Large-diameter Wells Research Articles

ANALYSIS OF TECHNOLOGIES FOR COLLARING LARGE-DIAMETER SECTIONS OF OIL AND GAS WELLS

The article considers issues on construction of deep oil and gas multi-column wells with an extended trunk. Drilling of such wells is relevant, for example, on offshore fields, where drilling from the shore with a vertical deviation of over 1000 m is much more economically feasible than from floating vehicles.For drilling wells with an extended trunk and passing through zones incompatible with drilling for geological reasons, it is necessary to begin construction of a well with landing a large diameter direction – often up to 720 mm. The article analyzes the emerging problems and ways to solve them by drilling large-diameter wells and landing casing string. The results of practical experience in conducting work in the Pechora Sea and at the Astrakhan gas condensate field are summarized, on the basis of which recommendations for carrying out similar works are given.When constructing wells, especially starting with a large diameter, it is not always possible to drill a well efficiently, even with drilling equipment ideally suited to specific geological and technological conditions. And this, in turn, can lead to losses of production time and materials, and in the extreme case – to emergency works. Based on the analysis carried out, the article proposes an algorithm of technological operations for drilling wells and landing large diameter directions, which increases the probability of successful drilling to the design depth.

Hydrogeological characterization of low permeability medium for conceptualization of a mine site in Eastern Turkey

The mining site in Eastern Anatolia of Turkey were encounter a significant risk of slope instability within the operational area. One of the processes that govern slope stability is the pore water pressure distribution. The conceptualization and characterization of porous media serve as fundamental prerequisites for the implementation of numerical methods aimed at predicting pore water distribution. This study aims to characterize the hydrogeological properties of water bearing rocks in the active mining site in Eastern Anatolia of Turkey. A total of 21 wells and drill holes were drilled in the study area to conduct in-situ tests, monitoring, and sampling. The large diameter wells drilled in surrounding the carbonate rocks were to determine the groundwater flow and boundary conditions and also wells tapped metasediments and diorite unit for hydraulic testing. The lugeon tests and installation of vibrating wire piezometers were carried out at small diameter drill holes to obtain localized hydraulic conductivity of metasediments and diorite at different depths and monitoring pore water pressure distribution along some critical cross-sections. The results obtained from these tests are used for developing hydrogeological conceptual model for groundwater flow. The results of in-situ tests show that the metasediment and diorite units act as a single hydrostratigraphic unit. The metasediments and diorite have high total porosity and low specific yield indicating that the pore water is retained by electrostatic forces in the medium and it resists flow due to low hydraulic conductivity. The vertical variation in hydraulic conductivity values indicates that the medium is highly heterogeneous.

Возникновение и методы ликвидации открытого фонтанирования морских скважин с подводным расположением устья

The article will be useful to the scientists and specialists in the oil and gas industry involved in the construction of offshore wells and the blowout response. The article provides a brief list of the implemented oil and gas projects on the shelf of the Russian Federation. The principle of two barriers, which must be observed throughout the entire life cycle of the well, is considered. The main reasons for the transition of kick into blowout are considered, and the most well-known cases of blowout of the offshore wells in the industry are given. According to the SINTEF Offshore Blowout database, the worldwide statistics of loss of well control from 2000 to 2015 are given. The reasons for the development of offshore fields with large-diameter wells are described, and the risk of increased complexity in controlling such wells in the event of blowout is noted. The peculiarities of subsea wells blowout are described. As is known, the blowout of a gas well is associated with the creation of a gas-water plume, which makes it difficult or completely deprives vertical access to the wellhead in shallow water during emergency rescue operations. The paper shows a model of gas-water plume in shallow water conditions, and describes the processes occurring when the gas plume floats up and its impact on ships (drilling rigs, supply vessels and etc.). An example is given related to the modeling a rising gas-water plume using computational fluid dynamics (CFD) at various sea depths and flow rates of a blowing well. The methods of well source control are considered, the current schemes for installing an capping stack at the wellhead of a subsea blowing well are described. The advantages and limitations of each well source control method are presented.

