12.2. Well operation with horizontal end.
The development of equipment, technology and new scientific methods in drilling contributed to the construction and operation of wells with horizontal end. In literature the definition of these wells is not quite correct - horizontal wells (HW), considering that the operation involves only the horizontal part of well, drilled up the productive reservoir. The definition of the production capabilities of the HW is devoted a large amount of works. There are several hydrodynamic models describing the flow of fluid from the reservoir to the horizontal part of the well, the length of which can reach 600 meters and more. As a rule, the HW are used to recover oil from the low-permeability differences, not involved in the development [2,15,26,27].
Let’s consider the most essential: why is the flow rate of the well with horizontal end more than with inclined or vertical wells. According to the Darcy law, the production rate is equal to the area of filtration, multiplied by the filtration velocity.
where
q
– flow rate of the well, S
– filtration area of the,
- mobility coefficient, gradp
–
pressure differential. Filtration area of the HW is equal to the area
of the cylinder lateral surface: the product of the length of the
horizontal part L
and the circle of radius rc:
(12.3)
Thus, the flow rate of HW is considerably higher than the rate of the vertical wells under similar pressure differentials and mobility factors. From (12.3) it follows that the rate of horizontal well significantly depends on the length of the horizontal part. Since L can reach several hundred of meters, then at the first glance, the rate of horizontal well should be many times higher than the rate of vertical or inclined wells. But, not the whole length L is involved into operation. The formations, drilled up by HW, are heterogeneous, anisotropic, with impermeable clay interlayers. Therefore, the geological structure of the reservoir and its physical properties influence the flow rate greatly. To determine the giving up zones, the areas, drilled up by HW, geophysical studies are carried out that can give information about the giving up intervals. The influence of anisotropy is considered in [13,26].
For enhanced oil recovery in horizontal wells there is usually carried out formation hydraulic fracturing. The intervals, scheduled for hydraulic fracturing, may be several, depending on the design of horizontal end of the borehole. In addition, chemical treatment of the bottom-hole zone is carried out in HW. Hydrodynamic enhanced oil recovery methods have not been used yet in HW, but the future of this technology is rather perspective.
12.3. Acoustic methods.
Acoustic methods can be divided into PSM (production stimulation methods) EOR (enhanced oil recovery methods) according to the usage technology. The first (PS) methods have an influence on the bottomhole zone, improve pool-reservoir properties and well productivity. The second (EOR) methods have an influence on the item, the part of the operational facility, involving into the process of development the uncovered areas and capillary – hold-up oil. Recently, with the development of technology there have been developed various technologies aimed at the development and improvement of acoustic methods of influence on the bottomhole zone. This paragraph deals with two tecnologies, related to PS methods: ARWR and UST.
The technology of the acoustic rehabilitation of wells and reservoirs (ARWR) allows to improve the filtration properties of low-permeability oil-saturated interlayer of the producing wells and to increase the intake capacity of the injection wells [28].
Acoustic wave is a fluctuation of tensile and compressive pressure of the specified frequency. Moreover, the acoustic pressure gradient exceeds stationary gradient used in the practice of oil production. In the result, the force, applied to the fluid, increases the filtration in the bottomhole zone.
The mechanism of the acoustic field influence on the oil reservoir is described in [29,30]. Under the influence of longitudinal waves, liquid tends to move to the direction of the pressure drop, flowing into neighboring pores, at the same time as the shear stress solid skeleton of the reservoir gives the fluid torque. The movement of the fluid is occurred in the form of the turbulent flow that creates intensive in and out movement. However, the fluid consumption in the direction of the wave motion is always higher than the opposite. It happens because some of the fluid, flowing into the neighboring pores because of viscous and inertial forces manifestation, "has no time" to come back in a full volume. "Vacant" pore space is filled by the pulling fluid from the neighboring pores. Thus, in the formation, subjected to acoustic impact, two types of fluid circulation take place: interstitial turbulent flows and inter-porous fluid movement.
It is known that the majority of the reservoir fluids, that in volume behave like Newtonian, but moving in low-permeability reservoirs show viscoplastic properties, characterized by plastic viscosity and structure strength, which depends on the ultimate shear stress [1,31,35]. It means that the smaller the diameter of pore channel, the more effort should be applied to the liquid to involve it in the movement. As a result, when there is a certain ratio of the pore channel size and current pressure gradient there is occurred full or partial “lock-out” of oil in the formation: and under the current parameters of the development oil left unrecovered.
Application of acoustic treatment creates an additional gradient of acoustic pressure and there is a movement of a liquid in pores, where under normal conditions of the development it was absent. The areas of the degraded filtration in the reservoir, as a rule, have oil saturation, as the oil has more resistance to the shift compared to water. In the result of the acoustic treatment oil is shifted to the larger pores and displaced by water.
The technology of ARWR was held on the producing well № 1116 of Zapadnoye - Surgutskoye field. Additional production for the 12 months has become 4646 tons of oil.
The main aim of the injection wells treatment was redistribution (change) of injectivity profile as according to the data of geophysical research, it was noted that the preferred injection of water into the layers lying in the bottom part of the perforated productive horizon, and, as a result, we have inadequate sweep by the process of flooding the interlayers, lying in the upper boundary of the reservoir. A typical example of uneven injectivity profile can serve a profile of the injection well 377/4 of Kustovoye field. After the works with the implementation of ARWR technology the injectivity profile has significantly increased.
Before ARWR After ARWR
Fig.12.1 . Injectivity capacity of the well.377 of Kustovoye field before and after ARWR implementation.
The method of ultrasonic treatment (UST) of the bottom-hole zone.
The works on the bottom-hole stimulation was held by the company OOO (limited liability company) "Center of ultrasonic technologies service". The use of ultrasonic influence on the bottom-hole with the frequency 15-59 kHz allows to:
- to reduce the surface tension on the boundary of the solid phase and fluid under the influence of vibrational energy generated in elastic field by ultrasound;
- to change physical - mechanical properties of high-viscosity fluids containing asphalt and resinous compounds, making them more mobile;
- to involve in the development the layers with low filtration properties, increasing the thickness sweep coefficient. Tests, conducted at the Samotlor field, have shown that the average oil production rate increased from 3.5 to 7.8 t/day., productivity index from 0,143 to 0.23 m3/(day. atm)
The advantages of the UST are:
Short duration of the processing (average processing time of one well is about 15 hours); the use of mobile small-sized equipment, low processing costs; the integrity of the production casing and cement sheath; technically, physiologically and environmentally safe process of stimulation; the success of the treatment, in the case of the right choice of wells reaches 80%; the duration of the effect is from 3 to 12 months.
Thus, the prospects of the using of the acoustic methods to increase the productivity of the producing wells and intake capacity of the injection wells are confirmed by field tests. The area of application of these methods is quite broad and covers both low-permeability reservoir and deposits of heavy oil. We should notice that the acoustic methods of processing of the bottom-hole zone are compatible with the other methods of the bottom-hole treatment.