Evaluation of fracture geometries in rock formations

Abstract

A method of evaluating the geometry of a hydraulic fracture in a rock formation comprises the steps of: obtaining measured values of electric and / or magnetic fields induced by the forward or back propagation of a fracturing fluid between the fracture and the rock formation; and determining the geometry of the fracture from the measured values.

Description

FIELD OF THE INVENTION The present invention relates to a method for evaluating the shape or geometry of hydraulic fractures in rock formations. It can be advantageously applied to determine the shapes of fractures surrounding oil well boreholes. BACKGROUND OF THE INVENTION Hydraulic fracturing is generally used to stimulate production of hydrocarbons from hydrocarbon wells. Hydraulic fractures are created in subterranean formations by injecting high viscosity fluid (also referred to as fracturing fluid) at a high flow rate into well boreholes. The tensile fractures thus-created can be about 100 m long. The fracturing procedure generally takes from about 30 minutes to 4 hours. In order to create a high conductivity drain in the formation, the fracturing fluid usually contains proppants, small particles which are added to the fluid to keep the fracture open once the injection is stopped and pressure is released. These particles can be sand grain or ceramic grains. The width of the...

AI Technical Summary

Benefits of technology

wherein the geometry of the fracture is determined by adjusting the model to minimize the differences between the expected and measured values. In relation to the particular embodiments mentioned above, the method may further comprise: providing a forward model of electric and / or magnetic field distributions for a given fracture shape and injection pressure by calculating such distributions as a function of a measurement position and / or time; measuring downhole the electric and / or magnetic fields at the same positions and / or times as used for providing the forward model to provide observed electric and / or magnetic field distributions; and evaluating the shape of fractures in the rock formations by minimizing errors between the forward model distribution(s) and the observed distribution(s).

In relation to the apparatus embodiments, the or each downhole tool may be movable along the borehole. The means for injecting the given fluid may be a surface pump. The apparatus itself may further comprise: at least one memory unit for storing expected values of the electric and / or magnetic fields for a given fracture shape and injection pressure according to a measurement position and / or time; means for evaluating the location of the downhole tool in the borehole; at least one processing unit for selecting electric and / or magnetic field values measured by the downhole tool at the positions and / or times for which the values are stored in the memory unit, and for minimizing errors between the stored and selected values; and means for outputting data on the evaluated shape of the fractures in the rock formation.

Problems solved by technology

This approach provides, however, only very general information about fracture length and fracture width and does not provide any information about the exact fracture geometry.

However, they do not provide direct quantitative information on the main fracture.

The approach is commonly used in the field and is particularly suited for the estimation of fracture azimuth and dip, but not for an accurate determination of the position of the fracture tip.

Another disadvantage of the approach is that the micro-earthquakes are spread around the fracture and produce a cloud of events, which do not allow a precise determination of fracture geometry.

Disadvantages of this technique are first of all in that surface tiltmeters cannot accurately resolve fracture length and height due to the depth of the fracture below the surface.

Although downhole tiltmeters placed in the treatment borehole can provide better information on fracture height they still cannot resolve fracture length.

Images