This important factor, directly affect the productivity inflow, has been virtually ignored in the Geo-physical academia and Oil and Gas Industry. Well is drilled, productive formations identified and opened, all is analyzed and calculated to the last detail, but there is no productive inflow. Why is this happening ?
WHY WELL DOES NOT FLOW ?
GEOLOGY, PHYSICS AND MATHEMATICS
During conventional well production life-cycle only 25 % to 35 % of oil is usually extracted from each well. More than half remaining reserves are unrecoverable hydrocarbons. It is well known, that the most of oil field sedimentary rocks has closed pores filled with oil and practically unattainable by conventional "traditional" methods. The untapped oil volume is estimated to be from 65% to 75% of known deposits. Thus, there are estimated sufficient potential of remaining reserves of oil still waiting to be recovered.
There are circular tangential stress conditions (stress-strain states) that occur in any vertical or horizontal well after drilling due to rock pressure created by a weight of the overlying rocks: they create annular zone with compression stresses near well-bore, thus collapsing rock’s permeability and restricting access to the prospected hydrocarbons.
Newly drilled well already has a negative potential, which does not disappear after subsequent opening the casing and productive formations:
● reduction in permeability
● reduction of porosity
● as a consequence: reduced productivity inflow into the well
A well completion by use a cumulative perforation in the hydrocarbon layer not relieve the compression stress zone and has shortcomings like: creating a small passages to the hydrocarbon layer, damaging the cement sheath, debris and colmatation.
"Traditional" methods of opening the productive formation did not consider this complicated situation in the nearwell 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 well-bore. The more depth - more stress-strain states, the lower permeability, and as a result the decrease of productive inflow efficiency.
WHY WELL DOES NOT FLOW ?
INCREASING PERMEABILITY AND IMPROVING RESERVOIR PROPERTIES
Reduce the negative impact of circular tangential stress strain states is possible. Technologies of unloading stress conditions in the near well-bore zone exist a long time already, well proven and successfully used in different countries around the world.
By analogy with Mining Industry for relieve extraction of rock they make a special mine workings - continuous slots. The similar technology has been applied in the Oil and Gas Industry.
The essence of technology is as follows. If in the wellbore make a continuous slots along the borehole and with depth of penetration equal at least five hole diameters and more, the circular tangential stress conditions (stress-strain states) partially will be redistributed to the ends of these slots. Thus in the near wellbore zone the circular stress conditions will be reduced, there will be unloading circular zone.
When unloading stress conditions in the near well-bore zone (50-100%), the reservoir properties are improving:
● permeability increase (30-50%)
● partial increase in porosity
● the resulting increase in productive inflow to the well (5-10 times)
The deeper the well productive layer the greater the effect.
The main problem was only invention of special device, capable to create a lateral continuous deep slots along the borehole, directly in the cutting place inside the well.
Movement or tubing or coiled tubing from the surface did not give positive results except irregular holes, because a lot of foot tubing or coiled tubing have own frequency and stretching. This is impossible to make a deep continuous slots.
Cutting is carried out with water jets and abrasive filler. Working jets move uniformly with constant speed along the well-bore cutting through the casing and cement into the productive formation.
First figure below shows that the borehole is surrounded by low permeable "cork" thickness which is approximately 50% of the borehole radius.
From figures 2 and 3 shows that two or four diametrically opposite vertical continuous slots is almost twice lower operating on the circuit hole shear stresses, and reduced permeability areas are significantly reduced in size and are pushed into the interior of the reservoir. Thus, the presence of vertical slots significantly improves the situation, and generally retains rock permeability.
Redistribution of circular tangential stress conditions to the ends of slots.