Drilling fluid performance is a key contributor to the success of a drilling operation. The purpose of this fluid is to keep the drill bit cool, carry the cuttings out of the wellbore and to seal the bore internally so that it does not collapse.
Gel strength is a property of drilling fluid that impacts its performance in terms of removing cuttings.
How to Measure Gel Strength
There is an approved American Petroleum Institute (API) method for testing gel strength and there are also more crude measures, which can be done on site. The site-based methods do not give an accurate result in terms of a gel number, but they do give a good indication of the properties of the drilling fluid relative to the specific soil and gravel present on site.
The API RB 13B-1 measurement of gel strength is performed as follows:
- The sample is sheared to break up the gel formation before any measurements start. This is done by stirring at 600 rpm.
- The sample is left to stand to allow gel to form. The standard times for measurement are 10 seconds and 10 minutes, but other wait times are also used, like 30 minutes or even longer.
- Stirring is restarted after the waiting period at a speed of 3 rpm. The Gel strength is measured as the force required to break the gel in lbf / 100 ft2. It can be measured using a Marsh Funnel or using the maximum dial deflection on a rotational viscometer.
Crude field measurements can help engineers and contractors to assess the mud characteristics and whether there is a potential gel problem, which should be explored further.
One method involves taking a sample of drilling fluid in a clear plastic container. Scoop some excavation material from the entry or exit pit and stir it into the sample. Allow the sample to settle for a few hours and check whether the dirt has settled out to the bottom.
If the dirt settles out, the mud does not have sufficient gel strength to keep the cuttings suspended. This may require further investigation and adjustments to the drilling fluid composition.
How to Adjust Drilling Fluid Gel Strength
The properties of drilling fluid are different to those of water or oil. While these materials change viscosity according to their temperature, drilling fluid changes viscosity according to its flow rate. The faster it is moving, the lower the viscosity and vice versa.
Bentonite content has a direct influence on gel strength of drilling fluid. For example, lower yield bentonite gives a higher gel strength because of the increased platelet density in the fluid.
Polymer additives can also be used to increase gel strength without making a significant change to viscosity. Xanthan gum is one polymer commonly used in the industry. Mixed metal oxides are also used as specialty additives to increase gel strength for extreme soil conditions.
Horizontal Drilling Vs. Vertical Drilling Applications
Horizontal and vertical drilling applications are quite different when it comes to the best gel strength to use.
Vertical drilling is a simpler process requiring the mud circuit to overcome the forces of gravity to pump out drilling fluid with cuttings from the well. If the flow rate is stopped for any reason, cuttings held in suspension have a long way to fall down the well before accumulating at the bottom.
A lower gel strength will therefore be sufficient to slow down this effect of gravity and prevent blockages or pressure problems.
On the other hand, horizontal directional drilling is much more susceptible to blockages caused by a buildup of cuttings. If the flow in the mud circuit is shut off, the suspended cuttings only have to drop a few inches before building up on the lower surface of the bore.
A high gel strength is very important to keep the cuttings suspended even if the mud circuit stops flowing for a short time.
The required gel strength for horizontal directional drilling is therefore higher than for vertical drilling.
Consequences of Getting Gel Strength Wrong
When the gel strength is too high, the drilling fluid tends towards solidification in static conditions. This is dangerous because it may require a very high pump pressure to get the drilling fluid moving again. High pressures could cause a frac-out leading to leaked drilling fluid into the ground.
Another problem with gel strength that is too high is that it makes separation of cuttings from the drilling fluid difficult on the surface. The removal of cuttings is essential to the performance of the mud system as the drilling fluid is recycled for re-use.
When the gel strength is too low, cuttings tend to drop out of the drilling fluid too easily. Drilling fluid is designed to hold cuttings in suspension. If this fails, cuttings build up on the lower side of the bore and eventually could cause a blockage. It can also result in stuck pipe.
This is a hazardous situation because releasing the pipe could damage the bore and the equipment.
What We've Learned
Drilling fluid properties play a critical role in the success of a horizontal directional drilling or vertical drilling project. Gel strength is the specific property related to how easily the drilling fluid will hold cuttings in suspension and how strong the gel state becomes when the fluid stops flowing.
The right balance must be struck for the specific soil conditions in order to prevent costly repairs and downtime due to blockages or fracking-outs.