DEVELOPMENT OF AIR SUPPLY DEVICE FOR DRILLING WELLS WITH BACKWASHING USING AIRLIFT

In the work, a comparative analysis of the main methods of creating backwash during well drilling was carried out: using a centrifugal pump and using an airlift. A comprehensive research method was used, including a review and synthesis of scientific and patent publications, analytical studies of methods for creating backwash during well drilling. Their advantages and disadvantages are revealed. The greatest attention is paid to drilling of large-diameter wells with backwash, which is widespread in foreign countries in the development of groundwater reserves. The presented materials showed a clear advantage of this method over others, both in terms of the quantity and quality of produced water, and in terms of well operation. It is shown that, under local conditions, the advantages of this method can have a particularly pronounced effect, and its disadvantages will manifest themselves to a minimal extent. The advantages of rotary backflushing over other drilling methods can be considered proven. It will provide a sharp increase in the utilization rate of local groundwater resources, which is the main problem of local groundwater supply. The need for widespread implementation of this method is obvious. The influence of the upward flow rate of the drilling fluid and its density on the maximum depth of drilling with backwash using an airlift has been studied. An improved device of the circulation system for drilling wells with backwash using an airlift has been developed. Thanks to its use, it becomes possible to use commercially available drill pipes with conventional connections. This allows you to increase the drilling speed by reducing the time spent on tripping operations.

UAV-borne LiDAR revolutionizing groundwater level mapping

In hydrogeological research, the systematic and periodic measurement of the piezometric level is fundamental to assess aquifer storage, identify recharge and discharge areas, define flow directions and to infer the balance between inputs and withdrawals. Furthermore, knowledge of this variable and its fluctuations is essential for the efficient management and protection of groundwater resources. In this work, a novel methodology is proposed for the remote acquisition of piezometric information from traditional large-diameter wells, using drone-borne LiDAR observations. The workflow developed consists of different stages, from flight planning and parameter setting, to point cloud generation, data processing and validation and its statistical treatment to extract piezometric information. This methodology has been applied in a small coastal aquifer with numerous wells that have served as monitoring points. The UAV-LiDAR has enabled the straightforward obtention of measurements of the piezometric level with very high vertical accuracies (RMSE of 5 cm) with minimum and maximum residuals of −8.7 and 7.9 cm respectively. Likewise, the method has shown vertical accuracies 3 times better than those inferred from the official DTM of best resolution available in Spain, which is usually used in hydrogeological works. Since the technique provides absolute values of the piezometric level, it eliminates the need for laborious levelling work prior to hydrogeological campaigns. This method has proved to be an effective alternative/complementary technique to traditional measurements of the piezometric level, allowing to monitor extensive or inaccessible areas over short periods of time and to potentially reduce gaps in hydrogeological databases.

Influence of on-site sanitation on groundwater quality from large diameter wells

Field work was conducted to understand the possible influence of on-site sanitation on groundwater quality, estimate minimum bore well volumes of water to be purged from large wells to obtain representative aquifer samples, and unravel possible trends in variability in physico-chemical water quality of water samples from wells lying between the hills and discharge areas. We applied a chemical pollution risk assessment of wells modified from microbial risk assessment to enable the quantification of risk to chemical pollution of wells from on-site sanitation. For two wells, 3.2 well volumes were purged with progressive sampling and physico-chemical quality of purged and recovered water. Physico-chemical quality of samples was analyzed for wells lying between hills and discharge areas. Commonly identified contamination sources are sullage drains, pit latrines, waste dumps and septic tanks, and 14 out of the 15 wells (93%) studied at risk to pollution. Anion concentrations and EC values increased during purging and were stable and high during recovery indicating inflow of water with high ion content possibly from identified risks to chemical anion pollution. Stabilised physical water quality (EC and pH) during recovery indicate representative aquifer samples from large diameter wells in the shallow aquifer of the area may be possible after removing at least three well volumes of water. Apart from Nitrate, average concentrations of anions and EC were high in water samples from wells in the low-lying area. Results of pump test and sampling between the hills and discharge areas, we, respectively, conclude that: the wells are hydraulically connected to contamination sources and the observed groundwater quality may due to effluents from identified contamination sources, and dissolution of minerals and increased infiltration of contaminants may be responsible for high electrical conductivity and anion concentration in the low-lying discharge areas of the study area.

