Method of monitoring unconventional hydrocarbon exploration and development sites

Abstract

The present invention is a method of monitoring recovery site hydrocarbons (HC) using an unconventional method which permits quantifying the HC present in zones above the exploration and development zone. The method according to the invention is based on the adjustment of a model describing the gas concentration as a function of time, using in-situ geochemical analyzes of rare gases and, when appropriate, of injected gas used for fracturing, contained in fluid phases of subsoil samples. By use of rare gas analysis, the method according to the invention allows anticipation of hydrocarbon leakage above the exploration/development site.

Claims

1. A method of monitoring an underground formation to recover hydrocarbons using simulation and drilling and at least one rare gas present in the underground formation, comprising: a) selecting a diffusion model of the at least one rare gas and a diffusion model the hydrocarbons to be recovered with each model including an evolution of concentration as a function of time and depth and a diffusion coefficient; b) taking at least a first sample of fluid present in a subsoil zone which is being monitored that is located above the underground formation prior to recovery of the hydrocarbons by using a downhole sampler and measuring a concentration of the at least one rare gas within the first sample of the fluid; c) fracturing the underground formation; d) at least one of during and after recovery of the hydrocarbons, taking at least a second sample of the fluid present in the subsoil zone and measuring a concentration of the at least one rare gas within at least the second sample; e) repeating d) at different times; f) determining when the concentration of the at least one rare gas increases in the at least a second sample from the concentration of the at least one rare gas in the first sample and modifying the diffusion coefficient of the diffusion model of the at least one rare gas to be coherent with the increased concentration of the at least one rare gas in the at least a second sample and determining a ratio between the diffusion coefficient before modification and the modified diffusion coefficient; and g) incorporating the ratio into the diffusion model of the hydrocarbons to be recovered and thereafter determining from the diffusion model of the hydrocarbons incorporating the ratio an amount of hydrocarbons in the subsoil zone that can be recovered at a time t.

2. A method as claimed in claim 1, wherein the subsoil zone is an aquifer.

3. A method as claimed in claim 1, wherein leakage of the hydrocarbons to be recovered from the underground formation is detected using an amount of the hydrocarbons which are determined to be recoverable in the subsoil zone.

4. A method as claimed in claim 2, wherein leakage of the hydrocarbons to be recovered from the underground formation is detected using an amount of the hydrocarbons which are determined to be recoverable in the subsoil zone.

5. A method as claimed in claim 1, wherein the fracturing is performed by injecting gas.

6. A method as claimed in claim 5, wherein an amount of fluoropropane which is injected into the monitoring zone or an amount of helium present in the monitoring zone at the time t is determined by incorporating the ratio into the diffusion model of the hydrocarbons.

7. A method as claimed in claim 6, wherein the injected gas comprises at least one rare gas.

8. A method as claimed in claim 5, wherein the injected gas comprises at least one rare gas.

9. A method as claimed in claim 8, wherein the at least one rare gas comprises at least one of helium and argon.

10. A method as claimed in claim 5, wherein the injected gas comprises fluoropropane or helium.

11. A method as claimed in claim 1, wherein the hydrocarbons contain methane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the method according to the invention will be clear from reading the description hereafter of embodiments given by way of non-limitative examples, with reference to the accompanying figures wherein:

(2) FIG. 1 illustrates an example of unconventional hydrocarbon recovery;

(3) FIG. 2 illustrates helium (He), argon (Ar), fluoropropane (FP) and methane (CH.sub.4) diffusion models calibrated by geochemical measurements;

(4) FIG. 3 illustrates the calibration improvement with subsequent geochemical measurements; and

(5) FIG. 4 illustrates the calculation of the time limit available for taking action in order to address future leakage issues.

DETAILED DESCRIPTION OF THE INVENTION

(6) The monitoring method according to the invention relates to an unconventional method of hydrocarbon exploration or development of a site.

