Infiltration Point Detection

The application is a national stage application of International Application No. PCT/EP2023/069718, which was filed Jul. 14, 2023, which claims priority to Netherlands Application Number 2032573 filed on Jul. 22, 2022, both of which are incorporated by reference in their entireties.

The present invention relates to a method for detection of infiltration points in the soil of the ground. In particular, the present invention relates to a method of determining a vertical infiltration profile over a depth interval of the ground, and to determining well capacity of the soil at the respective infiltration points. Unlocking insights from Geo-Data, the present invention further relates to improvements in sustainability and environmental developments: together we create a safe and liveable world.

Infiltration of liquid into the ground may occur both naturally, e.g. due to precipitation, or initiated by human activity. It finds various applications, such as fresh water storage, remediation of contaminated or polluted soil, return or recirculation of water which has been pumped away from the ground during construction works, e.g. during construction works involving a lowering of the ground water level, etc. Fresh water storage and fast removal of excess precipitation water or water resulting from flooding may increase in importance as climate changes, leading to periods of draught alternated with heavy rainfalls.

However, the ability to perform infiltration into the ground, i.e., the degree to which ground, or the soil thereof, is able to receive infiltration liquid, depends on various parameters, including the permeability of the ground and the infiltration rate, i.e., the pressure and/or flow rate of the infiltration liquid.

The permeability of the ground, in particular various layers of the soil, influences various properties of the ground, such as the infiltration capacity of the ground. This may vary throughout the different layers of the subsurface, and may hence be different at different depths throughout the ground.

Infiltration points are conventionally determined through trial and error, e.g., by using a ‘Düsensauginfiltration’ (DSI) technique (‘jet suction infiltration’ in English), which involves pushing an injection probe into the ground, while liquid at a set pressure, and pushing the probe down into the ground until infiltration is observed. However, injecting liquid at too high pressure, or too high flow rate, may lead to various types of disruption or distortion of the ground, such as matrix failure, hydraulic failure, and/or fluidization/liquefaction, in particular in and around the injection probe aperture and at the infiltration points of the ground. This may lead to the intended infiltration well becoming unsuitable for use, and the process having to be repeated at a different location.

To mention an example, for water remediation processes, conventionally infiltration points may be searched for by measuring injection flow rate and associated pressure response during well installation. However, this does not enable prediction of matrix failures, which may therefore occur during the well installation. If matrix failure occurs during well installation, water will not be infiltrated into the ground but move up to the surface, and the remediation process will not be efficient. Even more, well installation might have to be started anew at a new position.

Attempts have been made to predict, or estimate, various soil parameters relevant for infiltration, such as infiltration flow rate, infiltration cone volume, etc., from permeability values of the ground, wherein the permeability values are assumed to be either previously known or to be determined from measurements.

However, the conventional techniques described above do not allow for estimating or predicting the location of infiltration points, i.e., the depth at which infiltration points occur, nor the vertical infiltration profile of the ground. Consequently, the conventional methods do not allow for predicting matrix failure, nor to properly estimate well capacity.

Such lack of estimation of infiltration points leads to a detrimental effect on the accuracy and resulting uncertainty in asset design, such as wells. If matrix failure occurs, the resulting inefficient remediation process or new well installation leads to extra time and efforts being required. This leads to prolonged exposure of personnel to hazardous surroundings and negatively affects sustainability. There is thus a need for a solution to the abovementioned problems.

It is an object of the invention to provide a method of detecting or determining the position of infiltration points in the ground, which method reduces the risks of damages and/or disruption of the ground caused by the measurements processes used for detecting the infiltration points.

The term “infiltration point” for use in the invention is defined as a location, or depth range, at the surface within the ground (deeper than the surface) which is more permeable than its surroundings, and therefore such a location or depth range can take up a certain amount of pressure.

The term “infiltration location” for use in the invention is defined as the point at the surface at which one or more infiltration points vertically aligned penetrates the ground.

The terms ground, soil, and subsurface are used alternatingly throughout the application, denote the material or substance of a formation.

