A STUDY ON DETERIORATION OF SCARPS TUBEWELS DUE TO CORROSION - INCRUSTATION WITH RECOMMENDATIONS ON MAINTENANCE AND REHABILITATION INCLUDINCG DESIGN

Abstract

The ground water development operation through tubewells in SCARPS of Pakistan is the biggest of its kind in the world for the control of water logging and salinity and for supplementing the canal supply for irrigation purposes. During the last 18 years, '11000 tubewells ranging in capacity from 2 to 5 cusecs have been installed by WAPDA and installation of about 49000 new tubewells has been planned to cover various SCARPS in the northern and southern regions of the country. The very high capital cost of these high discharge deep tubewells was justified by the consultants by assuming their useful life as 40 years whereas in practice this has proved to be about 15 years. For economic considerations, recently the size of tubewell has been restricted to 2-3 cusecs in place of 2-5 cusecs. The success of ground water continuous dewatering operations in various SCARPS depends upon the continuing effective pumping capacity of these tubewells. A. rapid decline in specific capacity of the tubewell is a continueus economic set back to the success of the SCARPS. Earlier investigators on the basis of their studies showed that corrosion and mineral incrustation of tubewell's mild steel screens are the contributory factors responsible for the loss of well capacity. Further they observed that it was impracticable to overcome corrosion and incrustation through the use of known corrosion control methods such as protective coatings, chemical treatments, cathodic protection or chemical sterilization. According to them the fibreglass reinforced plastics in place of mild steel or other metallic screens appeared to be most economically suitable material for well construction which would minimize corrosion as well as incrustation problems. But even in the case of fibreglass screens which are undoubtedly inert as far as corrosion is concerned there are observed variations in the data which range from 2.6 - 9.0 regarding the values of annual rate of reduction (%) in specific capacity. There are instances where the tubewells have rapidly failed after a few years of their operation irrespective of the screen material and in the absence of corrosion menace in the case of non-metallic screens. Corrosion in fresh waters is a complex phenomenon and it is often difficult to predict the rate of corrosion even when the character of water and the metal are known because of other obscure variables. However the cause of corrosion can usually be explained after the damage has occurred provided that full knowledge of the facts is made available. The presence of oxygen is necessary for the rusting of iron as the corrosion process essentially consists of the formation of a hydrated oxide. Oxygen can occur in soil as part of the air in solution in the soil water, in addition it can be derived from certain salts e.g. nitrates. The supply of oxygen in the soil varies between two extremes. It is plentiful as in the case of pipe burried at a shallow depth in looselypacked soils above the ground water level. The supply of oxygen is low or nil at greater depths below the ground water level. In general, corrosion does occur which may be as a result of the action of certain bacteria which reduce the sulphates present in the aquifer to sulphides and thus provide the oxygen necessary for the corrosion process. Thus most of the waters are anaerobic and contain significant concentrations of ferrous and sulphide ions which are usually present in air-free ground water systems. There is enormous data available on the quality of tubewell waters. Using this data, Langelier Index determinations have failed to establish a definite relationship between the index values and incrustating/ corrosive intensity of the well waters. So far no break-through has been possible on the basis of corrosion investigations in the field. Electrode potential (Eh) measurements showed no definite relationship between the Eh values and corrosive nature of the waters and the performance of the tubewells. It may be mentioned that 19 % of the tubewells with Eh values less than 100 mv have had good performance record. This is contrary to expectations because corrosion is considered inevitable for Eh values less than 100 mV. Corrosion rate measurements using a "corrosometer" showed some qualitative significance regarding the relationship between inches penetration per year (ipy) values and the performance of tubewells. The fact that there are examples of good and poor performance tubewells in all SCARPS which are sometimes closely located and operating in water with similar general characteristics poses a challenging problem to the investigator regarding the interpretation of the field data to identify causes of decline in well performance. It has been established that the corrosion damage in tubewclls where mild steel screens have been used is concentrated almost ·entirely in the saw slots of the screens. The iron bacteria and other slime-forming organisms are found in some ground waters that may cause plugging of the pores in the water bearing formation and the openings of the well screens. These bacteria produce slimes and they oxidise and precipitate dissolved iron and manganese which also have incrustating effect as these slimes may entrap particles of other insoluble mineral salts. It is believed that these bacteria feed on carbon compounds including the bicarbonates and carbon dioxide present in tubewell waters. Sulphate reducing bacteria have been identified in one area of SCARP I where about one hundred tubewells with mild steel screens have been lost. T.P. Ahrens has reported that once sulphate reducing bacterial infection occurs, it is impossible to eradicate it completely. This infection may be minimized by suitable disinfecting and chemical treatments. Water quality is the key to the understanding of the problem of corrosion and incrustation. The nature and concentration of dissolved salts and gases in natural waters determine their corrosiveness and incrustating effect. The dissolved minerals in ground water are present in equilibrium and any change of conditions results in deposition of these minerals as an incrustation. In certain cases, during TV Camera inspections, the screen slots have been observed to be unincrustated and unchoked yet the specific capacity/discharge had considerably been reduced, thereby suggesting that decline in well efficiency originates from outside the screen in the gravel pack and tho water bearing formations. The failure of these tubewolls has