NEAR-WELLBORE DAMAGE ZONE
The major problem with fracing is that 10 foot diameter constriction zones form near the Hwell during the fracing process and continue to plug as shale wells are produced resulting in the rapid decline of the wells (Figure 1).
The fracs plug due to the following five mechanisms:
- Crushed proppants
- Proppant embedment
- Silica sand solution and precipitation
- Flow back closures
- Frac fluid visosifiers
CONVERGENCE OF FLOW AT HWELL
As the shale wells are produced, the oil flows along the 100 foot tall x 1000 foot long frac into the 10 foot diameter constriction zone (Figure 2)
1. FRAC PLUGGING BY CRUSHED PROPPANTS
The spherical proppants are present in the frac in a hexagonal pattern (Figure 3).
High contact stresses generated are generated in the proppants as the frac close at the end of the frac job (Figures 4 to 6).
These high contact stresses crush about ten percent of the proppants creating some very fine particles (Figure 7).
These fine particles migrate along the fracs as the wells are produced until they enter the near wellbore constriction zones and progressively increase plugging of these zones (Figure 8).
Many frac experts state that reduction of frac conductivity due to crushed proppants are the primary cause of reduced frac conductivity and the rapid decline of shale wells (50% first year; 70 percent second year).
Approximately 10 percent of the proppants are crushed resulting in 400,000 pounds of crushed proppants in a 4 million pound Eagle Ford frac job.
Crushed proppants is the most common type of frac plugging.
2. FRAC PLUGGING DUE TO PROPPANT EMBEDMENT
A second cause of plugging is due to embedment of proppants into the formation creating fine crushed particles (Figures 9 to 11).
This is an important frac plugging contributor, but not as important as crushed proppant plugging
3. PROPPANT PLUGGING DUE TO SILICA SOLUTION AND PRECIPITATION
Silica sand proppants are highly soluble in water at the high temperatures and pressures existing in deep shale formations (Figure 12).
The solution rates increase with increased temperature (Figure 13).
The solubility also increases with increased pressure (Figure14).
The silica dissolves fastest at the contact points (high pressure and temperature) and then precipitates out in open areas (lower pressure) in the frac. (Figure 15).
The precipitated silica precipitates plugs the fracs, especially in the constriction zones due to the small flow spaces. (Figure 16). This mechanism is less important than frac plugging with crushed proppants and embedment debris.
This solution mechanism takes place in some fracs, but is not as important as plugging by crushed proppants and embedment fines.
4. FRAC PLUGGING DUE TO FLOW BACK CLOSURES
Frac plugging can also take place due to the proppant flowback where the fracs close in near wellbore sections containing no proppants (Figure 17).
This flowback plugging mechanism is important in some fracs, especially those that are non producers due to sealing off the fracs completely at the wellbores.
5. FRAC PLUGGING DUE TO FRAC FLUID VISOCIFIERS
Although “slick water” fracing is gaining acceptance, most of the existing 300 to 400 thousand shale fraced wells were with frac fluids containing viscosifiers to help suspend and transport the proppants in the fracs,
An Eagle Ford frac typically uses up to four million gallons of water, so a large amount of viscosifiers were used in these wells.
The fine, solid crushed particles (proppants and formation) still leave some small flow areas that the viscosifiers seal, especially in the 10 to 20 percent non productive fracs encountered in most shale wells.
Read the reports on SYSTEM DESCRIPTION and SHALES PROPERTIES because it gives considerable insight into the mechanisms that control flow from shale wells and how to maximize flow rate and profits from these wells.
Persons interested in applying this technology should contact Dr. William Maurer at firstname.lastname@example.org.
READ SPE PAPERS
We recommend that readers get copies of the excellent SPE papers cited in this presentation at www.onepetro.org for more details on these frac plugging mechanisms.