Perforation Interpretation Module-B

Perforation Interpretation Module-B

Sand Management

Questions:

1. How do we differentiate ‘weak and unconsolidated’ from ‘weak but consolidated’ rocks on the standpoint of the completion strategy?

The essence of sand management is the quantification of the risk of sand production. The very first step is to analyze what this would entail in terms of economics. If this risk isacceptable (or in other words there is substantial cost savings involved), then one opts for sand prevention. However, if this risk is unacceptable, then one opts for sand control. When we use the term sand control we mean exclusion methods that are installed downhole to prevent the sand from flowing into the well bore along with the flowing fluid. Examples are gravel pack, frac pack, screens,resin consolidation etc. In the case of sand prevention, no exclusion methods are used. Generally speaking, the sand strength has to be higher for sand prevention to be applied. Conversely, the cost increases when opts for sand control (additional hardware, reduced productivity). The following sketch shows the complete Sand Management.

Fig 1: Sand Management

For Weak and Unconsolidated rocksthe completion strategy is for Sand Control, but for weak and consolidated rock the completion strategy is for Sand Prevention. To Summarize

• Weak and Unconsolidated Rocks: Go for Sand Control
• Weak and Consolidated Rocks: Go for Sand Prevention.

The Objective of perforating for sand control is to clean the perforation to avoid high skins in internal gravel packs (IGP), toavoid exposing the formation to damaging fluids, and to limit the initial production of sand. The perforating hardware and procedure are as follow for Sand Control:
•Use Big Hole charges and maximize the area open to flow (hole diameter and shot density).
•Use moderate underbalance to control the initial flow of sand.
•Perforate with open choke to sustain the post-shot flow.•Never use damaging completion fluids.

The Objective of perforating for sand prevention is to avoid collapse of the perforation tunnel, to remove the perforation damage, and to avoid any sand production. The perforating hardware and procedure are as follow for Sand Prevention:
•Use Deep Penetrating charges (not Big Hole).
•Maximize charge-to-charge distance.
•Limitthe underbalance.
•Limit the flow rate per perforation.

2. Describe the perforation damage components in weak rocks.

Before we describe the perforation damage components in weak rocks it is important to know the classification of weak rocks. The definitions become important as though the nature of the problem remains the same for all types but they are produced using differentmechanism and hence different completion strategy needs to be developed to arrest the potential problems.

Weak and Unconsolidated rocks can be defined into 4 categories:

1. Category 0- Zero Strength.
2. Category 1- Very very weak
3. Category 2- Very Weak
4. Category 3- Weak

The classification can also be done based on the rock strength value.
• Category1: Very Weak rocks will have UCS < 10 psi.
• Category 2: Weak Rocks ( 10 psi < UCS < 500 psi).

Once we have understood the classification of weak rocks we can now summarize the damage components in a weak consolidated rock in the following points.
• Perforating in weak consolidated sandstone creates a cavity full of rock debris. The fractured sand grains in the vicinity of thetunnel also increased the amount of fine particles.
• The perforation generated rock debris which is difficult to remove either during underbalance surge or during post perf flow.
• Intermingling of the perforation generated rock debris with the gravel inside the tunnel tends to reduce the overall permeability of the packed gravel. This can lower the PI by (a) gravel plugging due to...