№ 9 (September 2006)
How to Combine Well Killing and “Soft” Treatment of Bottomhole Zone
During well operation, the most frequently repeated procedure is well killing in the course of workover.
By V. Kirillin, D. Ashigyan , S. Grigoriev, K. Pisarev
Most often, water solutions of NaCl and CaCl2 are used for this purpose in Russia, resulting in a greater time spent on well completion operations in the post-workover period, a decrease in production rates, and an increase in oil water cut and oil-bearing formation clogging.
Formation of insoluble deposits both in the pore space and on the working elements of the downhole equipment also causes premature equipment failures. Taking into account the fact that the main part of reserves in the fields under development is of a hard-to-recover category, and thus oil companies must be more attentive of the so-called tertiary and quaternary methods of oil recovery, developing a new generation of workover fluids becomes all the more urgent.
A subsidiary of the Silvinit (Solikamsk) company, called Mineral Nefteprom, was entrusted with development of a new generation of potassium-containing compositions under the “RECAS” trademark. While fulfilling the task, Mineral Nefteprom specialists attempted to develop the cheapest high-tech compositions, allowing for a combination of well killing with a “soft” treatment of the bottomhole formation zone.
The main contents of “RECAS” are based on the products of potassium-containing mineral processing, with the addition of a multifunctional composition called “Aksis”. To ensure that all compositions posess the necessary technological properties, the base is dried (to prevent freezing), ground (to the main fraction size not more than 5 mm), and sieved. The “Aksis” multifunctional composition, designed and produced by a Moscow company called AKSIS at Mineral Nefteprom’s request, contains scale inhibitors, an oil-wetting agent, and a surfactant. The most challenging part of the composition’s creation was fulfilling the customer’s requirement of consistency: the composition should be in the form of a powder and should thus consist of powdered substances compatible with one another. This requirement is based on the aim to prevent the possibility of freezing oil-wetting agents and surfactants, which can happen to liquid forms of these agents, and to guarantee their specificly required concentration in the workover fluids. Two compositions have been developed at present: “RECAS 600-68” and “RECAS 600-43B”. Composition “RECAS 600-68” is based on sodium chloride (approximately 93 percent of its mass) and a small addition of potassium chloride (2.53 percent of its mass). Composition “RECAS 600-68” is developed for application at the fields with rock permeability exceeding 50 mD, where the clay swelling problem is not as pressing as that of insoluble deposit formation and pore spaces clogging with water. “RECAS 600-43B” contains no less than 30 percent of potassium chloride, and is designed for application in the fields that have mudded reservoirs, with permeability 2 mD and more. Density of workover fluids based on “RECAS” compositions varies from 1.03 g/cm3 to 1.19 g/cm3. “RECAS 600-68” compositions cost no more than technical salt (sodium chloride).
Research conducted at VNIIneft named after Academician A.P. Krylov showed that water solutions of “RECAS” compounds, which contain the multifunctional composition “Aksis”, have a 40 times lower surface tension at the kerosene boundary, compared to water solutions made of ground sylvinite “Liman 800,” calcium chloride, and sodium chloride (Diagram 1). It was determined that the “RECAS” water solution with a 1.18 g/cm3 density has hydrophobic properties (water is absorbed into the treated porous medium at the rate of 0.04 g/min). Simultaneously, the lack of a hydrophobic effect is characteristic of water salt solutions of ground sylvinite and sodium chloride of the same density, and a porous medium treated with these solutions is characterized as hydrophilic (water is absorbed into the treated porous medium at the rate of 0.46 g/min) (Diagram 2).
In addition, UfaNIPIneft performed research on inhibiting deposit formation while mixing a workover fluid based on the “RECAS” compounds containing the multifunctional composition “Aksis,” with a model of sodium-bicarbonate and calcium-chloride formation water. Study results showed that the composition “Aksis” contained in “RECAS” compounds prevents calcite precipitation (calcite precipitation does not occur at negative values of the SI saturation index ).
Specialists at UfaNIPIneft also investigated the corrosiveness of “RECAS” compounds containing the “Aksis” composition. Sodium chloride and “RECAS” compound solution corrosiveness was tested as per Russian Standards GOST 9.905-82 “Methods of Corrosion Tests. General Requirements” and GOST 9.502-82 “Metal Corrosion Inhibitors for Water Systems. Methods of Corrosion Tests”. Corrosiveness was evaluated utilizing a gravimetric method by sample weight loss. The test results are given in Table 1.
As is shown in the table, corrosiveness of “RECAS” solutions is higher compared to that of sodium chloride solutions. However, its values are not critical, and use of the “RECAS” compositions will not result in a significant increase in the background values of corrosion rates of liquids extracted from the well.
Full field tests of “RECAS” compositions have already been performed at Sibneft-Noyabrskneftegaz. The company tested the “RECAS 600-68” composition at the Sutorminskoye field (Table 2).
At present, Varyeganneftegaz, Surgutneftegaz, Yuganskneftegaz, and TNK-Nizhnevartovsk are performing “RECAS 600-68” and “RECAS 600-43B” field tests.
The following conclusions can be drawn based on the above mentioned:
1. Correct ways of preventing negative impact of workover fluids on the poro-perm characteristics of the bottomhole formation zone have been selected.
2. The “RECAS” compositions fully meet the technical and technological requirements of modern workover fluids.
3. Sources of raw materials used for the “RECAS” compound production are virtually unlimited.
4. Because of the multifunctional composition “Aksis’”presence in the “RECAS” compounds, it becomes possible to combine well killing and “soft” treatment of the bottomhole formation zone operations.
5. The “RECAS” compounds can be utilized virtually at all the fields in the RF.
6. The multifunctional composition “Aksis” can be used as an independent product by being added to any salt solutions used for well killing, or to the Cenomanian or fresh waters.