Oil/Gas Condensate Well Stimulation for Enhancing Oil and Natural Gas Recovery

Attachment to the information of VNIIGAZ about well stimulation with high explosives for enhancing oil and natural gas recovery (WSHE)

1. Brief information about WSHE procedure.

1.1 A cord torpedo (CT) is prepared by fastening a designed number of detonating cords (DC) of appropriate types on the lower end of old logging cable. The prepared CT is connected to a main logging cable.

1.2 The CT is run through lubricator or seal, depending on reservoir pressure, into the interval of hole perforation and the seal is tightened. The seal should be designed on working pressure being higher of that of hydraulic test.

1.3 Working fluid (WF) is injected by a typical cementing unit (with a working pressure of around 30 MPa) into the well up to a corresponding well head pressure but not higher compared to that of pressure test. The above operation is accompanied by reservoir (well) injectivity estimation.

1.4 After certain operations(depending on reservoir properties) detonating cords are fired without stopping WF injection.

1.5 Well head pressure is reduced down to atmospheric and the logging cord is pulled up with parallel estimation of a volume of injected WF.

1.6 Other explosions, i.e. stimulation stages, are carried out as items 1.1-1.5 but in accordance with a designed quantity of high explosives HE.

1.7 The work is finished by ordinary well completion operations. Individual plan of stimulation operation is drawn up for each well.

2. Initial data on reservoir geology, fluid properties, equipment and materials that are necessary for well stimulation operation.

In order to setup WSHE procedure schedule the following information is to be available:

2.1         Reservoir geology, fluid properties and well characteristics

2.1.1 Geological characteristics of reservoir for a given well (reservoir rock, porosity, permeability, injectivity, reservoir pressure and temperature, distance between oil formation and aquifer and dynamic level of liquid);

2.1.2 Basic physical and chemical properties of oil (density and content of  asphaltenes, resins and paraffin);

2.1.3 Initial, maximum and minimum product output with indicating the relevant dates;

2.1.4 Data on a last hydraulic test of well and condition of cement layer;

2.1.5 Bottom hole and perforation interval. Perforation type (cumulative, sand-jet or bullet) with indication of a number of perforation holes per 1 meter, their diameters, depth of penetration into reservoir and a date of the last well perforation;

2.1.6 The presence of tubing. If so, its inner diameter and length and the presence of down hole pump and other devices that can interfere the cord torpedo to be run down the well;

2.1.7 The presence or absence of packer.

2.2 Equipment

2.2.1 Drilling rig or truck crane (preferably with a lifting capacity of 50 ton);

2.2.2 Cementing unit, water carrier and 10-20 m3 tank;

2.2.3 Hoisting unit and laboratory for conducting geophysical works. The hoisting unit should be provided with logging cable that can be used in thermobaric conditions of the perforation interval;

2.2.4 Lubricator if reservoir pressure is high, or simply a seal (for working pressure not less than 15 MPa) if reservoir pressure is low;

2.2.5 Exploder.

2.3 Maintenance staff

2.3.1 Seismic team (4-5 persons);

2.3.2 Workover team (4-5 persons including operators).

The duration of works depends on a well depth and reservoir geology and accounts for from 0.5 to 5 days (usually one day).

2.4 Materials

2.4.1 Common commercially available detonating cords of different types (designed quantity) with operating characteristics which are acceptable for the application in thermobaric conditions. Hexogen, TEN and octogen can be used as high explosives during the stimulation operation;

2.4.2  Electric detonators acceptable for using in thermobaric conditions;

2.4.3 Working fluid is any low viscous solids-free fluid, including industrial water, reservoir fluid (preferably with a relative density of 0.15-0.17 g/cm3), light oil, gas condensate or hydrocarbon fractions within 100-3500C. A working fluid consumption is 6 – 10 m3 per one well;

2.4.4 Surfactant both of iogenic and non-iogenic types in the amount of 25-30 kg per well. Surfactant is applied depending on reservoir properties and service life of a well;

2.4.5 To fasten detonating cord on the logging cable 3-6 mm dia. and 8-10 m long textile (rubber-free) cords are needed for one well with a perforation interval of 25-30 m.