(7) According to a non-limitative embodiment illustrated in FIG. 1, recovery of the unconventional hydrocarbons HC contained in an underground formation Zhc such as a particular surrounding rock, a clayey type rock of very low permeability for example, is performed via a well P. Zone Zhc which contains the hydrocarbons is topped by an upper zone, notably a water-containing aquifer. Recovery of the unconventional hydrocarbons HC requires a specific treatment, notably fracturing of underground formation Zhc. This fracturing allows recovery of the HC from well P, but it can generate migration of the hydrocarbons to the upper zone. This migration can be either advective if fracturing is excessive and poorly controlled, or diffusive, which is the case for all the sites. Diffusion being independent of permeability and depending only on porosity, even in the case of a layer of very low permeability, the diffusion phenomenon takes place. In the case of diffusion, in FIG. 1, diffusion of the hydrocarbons HC (methane CH.sub.4, ethane C.sub.2H.sub.6, propane C.sub.3H.sub.8, butane C.sub.4H.sub.10, etc.) is symbolized by curved arrows. Methane is the predominant species in most cases and the one that diffuses the fastest among hydrocarbons because its molecule is the smallest. Diffusion to the upper zone, especially if it is an aquifer, is preferably avoided in order to limit environmental problems related to the exploitation of unconventional hydrocarbons. The method according to the invention allows monitoring this diffusion to the upper zone, referred to as a subsoil monitoring zone Zsur. Advantageously, this monitoring zone Zsur corresponds to a saline aquifer containing of non-potable water, followed by a potable water aquifer (higher (closer to the surface) in the water column).

(8) There are several types of fracturing methods, such as: (i) injection of water under pressure (known as hydraulic fracturing), for example associated with silica (sand) microbeads to keep the created fracture open, (ii) mechanical fracturing through explosion (dynamite), or (iii) injection of gas such as fluoropropane (non-flammable propane) or warm helium (notably for drilling/fracturing in arctic regions where water freezes too fast and fluoropropane appears to be ineffective, for the same reasons).

(9) The gas injected, fluoropropane or helium for example, used as the “fracturing agent”, may also leak through diffusion and pollute the zone overlying the fractured exploited rock. This diffusion can also be prevented to limit environmental problems related to the exploitation of unconventional hydrocarbons by fracturing via gas injection.

(10) The monitoring method according to the invention allows quantification of the hydrocarbons diffused in the upper zone and anticipation of hydrocarbon leakage to the zone by means of an analysis of the rare gases notably and possibly of the gas injected for fracturing (fluoropropane or helium for example) that are present in the upper zone. Indeed, the underground formation comprising the unconventional hydrocarbons also contains rare gases (helium or argon for example) that will also diffuse to the upper zone. Helium is naturally present in geological environments, the more so the deeper the zone. Furthermore, for fracturing via gas injection, the injected gas can also contain rare gases that will diffuse to the upper zone (notably in the case of warm helium injection).

(11) The monitoring method is based on the use of three interesting characteristics of rare gases in relation to hydrocarbons and, when appropriate, to the injected gas: faster diffusion in an aqueous medium, finer detectability by measuring tools, and inactivity with respect to the environment thereof from a chemical and biological point of view.

(12) The monitoring method essentially comprises the following stages:

(13) 1. Diffusion model selection for a rare gas and hydrocarbons

(14) 2. Rare gas concentration measurements prior to exploration and exploitation

(15) 3. Rare gas concentration measurements during exploration and after exploitation

(16) 4. Rare gas diffusion model calibration with the concentration measurements

(17) 5. Hydrocarbon diffusion model updating from the calibrated rare gas diffusion model

(18) 6. Determining the amount of hydrocarbons present in the monitoring zone at a time t from the updated model.

(19) These stages are detailed hereafter for a non-limitative example where methane CH.sub.4 is a hydrocarbon to be recovered that is contained in the underground formation. However, these stages are suited to any type of hydrocarbon contained in the underground formation, for example ethane C.sub.2H.sub.6, propane C.sub.3H.sub.8, butane C.sub.4H.sub.10, or mixtures thereof.

(20) 1. Diffusion Model Selection for a Rare Gas and CH.sub.4

(21) A diffusion model is selected for a rare gas, helium for example, which is naturally present in geological environments, as well as a diffusion model for a hydrocarbon, CH.sub.4. Each model describes the evolution of the concentration of the chemical species as a function of time, of depth and of a diffusion coefficient specific to helium and to CH.sub.4.

