Coiled Tubing Times - oil & gas service, fracturing, coil tubing, well intervention, workover

Journal

Issue 38, November 2011

KILLING AND WASHING FLUIDS PRESERVING RESERVOIR CHARACTERISTICS OF THE FORMATION

L.A. Magadova, M.A. Silin, E.G. Gaevoy, V.L. Zavarotny, D. Yu. Yeliseev, Gubkin Russian State University of Oil and Gas, Khimeko-GANG

Killing wells is the most frequent operation with the use of chemical reagents. The application of quality killing fluids (KF), inhibited for stabilizing clay, preventing corrosion, emergence of infusible salts and enduring inverted emulsions, allows preserving reservoir characteristics of the formation.

Complexes of killing fluids were developed in Gubkin Russian State University of Oil and Gas. It allows not only preserving the reservoir characteristics of the formation, but also to increase the safety of well workover and remediation jobs and decrease the costs of the killing fluids.

All killing fluids can be divided into two categories:

solutions with absorption control;
solutions with productivity control.

Solutions with absorption control allow limiting filtration of salt killing fluids in the formation and keep the well productivity after workover on the same level.

Solutions with productivity control is a standard salt killing fluid containing a certain amount of chemical additives providing for minimal negative impact of the killing fluid on further exploitation of the well. Usually such solutions cost as much as other salt fluids, while they bring down the risks of endangering well productivity and increasing the rate of water in the product.

WELL KILLING TECHNOLOGY WITH ABSORPTION CONTROL

The process of killing wells with low BHP and wells with natural or artificial bottomhole fractures (produced by hydrofracturing) is accompanied by intensive absorption of salt solution. It doesn’t only results in the increased consumption of killing fluid, but may also prompt irreversible production loss and increased watering.

Several technologies are used for killing wells:

full replacement of well fluid with killing fluid;
partial replacement of well fluid with blocking pack of KF closing 200–300 m of perforation interval. The rest part of the well is filled with formation or mineralized waters (blocking technology).

KILLING HIGH-PERMEABILITY WELLS IN FRACTURED RESERVOIRS AFTER HYDROFRACTURING

Fluids of high viscosity and low filtration are necessary for killing wells in high-permeability formations. Polysaccharide well killing fluids (PSKF) on water or salt-aqueous base are gels produced from modified guars [1]. When thickening agents are added, the polysaccharide gel turns into a single thick structure effectively blocking large pores and fractures.

Polysaccharide water gel is heat-proof with the formation temperature under 100 °С. If it is necessary, it may have low viscosity, which is important for undisturbed operation of the electric submersible pump. It has low filtration and the filtrate has low boundary tension, diminishing its resistance to flow of oil into the well.

Polysaccharide gels are produced from reagents of gel producing complex Khimeko V, manufactured by Khimeko-GANG, consist of gelling agent GPG-3, thickening agent SP-RD, borate thickening agent BS-1 and biocide Biolan.

Fresh industrial or bottom water with low content of multivalent cations (≤500 мg/l) is used as a water base for producing polysaccharide killing fluids. In order to make it denser, mineralizing salts with monovalent cations like potash chloride or natrium

may be added.

PSKF makes very little contamination in the productive formation, which was proved by experimental researches. Figure 1 suggests that filtration of PSKF gel into the porous medium begins only when the pressures fall exceeds 6.0 MPa.

PSKF has good clay inhibiting effect. The moisturizing property of PSKF (assessed according to RD 39-2-813-82) PO = 0, 05–0,1 cm/h, and clay swelling rate (according to Zhigach-Yarov) W = 0,01–0,02 cm/h, which is enough for keeping the

productivity OP>0,94.

Starting from 2002 the PSKF is applied in various oil and service companies at Russian fields: RNPurneftegaz, Varieganneft, Slavneft-Megionneftegaz, Urengoigazprom, Orenburggazprom, LUKoil – Western Siberia, NTC GEOTECHNOKIN and so on.

In 2002–2008 RN-Purneftegaz applied PSKF in over 800 production wells killing operations. It should be noted that the works were done in the wells, drilled in low-permeability terrigenous reservoirs with the formation temperature 80–95 °С and clays tending to swell. Salt-aqueous solutions used to kill such wells substantially decrease oil flow rate, increase the abundance of water in the formation, and it took one or several weeks to make the well operational. It should be noted that killing

with PSKF was successful in wells with high gas factors (from 300 till 1,000 m3/t) at Harampur Field (Northern and Southern domes).

Killing wells with PSKF was performed with the use of combined substitution of well fluid (fluid consumption 3–5 m3, 5–8 m3 per well in well with high gas factor). The density of the obtained solution was 1.02 –1.18 g/cm3. The wells are subjected to various types of remediation jobs: well development after hydrofracturing with ESP

installation, installation of sucker rod pump (SRP), replacement of SRP, installation of ESP, replacement of ESP, replacement of ESP by SRP and vice versa.Stable positive results were obtained in all these operations.

