GB2475966A

GB2475966A - Process for determining hydrogen sulphide in drill cuttings

Info

Publication number: GB2475966A
Application number: GB201020352A
Authority: GB
Inventors: Osvaldo Pascolini
Original assignee: Eni SpA
Current assignee: Eni SpA
Priority date: 2009-12-04
Filing date: 2010-12-01
Publication date: 2011-06-08
Anticipated expiration: 2030-12-01
Also published as: ITMI20092149A1; GB201020352D0; IT1396647B1; GB2475966B

Abstract

A process for determining the presence and/or quantity of H2S in the drill cuttings coming from the subsurface, includes the following phases:a) drilling the layers of subsurface and recovering the drill cuttings;b) degassing the drill cuttings from phase a), with the separation of a gaseous phase from the cuttings;c) determining the possible presence and/or quantity of H2S in the gaseous phase coming from phase b) using suitable the drill detection means.The process may include determining the quantity of hydrogen sulphide in the drill cuttings of at least two subsurface layers, the layers being positioned at different depths. Preferably, acoustic pressure waves are used in the degassing phase.

Description

PROCESS FOR DETERMINING THE PRESENCE AND/OR QUANTITY OF H2S IN THE SUBSURFACE AND RELEVANT EQUIPMENT The present invention relates to a process for determining, especially in a field during the drilling of hydrocarbons or water wells, the presence and/or amount of H2S in the subsurface and relevant equipment.

In particular, the present invention relates to the determination of the presence and/or amount of H2S in the subsurface, at different depths with respect to the surface, and to the equipment for implementing said process.

The present invention falls within the technical field of the exploration and exploitation of subsurface resources, in particular hydrocarbons resources.

In this field, the exploration of the subsurface

through drilling wells and the subsequent analysis of the drilling mud carried to the surface, is a common practice.

Drill mud is produced by the use of lubrication and cooling fluids during drilling operations. These fluids mainly provide lubrication and cooling of the bottom hole assembly, as well as for conveying to the surface, the fragments of soil and rocks (drill cuttings) dug by the drilling assembly. The drill cuttings reach the surface mixed with drilling fluids, forming drill mud.

In the oil exploration activities, geological mud logging, essentially consisting of the microscopy -1*-analysis of the cuttings and chemical analysis (mainly of the chromatographic type) of gases contained therein or carried to the surface, provides useful information on both the nature of the drilled rocks and on the characteristics of the possible oil resources trapped in the subsurface The subsurface gases are of both the hydrocarbon and non-hydrocarbon type. Among gases of the non-hydrocarbon type, hydrogen sulfide (H2S) is particularly important due to its potential toxicity for human beings. During drilling, when the presence of H2S in the subsurface is foreseen or detected, additives (scavengers) are added to the drilling fluids and relative mud with the purpose of making them react with gaseous H2S to form solid or soluble suiphides, thus neutralizing the possible harmful effects of this gas.

At the rig sites it is therefore essential to determine the presence of H2S in the subsurface, with accuracy and as quick as possible, in order to guarantee the maximum safety conditions for the crew.

The determination of the amount of H2S and its distribution in the subsurface is also of fundamental importance for the geological characterization of the explored area, as it can provide useful information for exploiting the oil resources, possibly present.

At present, by using the techniques and instruments available in the state of the art, it is not possible to obtain sufficiently precise information relating to the distribution and concentration of H7S in the subsurface during drilling the well.

In order to assure operational safety, it is normal practice to detect the presence of H2S in the air of the area surrounding the well being drilled, by means of specific sensors for this gas. The detection of H2S by sensors is normally communicated to the rig site personnel by means of acoustic and flashing alarms.

This type of measurement, which can also be insufficient from the point of view of safety as it warns operators of a danger which is imminent or already there, cannot be used however for estimating the amount of H2S actually present in the subsurface.

As a matter of fact, the amount of H2S detected in the air is strongly affected by the dissolution and dissociation reactions which take place in the subsurface between H2S in a protonated form and the drilling mud. The gaseous H2S in the subsurface reacts with the mud, generally characterized by a basic pH, forming the ionic species HS and S2 which remain dissolved or dispersed as insoluble salts in the mud itself. Furthermore, the fraction of H2S which remains in the protonated gaseous form, and which therefore, once it has reached the surface, can be detected by the sensors, is further reduced as a result of the action of the scavengers which are specifically added for neutralizing the H7S. The content of H2S in the air surrounding the well head, therefore, greatly underestimates the amount of gaseous H2S actually present in the subsurface.