(22) A 1D model of vertical migration of a constituent through diffusion is for example known, wherein the evolution of the concentration (C) of the constituent in space and in time (t) is defined by:

(23) $\begin{array}{cc}C\left(z,t\right)=C_0\mathrm{erfc}\left(\frac{z}{2\sqrt{\mathrm{Dt}}}\right)& \mathrm{Equation}1\end{array}$
with: z being depth t being time D being effective diffusion coefficient of the constituent (rare gas, hydrocarbons, injected gas) such that D=Dm*ratio, where Dm is the molecular diffusion coefficient of the constituent and ratio (initially equal to the porosity) a parameter to be updated in stage 4 of the method, C.sub.0 being a maximum concentration of the dissolved constituent (He or CH.sub.4), i.e. the initial concentration prior to injection. It is the concentration at the water/gas interface.

(24) According to an embodiment of the invention, where recovery of the hydrocarbons is achieved by fracturing with gas injection, a diffusion model is selected likewise for the injected gas, for example a 1D model as described in Equation 1.

(25) FIG. 2 illustrates diffusion models (curves) for helium (He), argon (Ar), fluoropropane (FP) (gas injected for fracturing) and CH.sub.4 (hydrocarbons).

(26) 2. Rare Gas Concentration Measurements Prior to Exploration/Exploitation

(27) Prior to exploitation and exploration (considering that exploration comprises a fracturing stage), that is prior to fracturing and hydrocarbon recovery, at least a first sample of a fluid present in a monitoring zone (subsoil zone likely to be reached by the hydrocarbons) is taken and the rare gas concentration is measured within this first sample. Sampling is performed using at least one monitoring well into which a downhole sampler is run in order to recover the fluid (water) present, without disturbing the physico-chemical equilibrium of the system.

(28) This well allows the sampler to be placed in a zone of the subsoil likely to be reached by hydrocarbons. This monitoring zone is located above the underground formation containing the hydrocarbons. It notably can be an aquifer.

(29) A first measurement characterizing the initial state of the subsoil zone prior to exploitation is then performed on this first sample. The rare gas concentration within this first sample is measured. The concentrations of other rare gases, of the hydrocarbons and of the injected gas can also be measured if it is possible.

(30) This stage allows constraining the compositions of the hydrocarbons (elements in gas form dissolved in the underground formation). Early detection of helium in relation to methane depends on the helium composition difference between the hydrocarbon-containing formation and the monitoring zone. Since helium is naturally present in geological environments, in a concentration which increases with depth the difference needs to be constrained by the preliminary analysis of the natural fluids.

(31) 3. Rare Gas Concentration Measurements During and after Exploitation

(32) During and after fracturing and exploitation (hydrocarbon recovery), at least a second sample of a fluid present in the monitoring zone is taken and the rare gas concentration within this second sample is measured.

(33) The mechanism described for the previous stage is used which is a monitoring well and downhole sampler.

(34) This monitoring can be repeated at different times, and possibly in different monitoring wells.

(35) A set of values relative to the ratio of the rare gas concentration at a time t during and after exploitation to the rare gas concentration prior to exploitation (one at each sampling and measuring time t) is thus obtained.

(36) 4. Rare Gas Diffusion Model Calibration with the Concentration Measurements

(37) When the rare gas (He) concentration increases, the effective diffusion coefficient of the model selected in stage 1 is modified so that the model is coherent with the measurements. The ratio of the molecular diffusion coefficient to the modified diffusion coefficient (Equation 1) is then determined therefrom.

(38) The simple assumption is made that the ratio is a characteristic of the medium (porosity and tortuosity) only. If several rare gases are used, an average ratio modulo one error is calculated.

(39) FIG. 2 illustrates calibrating the analytical solution (Equation 1) that characterizes the rare gas concentration evolution as a function of time and of the position of the measuring point with the measurements performed in stage 3. The measuring points are illustrated by points in FIG. 2. FIG. 2 illustrates helium (He) and argon (Ar) diffusion models (curves) obtained by calibrating the analytical solution (Equation 1) on the measuring points. These two calibrated curves allow defining an average ratio between the molecular diffusion and the effective diffusion. This ratio allows calibration, for example, of the diffusion models (curves) of CH.sub.4 and of the injected gas, fluoropropane (FP).

(40) Measurements performed later allow the results to be refined by modifying the ratio (FIG. 3).

(41) 5. Hydrocarbon and Injected Gas Diffusion Model Updating

(42) To update the CH.sub.4 diffusion model from the calibrated rare gas diffusion model, the ratio calculated previously is applied to the CH.sub.4 diffusion model selected in stage 1.