The methods described herein find application, in particular, in the design of injection and/or extraction wells. Injection and/or extraction wells may generally be formed by a substantially solid tubing extending through a well bore in the ground, the solid tubing interrupted by one or more filter regions positioned at infiltration points, such as to enable the injection, or extraction, of liquid into, or out of, the ground. Such wells may, for example, be used for reception of excessive rain water, fresh water reserves, storage of water having been pumped e.g. from construction sites or drained from fields, for example sport fields.

During Cone Penetration Testing, CPT, a probe is pushed into the ground while measuring the tip resistance as it is pushed into the ground and through the various subsurface layers over the depth interval, or penetration depth. This may be measured, e.g., by force sensors, strain gauges or piezo elements provided on or in the CPT probe, which measure forces relating to the tip resistance, the local friction at the probe, and friction ratios.

During Hydraulic Profiling Tool (HPT) measurements, a probe is pushed into the ground while injecting fluid from one or more openings provided on the probe and measuring the pressure response of the ground using one or more pressure sensors provided on the probe. The Hydraulic Profiling Tool (HPT) is used to measure the volume of flow and pressure required to inject water into the soil. The results are called HPT logs or HPT data. The liquid is generally injected at a preset, substantially constant flow rate. In certain applications, one or more additional probes may be provided, located at a lateral distance from the probe and pushed into the ground substantially simultaneously. The one or more additional probes comprise one or more additional pressure sensors, enabling determination of parameters related to the soil, such as relative permeability values, in three dimensions over the infiltration volume.

CPT probing and HPT measurements may be performed in conjunction, referred to as CPT-HPT measurements, using one single probe, which may be referred to as a HPT probe or CPT-HPT probe.

Alternatively, or additionally, during CPT-HPT measurements the probe may be stopped at one or more (set) penetration depths and pressure testing performed at these depths. For example, by performing a plurality, or series, of pressure tests at stepwise increasing flow rates at each penetration depth. A liquid, such as water, is injected into the ground, at each one of the one or more penetration depths, and the resulting pressure response of the soil is measured. Additionally, the injection flow rate may be measured. This may be referred to as mini pumping tests, MPT.

From the HPT and/or the MPT measurements, various parameters, including the permeability of the soil of the subsurface layers, can be determined.

The CPT, HPT and MPT measurements may be performed as described in WO 2017/222372 A1 and US 2021/0003492 A1.

In particular, it is an object of the invention to detect potential infiltration points over a depth interval of the ground.

This is achieved by a method as defined in claim.

Embodiments of the invention are claimed in dependent claims.

According to a first aspect, a method of detecting one or more infiltration points in ground is provided, the method comprising:

Infiltration points may be determined using a DSI technique which involves pushing an injection probe into the ground until infiltration is observed, while subtracting liquid near the groundwater level and injecting the subtracted liquid at a set pressure rate into the same borehole, but at a greater depth. That is, at an infiltration point, the ground is capable of receiving the injected liquid at the applied injection flow rate. Further, at these points, no ground disruption should occur during liquid injection.

The method of the invention is hence based on concept of detecting infiltration points from a combined, or conjunct, analysis of CPT measurement data and HPT measurement data, and a realization that infiltration points can be identified as locations within the ground, or portions along the depth interval, at which the ground is able to receive the injected liquid, while there being no ground disruption, or limited with respect to its surroundings.

Herein, CPT and HPT data may be obtained according to methods known in the art. For example, the CPT and the HPT measurement data may be obtained by a combined CPT-HPT probing using a CPT-HPT probe. Alternatively, one or both sets of measurement data may be provided as previously known data.

By combining CPT measurement data and HPT measurement data, insight of the ground properties is improved, leading to gain an insight of the likelihood of disruption, thereby well(s) installation is optimized. Indeed, the method according to the present invention prevents that liquid is injected incontrollably, for example at too high pressure, or too high flow rate and thus prevents various types of disruption or distortion of the ground, such as matrix failure, hydraulic failure, and/or fluidization/liquefaction, in particular in and around the injection probe aperture and at the infiltration points of the ground. Furthermore, the invention prevents overlooking infiltration points due to an oversight.

