What Works, What Doesn’t
Not all plays are equally suitable for a geosteering approach. While geosteering is the preferred method of reservoir navigation in unconventional shale and siltstone reservoirs, the approach is also used in conventional tight oil and gas clastic resource plays, as well as in some heavy oil plays and carbonate reservoirs. This is a look of applicability of geosteering and remote geosteering in the Western Canadian Sedimentary Basin.
A true geosteering process is in place when three criteria are met:
- Real-time data streaming
- Continuous supervision while drilling
- Use of a specialized correlation software
Based on MWD/LWD curve correlation, a geosteering interpretation delivers
- Exact position of wellpath relative to stratigraphic markers
- Apparent dips and their lateral extent along the wellbore
- Statistics and target projections
Advantages achieved through a geosteering approach include drilling efficiencies (better ROP, less sliding, longer lateral reach), better reservoir penetration (percentage in zone, position closer to targets), completions efficiencies (decreased tortuosity, uniform reservoir conditions), increased predictability and ultimately better rate of investment.
Deployment of geosteering
Chinook Consulting implemented and deployed geosteering supervision for most unconventional plays in the Western Canadian Sedimentary Basin, as well as some conventional plays.
In Canada, the service was used on a larger scale to drill Montney, Duvernay, Horn River and Cardium reservoirs, and on a smaller scale in the Spirit River, McMurray, Lower Mannville, Viking, Bakken, Exshaw and Glauconitic. Internationally, the approach is used in the Wolfcamp/Spraberry in the Permian Basin, in the Eagle Ford shale in Southern Texas, CBM in Australia and other international projects.
Geosteering in the Duvernay Formation is relatively straight forward, as the formation exhibits good gamma curve definitions, correlated over long distances. The organic rich shale has high gamma readings, while reading drop sharply in the more calcareous layers. Lateral continuity is generally very good, especially in the northern Duvernay trend. Geosteering issues can arise in the Southern Duvernay trend where lateral changes can occur in gamma signatures. Bulk resistivity is a viable option in that scenario.
The Montney can be very different from the shales in the West of the basin to the shaley siltstone (“shilts”) at the Alberta/BC border and the oil charged sandstone in the north-eastern reaches of the trend. Gamma signatures vary wildly from very busy to almost character-less. However, gamma curve character is very consistent across large distances, making it easy to correlate even when curve character is not very well defined in depth.
The Horn River Formation (including the Evi, Otter Park and Muskwa Members) have very good vertical definition on the gamma curves, similar in character to the Wolfcamp and Spraberry in Texas. Signatures are easy to identify and to follow along lateral sections of horizontal wells.
The Cardium Sandstone has a relatively monotonous Gamma signature, with significant lateral variation. Geosteering wells drilled in the Cardium Sand require a lot of attention, and several offset wells along the section are needed for accurate interpretation. Wells drilled below the Sandstone horizon (as are most horizontal wells targeting the Cardium oil in Central Alberta) show more vertical definition in gamma signature, as well as better lateral continuity. Wells drilled in the Cardium Siltstone are easier to steer than wells targeting the Cardium Sandstone proper.
Besides clean sand saturated with bitumen, the McMurray contains inclined heterolithic beds (IHS) and breccia, both with beds/clast scale of millimeters to centimeters. It is impossible to identify features of this small scale on logs (that have resolutions of 0.2 meters). Azimuthal Resistivity on its own may not be enough for effective steering, but in connection to resistivity modelling, it remains key to steering McMurray wells.
The Viking Sandstone is generally thin and gamma signature above and below the sandstone are relatively well preserved laterally. Drape features and other structural events along Viking wells are generally easy to identify. The difficulty in geosteering Viking wells is more related to the extreme rate of penetration recorded in this play. Data quality is also negatively impacted by extreme speeds of drilling.
Glauconitic, Falher, Wilrich
Channel plays are complex, with shale lenses and channel margins and incisions, making it more difficult to geosteer. Channel sizes vary from hundreds of meters to kilometers, with smaller scale lithological and sedimentary features that can throw off correlations. A combination between geomodelling and geosteering is better suited in steering these type of plays than pure geosteering based only on type logs. Other channel plays and shallow water reservoirs exhibit the same issues.
Deep, low energy depositional environments such as turbidite fans or deep basinal mudstone lend themselves to pure geosteering. Shoreface and other marine sandstone can also be geosteered, but may be more difficult to correlate and may require multiple type-logs. Shallow fluvial / tidal ecosystems and channel plays are more difficult to geosteered with only curve correlation.
Ultimately, it is the combination between clear markers and lateral continuity that makes a reservoir more suited for geosteering.
Combining remote geosteering with lithological observations provided by a wellsite geologist greatly improves the quality of a geosteering interpretation. Other low-cost aids to the geosteering correlation are gas chromatography/ gas ratio analysis and field geochemistry.
After all, geosteering is equal part geology and geometry.