PRE SATURATING FORMATION WITH VES

So, the wells have low productivity now.  Mostly of them produce 150 bpd and 7 bopd. Some of the wells initially produced 300 bopd with 20% WC. Previous engineers had tried to treat the wells with acid but unsucceeded. Maybe you don’t need acidizing, instead, just add more perforation.

Several questions must be answered before choosing an appropriate treatment for the well. How much is the remaining reserve around the well?  How about the decline rate? What kind of productivity damage is there? Stimulation or Fracturing?

Formation damage is complex. Sometimes, preventing formation damage is more expensive than doing a treatment to cure the damage. The compatibility test is important to avoid worst damage after treatment. Precipitation, scale build-up are some side effects due to incompatible treatment fluid with formation fluid. Also, be careful with the size of particle inside the treatment fluid. The particle can block pores and prevent the treatment fluid from touching the formation. Some companies use expensive equipment to filter completion fluid and treatment fluid.

Pre-saturating formation with VES (viscoelastic surfactant) is a way to prevent  formation damage. Before fracturing or matrix acidizing, the formation soaks with VES where the VES contain viscosity enhancer, temperature stabilizer, and internal breaker. The viscosity enhancer will enhance elasticity where at the static condition the VES has high viscosity but at dynamic condition has low viscosity. As of the name, temperature stabilizer is for stabilizing the VES viscosity against formation temperature.

The internal breaker is the improvement of the previous method of using VES. It is believed that without the internal breaker, the VES will stick onto the formation pore and can not be removed all by well unloading. The internal breaker will break the VES which will be easily unloaded during well flow back.

In fracturing, the VES will occupy high permeability area including fractured formation and prevent loss of fracturing fluid to that zone. The fracturing fluid will then frac low permeability area. The same situation is achieved during matrix acidizing. Without VES, the acid will acidize high permeability area and leave low permeability area untreated. VES will occupy the high permeability area and divert the acid to low permeability area.

FRACTURING USING A JETTING TOOL AND VISCOELASTIC SURFACTANT FLUID

Whether fracturing a formation or not must be answered before further step acquired. A comprehensive evaluation on the formation and fluid characteristic is important. Skin build-up evaluation behind casing must be assessed. Sometimes, the fracturing program has been planned before the well is drilled.

Many fracturing techniques have been developed.  The fracturing fluid, additives, proppant are varied depend on formation characteristic. Some companies use gel fluid for better fracturing and others combine the fracturing fluid with acid. Make sure no precipitation during acidizing.

Figure 1: Perforate First Zone

Isolation in multi-zone fracturing is an issue during the fracturing operation. After fracturing the first zone, high fluid loss into the first zone is encountered during fracturing second zone. Some companies use a bridge plug to isolate the first zone and open the bridge plug subsequently after fracturing the second zone. It is not easy to do so in high deviated wells or horizontal wells.

Figure 2: Frac First Zone

In Figure 1, the first zone is perforated by using a jetting tool. Jetting tool perforation is more preferably than shaped charge perforation for some reasons:

1. Jetting tool perforation can reach deeper penetration.

2. Jetting tool perforation can make slot or helical perforation instead of hole only.

3. Jetting tool can directly deliver fracturing fluid for fracturing.

4. Jetting tool can deliver isolation fluid to isolate fractured zone.

5. Jetting tool can do the same procedures on other zone without any trip.

Figure 3: Isolate First Zone

Subsequently after perforating  the first zone, the zone is fractured (Figure 2) by using the jetting tool.  Optionally, acid can be mixed with the fracturing fluid to etch the formation or do acidizing separately after fracturing. Another additive can be added to fracturing fluid for additional purposes such as corrosion inhibitor, scale inhibitor or paraffin depressant.

Figure 4: Perforate and Fracture Second Zone

In Figure 3, the first zone is isolated by using a viscoelastic surfactant.  Viscoelastic surfactant is more preferably as isolation fluid since the fluid can decrease formation damage, more stable against shear and no substantial viscosity loss due to temperature increase. By isolating the first zone, there is no fluid loss during any operation on the second zone.

Figure 5: Unloading the well

Perforating and fracturing or any other treatments can be repeated on the second zone (Figure 4). If there is no other zone to be perforated or fractured, the viscoelastic surfactant is unloaded by means of circulation or reverse circulation (Figure 5). If the reservoir pressure is strong enough, the well can be flowed back through the jetting tool or help the well to flow-back by decreasing the hydrostatic fluid within the well using Nitrogen.