Best Practice for Selecting Potential Candidate Wells and Methods for Repair and Insulation Works (RIW)
Gabdulov, R.R. (RN-Yuganskneftegas, LLC),
Nikishov, V.I. (NK-Rosneft, OJSC),
Slivka, P.I. (PN-UfaNIPIneft, LLC)
Introduction
Among the Geological and Technical Works (GTW) carried out in the process of oilfield operation at their latest stages of development, the main part of these operations comprises Repair-and-Insulation Works (RIW) on production and injection wells for the development, maintenance and improvement of well-injection capacity. The aforementioned details establish the feasibility for development of a logical design for the selection of candidate-wells for RIW. Most of the individual GTW are carried out in small volumes, some of them rarely performed in the conditions of the development of certain oilfields.
Planning the scope of the GTW to be carried out must be based firstly on their purpose – ensuring planned oil production levels are achieved from productive strata and planned targets for oil recovery, and must also resolve health, safety and environmental protection problems.
Many oilfields, in their late stages of development, are characterised by a significant current recovery of reserves, a high degree of water cut from well production and a significant number of non-working wells. Efficient Well Work-Over (WWO) in such conditions allows a significant decrease in the natural decline of oilfield production and results in high profits. RIW are a special major part of WWO.
Until now, RIW were limited to the installation of cement plugs or polymer injection. The main reason for the successive failures in the solution of water inflow problems was the insufficient understanding of the problems, leading to the wrong solutions.
The success of RIW largely depends upon the
following (Pic 1):
» The correct selection of wells for RIW and the quality of Geological-Geophysical Methods of Studying Wells (GSW);
» The selection of RIW technology;
» The selection of appropriate insulation materials.
The omission or underestimation of even one link in the chain of these procedures reduces the value and success of the whole RIW planning process.
In this article, the authors have tried to sum up these efficiency criteria on the basis of how all RIW are planned. Even negative results are still results for the solution of multi-factor tasks, requiring analysis, experiments and follow-up.
Analysis of the geological and technological efficiency of RIW
RIW planning includes:
– identification of casing leak intervals or cement bond
– establishing the selection of candidate-wells for RIW with the application of various methods, mostly based on the identification of the reasons for flooding:
» the identification of imbalance between the oil production rate and the water ratio in well products (degree of flooding);
» the evaluation of imbalance between productivity and the character of formation saturation of the formation under operation, etc;
» the RIW feasibility study;
» the performance of the series of direct geological-geophysical studies on selected wells with the aim of identifying:
» the profile and composition of liquid influx from the formation of non-flowing wells;
» the water cross-flow intervals, etc
» RIW procedures (the insulation of separate flooded formations/sub-layers, the elimination of water cross-flows behind the casing and casing leaks with the application of modern technologies and equipment, the use of qualitative backfill materials: good quality cement slurry with special chemical additives, resins and other insulation compounds);
» the sparing re-perforation of oil-saturated intervals and formations after RIW;
» geophysical and geodynamical studies on completion of the works, which provide information on the quality of the RIW performed, formation intervals work efficiency, the composition of influx, well productivity and formation filtration properties;
» the selective deeply penetrating re-perforation of low productivity and non-working intervals and the application of other impact methods on the bottom-hole formation zone.
In order to identify the reasons which have a negative impact on the current development, it is necessary to analyse the completed RIW. For this work, it is proposed to perform the evaluation of ROW on the basis of two-key criteria, which make it possible to assess the accuracy of the candidate-well selection and the effectiveness of the technologies applied:
» the achievement of design growth (geological success) and the efficiency/success of RIW (technological efficiency);
Geological success is determined by a relative limit of geological efficiency (the achievement of design growth), depending on the reliability of the geophysical data and subjective factors (the correct calculation of expected parameters: geological potential, watering level, etc). The key word here is “expected” because it reflects the degree of uncertainty in the evaluation. A calculation of expected parameters lower than this boundary is assessed as incorrect.
The evaluation of the technological success of performed RIW is connected with a number of problems, identified by the exclusive diversity and complexity of inter-conditioned events resulting from RIW and events not connected with RIW, such as oilfield water flooding of wells located in pure oil zones, while there are no water-bearing formations in the section, or if there are water-bearing formations in the section and the water produced is different from the formation water. The technological success of RIW will be confirmed by geophysical studies on completion of the RIW and by well operation indicators when the well is brought into stable operation.
Based on the number of RIWs performed, it is possible to assess the significance of RIW and the problems that occur. A low RIW efficiency figure (Gэф, Тэф) implies that it will be necessary to reconsider the accuracy of the selection of the candidate-wells and of the technology and composition of the insulation materials, etc.
The GTW may be considered successful provided that the operation mode of the well, when brought into stable operation, is cost-effective and the expenditure for GTW evens itself out during operation between overhauls.
Experience of repair works shows that some repairs fail (repair works not completed or the forecast operating practice of wells not achieved, etc). This being said, a proportion of failed repairs varies for different GTW, in particular when replacing a pump. A percentage of work success is 0.95 to 1 and, when performing RIW, it is possible to calculate the percentage of RIW success, etc.).
The proposed method of RIW analysis gives a clear demonstration of RIW success not only for subcontractors offering various technologies, but also provides a breakdown on the basis of various types of work.