Role of horizontal bores in improving the yield of large diameter wells in low permeability aquifers

Role of horizontal bores in improving the yield of large diameter wells in low permeability aquifers

Classification of hydrogeological structure along the Red River in the Hanoi area

Hanoi city has abundant groundwater, supplemented by the surface water (mainly Red River) all year round, and the extensive shallow aquifers, which are easily exploited by large-diameter wells. There always exists some open hydrogeological windows in the Red River area with an open structure; therefore, the groundwater has a strained hydraulic relationship with the Red River water system Along the Red River from Ba Vi to the end of Phu Xuyen district, there are nine regions with three types and four sub-types of different hydrogeological structures. In particular, the sub-type I-A of the groundwater has a tight hydraulic correlation with the Red River since the hydrogeological structure of the Red River bottom includes three aquifers: Holocene (qh), upper Pleistocene (qp2), and lower Pleistocene (qp1) that constructs a hydraulic system. The sub-type I-B is characterized by the hydrogeological structure at the Red River bottom, including the aquitard in Vinh Phuc and two aquifers qp2 and qp1, which form a hydraulic system. The sub-type II-A is distinguished by the fact that the Red River crosses the aquifer qh; there are no aquitards between the aquifer qh and qp2 to form a hydraulic system; the aquitard separates the aquifer qp1. The sub-type II-B is identified by the fact that the Red River crossing the aquifer qh; there is an aquitard between the aquifer qh and qp2; there are no aquitards between the aquifer qp2 and qp1 so that can create a hydraulic system. Type III has a solid existence of both aquifers and aquitards; thus, the hydraulic relationship between the Red River and the aquifers qp2 versus qp1 is inferior.

Design and Safety Considerations for Coiled Tubing Operations in Geothermal Wells

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30408, “Design and Safety Considerations To Perform Coiled Tubing Operations in Large-Diameter, High-Temperature Geothermal Wells,” by Ishaan Singh, SPE, Danny Aryo Wijoseno, SPE, and Kellen Wolf, Schlumberger, et al., prepared for the 2020 Offshore Technology Conference Asia, originally scheduled to be held in Kuala Lumpur, 17–19 August. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. The productive section in a high-pressure, high-temperature (HP/HT) geothermal Field A in the Philippines features shallow and deep reservoirs separated by a low-permeability formation. However, recent years have seen a reduction in production levels. To activate and enhance well production, coiled tubing (CT) nitrogen-lift operations were required. CT simulations were combined with simulations from the geothermal reservoir to overcome modeling limitations. The outcome helped the design of a new cooling-loop system and allowed optimization of the nitrogen-lift technique. As a result, two large-diameter geothermal wells were lifted safely with 2-in. CT. Introduction This study describes design and safety considerations in performing CT operations in high-temperature, large- diameter geothermal wells. The customized high-temperature-grade seal material was chosen to withstand high bottomhole temperatures (BHT) (600°F), and a heat exchanger riser system was designed and tested on the job to handle high-surface-temperature steam (350–400°F), thus mitigating potential well-control incidents. Challenges of Seal Damage Caused by High Surface Temperatures in Live Well Intervention The CT interventions in quenched HP/HT geothermal wells reduce the risk of surface equipment failure. The seal material readily available in the market is rated to 250°F, but, if quenching is not possible, the high-temperature steam (approximately 350–400°F) may flow into the pressure-control equipment, leading to seal damage and CT contingencies. At high temperatures (400°F), these seals are unusable. It becomes essential to use a surface heat exchange riser (HER) system to prevent this issue. Design and Execution of HER Systems in Field A To avoid any well contingency and to keep pressure-control equipment safe, HER systems can be used. Some basic designs for HERs are described in the complete paper. For this study, a customized 4.06-in. HER cooling system (Design 1, shown in Fig. 1) was designed to accommodate 2-in. CT pipe. Design 1 was chosen from an evaluation of three design candidates outlined in the complete paper. The wellhead stack featured seal elements rated to high temperatures (400°F). To prevent high- temperature steam from entering the wellhead stack, the blowout preventer, and other surface- equipment elements, an efficient HER system was designed wherein, while the CT is still in the well performing CT operations, the cold water can be pumped into the CT-stack annulus from the top flow cross through the cooling riser to the bottom flow cross and back to the return tank. The temperature of the cooling loop was continuously monitored to ensure that it was well below 212°F (the boiling point of water).

