to the revised interpretation, the presence of the existing, nearly depleted wells in the block would have discouraged any additional drilling. Results of two new wells demonstrate that the revised interpretation was correct. This 3-D interpretation would not have been made without the results of the Previous HitsimulationNext Hit, which suggested that additional faulting was present.

 

Selected Figures 

  

Poster 4: Case Previous HitStudyNext Hit #2 

In a different part of the model created in Case Previous HitStudyNext Hit #1, a well was proposed to offset a competitive drainage situation in a 60-acre fault block. The proposed well was expected to encounter similar pressure to a recently drilled updip well. An unexpected 80-foot fault was encountered in the objective Previous HitreservoirNext Hit when the well was drilled, but the pressure matched what had been predicted by the Previous HitsimulationNext Hit model. However, after fracture stimulating the well, the observed pressure had decreased by more than 2000 psi. The fault encountered in the well was apparently a seal between two compartments of vastly different pressure. The stimulation ruptured the seal and the frac job “went south.”

 

Selected Figures 

  

Lessons Learned 

  • Previous HitReservoirNext Hit Previous HitsimulationNext Hit does not need to take several months or years to add value. Simple models can generate significant value with a few days of technical work.

  • P/Z volumetric analysis applied to a compartmentalized gas Previous HitreservoirNext Hit cannot accurately predict pre-drill reserve potential, especially in low-permeability gas reservoirs.

  • Flowing pressure data is readily available as commercial data and can be used to determine drainage areas in gas wells, when accurate static pressure data is unavailable.

  • Simple numerical Previous HitsimulationNext Hit models:

    • Work well in an environment where there is sparse Previous HitreservoirNext Hit and temporal data.

    • Can be used routinely in daily Previous HitreservoirNext Hit engineering tasks and problem solving by ALL Previous HitreservoirNext Hit engineers.

    • Work nicely in an environment where rig moves take place very two to three weeks.

  • Inflection points on seismic events can sometimes indicate faulting, even if no offset is observed.

  • Sealing faults can be breached by fracture stimulations.

 

Best Practices 

  • Start simple—add complexity over time only if necessary. Simple models work.

  • Sparse data does not need to inhibit the use of Previous HitreservoirNext Hit Previous HitsimulationNext Hit.

Flowing tubing pressure (converted to bottom hole flowing pressure) can be used for the pressure match when other Previous HitreservoirTop pressure measurements are unavailable

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