The selectivity of the backfilling of water supply channels will be determined not only by the chemical content of the water insulation composition, but also by the variations in the penetration of water insulation material into oil and water, saturated intervals due to viscosity, density and permeability and also due to some technological specifics of treatment and well operation.
The priority for selecting technology and backfilling material is well watering. Based on the character of well watering, it is possible to divide RIW into the following types [1]:
» elimination behind-the-casing flows from the top and bottom of the water-bearing layers;
» limitation of bottom water influx (deposits with bottom waters);
» elimination of influxes of embedded and injected waters through mostly permeable layers inside the oil section;
» improved oil recovery from production formations by means of conformance control in injection wells;
» water insulation works, carried out simultaneously in production and injection wells;
» shutting down reservoirs (the requirement for this type of RIW occurs in wells where there are several formations under operation simultaneously);
» eliminating casing breakdowns;
» switching wells to lower layers/horizons, temporary well preservation and well abandonment (to be carried out in compliance with current regulations on switching wells to other horizons, temporary preservation and abandonment).
Furthermore, the following factors affect the selection of RIW technology and backfilling material:
» the specific geological structure of the oilfield and formation;
» the geological-oilfield specifics of formation, exploration conditions, etc;
» the well structure, technical condition, available surface and subsurface equipment, etc
Based on the specified factors, a technology for RIWs is developed and the most suitable insulation composition is selected.
Therefore, if guided by the establishment of the necessity and the requirement to identify the type of RIW required without specifying the method and technology, the task may be resolved on the basis of the information in the database for calculating reserves and analysing oilfield exploration.
The advisability of RIW will depend firstly on the general conditions and the efficiency of the exploration system in the oilfield. To date, a great number of methods/criteria for selecting potential candidate-wells for RIW (see Table 1) have been developed, each of those methods having its own advantages and disadvantages. It would be wrong to use only one method for planning RIW, since each method is a support tool for the selection of wells on the basis of a large amount of information and requires detailed examination well by well.
When developing the algorithm for identifying the RIW requirement for production and injection wells, tasks were set up to identify the general requirements for specified works. However, the methods for work in the development conditions of each individual formation are selected separately, allowing for such factors as experience, equipment, the level of WWO service, the availability of the required materials, etc.
For fast decision making, a simple, user-friendly procedure is needed which would not require time and complicated calculations. It should be borne in mind that when wells are selected a calculation of well potential is carried out (for lowering a standard-sized pump), the assessment of watering (one of the most difficult parameters in planning RIW) and a feasibility study.
To a greater degree, the methods/criteria for selecting candidate-wells are based on the following:
» the comparison of their development parameters with the condition of a reservoir’s management in general;
» the application of validity criteria, taking into account the geological-physical characteristics of the reservoirs, the physical-chemical properties of the formation liquids and gases, the exploration system and the reserve recovery conditions;
» a study of the reasons for closing errors/reasons for watering (Pic. 2).
In order to establish the type of RIW, the reasons for closing errors must be identified, i.e. the reasons for well watering: breakdown of casing, non-watertight cement ring, watering of some intervals of production formation, etc. Watering reasons may be identified on the basis of an analysis of the geological and operational data concerning the well and data on the development of separate deposit sites and formations in general.
Sometimes, the requirement for a certain type of RIW may be identified and the selection of the method, flow diagram and insulation material or isolation medium established. If the analysis of the data listed above does not allow clear conclusions to be reached about the reasons for well watering and the water influx routes, it will be possible to identify a set of additional studies to clarify the reasons for watering.
Summary
1. The modern exploration conditions of many oil deposits are characterised by the increasing requirements for well RIW, which are one of the main means of ensuring the cost-effective development of deposits. The key factor in the modern exploration and operation of deposits is the achievement of an understanding of watering problems and their solution.
2. The timeliness and efficiency of RIW largely depends upon technical and economic performance indicators concerning deposit development in general.
3. The success of RIW largely depends upon the fulfilment of the following tasks:
» the accuracy of well selection for RIW and the quality of the geological and geophysical studies of wells (GSW);
» the selection of the appropriate insulation material and RIW technology.
4. In order to identify problems in RIW planning it is necessary to analyse the completed works so as to identify the area of the tasks (geological and technological) to be carried out.
5. The priority guideline for the selection of technology and backfilling material is the character of the well watering.
6. To date, a great number of methods/criteria have been developed for selecting potential candidate-wells for RIW, each with its own advantages and disadvantages. The selection of methods/criteria depends on the completion of the relevant task (time, scale, labour cost, etc).
7. The majority of the RIW problems under review require further development and more thorough studies so as to establish the conditions required for RIW and to improve their technology and efficiency.
List of References
1. Blazhevich, V.A., Umrikhina, E.N., Umrtbaev, V.G., Уметбаев В.Г. Oilfield Operation Repair & Insulation Works. Published: NEDRA, 1981. – page 236: illustrated.
2. Kulikov, A.N., Stroganov V.M., et al. Best Practice and Study of Validity Criteria for the Selective Insulation of Water Influx Technology Using Organosilicone Compounds // Petroleum Engineering, No 9. – 2005. pages 36-45.
3. Pasinkov, A.G., Gabdulov, R.R., Nikishov V.I., Slivka P.I., Selective Insulation of Water Influxes during Development of Multilayer Deposits // Neftjanoe Khozaistvo, No 5. – 2008. Pages. 64-66.