Hydraulic fracs provide a conduit for oil to flow from the shale reservoir to the Hwell, but the oil must first flow 200 to 500 feet through the shale to reach the frac (Figure 1).
Numerous small diameter spherical voids and tunnels in the shale provide the 1 to 50 mD shale permeability required to produce economical flow rates. The high vertical stresses close the natural fracs so they do not provide flow paths as presumed by engineers.
Frac engineers have found that when shale wells are fraced from Hwells, “near-wellbore” constriction zones are formed within ten feet of the Hwell.
As oil is produced, the fracs continue to plug resulting in a high decline rate (70 percent in two years) that severely reduces shale well economics.
The oil flow is proportional to the drawdown pressure between the formation fluid pressure and the pressure at the frac.
The pressure drop across the near-wellbore constriction zone reduces the drawdown pressure and flow rate and thus is responsible for most of the rapid decline.
A good understanding of these mechanisms by drilling, completion, fracing and reservoir engineers is important in maximizing production rates from shale wells.
FLOW IS NOT THROUGH NATURAL FRACS
It is widely believed by most shale well engineers that the primary mechanism of oil flow from the reservoir is through natural fractures in shale.
They strongly believe that the key to increasing production from depleted wells is to create new hydraulic fractures with refracing since the existing natural fractures are depleted.
We will show later that open natural fractures cannot exist in shale wells because the high vertical and horizontal stresses close them. Fracing cannot open them because the fluid pressure required to create a vertical frac is lower than the pressure required to open a natural fracture.
FLOW THROUGH SMALL CYLINDRICAL VOIDS IN THE SHALE
Fluid flow in shale wells is through small cylindrical voids and cavities that are strong enough to withstand vertical pressures up to 14,000 psi without collapsing (Figures 2).
Figure 3 shows that the Eagle Ford shale has permeabilities up to 50 mD and porosities up to 10 percent in commercial wells Technical papers state that the Eagle Ford porosity is about 5 to 7 percent.
Most shale data are from commercial wells so they tend to be in the “SWEET” spots. The permeability of very small shales with no flaws in them is about 0.1 nD which corresponds to 1/10000 Md = 1/10 million D
In shale wells water or gas drive provides the force and energy to push the oil through a few hundred feet of shale to the hydraulic fracs.
The Eagle Ford shale contains up to 60 percent calcium so it is more of a “shaly limestone” than a “limey shale”. That is why it has the high strength needed to keep the voids open with vertical pressures up to 14,000 psi.
Figure 4 shows void spaces that are about 0.2 microns wide. This is why large drawdown pressures are required to force oil through these small void spaces.
To flow hundreds of feet requires these void spaces to be continuous over long distances so they must connect up with numerous other void spaces to be productive.
Figure 5 shows that this Devonian Shale has 48.7 percent calcite, 27.7 percent clay, and 7.2 percent organics. These materials tend to be evenly distributed though the shale.
Shale strength increases with increased calcium content; therefore high calcium content is required to provide sufficient strength for voids to remain open in deep shale wells.
OPEN VOIDS IN SHALE
Voids in shale are basically elliptical horizontal tunnels that provide the 1 to 50 mD permeabilities and 2 to 10 percent porosities need to make shales commercial.
They are similar to the tunnels in 10,000 foot deep gold mines in South Africa.
The voids are small since they must be between flaws in the shale.
ROCK STRESSES AT 10,000 FEET
Vertical Stress = 1 psi per foot = 10,000 psi
Horizontal Stress = MU/ (1-MU) Vertical Stress
MU = Poisson’s Ratio = 0.40 inch/inch for shale
Horizontal Stress = 0.40/ (1-0.40) Vertical Stress = 0.67 Vertical Stress = 6700 psi
The horizontal stress in shale is nearly double that in sandstone and limestone which increases the frac pressure in shale wells.
These high vertical and horizontal stresses close all natural fractures in shale and prevent hydraulic fracs from opening them (Figure 6).
Therefore no oil is produced from natural fractures in shale wells, contrary to common belief.
LIMESTONE AND SANDSTONE
Vertical Stress = 10,000 psi (Same as shale)
MU = 0.25 to 0.30 for Limestone and Shale
Horizontal Stress = (0, 33 to 0.43) Vertical Stress = 3300 to 4300 psi
This shows that horizontal stresses in limestone and sandstone are much lower than in shale which reduces the frac pressures in these formations
When the shale wells are fraced, the frac pressure required to produce a vertical hydraulic frac is always lower than the pressure required to open a natural fracture, so fracing cannot open any natural fracs as assumed by most shale well engineers (Figure 7).
THEREFORE THE WIDELY HELD BELIEF THAT REFRACING REJUVINATES SHALE WELLS BY OPENING NATURAL FRACTURES IS INCORRECT.
Figure 8 shows an elliptical opening in the shale that is strong enough to withstand 10,000 psi vertical pressure without collapsing. The void spaces are actually tunnels in the shale that connect up to provide continuous flow paths to carry through the shale to the frac.
They voids are small diameter since they have to be between flaws in the shale.
The voids in shale tend to be elliptical (Figure 9) because this is the strongest shape to withstand high compressive stresses in deep shale formations.
INCREASE FRAC SPACING
A recent World Oil article stated that Sanchez recently drilled an infill shale well and after they fraced it, five surrounding wells produced frac water for 30 to 60 days and then started producing oil at the previous rates with no loss of reserves.
This shows that there is probably much more fluid communication in most shale formations than previously thought.
With this type of communication, it may be possible to double frac spacing, reduce fracing costs by 50 percent, and still get the same recovery from these wells.
The drain holes interconnect a large number of fracs so production from low and non producing fracs can be produced through interconnected high productivity fracs.
This could be very important since Schlumberger (Generon, 2016) and Microseismic (2015) recently stated that 30 to 40 percent of the fracs in many shale wells are non productive and Mike Vincent stated that 10 to 15 percent of fracs in most wells are non productive.
All hydraulic fracs are vertical because they are perpendicular to the smallest horizontal stress and they are all parallel to each other for the same reason.
Open horizontal or inclined fractures (natural or hydraulic) cannot exist in shale since the high vertical stresses (approximately 2X horizontal stresses) will always close them.
The concept of using refracing to open new natural fractures is therefore incorrect.
Refracs mainly increase flow by increasing the conductivity of existing vertical hydraulic fracs or by creating new vertical hydraulic fracs, not by opening existing natural fractures or creating new ones.
Read the reports on SYSTEM DESCRIPTION and FRAC PLUGGING 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.cyl
Persons interested in applying this technology should contact Dr. William Maurer at email@example.com.
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.