(43) This ratio is therefore applied to the CH.sub.4 molecular diffusion used in the CH.sub.4 diffusion model selected in stage 1. A new effective CH.sub.4 diffusion is thus obtained, which allows obtaining a new CH.sub.4 diffusion model based on the rare gas diffusion model calibrated on experimental data in stage 4.

(44) For the embodiment of the invention where hydrocarbon recovery is achieved by fracturing with gas injection, the injected gas diffusion model can be updated by applying the same ratio, which allows obtaining a new injected gas diffusion model based on the rare gas diffusion model calibrated on experimental data.

(45) This stage is illustrated in FIGS. 2 and 3.

(46) 6. Determining the Amount of Hydrocarbons in the Monitoring Zone from the Updated Model

(47) In this stage, the amount of hydrocarbons present in the monitoring zone is determined at any time t in order to determine a hydrocarbon leak in the monitoring zone. The following stages are therefore carried out:

(48) a) determining the amount of dissolved CH.sub.4, and

(49) b) determining CH.sub.4 leaks.

(50) a) Determining the Amount of Dissolved CH.sub.4 at a Time t from the Updated Model

(51) The CH.sub.4 diffusion model thus updated is then used to determine the amount of dissolved CH.sub.4 at a time t.

(52) Through volume integration of the model (updated Equation 1), we deduce the mass of dissolved CH.sub.4 at a time t is determined by the relationship:

(53) $\begin{array}{cc}M\left(t\right)=2\varphi {\mathrm{SMC}}_0\sqrt{\frac{\mathrm{Dt}}{\pi }}& \mathrm{Equation}2\end{array}$
with:

(54) φ being porosity of the medium,

(55) S being water/gas contact surface, and

(56) M being molar mass of CH.sub.4.

(57) b) Determining CH.sub.4 Leakage into an Overlying Aquifer

(58) According to an embodiment, it is also possible to determine injected CH.sub.4 leaks from the storage zone (reservoir). According to this method, the subsoil zone, that is the zone where samples are taken from a monitoring well and a sampler, is an aquifer located above the subsoil zone into which the CH.sub.4 is injected. CH.sub.4 leakage out of the injection zone is detected by determining the amount of dissolved CH.sub.4 in this aquifer (using the CH.sub.4 diffusion model (stage 6a) of the invention).

(59) This type of subsoil monitoring in an overlying aquifer avoids having to set up a monitoring well through the geological cover that keeps the CH.sub.4 in the underground formation. Furthermore, the diffusion phenomenon is by far the predominant phenomenon within the cap rock, which is all the more interesting regarding rare gases, and therefore the method according to the invention.

(60) This method allows quantification of the time limit provided before CH.sub.4 leakage can be detected through methods and to establish leak remediation and sealing protocols. It is based on the fact that rare gases have a much lower detection threshold than CH.sub.4 and they diffuse faster. The method according to the invention thus detects a CH.sub.4 leak before a CH.sub.4 concentration increase in the aquifer is detectable through geochemical measurement.

(61) FIG. 4 illustrates the helium and CH.sub.4 diffusion models (curves) calibrated on measurements. SHe indicates the detection threshold, by geochemical measurement, of a helium concentration increase. SCH.sub.4 indicates the detection threshold by geochemical measurement of a CH.sub.4 concentration increase. Thus, TDHe indicates the date when a helium concentration increase is detectable and TDCH.sub.4 indicates the date when a CH.sub.4 concentration increase is detectable. The time limit DEL provided for taking action and fixing a potential CH.sub.4 leak can then be calculated.

(62) For the embodiment of the invention in which hydrocarbon recovery is performed by fracturing with gas injection, the amount of gas injected into the subsoil monitoring zone can be determined in order to detect an injected gas leak in the monitoring zone. This determination can be achieved using the updated injected gas diffusion model by carrying out in the same manner stages a), b) and c) described above.

(63) Furthermore, through analysis of the injected gas used for fracturing (in considerable volume), the method according to the invention allows anticipation of a hydrocarbon leak above the exploration/development site.

(64) Fracturing with helium injection is the method according to the invention because helium (rare gas) is injected to fracture the rock in large amount and thus, in case of leakage, the helium is present in larger amount in the monitoring zone, which allows easier and faster leak detection.

(65) The method according to the invention is particularly suited for monitoring an underground formation from which shale oil and/or shale gas is extracted and wherein fracturing with injection of a gas such as fluoropropane or helium is performed.