Johnson, Carl (Schlumberger) | Gai, Alessio (Schlumberger) | Ioan, Tiberiu (Schlumberger) | Landa, Julian (Schlumberger) | Gervasi, Giuseppe (Ital Gas Storage S.p.A.) | Bourgeois, Benjamin (Geostock) | Bouteldja, Mohammed (Geostock)
With today's low energy prices and with the increasing drive towards sustainability, it is essential to develop more economically efficient and ecofriendly technologies in oil and gas field development. Such a technology is self-healing cement, which was successfully applied in a large project in northern Italy in the conversion of a gas field to a gas storage field.
During the construction phase of gas production and storage wells, one of the critical goals is to achieve competent hydraulic isolation between the surface and the casing to reach the reservoir. There are several cases documented in the literature where poor isolation has resulted in gas flow to surface, thereby polluting water reserves, greenbelts, and populated areas. Improper isolation can also result in interzonal communication, production of unwanted fluids, gas migration, casing corrosion, and sustained casing pressure. These can have significant health, environmental, and economic impact. Additionally, the impending need for well intervention, along with high re-entry costs, will further weaken revenue margins.
Breaking through conventional cementing solutions, a global oilfield service company had established an active cement technology to improve annular isolation in gas wells. This technology is capable of self-healing when exposed to hydrocarbons of any type, unlike other self-healing systems that are limited by the level of methane (CH4) in the gas reservoir. The new system allows universal coverage for any concentration of CH4. Because the concentration of CH4 in different gas reservoirs can vary significantly, the self-healing "protection" against different levels of CH4 is tailored to suit different reservoirs.
This field-proven technology, in use for more than 10 years, stemmed from the original self-healing technology commercialized more than a decade ago. Subsequently, an opportunity arose to apply this technology in a large project in the north of Italy. The project would exploit a depleted gas field by conversion to a gas storage field with the drilling of 14 wells from two clusters above the reservoir.
The product testing and implementation, job execution, and results evaluation brought several benefits with positive impact to the service company and the owner/operator of the field. A higher level of isolation significantly decreases the need for future well integrity and repair, which provides medium- to long-term benefit for the operator—an added value that is sometimes omitted in well construction design.
Using a zonal isolation technology, such as the self-healing cement system described here, inherently places the service company and operator in a much more secure position for the future. Furthermore, in the current industry climate, saving 30 to 40 days of rig time and the cost of remedial operations and achieving important mitigation against health and environmental impact pose a significant economic advantage.
After clarifying the terminology of man-made caverns and the advantages of mined caverns compared to other storage types, the concept of mined cavern is presented and detailed within a historical frame. The major evolutions of the concept are developed along with the main technical steps, from mining to tunnelling technology. Emphasize is given to the hydrodynamic containment allowing liquid hydrocarbons and LPGs to be stored in unlined caverns for a large range of geological conditions. More recently stability issues of megawedges have been considered in parallel with the general increase of cavern size in the last decades. The conclusion focusses on the contribution of structural geology in this domain.
Underground storages of hydrocarbons are conventionally classified into three major types corresponding to the three geological contexts in which they are developed: storages in porous media, storages in leached salt caverns and storages in mined cavities (Gomez-Montalvo, 1997; Londe, 2017)
Storages in porous environments include aquifers (porous geological structures, sealed by a rock cap) and depleted fields (old oil or gas fields, production phase being terminated). The latter type, for which the tightness of the geological structure has not to be demonstrated, is the oldest underground storage of hydrocarbons, with the 1st facility put in service in 1916 in Zoar, New York (USA).
The second type, storage in salt caverns, represents an adaptation of the cavities leached by fresh water in order to dissolve salt arranged in layers or domes and create a stable volume. This type of storage is the cheapest one by working volume (barrel or cubic-metre).
The third type, mined caverns, have been first designed and developed in the USA and 73 caverns were created between the years 1950 and 1984 in this country. At that time, the technology was a pure mining one, adapted to and modified for the purpose of underground storage. The room-and-pillar design (Fig. 1, left), classical in mining industry, represents this obvious mining heritage. The excavation used compressed air hammers and then explosive (drill-and-blast method) and even the name “mined caverns” refers to mining industry. Caverns were small even very small and the total volume of the 73 American caverns reached a bit less than 3 300 000 cubic-meters (as compared to present caverns which frequently reach 1 000 000 cubic-metre per cavern). Along with the technological developments, the room height increased as well as the room width, giving higher extraction ratios. The extraction ratio which was originally 0.3 to 0.5 in the 50s reached much higher values, 0.60 to even 0.75 at the end of the 70s. Despite a very interesting development, these values represented a technological limit which led to the abandon of room-and-pillar design for underground storages at the beginning of the 80s. A few room-and-pillar mined caverns have been excavated using road-headers, a technological development which increased productivity but with limited sections.
Underground hydrocarbon storage is a mature technology, born in the early 20th century and very widespread today. The success of this technology lies in its economic efficiency, its safety and, ultimately, its excellent environmental track record.