Biological and Physical Clogging in Infiltration Wells: Effects of Well Diameter and Gravel Pack.

Gravity-driven infiltration into the shallow subsurface via small-diameter wells (SDWs), i.e., wells with an inner diameter smaller than 7.5cm (3inches) and no gravel pack) has proven to be a cost-efficient and flexible tool for managed aquifer recharge (MAR), as it provides relatively high recharge rates with minimal construction effort. SDWs have a significantly smaller open filter area than larger diameter wells with gravel pack, making the infiltration of low-quality waters through these wells more at risk clogging. To investigate their susceptibility for biological and physical clogging, 24 physical models with different well setups were evaluated by infiltrating either nutrient-poor but turbid water or nutrient-rich but clear water. The experiments showed that smaller diameters and the lack of a gravel pack increase the well's susceptibility to both kinds of clogging. However, this effect was observed to be much more pronounced for physical than for biological clogging. Our conclusion is that SDWs show severe disadvantages with respect to the infiltration of highly turbid waters in comparison to large diameter wells with a gravel pack. Nevertheless, this disadvantage is much less severe when it comes to the infiltration of clear but nutrient-rich waters (e.g., treated wastewater). Depending on the economic and geological circumstances of a MAR-project, this disadvantage could be outweighed by the significantly lower construction costs of SDWs.

Risks involved in vertical shafts construction for the eastern drainage tunnel in Mexico

This article describes the main stages involved in the construction of the vertical shafts (large-diameter vertical wells), which are necessary for the subsequent construction of the tunnel´s sections. The different risk situations existing during the construction of the Eastern Drainage Tunnel in the valley of Mexico City (in Spanish, "Tunel Emisor Oriente") are analyzed. In order for this 52 km-long and 7.5 m-wide tunnel to carry part of the city’s sewage, 25 shafts must first be built, ranging from 55 to 150 meters deep. The magnitude of such a project implies working in different geographical areas and varied geological strata involving the presence of groundwater, which increases the risks due to possible landslides or flooding during excavation. As digging will occur in different types of soil, varying procedures must be used depending on soil type. Likewise, due to the magnitude of this kind of project, detailed scheduling and planning are required as simultaneous works on different fronts are necessary to meet deadlines. The study mentions that, while projects like these involve high risks for workers, analysis of activities and situations are conducted precisely to demonstrate that such risks can be considerably reduced.