By the method as described herein, potential infiltration points can be detected, e.g. during injection well design, while reducing the risk of ground disruption or even well explosion, also commonly designated as blowout.

The permeability of the ground, in particular various layers of the soil, influences various properties of the ground, such as the possibility or capacity of infiltration liquid into the ground.

However, often the permeability of the subsurface varies throughout different layers in the subsurface, which influences the ability of the ground, i.e., the soil, to receive the infiltration liquid. If liquid is injected with too high pressure at a depth where the permeability of the ground does not allow infiltration, the infiltration liquid will not be received by the soil, but will flow up through the well to the surface of the ground.

The pressure applied during injection will also influence the soil. The increase in water pressure in the ground caused by the injection of water reduces the effective stress in a formation, which can lead to various types of ground disruption. Typically, during DSI, a pressure of 1 bar, or on the order of magnitude of 1 bar, is used. If injecting liquid at this pressure outside of an infiltration point, i.e., at depths where the ground is not able to receive injected liquid, ground disruption, such as matrix failure, hydraulic fracturing, or even fluidization, may occur around the point of injection. If this happens during installation of an infiltration well, the infiltration well becomes unusable and a new well may need to be installed.

As will be discussed in detail further below, from CPT data various types of ground disruption, such as matrix failure, hydraulic fracturing, and fluidization, can be predicted.

The method may further comprise the steps of:

The corrected overpressure is the pressure measured during HPT probing, corrected for groundwater level. That is, it represents the pressure that is due to the HPT injection.

The increase in water pressure, which occurs in response to the injection of water into the ground, reduces the effective stress in a formation and may lead to ground disruption. Three different types of ground disruption can be distinguished: matrix failure, hydraulic fracturing, and fluidization or liquefaction. These processes are dependent on, e.g., the increase in water pressure as a result of injection, the overload (i.e., the depth), the effective horizontal stress and the effective vertical stress. These types of ground disruptions, and theory behind them, are known in the art. A summary of theory behind these, as well as equations defining where these failures or disruptions occur, are known, e.g., from Chapter 13 ofF. C. Payne, J. A. Quinman, and S. T. Potter, CRC Press, 2008.

Hydraulic fracturing occurs when the horizontal effective stress of the ground matrix is reduced to zero due to the injection pressure. During hydraulic fracturing, the water pressure is increased by an increase in water pressure which is equal to the effective horizontal stress. Hydraulic fracturing may lead to the uncontrolled development of preferred flow paths and fracturing/cracking.

Mathematically, the onset of the risk of hydraulic fracturing can be expressed as

wherein σ′h is the effective horizontal stress, σ′hi is the initial effective horizontal stress, and Aou is the increase in water pressure due to injection.

According to the present invention, it has been observed that the initial effective horizontal stress, σ′hi, can be obtained from CPT data, and the increase in pressure, Δσu, caused by the liquid injection, can be obtained from HPT data.

Fluidization/liquefaction occurs when the vertical and horizontal effective stress of the ground matrix are reduced to zero by the applied injection water pressure reaching a value where the overload of the matrix is exceeded. Fluidization/liquefaction hence occurs when the water pressure is increased by an increase in water pressure which is equal to the effective vertical stress:

wherein σ′v is the effective vertical stress, σ′vi is the initial effective vertical stress, and Δσu is the increase in water pressure due to injection.

According to the present invention, it has been observed that the initial effective vertical stress, σ′vi, can be obtained from CPT data, and the increase in pressure can be obtained from HPT data.

Prior to hydraulic fracturing or fluidization/liquefaction, matrix failure occurs. Matrix failure is initiated when the horizontal effective stress is reduced to a level when the ratio of vertical and horizontal effective stress reaches a critical limit. Furthermore, at limited overload (depth) and/or poor well design, water injection may lead to short cuts.

Therefore, differently expressed, the critical limit of applied water pressure caused by injection, at which matrix failure may occur, can be expressed as

wherein Ø is the angle of internal friction (expressed in degrees), σ′vi is the initial effective vertical stress, and Δσuf is the critical limit of the increase in water pressure due to injection. Again, according to the invention, the initial effective vertical stress can be determined from CPT measurement data.