№ 10 (October 2011)
New Approach to Purification of Kill Fluids
In recent years, oil producers became more attentive to protection of pay zones against contamination during well servicing operations (WSO), in particular during well killing operations. To some extent, this was facilitated by the RF Committee for Mining and Industrial Supervision, Enactment No. 71 dated 06.06.03, “On Approval of Mineral Resources Conservation Regulations”.
By M.A. Terentiev (3М Russia), Candidate of Biological Sciences; V.I. Nadenov,Yu.V. Danchenko (Novomet-Perm), Doctor of Engineering,
Article 108 of these Regulations states that during well servicing operations it is prohibited to apply working fluids reducing permeability of the bottomhole formation zone. In this connection, removal of mechanical impurities that are contained in kill fluids (KF) and cause deterioration of the reservoir’s porosity and permeability (P&P) has become one of the most significant problems. Operators and service companies make their best to enhance maintaining purity of KF and other working fluids not only in deference to the Mineral Resources Conservation Regulations. In our opinion, their main incentive is hard benefits.
According to standards of west and leading Russian oil producers, a fluid entering a well must not contain suspended particles exceeding 5 µm in size. But most Russian companies do not regulate this parameter; therefore less severe requirements are set for the KF, which limit only a total content of suspended solids (SS) as applied to properties (permeability and pore size) of reservoirs under development. To comply with mentioned standards, companies and their well servicing contractors must undertake the following steps:
– improve arrangements for a mixing system, in particular, equip it with KF filtration units;
– monitor quality of the KF being mixed;
– treat the tank trucks’ tanks to achieve the required purity for fluid to be delivered to the well.
In technical terms, the first two actions are most feasible though quite expensive. Modern mixing units ensure preparation of high quality KF with SS content equal to 10-30 mg/l and its systematic monitoring. Mixing units using obsolete technologies for KF preparation (purification is based on particle sedimentation), despite their disadvantages (a long operating cycle due to low sedimentation velocity of fine particles; impossible sedimentation of 5-8-µm particles due to Brownian movement), also succeed in preparation of acceptable quality fluids. At the same time, results of numerous audits of killing crews in Noyabrsk, Nizhnevartovsk, and at other fields showed that SS content in the kill fluid delivered to the well by tank trucks is practically always higher than in the initial fluid from the mixing unit. A reason for this is not in deficient tank cleaning operations (appropriate regulations are fulfilled) but in practical failure to prepare the kill fluid immediately before each filling. Besides, being a corrosive medium for carbon steel, the kill fluid causes corrosion damage of tanks during its transportation from the mixing unit to the well. Moreover, numerous particles of corrosion products emerge in the fluid.
Certainly, maintaining stable high purity of the KF requires expensive actions mentioned above, which also can be supplemented by anticorrosive protection of mixing unit tanks and tank truck’s tanks. This also requires serious arrangement efforts. Nevertheless, there is a drastic remedy to overcome the existing, to be honest, unacceptable situation. In any industrial technology with mechanical impurities to be removed at several stages, the most efficient approach to their removal is to monitor the impurities in the possible closest point to their source (filtration at the mixing unit). However, in technologies where zero mechanical impurities are a key quality component, multiple filtrations are applied with final post-filtration before application.
To prevent contamination of the bottomhole formation zone with various mechanical impurities from the KF, the fluid final filtration is proposed to be carried out directly during the process of its pumping into the well. For this purpose, a portable suction filter is installed between the tank truck and the pumping unit. The filter must be small in size and must have an acceptable weight for easy transportation, installation, and servicing, as well as sufficient throughput performance and service life with high fineness purification for compliance with pumping process parameters.
Filtration unit DuoFlo™ by 3М, with improved geometry of a bag filter (increased by 62 percent filtering surface area) and supporting double-walled perforated basket (the filter is fully supported by the basket support surfaces) meets the listed requirements (Fig. 1).
Filter features are described below.