possibly occurred due to invasion of the water bearing formation and gravel pack by fines, and other factors of near-well hydraulics. As far as possible, a general well design and methods of construction and well development have been standardized. However, the adoption of a standard design including gravel pack gradation and screen slot size based on empirical calculations and bold assumptions has not solved the problem of well failure. Experts believe that tubewell design specifications though standardised are generally not tenable in all areas. The shrouding material of the screen, if not properly designed to the stratigraphy of the water bearing formation of the representatives strata crossed, the failure of the well is inevitable. Fine to medium sands with relatively high silt and clay fractions predominate in aquifers in Pakistan. The tubewells are drilled through reverse circulating drilling rigs with the result that precise detailed logging of the data is not possible, hence the sampling technique has been considered as poor and data based on better sampling methods from a smaller hole at the site for each well to be designed should be used by the design engineer. For well screen design, according to T.P. Ahrens (1975) the laboratory experiments have shown that an4 open area of 15% is about the minimum acceptable for large capacity tubewells which is almost impossible to obtain except for wire-wound type screens. According to Barker and Grey (1978) the mechanical deterioration of the well by sand pumping, chemical deterioration by corrosion and incrustation, and biological deterioration by bacterial growth are to some extent controlled by ground water velocity which is further regulated by the design parameters of the well. There exists a major gap in hydrogeological literature on the subject and thus there are outstanding problems in rationalizing well design. There are many areas of uncertainty such as critical entrance velocity, frictional resistance within the screen, slot size and percent open area, aquifer stratification and gradation etc. Barker and Grey have tried to present theoretically a preliminary solution to the problem of rationalizing the design of tubewell and have concluded that no general solution appears to be available and have suggested that each case of well design is a unique problem in itself. They have identified area of uncertainty for further investigation. According to them the solution of many of the problems is of fundamental importance for well an understanding of the near well-hydraulics. According to them actual well loss measurement in the field is a difficult and somewhat confusing problem and that interpretation of such observation data poses further complications. These authors are of the view that an effective theoretical approach to tubewell design is economically viable and there are pressing needs for future work on many of these problems, some of which are as under:- 1. The importance of assessment of turbulent flow in the aquifer and the gravel pack. 2. Critical examination of design entrance velocity and the design parameters. 3. Examination of other sources of well loss which undoubtedly are prevalent and need careful attention. 4. Precise understanding of frictional resistance within a slotted pipe. Thus concerted efforts are recommended to be undertaken to review scientifically the aquifer conditions, incrustating-corrosive effects of water as well as tubewell design, construction, development practices, maintenance and rehabilitation methods including the materials for well construction. For the existing tubewells, research on high priority basis has also to be vigorously pursued relating to their maintenance and rehabilitation measures because the maintenance and rehabilitation of the screen and the gravel pack is as important as the maintenance of the mechanical and electrical installations of these tubewells. A combination of chemical and mechanical maintenance and rehabilitation methods shall have to be optimised to restore the well efficiency at various stages of the operation of the SCARPS tubewells. Chemical methods include treatment with polyphosphates, chlorination and acidization of a tubewell with hydrochloric or sulfamic acid whereas mechanical methods include surging, detonating and re-development of the tubewell. Efforts have to be made in this direction by adoption of proper maintenance and rehabilitation procedures such that the tubewell specific capacity may not be allowed to deteriorate beyond 25% in any case. Experts have recommended that rehabilitation measures may be carried out on those wells whose specific capacity has reduced beyond 30%. But the rehabilitation operations so far carried out in various SCARPS have met with little success. The recovery was only temporary. The above said recommendation loses its significance. What is important is that proper maintenance of a tubewell screen should be carried out at regular intervals to check well deterioration even in its initial stages. It is therefore suggested that rehabilitation operation if carried out at an earlier stage of deterioration for tubewells subjected to proper maintenance checks would be accomplished successfully. CMO, W.APDA has undertaken a systematic programme of monitoring and research in the field to feed-back the information to the design, construction and operation organizations for improvements in well design and construction. The way in which different hydraulic engineers and hydrologists interpret and co-relate field data is a controversial issue hence more laboratory work on scientific lines is required with simple model experimentation to arrive at precise relation-ship which can be demonstrated more clearly with much lesser cost as compared to field observations and experimentation regarding the performance of tubewells. Enormous data covering various aspects of tubewell deterioration based on field research is being collected by WAPDA and the critical analysis of the data reveals high complexity of the phenomenon of well deterioration. An elaborate and fundamental research is now needed to investigate the factors identified through field work, theoretically and experimentally at the laboratory level with close collaboration of corrosion technologists, experts in the tubewell industry, applied mathematicians, chemical engineers, chemists and material engineers drawn from the Universities. The Pakistan Science Foundation is requested to consider this proposal for funding so that the nationally important problem of tubewell deterioration is solved without further economic set-back to SCARPS in the country.