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Reservoir Navigation With Resistivity Forward Modeling

A workflow for geosteering thermal oil sands development wells

Chinook is one of the more active services companies in Canadian thermal oil sands developments. Services provided include wellsite geology, coring supervision, core logging, geochemical analysis and now also geosteering, performed remotely and in the field.

Since the launch of capable geosteering software packages ten years ago, we tried to apply the concepts of curve correlation to oil sands project. We found that applicability of traditional correlation techniques is very limited in thermal oil sands reservoirs. Among the reasons for this: lateral variability in formations such as McMurray and Clearwater, presence of heterolithic inclined beds associated with fluvial and deltaic depositional environments, small scale of lithology changes, complex geometry of reservoirs. For these reasons, thermal developments typically employ the use of resistivity logged while drilling as the main tool for reservoir navigation.

SAGD model: correlation and mapping.
SAGD model: correlation and mapping.

While geosteering with gamma relies on simple and robust trigonometry, measuring and interpreting resistivity logs is a lot more complex. Physical characteristics or the reservoir, tool configurations, wellbore geometry, all have significant influence over resistivity readings. Different tools have different readings in the same lithology (based on sensor configuration, induced current frequency, phase vs. amplitude, electrode spacing). The same tools have different readings at different inclinations of the probe inside the same reservoir. And of course, tools will have different readings when the wellbore and the LWD tool is placed at different depths within the reservoir.

The combination of so many variables made it very difficult to simulate a theoretical reservoir response from resistivity tools. Math and physics computations of these complex equations were the exclusive  domain of large oilfield service companies such as Halliburton, Schlumberger and Weatherford. Recently tough, ROGII released a resistivity add-on module to their Starsteer geosteering software product. Starting in late 2019, third party vendor neutral resistivity forward modeling became a viable option for navigating thermal reservoirs.

Resistivity forward modeling

Forward modeling of resistivity curves combines the following variables in the calculation:

  • LWD vendor tool string specifications and set of resistivity curves
  • A theoretical earth model derived from one or more resistivity profiles based on offset type wells
  • Location and position of wellpath inside the reservoir

The last element is determined in large part by the geosteering interpretation. Correlation between LWD readings and forward modeled curves is at the center of this geosteering interpretation. This interpretation leads, as usual, to the identification of blocks of constant apparent dip. The value of that apparent dip, along with determination of wellpath position relative to stratigraphic markers leads to precisely calculated targets ahead of the bit.

A particularity of many thermal oil development wells is determination of stand-off to water. SAGD producer wells are drilled as close to reservoir bottom as possible, in order to maximize recovery of the gravitationally drained heated bitumen. Many times, the bitumen lies on top of a water leg present inside the same geological formation. The resistivity contrast between bitumen saturated sand and water sand is what is being chased while steering most SAGD producer well.

SAGD model: correlation and mapping.
Steering with forward modeling of Halliburton ADR curves; a combination of deep, medium, shallow; high and medium frequency; and azimuthal resistivity readings

Workflow

When conceptualizing a geosteering process, we mostly think about curve correlation. However, there is a lot more that goes into the planning and execution of a geosteering routine.

The workflow includes a series of steps:

  1. Preparation
    • Data loading
    • Top picking and correlation
    • Mapping
    • Referencing
  2. Set-up
    • Log squaring
    • Earth Model generation
    • Display calibration
    • Resistivity forward modeling
  3. Geosteering interpretation
    • Curve correlation (LWD vs modeled curves)
    • Identification of blocks of constant apparent dip
    • Apparent dip and stand-off determination
    • Reservoir navigation and target prediction
Top picking and correlation: McMurray sequence

A more complex workflow involves the use of combined earth models whereby the resulting model used is much closer to “real” reservoir geology and will reflect the variable nature of the reservoir and changing resistivity profiles along the lateral length of the well.

It is worth mentioning that while a single profile interpretation is easy to perform (just run the resistivity module in Starsteer), using combined models is more laborious. This is where thorough geological understanding of the play is leveraged alongside Starsteer’s heavy mathematical computations.

Combined Earth Model

Preparation (including mapping, top picking, display setup, format and colors) and then log squaring, earth model generation, tool and curve setup, is all done prior to the core processes of geosteering and curve correlation. Correlation is where we spend most of our time as geosteeres. We perform curve correlations in order to project targets ahead of the bit by using calculated apparent dips and stand-off determination.

Use of resistivity modeling gives precise stand-off values, while correlation on the MD scale gives apparent dip a precision within 0.5 degrees in our evaluation (compared with 0.1 degrees precision in traditional correlation on the vertical scale).

Results are integrated in the area model and can be used for statistical analysis at pad or pool level.

Thermal development 3D
Thermal development

Limitations

The approach has limitations related to extreme reservoir heterogeneity, drilling in depleted zones where the resistivity profile was altered during production, or resistivity drift due to change in reservoir temperature. Additional techniques are deployed to mitigate such factors, depending on reservoir, pool and development specifics.

Chinook Experience

At Chinook, we applied geosteering with resistivity forward modeling in the field and from our Remote Operations Space, using tools such as Halliburton ADR, Weatherford GuideWave and Schlumberger Periscope, on diverse thermal reservoirs (McMurray, Clearwater, Mannville). The approach allows us to steer the wellbore of producer wells more aggressively, closer to reservoir bottom, in turn maximizing recovery and improving return on investment.

Thermal experience in the WCSB
Chinook thermal development experience in the WCSB (in-situ)

See also:

 

© 2020 Chinook Consulting Services

Author:
Calin Dragoie

Posted On:
December 26, 2020

Category:
Geoscience, Technical Articles