Quantifying Focused Groundwater Recharge Induced by Irrigation Surface Water Reservoirs in Crystalline Basement Areas for Complementary Irrigation

Through the practice of irrigation, surface water reservoirs (SWRs) contribute to the socio-economic development and food production activities of populations in Sub-Saharan Africa (SSA). However, they tend to dry up prematurely. One solution to circumvent these irrigation water shortages is to ensure their conjunctive use with groundwater. The objective of this study is to better understand the contribution of SWRs to groundwater recharge and to determine if groundwater may be considered as a complementary local resource for irrigation. The study was carried out on two watersheds in Burkina Faso, Kierma and Mogtedo. The spatiotemporal analysis of piezometric and SWRs level records coupled with physico-chemical analyses of water was used to characterize exchanges between SWRs and groundwater. The regional groundwater recharge at the scale of the watersheds was assessed. At the SWRs scale, a water balance methodology was developed and used to estimate focused recharge. The results show that SWRs interact almost continuously with groundwater and contribute focused recharge. The magnitude of this recharge is a function of the geological context and the sediment texture of the SWRs. It is estimated at 5 mm/day in Kierma and 4 mm/day in Mogtédo. These values are higher than the natural recharge estimated at 0.2 mm/day in Kierma and 0.1 mm/day in Mogtédo. Additionally, the values of hydraulic conductivity are between 0.01 and 2 m/day in Kierma and between 1 × 10−4 and 0.2 m/day in Mogtédo. These conductivities could allow pumping in large-diameter hand-dug wells with a significant yield between 0.5 and 120 m3/day in Kierma and between 0 and 10 m3/day in Mogtédo to palliate the early drying up of the SWRs.

Status of the Use of Groundwater during the Dry Season at 5 and 7 Blocks of the Irrigated Rice Plain of Bama, Burkina Faso

Water insufficiency in the irrigation canals during the dry season in the rice-growing plain of Bama means that barely 20% of the plots is cultivated with rice during this period, causing losses of over 1,500,000. US $ per year. So, groundwater is used as an alternative by some producers without control. This study aims to assess the current use of groundwater for off-season production in blocks 5 and 7 of the Bama plain. The diagnosis was made during the 2019 dry season by means of focus group, then an individual survey in order to inventory the sinks and speculations produced. Then, around twenty large-diameter rice irrigation wells were followed to determine the volumes of water supplied. The results showed that, the small wells are mainly intended for manual irrigation of market gardening during the cold dry season and large-diameter wells for pumping rice cultivation in the hot dry season. Finally, over-irrigation of more than 2.5% of rice needs was observed.

Contradictive mining–induced geocatastrophic events at open pit coal mines: the case of Amintaio coal mine, West Macedonia, Greece

For open pit coal mines, subsidence of the surrounding area and landslides are among the most common mining-induced catastrophic geohazards. These large-scale geohazards are related to both hydrogeological and geotechnical parameters, and they cause substantial damage. A major challenge to managing these hazards is that the factors that trigger these two phenomena are diametrically opposed. Land subsidence is triggered by the overexploitation of the aquifers conducted for the protection of the slope’s stability. Thus, the reduction of groundwater pumping from the draining wells reduces land subsidence deformations but at the same time decreases the stability of the slopes. In the present study, the investigated site is the area at the perimeter of the Amyntaio opencast coal mine at Florina Prefecture, Northern Greece. The overexploitation of the aquifers for the dewatering of the slopes turned the mine into a large-diameter well. The land subsidence caused by the dewatering extended 1–3 km around the mine, affecting the villages of Anargiroi and Valtonera. Furthermore, on June 10, 2017, a massive landslide of approximately 80 million cubic meters occurred at the working slopes of the mine, burying 25 million tons of lignite, resulting in severe damage to a large amount of mining equipment and causing the evacuation of the nearby Anargyroi village. The geometry of the slopes, the preexisting sheared zones, the extensive faults intersecting the site, and the high piezometric loads underneath the floor of the open pit as well as the high groundwater level of the shallow aquifers are the main causal factors of the landslide. The current research, besides presenting the two catastrophic events, aims to correlate the geological, hydrogeological, and geotechnical factors affecting both phenomena, outlining their opposing mechanisms. The study proves the necessity of the establishment of a holistic development plan supported by focused geotechnical and hydrogeological studies. This way, the occurrence and the diametric drivers of both types of catastrophic events can be evaluated and predicted, minimizing their effects on the mining activities and the surrounding environment.

