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.

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