For similar reasons, the traditional analyses which can be carried out on the drilling mud for characterizing the subsurface do not provide precise information concerning the amount of H2S present.

The lack of measuring methods capable of determining with sufficient precision the quantity of H2S present in the subsurface, makes it extremely difficult to investigate the distribution of this gas during the drilling of a well.

As of today, the only direct measurement systems in depth which enable to quantify gaseous H2S in the subsurface during drilling, are those based on the use of special samplers called "wirelirie formation testers" which enable the accurate sampling of the fluid trapped in the drilled rock formations. These measuring systems, however, require the drilling phase to be discontinued and are costly due to the purchase and maintenance of the sampling equipment and analyses of the samples.

An objective of the present invention is therefore to provide a process for determining the presence and/or quantity of H2S in the subsurface which overcomes the above-mentioned drawbacks of the state of the art.

This and other objectives of the present invention are obtained by means of a process for determining the presence and/or quantity of H2S in the drill cuttings of layers in the subsurface, including the following operational phases: a) drilling a layer in the subsurface and collecting the drill cuttings of said layer; b) degassing the drill cuttings coming from phase a), with the separation of a gaseous phase from said cuttings; c) determining the possible presence and/or quantity of H2S in the gaseous phase coming from phase b) using suitable detection means.

A further object of the present invention therefore relates to a process for determining a distribution profile of H2S in the subsurface, along the well path, comprising the determination of the amount of I-i2S in the drill cuttings of at least two layers in the subsurface, said layers being collected at different depth levels from the surface, said determination being effected by means of the following operative phases: a) drilling a first layer in the subsurface and collecting the drill cuttings of said layer; b) degassing the drill cuttings coming from phase a), with the separation of a gaseous phase from said cuttings; c) determining the quantity of H2S in the gaseous phase coming from phase b) using suitable detection means.

d) repeating phases a) to c) on one or more further subsurface drill cuttings, preferably collected at regular intervals.

The process according to the present invention comprises a first drilling phase (phase a)) of a subsurface layer of the area interested by the geological exploration. This phase is carried out using the drilling means normally adopted in the field. The drilling of a layer in the subsurface generates cuttings, which are carried to the surface by the drilling mud.

Drill cuttings, essentially consisting of crushed rock, contain variable quantities of gaseous substances present in the drilled subsurface layer, which have been entrapped in the form of small bubbles inside the rock pores. The gases occluded in the cuttings, including possible H2S, can be extracted and separated from these by subjecting the cuttings to a degassing process (phase b) . In order to degas the cuttings, any possible device can be used, which is capable of extracting the gases entrapped in the cuttings, forming a gaseous phase separated from these ones. In order to achieve an effective separation of the gases from the cuttings, it is preferable to use degassing techniques which exploit the cavitation effect induced by the application of acoustic energy fields. Phase b), for example, it is applied to the drill cuttings by means of acoustic pressure waves. Phase b) is preferably effected by bombarding the drill cuttings with acoustic pressure waves which are high-frequency ultrasounds, i.e. frequencies from 2 MHz to 200 MHz; in this way, sufficient energy is supplied to the gas bubbles entrapped in the cuttings to allow their release from the pores and the formation of a gaseous phase separated from the cuttings, including H2S possibly present in the drilled subsurface layer.

According to the present invention, the degassing phase of the drill cuttings separated from the drill mud, can be carried out on the cuttings, without necessarily perform any other preliminary treatment (e.g. washing).

The gasous phase which is separated from the cuttings is collected in a suitable container and subjected to analysis for determining the possible presence of H2S and/or measuring its concentration using suitable detection means (phase c)) In a preferred embodiment of the present invention, the presence of H2S alone is determined, directly at the drilling site, preferably in a laboratory-cabin, using a specific sensor for this gas (qualitative determination) . Alternatively, the qualitative determination of H2S can be done through any type of chemical analysis capable of detecting the presence of H2S in the gaseous phase (chromatographic analysis, for

example)

The possible positive response of the sensor with respect to the presence of H2S in the gaseous phase, can also be suitably used as a signal for activating an alarm device, for example, by suitably connecting the sensor to an electronic processor capable of processing a signal transmitted by the same sensor and consequently activating an alarm device (e.g. acoustic and/or flashing type) Consequently, the detection means of H2S in the gaseous phase can be connected to an alarm device (for example of the flashing and/or acoustic type), by means of an electronic processor which, in the case of a positive signal transmitted by the same sensor, activates the alarm device.