Analysis of growth drive breakdown by the increase of the Dashav UGS fountain pipes

Scientists who were analyzing the results of the underground gas storage (UGS) complex of Ukraine, say that for modern conditions the maximum daily productivity should be increased by 30-35%. Since the beginning of the creation of gas storage facilities passed than 40 years, work is carried on outdated and physically worn out equipment. Work of the wells (which were build based on the old technologies at low reservoir pressure) are characterized by low productivity because their bottomhole zone is partially occluded by the mud filtrate, partially contaminated with compressor oil; due to the selective placement of wells in the underground storage area, some of them were formed so-called "stagnant" zones; extraction of all active gas volume which takes 100-180 days. Need for Ukraine to further develop underground gas storage should be aimed at increasing both the active volume of gas and the daily productivity of gas storage facilities. The main directions of increasing the productivity of gas storage are the drilling of horizontal shafts in both new wells and old ones in experience [9], the formation of open holes in the drilling of new wells and the milling of operational columns in the "old" drilled holes, filters, hydraulic fracturing, construction of large diameter wells, additional perforation of gas-saturated intervals. Nowadays, the most important problem is the reconstruction and modernization of existing underground storage facilities through replacement of equipment. This will not only save the achieved performance, but also can improve it. Due to the reconstruction of the equipment, gas preparation will be improved, gas storage capacity will be increased and operating costs will be reduced. Therefore, an increase in the daily productivity of UGS can be achieved, first of all, due to the improvement of the main element in the gas storage facility - the operational well. In order to analyze this issue, the paper analyzes the efficiency of replacing the fountain pipes of the Dashavsky UGS wells with a larger diameter, and presents the results of the hydraulic, technical and economic calculation of the implementation of this UGS reconstruction.

Dewatering–Induced Building Settlement around a Deep Excavation in Soft Deposit in Tianjin, China

This case study presents an investigation on building settlement induced by dewatering for a deep excavation constructed in a deposit in Tianjin, China. The excavation site is located in the Tianjin urban center, around which there are many existing buildings. Geological conditions at the site involve a multiaquifer system with one phreatic aquifer over three confined aquifers. To maintain the stability of the bottom of the excavation, a cutoff wall was used to block groundwater seepage with large diameter wells to lower the groundwater level. Field instrumentation was installed during construction to monitor (1) the quantity of withdrawn groundwater, (2) the groundwater head, and (3) the settlement of surrounding buildings. The field observations yielded the following results and implications: (1) pumping of groundwater could lead to large building settlement and a large area of ground settlement; (2) most of the settlement caused by dewatering did not rebound; (3) the cutoff wall played a significant role in reducing the building settlement during dewatering; and (4) the dewatering-on-demand procedure should be implemented during excavation.

Numerical investigation of small- and medium-diameter drain wells to stabilise deep landslides

The stabilisation of deep landslides is still a challenge for geotechnical engineers, mainly due to the high cost of structural control works that generally lead engineers to opt for drain systems. Unfortunately there are operative difficulties affecting excavation of deep drain trenches in soils dislocated by landslides (especially as regards the provisional support of the excavation walls). Small- and medium-diameter wells are a valid alternative in deep landslides, being less expensive than large-diameter wells equipped with horizontal pipes and simpler to construct than deep trenches. However, no technical procedures for their proper design are reported in the literature: design criteria commonly used in practice are derived from pumping problems and are largely inadequate, being based on Dupuit’s hypothesis that presumes stream lines to be horizontal. This paper presents numerical solutions obtained from three-dimensional seepage and consolidation analyses of a slope drained by small- and medium-diameter wells, arranged according to various positions, showing that the efficiency of the well system decreases as the depth of the landslide slip surface increases. Comparison is also made between well systems and deep trenches occupying the same draining volume in the landslide body, showing that the former attains much greater efficiency than the latter. Non-dimensional abaci for the square grid of wells (which is the most effective arrangement) are reported herein to be used for practical purposes.

