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№ 4 (April 2012)

Wotasoft™ Technology from Zirax as Best Method to Retain Reservoir Properties

   At present, the petroleum science is facing an important task of the most efficient and complete extraction of hydrocarbons from the oil and gas fields in operation.  One way to meet this challenge is to introduce some novel technologies aimed at preservation of reservoir properties within the cored interval, and especially the bottomhole formation zone (BFZ).

By Sergey A. Demakhin, Candidate of Geological and Mineralogical Sciences, is a Technical Consultant at Zirax

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   Once the well has penetrated the cored interval, there is a bottomhole formation zone, where drilling, exploration and exploitation of the wells involve various processes that break the initial equilibrium between the mechanical and physical and chemical state of the rock. Through the bottomhole formation zone, there is some fluid flow from reservoir into the well, and the state of permeability has a significant impact on the wells productivity. The reduction of the BFZ permeability as compared to any remote reservoir area can be 10 times or more, thus dramatically reducing the flow rate per well, and sometimes leading to a complete stop of fluid filtration.

   The well killing operation is one of the many possible causes of the BFZ deterioration. When operated, almost every producer well is subject to killing due to workovers during the year, change of pumping equipment, washing of bottom hole and other operations. Killing is the most popular type of well impact. It represents a set of measures for the choice, preparation and injection of some special killing fluids (KF) into the well to ensure safe and trouble-free preventive maintenance.

   Traditionally, the killing fluids have had to meet some specific requirements (see the “Requirements to Killing Fluids”), whereby one of the most elusive indicators is the requirement of the maximum retention of collecting properties in the cored interval. More often than not, well killing has resulted in a drop of productivity, taking it longer to get the production rate stabilized. One reason for this is the use of the low-quality mineral salts that contain particulate matter (white halite, potassium chloride production from dumps or calcium salts obtained as waste from the major process technologies) as well as the use of the aqueous phase contaminated with impurities, and/or incompatible with the formation fluids (field waste water, industrial wastes, saline water from aquifers, process and sweet water).

The result is the reduction in the natural reservoir permeability due to the following reservoir processes:
The reservoir pores get clogged by particulate matter, penetrating the reservoir along with the filling fluid.
The pore volume generates some insoluble precipitates formed by the interaction between the killing fluid and reservoir fluids.
The reservoir forms the stable water-oil emulsions.
There is a “water block” effect caused by the increase in water saturation of the BFZ rocks due to the capillary and surface phenomena. The effect is mainly manifested in the hydrophilic reservoirs’ characteristics; for instance, in the Western Siberia;
The argillites contained in the rock are swelling.
The application of the Zirax killing fluids based of pure synthetic salts with the addition of Wotasoft™ allows the industry to reduce the intensity of those negative processes and to assure good preservation of reservoir properties (Table 1).

   To this end, compounds based on the evaporating NaCl (SolmixWotasoft™) salt and synthetic CaCl2 (PelletOil WotaSoft™) with the basic substance content of 98-99 percent and a minimum content of iron salts. The choice of the basic salt, depending on the chemical composition of formation water prevents salting-up, while the purity of salts helps avoid clogging of the bottomhole formation zone.

   To solve the remaining problems, a specially synthesized additive called Wotasoft™ is used. It is a package of the surface-active agents (surfactants) that provide a gentle killing fluid impact on the cored interval conditions. The demulsifying ability of the Wotasoft™ additive contributes to the destruction of stable emulsions, never allowing them to form again. The reduced surface tension provides for a faster and fuller removal of the killing fluid from the reservoir, when the well is put into operation.

   The effect is enhanced by the hydrophobization effects of the Wotasoft™ additives, affecting both capillary and the surface effects in the pore volume of the reservoir. Due to the hydrophobizated surface of the argillites available in the reservoir, it can be possible to prevent their hydration and subsequent swelling.

   A peculiar feature of the Zirax-made killing fluids is the addition of Wotasoft™ at the production stage by spraying it in a fluidized bed right on the surface of the salt granules with varying density (Fig. 1). Due to it, the customers get a salt system already containing the Wotasoft™ additive in the required quantities and the process of applying a killing fluid is no different from the usual one. After dissolving, the Wotasoft™ additive gets dissolved in water and during the killing it penetrates the layer, providing its beneficial effects.

   It should be noted that the addition of Wotasoft™ maintains its stability in the composition within a wide range of temperature and density values. It is compatible with any reservoir and process fluid and has an inhibitory effect on the corrosion of oilfield equipment.

   Therefore, the application of the modified killing fluids, manufactured by Zirax using the Wotasoft™ technology helps reduce the costs associated with the development of wells. In addition, it stabilizes the production dynamics in the post workover period, while increasing the oil production rate and the productivity index of wells. Gentle killing aimed at the preservation of formation characteristics in the reservoir should extend the life of the wells and increase their ultimate recovery.

   Moreover, the application of the salt compositions obtained with the addition of Wotasoft™ and supplied in a solid, ready for sale and use form, cuts down the need for procurement of other agents associated with the well killing like corrosion inhibitors, demulsifiers, hydrophobic additives. There is no need to buy them separately to face various problems with their compatibility, transportation, dosage, or application peculiarities.

   The Wotasoft™ technology is already being widely used for the wells located in Russia and the CIS, with the results demonstrating a two-fold decrease in the production stabilization time as compared to the traditionally used killing fluids.

Requirements to Killing Fluids
Sufficient density value to provide the necessary back pressure;
Maximum preservation of reservoir properties of the cored interval;
Produceability of preparation and use;
Minimal corrosive effects on the casing and production equipment;
No effect on the performance of geophysical research in the well;
Compatibility with other process fluids used in the well workover;
Thermal stability under specific conditions of its application;

Adjustability of technological properties within a wide range of mining and geological conditions of wells operation;
Explosion- and fire safety.

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