In a second preferred embodiment of the process, the qualitative determination of the presence of H2S in the gaseous phase separated from the drill cuttings can be replaced or coupled with a quantitative determination of its concentration.

For this purpose, it is possible to use a high resolution detector capable of also determining very low concentrations of H2S (detectability limit in the order of ppb) . High resolution detectors are available on the market.

Alternatively, the concentration of H2S can be determined by chemical analysis of the gaseous phase (e.g. chromatographic or mass spectrometry analyses) The quantitative determinations of H2S in drill cuttings which can be obtained with the process according to the present invention are a representative measurement of the concentration of gaseous}-12S actually present in the subsurface at a certain depth.

Unlike what takes place in drilling mud where the H2S dissolves forming the ionic species HS and S2, in drill cuttings the above dissociation equilibriums do not occur and consequently the H2S is exclusively present in its protonated form (gas) . Furthermore, the quantity of H2S present in the cuttings in protonated form is not, except only marginally, influenced by the action of the scavengers used during the drilling phase.

The amount of H2S which can be measured with the process according to the present invention therefore reflects the quantity of H2S actually present in the layer of subsurface from which the cuttings derive.

The possibility of quantifying the presence of H2S in a subsurface layer by analysis of the H2S withheld by the drill cuttings can also be conveniently exploited to obtain information on the distribution of H2S in the subsurface.

As aforesaid, actually, the present invention also relates to a process for determining a distribution profile of H2S in the subsurface along the drilled well path, comprising the determination of the quantity of H2S in the drill cuttings of at least two layers in the subsurface, said layers being positioned at different depth levels with respect to the surface, said determination being effecting by means of the following operational phases: a) drilling a first layer in the subsurface and recovering the drill cuttings of said layer; b) degassing the drill cuttings coming from phase a), with the separation of a gaseous phase from said cuttings; c) determining the quantity of H2S in the gaseous phase coming from phase b) using suitable detection means.

d) repeating phases a) to c) on one or more further drill layers of the subsurface, preferably collected at regular depth intervals.

By repeating the quantitative determination of H2S on the cuttings deriving from two or more layers of subsurface situated at different depths with respect to the surface, it is in fact possible to correlate the concentration of H2S with the depth of the subsurface layer along the drilled section.

The distribution profile of H2S in the subsurface is preferably obtained by performing the quantitative determination of H2S on cuttings deriving from different subsurface layers at regular depth intervals from each other, varying from 1 to 10 metres, preferably 3 metres.

A further object of the present invention relates to equipment for executing the process according to the present invention, including: -a container suitable for housing drill cuttings and equipped with appropriate loading and unloading means of the cuttings; -means for degassing the drill cuttings contained in the container; -means for collecting the gaseous phase generated by the degassing of the drill cuttings in a specific second container; -means for detecting and/or quantifying the quantity of H2S possibly present in the gaseous phase, said gaseous phase deriving from the degassing of the drill cuttings.

The equipment comprises at least one container capable of housing drill cuttings recovered during the drilling of the subsurface layer object of the exploration. The container is equipped with suitable loading and unloading means of the cuttings.

In order to proceed with the degassing of the cuttings, the above container is equipped with suitable degassing means capable of striking the cuttings with acoustic pressure waves adequate for inducing the evolution of the gases contained therein due to cavitation. The degassing means, for example, can be transducers capable of generating acoustic energy fields, preferably transducers capable of generating high-frequency ultrasounds.

The gaseous phase which is developed by the cuttings is collected in a specific container capable of containing gaseous substances, without dispersions.

The container of the gaseous phase is equipped with suitable detectors for determining the possible presence of H2S in the gaseous phase.

In a preferred embodiment of the equipment according to the present invention, the detection means comprise a specific sensor for H2S (qualitative determination) In a second preferred embodiment, the detection means comprise an analysis system capable of quantifying the H2S present (quantitative determination) . The quantitative determination of H2S in the gaseous phase can be carried out, for example, with a high resolution detector capable of also measuring very low concentrations of H2S (detectability limit in the order of ppb) The present invention has several advantages. A first advantage lies in the possibility of precociously identifying the presence of H2S during the drilling of the subsurface. The above process, in fact, also allows extremely reduced quantities of gaseous H2S entrapped in the cuttings to be detected, allowing an improvement in the safety measures and operative field conditions.