The impact of urban development on aquifers in large coastal cities of West Africa: Present status and future challenges

This paper investigates the coastal cities of Abidjan, Cotonou, Lagos and Douala in West Africa. Published data on these areas were aggregated in order to compare the urban development of some African huge cities and assess their impacts on groundwater. Those urban centers have experienced an exponential demographic expansion since the 1950s, with increased population densities and a geographical coverage expansion as well. The Continental Terminal aquifer, major groundwater resource taped in this region by the national water companies and local populations, shows a continuous downward trend in piezometric levels. Concerning water quality, the evolution up to the current state (saline intrusion, nitrate pollution) and the natural geochemical process (dilution, redox reactions) affecting the aquifer have been highlighted. The results confirm the urgent need to consider groundwater development relatively to demographic and economic growth. Some management approaches have been proposed including monitoring of contamination, protection of the resource and the use of shallow large-diameter wells, which have proved to be less saline and more sustainable than deeper small-diameter boreholes. The results and discussion of this paper have provided a considerable new insight of West African coastal cities. This will help stakeholders involved in local development to face the urban pressure.

SlugIn 1.0: A Free Tool for Automated Slug Test Analysis.

The correct characterization of aquifer parameters is essential for water-supply and water-quality investigations. Slug tests are widely used for these purposes. While free software is available to interpret slug tests, some codes are not user-friendly, or do not include a wide range of methods to interpret the results, or do not include automatic, inverse solutions to the test data. The private sector has also generated several good programs to interpret slug test data, but they are not free of charge. The computer program SlugIn 1.0 is available online for free download, and is demonstrated to aid in the analysis of slug tests to estimate hydraulic parameters. The program provides an easy-to-use Graphical User Interface. SlugIn 1.0 incorporates automated parameter estimation and facilitates the visualization of several interpretations of the same test. It incorporates solutions for confined and unconfined aquifers, partially penetrating wells, skin effects, shape factor, anisotropy, high hydraulic conductivity formations and the Mace test for large-diameter wells. It is available in English and Spanish and can be downloaded from the web site of the Geological Survey of Spain. Two field examples are presented to illustrate how the software operates.

Slim hole drilling and testing strategies

The financial and geologic advantages of drilling slim holes instead of large production wells in the early stages of geothermal reservoir assessment has been understood for many years. However, the practice has not been fully embraced by geothermal developers. We believe that the reason for this is that there is a poor understanding of testing and reservoir analysis that can be conducted in slim holes. In addition to reservoir engineering information, coring through the cap rock and into the reservoir provides important data for designing subsequent production well drilling and completion. Core drilling requires significantly less mud volume than conventional rotary drilling, and it is typically not necessary to cure lost circulation zones (LCZ). LCZs should be tested by either production or injection methods as they are encountered. The testing methodologies are similar to those conducted on large-diameter wells; although produced and/or injected fluid volumes are much less. Pressure, temperature and spinner (PTS) surveys in slim holes under static conditions can used to characterize temperature and pressure distribution in the geothermal reservoir. In many cases it is possible to discharge slim holes and obtain fluid samples to delineate the geochemical properties of the reservoir fluid. Also in the latter case, drawdown and buildup data obtained using a downhole pressure tool can be employed to determine formation transmissivity and well properties. Even if it proves difficult to discharge a slim hole, an injection test can be performed to obtain formation transmissivity. Given the discharge (or injection) data from a slimhole, discharge properties of a large-diameter well can be inferred using wellbore modeling. Finally, slim hole data (pressure, temperature, transmissivity, fluid properties) together with reservoir simulation can help predict the ability of the geothermal reservoir to sustain power production.