INITIAL FACTORS THE FALL OF PRODUCTION RATE
During the drilling of any well (perforation of any horizontal or vertical holes) under the action of hydrostatic, mining and high overburden pressures in the annulus are formed the circular tangential stress conditions (stress-strain states).
Not to be confused with the hydrostatic or rock pressure.
It is known that one of the main causes of the declining production rate of oil and gas wells is the deterioration of the permeability of rocks in the near-wellbore zone directly from the borehole wall.
This occurs both during drilling and during their operation (figure on the right).
Study the role of this factor is still neglected.
Under the action of these stress conditions occurs a significant reduction in permeability in the near wellbore zone. Oil or gas flow cannot penetrate into the well, and in some cases close to zero.
Traditional methods of opening the productive formation did not consider this complicated situation in the near-well zone and therefore was not effective. Porous and fractured formations are subjected to compression, that deform the rock mass and reduce its permeability.
The depth has a significant influence on the stress-strain state of the rock mass around the wellbore. The more depth - more stress-strain states, the lower permeability, and as a result the decrease of productive inflow efficiency.
Newly drilled well already has a negative potential, which does not disappear in the subsequent opening of the casing (column) and productive formations:
reduction in permeability
reduction of porosity
as a consequence: reduced productivity inflow into the well (in some individual cases to zero)
How often are taken into account the above-stated facts ?
Take as an example producing formation at a depth 3000 ft. Hydrostatic pressure 1350 psi, layer pressure 1620 psi, rock pressure 3350 psi, compressive stress conditions around the drilled hole (according to the laws of Geomechanics) almost twice as much rock pressure 5370 psi.
After cutting longitudinal slots along the borehole, load in the cutting slot's zones decreases. Theoretically stresses aspire to balance the pressure with the pressures of the producing formation. Compressive stresses redistributed to the ends of the slots, respectively unloading near wellbore zone. Respectively happening an increase the permeability/porosity, and accordingly increase productive inflows in the unloading area. As can be seen from the above table, when creating a lateral extension slots along the wellbore at a depth 3000 ft. compressive stress conditions decreases (unloading) form 5370 psi (rock pressure) to 1950-2100 psi (at effective pressure in the productive layer 1620 psi).