Delivering a healthy well that's producible for the next 20 years is always better than skimping a few thousand dollars merely drilling a hole today. Geo-steering is one of the techniques to achieve that.
Geo-steering helps to adjust our wellbore position inside the target reservoirs based on real time geological data gathered while drilling. It's like driving across a very narrow bridge, with just enough width to fit one car, safely. You have judge your next move based on what happened in the moment. Veer just 0.5-ft to the left, you'll fall into the river.
We don't use geosteering all the time as it costs more to drill a hole. So In this article, let's talk about why, when and what to consider for geo-steering.
Geo-steering achieve three goals at once:
- Proactively adjust - you are monitoring the wellbore trajectory all the time until TD.
- Borehole position - you are making sure that you stay within your geological target.
- Gather data while drilling - you get real time data log data to steer the well and well log evaluation later on.
It's particularly useful if you are trying to drill a 9000-ft horizontal hole inside a 6ft reservoir. For a short target in a thick reservoir, maybe it's not that useful.
- You need the target reservoir, it's reservoir properties and the planned reservoir trajectory.
- Build a forward model using your offset wells data, geological models and other data. We try to predict the response as if we are drilling the well now. As we drill and get new data, we compare to our model, update the model, and adjust the tractor accordingly.
3. Use the right sensors that:
- give you sense of direction - up, down, left, right;
- are sensitive to geological changes; and
- have the right depth of investigation for your reservoir boundaries.
Pick from any azimuthal LWD sensors like GR, Density, Sonic, and Resistivity or distance-to-boundary resistivity tools for help steer you well.
4. Know the sensors disctance to bit. You want the right sensor to be as close as possible to the bit, depending on the objective. If you are looking for a porous zone, density maybe put as close as possible. If you are avoiding water leg, then probably you want to set the resistivity sensors closer to the bottom. You may have to work together with the service provider for the best tool configuration.
You can proactive steer and avoid exiting the reservoir before it’s too late.
The question is, when do you use geo-steering?
Assuming that you got the resource you need for geo-steering (contract, budget, manpower) covered, then you can use there are six situations where geo-steering could help.
- When you are in a thin reservoir and you need to stay inside that reservoir, regardless.
- When your well length is sufficiently long.
- When you have complex reservoir geology due to heterogeneity of some reservoirs. In this type of reservoir, you need to closely monitor the reservoir trajectory because you could potentially exit the reservoirs, encounter faults, see reservoir anomalies, and many more during drilling. Mother nature can be moody at times and she can throw in nasty surprises too!
- When you need real time data for on-the-spot reservoir evaluation. Geosteering with with resistivity and porosity allows for real time evaluation of logs.
- When you want to stay in the oil zone. It's crucial when you want to stay in the oil zones, to steer away from the water leg or, even to avoid penetrating the gas zones.
- When you need support. Drilling in a thin reservoir with long horizontal reach takes a lot of attention. It may take weeks to finish drilling. One person might not be able to do it alone and to monitor 24/7.
The next time you think about Geo-steering, then remember this poem;
I would geosteer in a sand
I would geosteer drilling on land
I would geosteer to produce more
I would geosteer drilling offshore
I would geosteer for oil or gas I would geosteer with witsml or las
I would geosteer with GR or images
I would geosteer sliding up and down or with nudges
I would geosteer with deep directional resistivity
I would geosteer using the measurement with best contrastivity
Doesn’t matter the well, the field or the goal,
If you’re not geosteering, you’re just making a hole.
Credit: Joe Gremillion