The market is divided among three techniques: (1) storage in porous media, by far the most widespread technique for storing natural gas; (2) storage in salt caverns, a technique that is suitable for all hydrocarbons; and (3) storage in mined caverns, a technique that is used for liquid or liquefied products.
The market is currently changing. Underground storage facilities are under increasing stress from operators that are seeking to obtain the most from them. Although some storage facilities still have few movements, since they are used for strategic reserves, many storage facilities today have to provide an optimised working volume and higher cycling and deliverability rates than those for which they were created. Monitoring and modelling are helping designers of storage facilities to improve their efficiency. Safety enhancements are the second mechanism by which underground storage facilities are currently being upgraded. This involves design improvements, with the addition of new safety equipment such as fail-safe valves. It also includes the implementation of best practices. Therefore, tools are available that comply with regulatory requirements and meet the economic efficiency needs of storage facilities. The storage facility integrity management system is one of these tools. Diagnosis and tracking of induced seismicity is another example.
The quality of input data for geomechanical models and design has been a real issue since models exist. Publications have been written and recommendations released on this topic yet problems still remain. This article analyses the origin(s) of poor quality data and subsequent models, based upon the analysis and experience of many site investigations. It aims at providing effective solutions. A review is conducted and the most frequent causes of non-reliable data are presented. Even if laboratory testing procedures are adapted, most problems come from a poor description of the tested samples (weathering, alterations, weakness planes, pre-existing incipient fractures) and from the fact that too often the selected samples cannot provide the required or anticipated results. We insist on the adequacy between the technical objectives of a site investigation and the equipment / procedures adapted for getting data to reach the desired goals. Depending on the required data, practical aspects are addressed such as i) the type of core drilling and/or the core barrel to use, ii) the importance of geometrical information for fractures and the use of acoustic televiewer versus optical imagery, iii) the selection of the most suitable method for measuring in situ stresses depending on rock type and structural conditions, etc. Finally, we suggest a procedure to perform core sampling for classical geomechanical laboratory tests (UCS, tensile, triaxial tests). The aim is to identify abnormal data or data sets and validate the overall quality of data. To conclude, the emphasis is on the constant interactions between structural geologists and rock mechanical engineers to ensure a common understanding of the geological conditions, subsequent adapted testing and finally better geomechanical models using reliable input data.
Within the European context, CO2 storage operations shall address the potential impacts of large scale CO2 storage through risk assessment. The key risks identified for this onshore CO2 storage site were the migration through faults and ground deformation.
To quantify the CO2 migration along a fault, flow modelling and uncertainties management codes are coupled to compute the failure probability i.e. the probability of CO2 migration towards a control aquifer. Such probability of failure is characterized by low to very low probability of occurrence which requires a large number of simulations to enable its evaluation. Each failure scenario models the CO2 migration from a storage aquifer to a control aquifer when altering the flow properties of the fault zone. Fault failure analyses are performed on the surrogate models. They show that limited CO2 migration is occurring along the fault but no breakthrough in the control aquifer. The injection induces some pressure disturbance in the control aquifer in about 30% of the cases which lead to effective stress changes.
To quantify effective stress changes due to CO2 injection and the subsequent ground deformation, the mechanical responses of the different sediment layers are modeled coupling flow and geomechanics. The impact of the stress changes on porosity and permeability of the storage reservoir is modeled along with the impact of uncertainties of the mechanical parameters. For this onshore CO2 storage site study case, the expected ground displacement is negligible (below the limit of the measurement capabilities).
In the fifties and sixties, some underground nuclear power plants, UNPP, have been built and operated in Europe and Siberia. It is clear that underground siting of any sensible and/or dangerous plant and depot will improve safety against external menaces (plane crash, bombing, etc.) as well as against consequences of internal accidents. The technical and economic feasibility of UNPP depends on various criteria, which are tentatively proposed herein, under main headings: i) Physiography (relief and water); ii) Geology (nature and structure of rocks, distance to geological hazards such as active faults, volcanism, glaciers, slope movements, tsunamis, etc.); iii) Population and Human activities (distance to large towns, industrial plants, agriculture, etc.); iv) existing service networks (harbors, roads and railways, transmission lines, etc.).
All along the nuclear era, many advisers have called for siting reactors underground as had been often done for explosives and now, more and more, for sensible and nuclear wastes. Long before the accidents at Three Mile Island, Chernobyl and Fukushima, a partial core melt occurred in the underground Lucens plant, Switzerland, without any harm to people and environment. The practice of underground hydropower plants together with some other uses of man-made caverns, from hydrocarbon storage to the Norwegian ice rink and caverns needed for neutrino research are shortly recalled as they provide the bases for the design of caverns. The senior author has followed the cavern construction of the French plant Chooz (only second to Siberian plants as an underground nuclear plant with a significant output) and has attended the only international conference on the topic, organized by the government of Germany in Hannover in 1981, soon after the TMI accident.
