The most noticeable effect of drilled solids is an increase in trouble costs when they are not removed from drilling fluid. Unfortunately, these costs are usually hidden because they are not obvious while drilling or in post-well analysis.

Poor filter cake quality can cause differentially stuck pipe and lost circulation. Filter cakes become incompressible when excessive drilled solids are present. This allows pressure to seal drill collars against the side of the well bore. Thick filter cakes caused by drilled solids also cause large surge and swab pressures.

Solids are indicated in a drilling fluid by retort solids and plastic viscosity. The retort indicates the total quantity of solids; the plastic viscosity responds to more than that. Plastic viscosity is a drilling fluid’s viscosity at a very high shear rate and depends upon four factors:
1. Liquid phase viscosity;
2. Size;
3. Shape; and
4. Number of particles.

The founder point

The concept of the founder (or flounder) point was presented initially by Rubin Feenstra in the ’50s. For many years the full significance of this contribution was not appreciated. The founder point is the point at which the drilling fluid ceases to remove all of the drilled cuttings from the bottom of the hole and the drill bit is drilling cuttings instead of new rock. Not knowing whether drilling is taking place above or below the founder point renders data totally incomprehensible. Below the founder point for example, increasing drilling fluid flow rate might decrease drilling rate because of the increase in ECD. Above the founder point, better bottomhole removal of cuttings will show a significant increase in drilling rate. So, does improved hydraulics improve drilling performance? Obviously, it depends upon whether the drilling is above or below the founder point.

Cleaning beneath the bit is more effective with low plastic viscosity. The founder point is higher with low plastic viscosity. If all of the cuttings are being removed and the bottom of the hole is clean, increasing the hydraulic cleaning of the bottom of the hole will not increase penetration rate in hard rock. Decreasing the plastic viscosity will provide the capability to increase bit loading.

Removal of drilled solids from a drilling fluid will decrease plastic viscosity. If drilled solids remain in the drilling fluid, they will grind into smaller and more numerous particles, which increases plastic viscosity and decreases drilling performance. Cleaning cuttings from the bottom of the hole without regrinding will provide the largest solids possible entering the annulus. The solids must be brought to the surface as quickly as possible without tumbling and grinding in the annulus. Failure to keep the cuttings moving upward as they are brought to the surface, will result in smaller solids and a larger number. Both of these effects will increase plastic viscosity.

Drilled solids effects

To remove cuttings as quickly and efficiently, the solids control equipment should receive cuttings that are as large as possible. Good solids control starts with rapid removal of solids from beneath the drill bit and good transport up the annulus. Good transport can be verified by examination of cuttings removed from end of shale shaker. They should have sharp edges. Large cuttings will also appear if they are in the annulus. The quantity of solids on the shale shaker screen does not indicate whether cuttings are being transported correctly.

Cuttings will reach the surface if a thin fluid is pumped fast enough and long enough. These cuttings will tumble in the annulus and assume a well-rounded shape. The detritus from this tumbling increases the concentration of small solids and will increase plastic viscosity. Surprisingly, removal of drilled solids will also increase the very low shear rate viscosity as measured by the K value of the Power Law Model of drilling fluid.

As drilled solids are removed, the plastic viscosity decreases. Decreasing the plastic viscosity will increase the low shear rate viscosity (K), which will bring larger, more easily removable solids to the surface. It also works the other way. Failure to bring cuttings to the surface while they are large enough to be removed by the equipment will increase the plastic viscosity. Increasing plastic viscosity will decrease the ability to bring cuttings to the surface and allow them to grind into smaller, more numerous particles. This increases the plastic viscosity, which decreases the carrying capacity.

When this happens, the normal reaction is to increase the yield point. When the yield point increases significantly, the shale shaker screens may be too fine to handle the more gelled fluid. The screens will need to be changed to a coarser screen. This decreases the quantity of drilled solids that can be removed which again increases the plastic viscosity. This cyclic change requires careful attention and some drastic action to reverse the cycle. This is a “solids problem” that arises without always having an increase in total solids content.

Trouble costs

Surge and swab pressure — Thick filter cakes make good seals around a drill bit. As the bit is raised, drilling fluid will have difficulty flowing down the annulus to fill the hole beneath the bit. This can swab a kick into the well. As the bit moves into the hole, the fluid from beneath the bit must be displaced up the hole. With a thick filter cake sealing or restricting the flow, the pressure beneath bit increases and can cause lost circulation.

Cement placement — If filter cakes are thick, cement will not move the filter cake to make a good seal with the formation. With contact with the calcium in the cement, filter cakes become very permeable and can cause flow behind the casing. Expendables wear much faster when impacted with abrasive solids. Large drilled solids act like sandpaper and erode metal.

Disposal costs — Poor solids removal equipment and arrangement results in excessive quantities of dilution. Failure to remove enough solids with the removal equipment means that additional clean drilling fluid must be added to keep the drilled solids under control. This will increase drilling fluid costs and disposal costs.

Formation damage — Thick filter cakes are an indicator of large quantities of filtrate entering the formation. Filter cakes containing drilled solids are not compressible. Fluid loss may appear satisfactory for the API 100 psi test, but at higher pressures fluid loss can increase. When the filtrate interacts with the formation fluid, some insoluble products may be formed and precipitate in the pore space. This will plug the formation. If the damaged region next to the well bore is thicker than the penetration expected from a perforator, production may be permanently diminished.

Log interpretation — Thick filter cakes usually mean that a large quantity of filtrate has entered the formation. Now instead of the original fluid, the formation is filled with filtrate. Any measurement must be made through this invaded zone and may hide the true concentration of fluids in the formation. Some production might be lost.

Trouble costs caused by drilled solids are frequently not attributed to drilled solids. Drilled solids removal results in wells that are not only drilled faster and cheaper but also without much nonproductive time. The most prevalent trouble created by drilled solids is caused by thick poor filter cakes. They are responsible for stuck drill pipe, kicks and lost circulation from excessive surge and swab pressures, poor cement placement and difficulty with log interpretation.

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