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On 26 November 2009, a 2.8-magnitude earthquake (henceforth referred to in this paper as the De Hoeve tremor) occurred in southwestern Friesland in the Netherlands at an approximate depth of 2 km. This relatively shallow depth indicated that the event was induced. The closest mining activities were believed to be in the Weststellingwerf field. A study was required to determine the origin of the tremor, evaluate if it could be followed by other tremors in the future, and estimate its magnitude. The tremor's epicenter was identified by the KNMI (Koninklijk Nederlands Meteorologisch Instituut: Royal Netherlands Meteorological Institute) (Figure 1).
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 166430, ’Inducing Earthquake by Injecting Water in a Gas Field: Water- Weakening Effect,’ by Axel-Pierre Bois and Mehrdokht Mohajerani, CurisTec; Niek Dousi, SGS-Horizon; and Stijn Harms, Vermilion Energy, prepared for the 2013 SPE Annual Technical Conference and Exhibition, New Orleans, 30 September-2 October. The paper has not been peer reviewed. On 26 November 2009, a 2.8-magnitude earthquake (henceforth referred to in this paper as the De Hoeve tremor) occurred in southwestern Friesland in the Netherlands at an approximate depth of 2 km. This relatively shallow depth indicated that the event was induced. The closest mining activities were believed to be in the Weststellingwerf field. A study was required to determine the origin of the tremor, evaluate if it could be followed by other tremors in the future, and estimate its magnitude. Introduction The tremor’s epicenter was identified by the KNMI (Koninklijk Nederlands Meteorologisch Instituut: Royal Netherlands Meteorological Institute) (Fig. 1). Three wells are located near the epicenter: Weststellingwerf-01 (WSF-01), Noordwolde-01 (NWD-01), and De Hoeve-01 (DHV-01). They all are producing from formations at a depth of approximately 2 km. Well WSF-01 was a gas producer between 1998 and 2004; cold-water injection started in 2008 with no tremor reported during the gas-production phase. Well NWD-01 is immediately outside of the epicenter perimeter. It was producing gas at the time of the event, with no tremor measured before November 2009 and none after. Well DHV-01 is located in the middle of the estimated epicenter area, but had not been drilled at the time of the event. The observed tremor, because of its shallow depth, was assumed to be induced instead of having occurred naturally. It was attributed by KNMI to displacements along the Weststellingwerf fault because the seismograms were in agreement with a normal west/eaststriking fault. It was thus necessary to identify which mechanism was at the origin of the fault movement, to investigate whether an earthquake could occur in the future, and to determine the magnitude of the event. Five mechanisms were studied: Salt dissolution Reservoir depletion during gas production Reservoir repressurization caused by water injection Temperature variations caused by cold-water injection Fault weakening caused by lubrication or annihilation of capillary pressure during water injection However, salt dissolution during water injection was quickly discarded as being at the origin of the tremor because the reservoir intervals do not contain halite. Carbonate dissolution was not pronounced and could not explain fault weakening caused by water injection, either.
Abstract On November 26, 2009 a 2.8-magnitude earthquake occurred in southwestern Friesland in the Netherlands at an approximate depth of 2 km. This relatively shallow depth indicated that the event was induced, instead of being natural. Closest mining activities were supposed to be the Weststellingwerf field with well WSF-01 and the exploration well DHV-01. The event was named De Hoeve. However, production from DHV-01 started after the event was occurred. Thus, a study was required to determine the origin of the tremor, evaluate if it could be followed by other tremors in the future, and estimate its magnitude. The objective of this paper is to present the study that was conducted in order to identify whether the gas production and water injection in well WSF-01 could have caused the so-called Weststellingwerf fault to slip leading to the earthquake as observed in November 2009. The following work plan was devised: 1) Perform seismic interpretation; 2) Construct fit-for-purpose geological model of Weststellingwerf area; 3) Construct reservoir simulation model; 4) History match production data of two wells and record pressure and temperature distribution as a function of time; and 5) Build 2-D geo-mechanical model, feed geological data and structure into model, feed pressure and temperature from dynamic model into geo-mechanical model, assign reasonable geo-mechanical properties, perform stress calculations at critical time steps, and determine potential slip displacement along Weststellingwerf fault. Five triggering mechanisms were tested: 1) Salt dissolution; 2) Pressure depletion during gas production; 3) Re-pressurization due to water injection; 4) Thermal stresses due to cold water injection; and 5) Fault weakening due to water injection (chemical alteration) Geo-mechanical simulations showed the causes of the earthquake should be looked for, not in stress variations along Weststellingwerf fault, but instead in a fault weakening upon first contact with water. Two mechanisms can be at the origin of this reduction: fault lubrication or decrease in capillary pressure in the fault gouge material when it was first contacted by water. The simulations also showed that no earthquake should occur in the section of the fault where displacements occurred during de Hoeve tremor. Sections that did not show any displacement could be at the origin of new earthquakes when first contacted by water, but probably of lower magnitude.