3. Dependence of stimulation efficiency on reservoir rock

The stimulation technology concerned is not limited by reservoir rock. The efficiency of the technology increases with the increase in porosity and permeability of rocks. The technology is especially effective in the formations where the use of standard stimulation is not effective and in the formation of high strength (carbonate) and low porosity. As for the action on pores plugged with adsorbed heavy oil components (asphaltenes, resins and paraffin), the offered technology practically has no rivals. In this case a governing factor lies in the fact that so-called back wave of the explosion creates the depression of hundred thousands of bars in working fluid in fractures (within fractions of a second). This results in removing heavy components of oil as well as residual mud from pores and thus in increasing oil, condensate and gas inflow.  In our opinion, the offered well stimulation technology can increase oil and gas recovery up to 80-90 % with regard to a reservoir geological potential.

4. Crack propagation depth and maximum permissible depth of well treatment

4.1 It is known that there is no a single method of direct physical measurement of crack propagation depth during formation stimulating (by any technology) in time being. All companies involved in fracturing activity use indirect methods and their own know-how in crack propagation calculations. So, at present I can not answer your question concerning crack propagation depth. If you are satisfied with data obtained by an indirect method, then for the terrigenous deposits of Western Siberia (the town of Nizhnevartovsk) a crack propagation depth reached finally around 64 meters. The calculations were performed on the basis of working fluid absorption by formation, initial number of perforation channels etc. According to my data, a crack propagation per one explosion (one fracturing operation) in carbonate formations is 0.25-0.4 m. For more weak rocks this propagation increases (in my opinion up to 1 meter per one operation). A total number of stimulation operations is 6-8. Besides, due to the fact that cracks develop usually along of perforation channels and by small single penetrations into the formation (in contrast to injection technology), a high selectivity level can be achieved. For example, during one of my tests conducted on the well drilled in the terrigenous deposit (Nizhnevartovsk, Siberia) with a distance of aquifer from oil-bearing bed of 1.2 m, no water was observed (at zero initial well output). In all cases we did not inject parapant into the formation since a high dynamic impact of explosive shock wave on crack walls and reservoir pores leads to a considerable strengthening of cracks and pores walls. (As it was mentioned before, the blast wave is passed through working fluid and can reach several hundred thousand atmospheres of pressure.)

Based on the stated above, the main criteria of the efficiency are as follows: increase in well output, restoration of well production in case of abandoned wells and considerable reduction of completion time of new wells after drilling works.

In order to reach maximum oil and gas recovery a periodical repeat of stimulation operations (as well output declines) is required. This approach will determine a fundamentally new method of workovers, so called planned-preventive reservoir repair.

4.2 A depth of well treatment is limited only by thermobaric conditions within the zone of cord torpedo before its explosion. The detonating cords we use in our work allow to conduct fracturing at reservoir temperature and pressure of 2500C and 1000 bar respectively.

Note: The above material is given with regard to know-how and confidentiality limitations.

6. Results of preliminary works on applying the offered stimulation method on some fields of Turkmenistan, Uzbekistan and Kazakhstan

Below are some results of the preliminary works (not completed) on the stimulation technology application:

6.1 Turkmenistan, carbonate producing horizon of the Kirpichli field. After two stages of stimulation operation the output of gas well No 135 being drilled up to 2989 m and having zero output before the stimulation operation, amounted to 25000 nm3/d (after 09.1990).

6.2 Uzbekistan, carbonate horizon of the Karaktai field. The oil well No 39 drilled up to 940 m was abandoned (excluded from the well stock) in 1975 due to zero output and 100 % water influx. Since then the well had been repaired several times to isolate water-bearing interlayers, however, without success. The stimulation that was performed in 1989 allowed to reach oil output to 4.7 m3/d with water encroachment being 90 %. During 24 days period the well was under operation, around 100 tons of oil were produced.

6.3 Kazakhstan, terrigenous deposits of the Karachaganak gas-condensate and oil field.

The offered stimulation technology was used for increasing water wells injectivity (the wells for injecting waste water). A well No 2 can be cited as an example. The perforation intervals of the well were 1419-1471 and 1636-1728. Well head injection pressure of waste waters taken from a gas and condensate conditioning unit reached 180-190 bar. After stimulation works in 1990 carried out in a volume of only 20 % of design scope (by the Customer’s reason) the well head injection pressure lowered to 10-10.5 MPa, i.e. the injectivity was doubled.

The stimulation technology has enhanced considerably (by more than 2.5 times) the injectivity of few water wells in Turkmenistan.