More traditional methods of drilling an oil or gas well involve drilling a vertical hole in the ground, straight down and through the reservoir. Jokingly referred to as “post holes,” these types of wells are easy and cheap to drill; but when you intersect the (mostly) flat rock layer at 90 degrees, the amount of exposed reservoir in your well is pretty small. With the advent of directional drilling practices, drillers were able to get creative, intersecting the reservoir layer at different angles in order to maximize exposure and production.
As directional drilling techniques advanced, it became possible to drill wells that are completely horizontal. Using seismic surveys and surrounding well information, geologists are able to predict where and at what depth the reservoir layer will appear. The well planners are then able to set geometric targets for the well to intersect so that the horizontally drilled well bore will remain within the target zone for huge distances.
Geosteering is the next level of directional control. The drilling crew uses a combination of real-time electronic logs, analysis of drilled cuttings, and even some paleontology to identify the exact rock layer being drilled. This means that rather than relying solely on pre-calculated geometric targets, which have their own inaccuracies, they can pinpoint the wells position exactly and steer the drilling assembly to keep it in the best possible part of the reservoir, finally reaching that “sweet spot.”
Successful geosteering is a real team effort involving well planners, drilling engineers, geologists, the directional team, and the rig crew. All of these people must work together as the oil or gas well is being drilled in order to keep the process on track. It also involves additional high tech equipment, particularly the advanced logging tools in the drilling assembly which measure the properties of the surrounding rock and relay it back to the surface in real time.
All the additional personnel and costly equipment mean that a geosteered horizontal well can be very expensive to drill, up to three times more than a conventional well. However, the increased reservoir exposure, production levels, and oil recovery makes this return on this initial investment worth it. A lot of wells drilled offshore and in deep water are geosteered in order to maximize the efficiency and offset the huge additional cost of an offshore drilling operation.
Improvements in drilling equipment, tools, and techniques combined with the increased accuracy that geosteering brings, means that some extended reach horizontal wells are being drilled to incredibly long distances within the reservoir. The technical definition of extended reach is any well where the horizontal to vertical ratio is more than 2:1, step out is more than twice the depth of the well under the surface. Currently, record wells are being drilled with a horizontal to vertical ratio that is more like 7:1 or even 8:1 with extremely long geosteered reservoir sections that are close to 40,000ft (12,000m) in length!
In order to achieve such long horizontal step outs, successfully geosteering to keep the well within the sweet spot of the reservoir takes teamwork, a high level of skill, and the best possible equipment. Certain downhole tools are an essential part of the drilling assembly in order to accurately geosteer the well:
- Logging While Drilling (LWD) Tool – This is a segment of the drilling assembly packed full of electronics and sensors and is one of the most important parts of the geosteering The LWD tools sensors can measure a wide range of properties of the surrounding rock such as resistivity/conductivity, acoustic properties, porosity, density, and magnetic resonance. The LWD tool sits just a short distance behind the bit and communicates with computers at the surface to relay data in real time, meaning that geologists can see the properties of the rock as it is being drilled while geosteering.
- Measurement While Drilling (MWD) Tool – Also a segment of the drilling assembly packed full of electronics and sensors, the MWD tool serves a very different purpose. The MWD tool measures and surveys the geometric properties of the wellbore and relays that information to the surface computers. Obviously essential to the geosteering process, we need to know the exact position of the drilling assembly at all times in order to make decisions on where to direct it to go.
- Drive System – In order to change the course of the bit and geosteer the well in to stay in the right reservoir layer, we must have a device at the bottom of the hole which points or pushes the bit in the right direction. There are many types of drive systems: positive displacement motor or rotary steerable, push or point. These all allow the directional driller to change the direction of the bit and close the loop on the geosteering
The Drill Bit
At the end of the drilling assembly, and another key geosteering component is the drill bit. This is the only component that actually cuts rock and as such, its design and properties can have a profound effect on the success of the geosteering operation. In order to accurately steer and stay on track, it is essential that the bit is balanced and smooth in order to minimize drilling dysfunction and help all of the other tools do their job. The bit should also be compatible with the drive system and respond predictably to steering inputs so that the well can be placed accurately.
As the number one drill bit provider, Ulterra has the knowledge, technology, and products that our clients need for successful geosteering. At Ulterra, we view ourselves as a trusted partner in drilling operations committed to a long-term mutually beneficial relationship. This attitude is invaluable to the team effort that is involved in the geosteering process. We work closely with our clients and the other service personnel to ensure that they have the best and most innovative drilling solution for their geosteering operation.