A second advantage consists in the possibility of obtaining much more precise quantitative measurements of H2S present in the subsurface with respect to what can be obtained with the methods known in the current art.

This enables important information to be acquired on the geological nature of the subsurface and also to determine the distribution profiles of H2S in the subsurface, also offering an innovative instrument for the characterization of the subsurface. The distribution profiles of H2S in depth provide complementary information to that of traditional stratigraphic and petrophysical profiles and contribute to improving the results of so-called "we1l-to-well" correlations, i.e. correlations of information which can be obtained through the drilling of a series of wells within the same geological exploration area.

In particular, the process according to the present invention enables a continuous direct measurement to be obtained with a semi-quantitative concentration profile of H2S in depth.

A third advantage offered by the quantitative determination of H2S present even only in traces in the subsurface is linked to the possibility of characterizing the efficiency of the cap rocks, at certain levels of depth.

The cap rock consists of lithologies having an extremely low or zero permeability (generally shale) which, by acting as a "seal", assure the trapping of hydrocarbons (volatile compounds) in the underlying porous and permeable layers, thus forming the reservoir.

Finally, the higher precision of the quantitative determinations of H2S in the subsurface also enables the use of scavengers to be optimized as correctives of drill mud, thus avoiding problems of corrosion of the drill pipes connected to the use of these additives.

Claims

C LA I N S1. A process for determining the presence and/or the quantity of H2S in the drill cuttings of a layer of subsurface including the following operational phases: a) drilling the layer of subsurface and recovering the drill cuttings of said layer; b) degassing the drill cuttings coming from phase a), with the separation of a gaseous phase from said cuttings; c) determining the possible presence and/or quantity of H2S in the gaseous phase coming from phase b) using suitable detection means.
2. The process according to claim 1, wherein phase b) is performed by subjecting the drill cuttings to acoustic pressure waves.
3. The process according to claim 2, wherein the acoustic pressure waves are high-frequency ultrasounds.
4. The process according to any of the claims from 1 to 3, wherein the detection means comprise a specific H2S sensor, or a high-resolution H25 sensor.
5. The process according to any of the claims from 1 to 4, wherein the determination of phase c) is effected by subjecting the gaseous phase to chemical analysis, for example chromatograph analysis or mass spectrometry.
6. A process for determining a distribution profile of H2S in the subsurface along the drilled well path, comprising the determination of the quantity of H2S in the drill cuttings of at least two subsurface layers said layers being positioned at different depth levels with respect to the surface, said determination being carried out through the following operational phases: a) drilling a first layer in the subsurface and recovering the drill cuttings of said layer; b) degassing the drill cuttings coming from phase a), with the separation of a gaseous phase from said cuttings; c) determining the quantity of H2S in the gaseous phase coming from phase b) using suitable detection means.d) repeating phases a) to c) on one or more further drill subsurface layers, preferably collected at regular depth intervals.
7. The process according to claim 6, wherein the quantitative determination of H2S is performed on cuttings coming from different layers of the subsurface and sampled at regular depth intervals from each other, varying from 1 to 10 meters, preferably 3 meters.
8. The process according to claim 6, wherein phase b) is executed by subjecting the drill cuttings to acoustic pressure waves.
9. The process according to claim 8, wherein the acoustic pressure waves are high-frequency ultrasounds.
10. The process according to any of the claims from 6 to 9, wherein the detection means comprise a specific H2S sensor, or a high-resolution H2S sensor.
11. The process according to any of the claims from 6 to 10, wherein the determination of phase c) is carried out by subjecting the gaseous phase to chemical analysis, for example chromatograph analysis or mass spectrometry.
12. The process according to any of the claims from 1 to 11, wherein the H2S detection means in the gaseous phase are connected to an alarm device (for example of the flashing and/or acoustic type), by means of an electronic processor which, in the case of a positive signal transmitted to the sensor itself, activates the alarm device.
13. Equipment for carrying out the process according to any of the previous claims, comprising: -a container suitable for housing drill cuttings and equipped with appropriate loading and unloading means of the cuttings; -means for degassing the drill cuttings contained in the container; -means for collecting the gaseous phase generated by the degassing of the drill cuttings in a specific second container; -means for detecting and/or quantifying the quantity of H2S possibly present in the gaseous phase, said gaseous phase deriving from the degassing of the drill cuttings.
14. The equipment according to claim 13, wherein the degassing means are transducers capable of generatingfields of acoustic energy.
15. The equipment according to claim 13, wherein the degassing means are transducers capable of generating high-frequency ultrasounds.