Changes

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.

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.<ref>Beliveau, D., 1995, Heterogeneity, geostatistics, horizontal wells, and blackjack poker: Journal of Petroleum Technology, v. 47, no. 4, SPE Paper 30745, p. 1068–1074.</ref> The record may have improved since then; nevertheless outright failures still occur today.

Frequently, when drilling new well locations, the geology will turn out quite different from what was expected and this reflects the nature of reservoir uncertainty. Even so, the outcome from the vertical penetration of a reservoir interval is a lot more predictable than when a horizontal well is drilled. Random geological uncertainties that will have a relatively trivial effect on the drilling outcome of a vertical well can cause serious problems with a horizontal well operation.

Frequently, when drilling new well locations, the geology will turn out quite different from what was expected and this reflects the nature of reservoir uncertainty. Even so, the outcome from the vertical penetration of a reservoir interval is a lot more predictable than when a horizontal well is drilled. Random geological uncertainties that will have a relatively trivial effect on the drilling outcome of a vertical well can cause serious problems with a horizontal well operation.