Abstract The potential for the exploration and exploitation of geothermal energy for greenhouses in Franekeradeel in the north of the Netherlands has been investigated. Available borehole and seismic data have been used for the evaluation of the subsurface. These data show that the Slochteren Formation sandstone reservoirs are very likely present at a depth of ca 3,000 m. These reservoirs are deemed to be suitable for the production of hot water. From interpreted 2D seismic lines could be inferred that the Slochteren Formation is around 200 m thick at the proposed location. Based on the local geothermal gradient, the temperature at the top is estimated to be around 100 ºC. At the surface location a fault zone is present in the subsurface complicating the siting of the injection and production wells. Towards the northeast the proposed reservoir is seemingly less faulted. A minimum transmissivity of 11 Dm is expected, based on the measured porosity in nearby boreholes and porosity-permeability relationships in the surrounding area. A potential flow rate of 160 m3/h is hence inferred, leading to a thermal capacity of 11 MWt which can be delivered with one doublet. Different well configurations were studied. The conclusion is that deviated wells from the edge of the surface location towards the northeast carry the lowest risk at still acceptable costs. From financial analyses taking different options into account, it was concluded that geothermal energy can deliver significant amounts of renewable energy for heating of greenhouses. In the optimum situation, the cost price of heat amounts to 6,20 Euro/GJ delivering annual 202 TJ of energy. This price is below the current price (January 2008) of natural gas at 0,20 Euro/m. Introduction In Franekeradeel, the province Friesland in the northern Nertherlands, near the village of Sexbierum an expansion of greenhouses is foreseen for the nearby future. The ambition of the regional government is a reduction in the water- and energy-consumption compared to exisiting greenhouses. One of the options to reduce the amount of fossil fuels is to use geothermal energy for the heating of the greenhouses. Within this paper, the opportunities for geothermal energy are discribed. Geologically speaking, the area is located within the Northwest European basin and its history has been influenced by sedimentation rates, tectonic phases and sea level changes. The oldest penetrated sediments in the area are of Carboniferous age. The geology of the area consists of different types of sediments with unconformities and NW-SE oriented faults. An overview of the different formations, their depth, thickness and temperature are given in table 1. The reservoir temperatures have been estimated by uncorrected bottomhole temperatures, from which the geothermal gradient could be estimated. The number of log-runs in the wells was insufficient to correct bottomhole values to the real temperature of the reservoirs, leading to an underestimation of gradients.
Abstract Besides curing wax, hydrate or viscous flow problems, thermal insulation should also improve lift performance and producing time of wells and risers, since it helps to reduce by various ways the liquid content in a gaseous column of production. "Thermogelf" project objectives were to prove benefits of thermal insulation on well performance and to test one specific method to set a super insulation (an aerogel) in a producing well, avoiding a work-over. Thus, a silica aerogel has been processed into a self-killing gas well in the Netherlands, in autumn 1999. Although the process was not completed, because not so fast as expected, this field trial enhanced significantly the well production, and produced a decisive experience to improve the implementation method. Therefore, this project demonstrated that "Thermolift" methods could be lucrative on numerous mature and new fields, and aerogel processing in the annulus of wells should be next developed as a relevant technology. Introduction Production problems due to wax, hydrates or viscous oily mixtures are well-known as depending on the fluid temperature but, although numerous natural producing wells suffer from self-killing, no effective attention seems usually paid about effects of heat on lift performance. Having for long time to solve such problems in field operations and aware of their relation to energy, Total Fina Elf teams have been working on heat management throughout the production chain, since 1993. They developed an innovative concept, the "Thermolift", for well performance engineering, which takes advantage of heat to enhance production and recovery of wells and risers. Then, searching for cost-effective methods to insulate wells and risers efficiently, recent advance in aerogel processing has allowed Total Fina Elf and partners developing new methods to reach low thermal conductivity in annuli. Resulting from these works, the "Thermogelf" project had two objectives: to demonstrate benefits of thermal insulation on well performance, and to test an innovative method to highly insulate a well without work-over, processing a silica aerogel directly into the annular space, between the tubing and the casing. The NGA6 (Nijega-6) gas well selected for that demonstration is producing from a depleted sandstone reservoir of the onshore Nijega field, operated by Total Fina Elf E&P Nederland B. V., in the onshore Friesland, North of the Netherlands. This well self-killed periodically only due to condensed water. Numeric simulations of the well performance predicted that it would be possible to avoid self-killing by using an aerogel in the tubing-casing annulus. But not by using a wet gel, because of still too conductive. Expected benefits with an aerogel were a 15% increase of gas rate and at least a one-year postponing of self-killing problems (not more because of further reservoir depletion). The Thermogelf Project was operated by Total Fina Elf, Halliburton b.v., Air Liquide and PCAS (chemicals), with strong expertise and skill from Atochem(Atofina). In addition, this innovative project obtained funding from the European Union ("Thermie" Program). Heat and well performance Performance of wells is commonly engineered by analysing the nodal relation linking inflow performance relationship, said IPR, and the vertical lift performance outflow curve (VLP).
Abstract To date, several wells have been drilled in the Friesland/Groningen area of northern Holland. Historically, catastrophic cutter damage/failure in the 12 1/4" Triassic section resulted in inconsistent and unacceptable bit performance. Here, the Triassic is an interbedded formation containing hard dolomite and anhydrite stringers and abrasive sands. The transition from a softer to harder formation at high inclination evidently initiated bit and drillstring vibrations, thus causing premature cutter failure of the PDC bits used in the program. This paper presents a detailed analysis of the 12 1/4" section conducted after the drilling of Well No. 1. The recommendations arising from the study concentrated on the use of a real-time downhole vibration monitoring or VSSMWD tool (Vibration Stick Slip) in combination with revised drilling practices to reduce vibrations when encountering the interbedded formation. Thus, on Well No, 2, nearly all of the 2358-m Rijnland and the hard and abrasive Triassic forrntions were drilled with one PDC bit on a rotary assembly, where previously six bits were required. The authors will illustrate the benefits of a detailed interval analysis and discuss the role of the VSS system and the dring crew in addressing catastrophic drillstring and bit vibrations. P. 925