The analysis of making the wells operational after filling them with PSKF at 102 wells of Purneftegaz fields showed that the average oil flow rate per 1 well increased by 3.6 t/day. The average water rate per 1 well increased just by 0.7% and the average

term of making the wells operational was 2.2 days per 1 well. Figure 2 represents changes in the content of water in the production (vol. %) before and after service in 2002–2007 (the analysis includes 569 well killing operations).

KILLING GAS AND GAS CONDENSATE ABNORMALLY LOW FORMATION PRESSURE WELLS

While killing abnormally low formation pressure wells the killing fluid should have low density and at the same time provide for back pressure during the whole service period.

In order to manage this problem Khimeko-GANG and Orenburggazprom designed an aerated gel (“solid foam”), having low density and filterability, which allows using it on gas and gas condensate fields abnormally low formation pressure (ALFP) [4].

The technology of killing with solid foam implies simultaneous pumping of gelling agent with nitrogen (or other inert gas) and thickening agent. It results in a bulk structure with high structuralmechanical properties and low density producing

highly efficient killing of ALFP gas and gas condensate wells.

Composition of gelling agent:

biocide Biolan 3 l per 50 m3;
gelling agent GPG-3 6.0 kg/m3;
complex surfactant Neftenol VVD 1.0 l/m3.

Composition of thickening agent:

thickening agent SPRD 6.0 l/m3;
borate thickening agent BS -1.3 4.0 l/m3.

When there is hydrogen disulfide present in the well, it is necessary to introduce 10–20 l/m3 of hydrogen sulphide scavenger – DEA. Test on gas fields of Orenburggazprom held according to solid foam killing technology, proved the efficiency of the technology. During the killing the borehole of the well was completely filled with solid foam. After specified sedimentation during 12 hours and bleeding gas-cap, the pressure in annular and tube space was 0 atm.

KILLING TECHNOLOGIES IN WELLS WITH HIGH FORMATION PRESSURE AND TEMPERATURE

Solution to the problem of keeping reservoir properties of the formation should be all-inclusive. There is no point in managing the problem on the completion stage and neglecting it at the operational stage.

One of the ways of improving the quality of well service works is science-based choice of killing liquid, which can lead simultaneously to the restored permeability of oil formation, better productivity and additional recovery.

It is known that productivity is going down by 10–20% after killing wells with formation waters, by 14% after using СаСl2 and increases by 16–30%, when solutions on hydrocarbon base are used. Hydrodynamic researches state that in the first 2 cases the reservoir permeability goes down by 10–25% and increases 11–31%, when fluids on hydrocarbon base are used.

Khimeko-GANG devised a killing fluid based on invert emulsions – KF-IER.

When a killing fluid based on invert emulsions KF-IER is forced into the formation, it is possible to create an impermeable barrier against salt solutions during killing oil and gas wells. It also provides for bottom washing and other operations involving

technological fluid on water base.

KF-IER consists of hydrocarbon phase – oil or diesel fuel containing emulsifiers Neftenol NZ or Neftenol NZb or MR and water phase – water, mineralized with various salts (chlorides of sodium, kalium, calcium etc.). Viscosity and density

of invert emulsion depend on correlation of phases and degree of mineralization [5].

The composition, density and technical parameters of KF- IER are specified for each field. Unlike PSKF, KF-IER can be used in temperatures over 100 °С.

More than 2500 well killing operations with the use of KF-IER have been performed in various regions of Western Siberia (Gubkinsky, Noyabrsk, Surgut, Megion and Nizhnevatorsk) from 2000 through 2008. It should be remarked that the successful killing operations with KF-IER were held at the wells of Tarasov field in winter time.

WELL KILLING TECHNOLOGIES WITH PRODUCTIVITY CONTROL

Water-salt solutions penetrate the bottomhole zone during the conventional well killing. While contacting rock minerals, oil and formation water, they may seriously affect the well productivity. The principal complicating factors after killing wells

with water solutions include:

water blockades;
built-up of water insoluble salts;
build-up of steady water-in-oil emulsions;
swelling clay minerals.

The use of various inhibitors and chemical additives cut the risk of decreased productivity after well killing and make the well operational soon after service.

Killing wells in low-permeability formations that often contain a large amount of clay

minerals should be carried out in a thorough manner, since the use of conventional water killing solutions in such formations brings about serious productivity slump. Its restoration requires additional time and resources.

The application of multifunctional surfactant Neftenol K as inhibiting additive allows solving the following problems:

reduce clay swells;
prevent the emerge of steady water-oil emulsions;
curb the corrosion activity of salt solutions.

Filtration tests for assessing the permeability restoration in porous mediums were made during well killing modeling. The researchers took samples of cores with hydrophilic and hydrophobic behavior and the air permeability of the sampleswas measured. Then the samples were filled with water, which then was pushed

out by kerosene, oil, surfactant and oil once again. During the whole period the coefficients of restoring permeability for oil were calculated. The results of the experiment are summed up in Table 1.

