Sand Control, Pumping and Design - webinar

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The webinar was presented by Amro Hassan, who spent 13 years with Haliburton as an Associate Technical Professional in hydraulic fracturing, acid stimulation, sand control, water/gas shut-in and coil tubing services.

UCS (Uniaxial Compressive Strength)

The first thing we need to look at is the rock we defined something which is called the UCS or the uniaxial compressive strength (The uniaxial strength, also known as the unconfined compressive strength, of a rock may be regarded as the highest stress that a rock specimen can carry when a unidirectional stress is applied, normally in an axial direction to the ends of a cylindrical specimen.). This is a test, done on the core sample: if it's above 800 psi, we consider this as a weak sand; between 800 and 300 psi this is a very weak; zero cohesion would be less than 300 psi which is a loose sand.

Note:
weak sand:       USC > 800 psi
very weak sand:  800 psi > USC > 300 psi
loose sand:      USC < 300 psi

Rock failure causes

So how does the sand production process happens? First - the rock has to fail somehow, then after the rock fails we need something to transport this rock failure (or the sand) into the wellbore.

The first thing that causes the rock failure is the stresses in the bottom hole. There are a lot of changes in the stresses in the bottom hole either from the drawdown or from the tectonic movement in general, this could cause a rock failure specifically in a fragile rock. Then after that the flow rate, once you start production, would manage to transport the rock (or the loose particles from the rock) into the wellbore.
Also the flow rate is affected by the drawdown as well as the viscosity of the fluid being transported.

We can classify the causes as natural causes and operational causes.

Natural causes

The natural causes could be unconsolidated sand. There are some formations that are already unconsolidated, that don't have good compressive strength.

The capillary forces in some formation could cause the rock failure and could affect the stress regime in bottom hole. And the reservoir depletion affect the stress profile: sometimes at the early state of the well it does not produce as much sand and all of a sudden at the end you start seeing loads of sand coming in and this is because of the depletion.

Water production

The water production does several things.
In many of the sandstone formation the cementing material between the grains is actually a clay. Once the clay is contacted with water production it starts getting dissolved and then the grains start to get loose and you start getting sand production. So normally you would see sand production closer or right after the water production or the water breakthrough.
Water as well increase the drag forces because the in some of the gas ones, once you start watering out, the viscosity of the water would manage to transport loads of sand into the formation; it does disturb the cohesive forces between the grains and each others and it does disturb the capillary forces.

Operational causes

• Drilling is already a failure to the rock, we try to break the rock.
  
• Perforation also is one of the other aspects of breaking the rock.
  
• Acid treatment as much enhancement as it gives to the production it actually dissolves the cementing material which didn't dissolve from the water production. In some areas the clay is not water soluble, so the acid would manage to dissolve it and start producing fines from the acidized formation.

Consequences

• Loss of production due to wellbore filling, skin, facility downtime;
• Equipment problems due to sand fill;
• Erosion of downhole and surface equipment;
• Casing / liner collapse;
• Environmental concerns due to disposal.

Prediction

• Experience. Past experience in the area may indicate potential for sand production.
• Offset well production. If there are sand production problems within the same reservoir, new wells will have similar problems.
• Drilling data. Sand production also occurs during drilling.
• Core sampling. Compressive strength tests.
• Logging. Rock mechanical properties can be calculated from logs.
• Computer modeling. Sand studies can predict potential dispatch based on data provided from all of the previous items.

Sand control techniques

Any sand control technique would need to:

1. stop the sand as a first priority;
2. maintain the productivity;
3. minimize the downtime caused by the sand production.

Sand control methods:

• Production: choke back production, selective perforation, reperforation
• Mechanical: stand alone screens, gravel packs
• Chemical: resin consolidation, polymer coating

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