Changes

The alternative is to geosteer a horizontal well, particularly where there is less confidence in predicting the reservoir geology. Geosteering involves using geological information obtained as the well is being drilled to try and keep the well path within the target. This can involve the use of real-time log data but may also include input while drilling from well-site biostratigraphy or from examination of drill cuttings if the lithologies at the top and base of the reservoir are distinctive.

The alternative is to geosteer a horizontal well, particularly where there is less confidence in predicting the reservoir geology. Geosteering involves using geological information obtained as the well is being drilled to try and keep the well path within the target. This can involve the use of real-time log data but may also include input while drilling from well-site biostratigraphy or from examination of drill cuttings if the lithologies at the top and base of the reservoir are distinctive.

The main technique in geosteering involves the use of a real-time log data display while the horizontal well is being drilled. The downhole log data can be directly transmitted to a computer screen in the geologist's office from the well site. This allows the geologist to establish which part of the reservoir is being drilled through and then decide where the well should be steered to next. This is done by comparing the real-time logs with data from nearby wells. Log responses in horizontal wells can look different from that in conventional wells (Meehan, 1994). A catalog of expected log responses, as they would appear in a horizontal trajectory, can be created by computer modeling. If the geologist thinks that the well is above the target zone, they will ask the directional driller at the rig site to steer down; if the geologist believes they are below the target, they will ask the driller to steer up.

The main technique in geosteering involves the use of a real-time log data display while the horizontal well is being drilled. The downhole log data can be directly transmitted to a computer screen in the geologist's office from the well site. This allows the geologist to establish which part of the reservoir is being drilled through and then decide where the well should be steered to next. This is done by comparing the real-time logs with data from nearby wells. Log responses in horizontal wells can look different from that in conventional wells.<ref>Meehan, D. N., 1994, Geological steering of horizontal wells: Journal of Petroleum Technology, SPE 29242, v. 46, no. 10, p. 848–852.</ref> A catalog of expected log responses, as they would appear in a horizontal trajectory, can be created by computer modeling. If the geologist thinks that the well is above the target zone, they will ask the directional driller at the rig site to steer down; if the geologist believes they are below the target, they will ask the driller to steer up.

Geosteering is at times a high-risk operation, and it can be stressful. In the early days of drilling horizontal wells, it was found that just under half of all the horizontal wells that were drilled ended up as failures or underperformed compared to expectation (Beliveau, 1995). The record may have improved since then; nevertheless outright failures still occur today.

Geosteering is at times a high-risk operation, and it can be stressful. In the early days of drilling horizontal wells, it was found that just under half of all the horizontal wells that were drilled ended up as failures or underperformed compared to expectation (Beliveau, 1995). The record may have improved since then; nevertheless outright failures still occur today.