In hydrophilic reservoirs the coefficient of restoring permeability, after the killing fluid with 4% of Neftenol K was pumped, was 0.72. This is explained by hydrophobic properties of the surfactant. Surface hydrophobization of large and small porous channels significantly reduce the amounts of water, accumulated in them during the drainage and contribute to better restoration of phase oil permeability.

In hydrophobic reservoirs coefficient of permeability restoration after pumping killing fluid with 4% Neftenol K added was 0.875.

Table 2 demonstrates the impact of Neftenol K on the corrosion activity of killing fluid. The solution of sodium chloride with the density of 1.18 g/cm3 was applied as a well killing fluid.

Table 2 suggests that Neftenol K produces a protecting impact diminishing the corrosion activity of sodium chloride solution by more than 2 times.

Test application of killing fluid with Neftenol K on the fields of Western Siberia provided for effective prevention of water accumulation in the production wells after service. Figure 3 represents changes in water share in the product before and

after treatment.

The analysis included wells, where the downhole equipment was replaced without changing the type and productivity of the pump.

Khimeko-GANG produced a dry brand Neftenol K (SNK-30) for working in hard-toreach fields, where there are no stock facilities for keeping chemical agents.

TECHNOLOGY OF WASHING AND KILLING WELLS WITH SELF-GENERATED FOAM SYSTEM

One of the methods of developing gas, gas condensate and oil wells is application of selfgenerating foam systems (SGFS) [6].

The application of SGFS for forcing out the washing fluids provides for effective well development without a nitrogen unit. The selfgenerating foam system is produced from SGFS composition and fresh technical water.

SGFS consists of inorganic salts solutions (a foaming agent and an activator), which catalyze nitrogen emission. Addition of the foaming agent surfactant allows catalyzing nitrogen emission without harmful additives (nitrogen oxides), obtaining steady fine foam and thus improving well development.

The water base of SGFS consists of fresh technical water (рН=6–7).

ABSORPTION WELLS WASHING TECHNOLOGY

When the walls are washed with standard solutions after hydrofracturing, an intensive absorption is observed. Besides, the conventional washing solutions don’t have enough viscosity and sand retaining ability and can’t effectively remove the contamination from the well bottom.

In order to solve the problems, Khimeko-GANG developed a washing fluid for removing proppant plugs from the bottomhole. The washing fluid has the following advantages:

high sand retaining ability;
low filtration in the formation;
low friction losses while pumping through low diameter pipes.

The composition of washing liquid per 1 m3:

gelling agent GPG-3 4.00–6.00 kg;
thickening agent SP-RD 4.00–6.00 l;
biocide Biolan 0.06 l.

The following formula is used to calculate the volume of liquid necessary for washing the wall from proppant:

Vwf = Vivps+ Vhff+ Vreserve

where Vwf – the necessary amount of washing fluid, m3;

Vhff – the amount necessary for filling the HF produced fracture, which can be calculated by using the formula: Vhff = (0,3 ÷ 0,4) × weight of proppant, m3;

Vivps– inner volume of the production string, m3;

Vreserve – the volume of liquid for unprecedented cases, which can be calculated by the formula: Vreserve= (0,1 ÷ 0,2) × weight of proppant, m3.

The density of the washing fluid is specific to the BHP. When the density of the washing fluid is selected, it is necessary to take into account that hydrostatic pressure should exceed BHP by no more than 5–10%. The density of washing fluid may be regulated within 1.00–1.15 g/cm3.

Washing liquid has the following properties:

losses of pressure during pumping through the coiled tubing with the diameter of 23/8 mm shouldn’t exceed 0,007 atm/m;
early filtration pressure differential on a porous medium model with permeability of 2 mkm2 is 20 MPa/m;
sand bearing ability of the washing fluid is 100 – 300 kg per 1 m3 of the washing fluid;
the fall velocity for proppant with the diameter under 1 mm is no more than 0.01 m/min.

After the washing fluid is separated from the proppant it can be used again.

The application of the technology on the fields of Surgutneftegaz raised the efficiency of proppant backflow after hydrofracturing.

REFERENCES

1. Composition of Polysaccharide Gel for Killing Wells and Its Production Method. Patent of the RF №2246609 / L. Magadova et all // Published on 20.02.2005, bulletin №5.

2. The Experience of Using Khimeko-V Complex in Hydrofracturing and Well Killing Technologies / E. Kuryatnikov et all // Oil and Capital Journal. – 2004.– №2. – Р. 2–3.

3. Research of Killing Fluids and Acid Solutions on Mudded Terrigenous Reservoirs / T. Khismetov et all // Oil Industry Journal. – 2007. – №3. – P. 92– 95.

4. Killing Abnormally Low Formation Pressure Wells. Patent of RF №2330942/ L. Magadova et all // Published on 10.08.2008, bulletin №22.

5. Emulsifier of Invert Emulsions. Patent of RF №2200056/ R. Magadov et all // Published on 10.03.2003, bulletin №7.

6. Foaming Agent for Development and Washing Wells and Its Production. Patent of RF №2250364 / L. Magadova et all // Published on 20.04.2005, bulletin №11.

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