Mega wedges represent adverse conditions during the excavation work of large underground caverns for storing hydrocarbons. Even in “Very Good” rock (assessed using an empirical approach such as the Q system or the RMR), the potentiality of wedges involving atop heading and one or several benches is a recurrent problem that cannot be neglected in unlined rock caverns, only stabilized using rock bolts. The empirical approaches have not been developed to define the necessary support in this very specific context which requires detecting and stabilizing such very large wedges (mega wedges)using a deterministic approach, based on structural geology. Although minimization of the risk and impact of mega wedges is considered by the cavern basic design and the adaptation of the layout and support, a deterministic re-assessment of the areas where mega wedges may occur shall be done during the excavation phase at a more precise scale, on realistic structural conditions. Established on real cases in granite, this paper presents a practical and balanced methodology that is used from the design to the excavation and support phases. For that, as a first step, structural geology aims at identifying location, geometry and potentiality of mega wedges and then rock mechanics helps to characterize the shearing capability of the identified mega wedge and its putative failure. In addition to the support recommendation, the phasing of the support installation with respect to the excavation work to achieve stable condition of the mega wedges is discussed. Results are promising in term of applicability, even if a better estimation of the shear strength parameters of the discontinuities involved in the failure mechanism can be necessary for local fine tuning.
More than 70 underground mined caverns for LPG storage have been constructed in the USA from 1950 to the early 1980’s. Several decades later, a revamping, or at least a review of some of those underground facilities becomes necessary regarding their long term stability and hydrogeological integrity.
A specific approach has been developed for evaluating the geomechanical stability of such old underground storages. This paper presents this methodology. Requirements for hydrogeological integrity are briefly presented. The article focuses on geomechanical stability. The different phases of the studies from the definition of a complementary site investigation to the realization of analytical and numerical calculations are detailed. Finally, the evolution across time of the best practices for underground storage concept design is presented.
Structural geology (a branch of geology aiming at describing the structures – joints, faults, folds, etc. – at various scales) can be used in the field of rock mechanics and rock engineering, and particularly in underground engineering works (tunnelling and rock caverns) to gather more reliable data for empirical stability analyses and deterministic calculation models. Methods of structural geology are presented and their applications in rock mechanics/rock engineering are highlighted in particular through the observation of faults and joints arrest. Structural geology allows a better understanding of the origin, the chronology and the mechanical behaviour of discontinuities, and therefore a more accurate rock mass characterization and rock mass classification, as well as a validation of the actual stress regime. Examples selected from different countries of using structural models are also given with emphasize on the necessity and the way to build a 3D model at each stage of an underground project, from site selection to investigation and construction, to ensure the quality and validity of rock mechanical data and assumptions.
In the last five years, several publications and oral presentations insisted on the importance of structural geology in the field of rock mechanics (e.g. during Sinorock 2009 in Hong Kong or the ISRM congress in Beijing in 2011). They highlighted the necessity of using structural geology and structural data as input parameters for rock mass characterization and rock mass modelling.
This wish is praiseworthy and corresponds to a real need.The situation is due to the lack of structural geologists on the market: structural geology being less taught in university and often replaced by engineering geology, geologists or engineering geologists are easily involved in big projects (e.g. dams, hydropower plants, tunnelling, etc.) whereas it is nowadays quite uncommon to meet structural geologists in teams in charge of design, modelling or even supervision of site works.
In addition, most structural geologists are academics and very few are full-time practitioners, especially in the field of rock mechanics.
This short paper cannot be exhaustive but set out the various possibilities offered by structural geology. The objective is less ambitious and is limited to the presentation of practical examples in different geological situations in which a structural knowledge can provide a real help for modelling or for characterizing the rock mass through the two most geology-based rock mass classifications, the Q system (Barton et al. 1974) and the RMR (Bieniawski 1989).
Since the vocabulary also acts as a brake, this paper deliberately uses simplified technical vocabulary and concepts, hoping that this will bring rock mechanical engineers to working closer with structural geologists.
During feasibility and design stage, for large underground rock caverns, through an integrated investigation approach an interpretative geological model is developed with geologically critical areas predicted as geological hotspots for special interventions during construction. The initial geological model developed - which is essentially a structural model evidencing major faults, fractured zones and dykes - is updated continuously during construction of water curtain system and main cavern with focus on geological hot spots through an active design process. This active design process involves additional investigations during construction for hot spots, 3D geometrical analysis, geotechnical assessment of the cavern and verification of rock supports by detailed geotechnical back analysis using actual excavated data from water curtain and cavern heading. The geological model is improved and updated further after cavern heading and at every stage of benches for prediction of likely adverse conditions, if any, in the next benches. The predictive assessment is corroborated by observed geotechnical monitoring results; and for segments with abnormal deflections, FEM analysis is performed. The present article outlines the review and updation process of predictive structural analysis and geological models for large underground rock cavern excavations being practised in execution of one such storage project in India with an objective to reduce exposure to underground risks and ensure preparedness to address adverse scenarios.