Length, cm 72.6
Diameter, cm 17.8
Filtration area, m² 0.62
Filter cake volume, l 5.3
Maximum flow, m³/h 34
Maximum differential pressure, MPa 0.24
Purification fineness, µm 1, 5, 10, 25
Maximum temperature, С up to 149
DuoFlo™ filter body is made of chemically resistant stainless steel. A changeable filtering element (cartridge) is made of material based on thermally bonded polypropylene or polyester fibers that are chemically resistant to salt, acid, and alkali solutions and organic solvents. A cartridge material has a gradient porous structure ensuring deep filtration and high dirt-holding capacity.
3M Russia jointly with Novomet-Perm, at different year seasons, carried out pilot testing of the filter in several oil-and-gas production departments (OGPD) of West Siberia, geographically remote from each other. See Fig. 2 for DuoFlo™ filter in the system “tank truck – pumping unit – well”.
During testing, cartridges with filtration rating of mainly 5 µm were used, on the a priori presumption that they would ensure a high degree of the kill fluid purification with an acceptable volume of pumped fluid to be achieved. In the course of testing, SS content and fractional composition of mechanical impurities in the initial (after tank truck) and purified (after filter) fluid, and a volume of pumped through cartridges fluid were analyzed, and cartridges were inspected visually. The table below presents results of DuoFlo™ filter testing in one of the OGPDs. Let us cite some of the killing results taking into account their similarity (over 40 killings were carried out with the filter application).
Due to significant contamination content (SS=89 mg/l), unpurified fluid was brown in color and after filtration became nearly clear (SS=17 mg/l). A degree of fluid purification was 80 percent. See Fig. 3 for typical mechanical impurities filtered from the specified fluid samples.
At the filter suction, sediments consisted of mixed sand particles and tank corrosion product scales with a size from several hundred micrometers to several millimeters, as well as aggregates of yellow fine particles being ferrous oxide or hydroxide particles according to data of roentgen-fluorescent and chemical analyses. After the filter, the fluid contained only colloid ferrous particles, sizes of which met the filtration rating of the cartridges applied.
For visual presentation of contaminations getting to the reservoir in case of absent filtration only with 10 cubic meters of the fluid, see Fig. 4 with a picture of a cartridge with coarse particles partially removed from it by simple shaking.
A total weight of particles varied from 50 to 140 grams depending on the tank condition. Mechanical impurities of smaller size were firmly retained by the cartridge hairy surface. Their weight was determined by comparing weights of a spent cartridge (after drying) and a clean cartridge and varied between 700-1,500 grams. Performance of cartridges in terms of pumped fluid (before unacceptable fluid head losses occur at the pumping unit suction) was 120-190 cubic meters, i.e. one cartridge was used for killing four to six wells. Oil contained in the fluid resulted in reduced performance of the cartridge. Due to oleophilic properties of porous polypropylene, oil was well soaked in the material and in combination with mechanical impurities formed a low-permeable layer on the cartridge surface. If oil content was 25-30 mg/l, one cartridge can be used for filtration of up to 50 cubic meters of the fluid, with a high purification degree achieved: SS content reduced from 59 to 2 mg/l.
At present, several tens of WSO crews in Noyabrsk and Nizhnevartovsk continuously and successfully apply portable DuoFlo™ filters. Their introduction was facilitated, on the one hand, by ensuring high quality filtrate and on the other, by serviceability, maintainability, and time saving. The technology of the final fine filtration at the wellhead demonstrated that it could be applied even under extreme winter conditions. According to well operators who already tried well servicing with application of such filtration, the well returns to the pre-repair flow rate much faster. Sometimes, flow rate levels significantly increase. This is the main benefit of maintaining high purity of working fluids and protection of the bottomhole formation zone against contamination.
3М Russia, Filtration Equipment Department
17, bldg. 3, Krylatskaya street, Moscow
Tel.: +7 495 784 7474