METHOD AND RELATIVE SYSTEM FOR THE EXTRACTION OF THE GASES CONTAINED IN DRILLING MUD

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Italian patent application no. MI2014A001647 filed on Sep. 23, 2014 with the Italian Patent and Trademark Office in the name of GEOLOG Srl. The earliest priority date claimed is Sep. 23, 2014.

FEDERALLY SPONSORED RESEARCH

None

SEQUENCE LISTING OR PROGRAM

None

BACKGROUND

The present invention relates to a method and relative system for the extraction of gaseous components contained in drilling mud, preferably, in a managed pressure drilling system. The main feature of the method that forms the object of the present invention is that of extracting from the mud a gas sample on which to subsequently perform quantity and quality measurements which are accurate, repeatable and, at the same time, independent of temperature, pressure and volumetric flow rate conditions of the mud itself.

The state of the art to which the present invention relates is that of exploration and exploitation of the subsoil's geothermal and oil resources. During drilling of a well for the extraction of hydrocarbons, fluid is made to circulate inside the hole to lubricate and cool the auger, referred to as drilling mud. The drilling mud is fed into the hollow interior of the borehole and rises up into the annular space, known as “annulus”, between the borehole and the walls of the well. The return mud, after having risen to the surface, flows along return piping at atmospheric pressure, also known as “flow line;” at the end of which, solid debris carried by the mud is separated from the fluid component by means of a system of vibrating screens. The fluid component of the mud is then fed again into the starting tank. During passage in the return piping, a small part of the fluid component of the return mud is further taken by means of a pump, provided with a filter, which prevents the passage of the solid debris, and fed into a degasser where extraction of the gas dissolved in the mud takes place. The volatile components extracted from the degasser are then analysed in order to obtain information on the features of the possible resources of hydrocarbons and, more generally, of gas, present in the subsoil. The purpose of the circulation of the drilling mud is, therefore, not only lubrication and cooling of the auger, but also the conveying rocky solid debris, and fluid products contained in them, to the surface.

Last but not least, a further function of the drilling mud, is to support the part of the well already drilled. Due to the hydrostatic pressure exerted by the mud, it is possible, in fact, to distinguish the phenomenon of the non-controlled entry in the well of the fluids contained in the rocky formations during drilling. More particularly, so that this phenomenon does not occur, it is necessary that the pressure on the base of the well, known as “bottom hole pressure”, succeeds in distinguishing the pressure exerted by the fluids contained in the pores of the rock during drilling. The value of the BHP will depend both on the hydrostatic pressure of the mud and on the load loss caused by the rise of the mud along the annulus. Due to particular conditions of the subsoil, the pressure of the fluids contained in the rock can take on values that are significantly and unexpectedly different from the standard ones. Therefore, it is not always possible, with conventional drilling techniques, to control the BHP so as to distinguish the pressure of the fluids contained in the rock.

In order to guarantee a control of the BHP, also when sudden changes occur in the pressure of the fluids of the rock formations, drilling methods have been developed known as managed pressure drilling (MPD) or ENI near balance drilling (E-NBD) methods. These methods allow for monitoring and to rapidly modifying the BHP in an adaptive manner.

The MPD and E-NBD methods provide for the use of automatic valves, said valves being controlled continuously by a programmable logic controller (known as PLC) by means of a software which analyses, in real time, the data coming from the various sensors. On the basis of the results of the processing of the data coming from these sensors, the valves are regulated by the PLC so as to control the variations in pressure on the bottom of the well. In the MPD drilling systems, the return mud will flow in a closed and pressurised piping towards a mud gas separator (MGS) to then be conveyed towards the vibrating screens at atmospheric pressure. In traditional drilling systems the degasser which takes gas from the mud sample is placed in proximity of the vibrating screens. Contrarily, in managed pressure drilling systems, it is not possible, by installing the degasser in this position, to obtain accurate measurements on the gases dissolved in the mud.

Taking samples of gas after the separator involves, in fact, several limitations. One of the most serious ones comes from the fact that inside the separator a consistent loss may occur of the lighter gaseous components dissolved in the mud. The consequent alteration in the ratios between light hydrocarbons and heavy hydrocarbons can make a correct interpretation of the measurements impossible. To obviate this problem, at least partially, it is customary to install the operation of degassing on the pressurised piping which carries the mud from the drilling control system (MPD) to the separator (MGS).

Again for the same purpose, i.e. that of overcoming the difficulties linked to the alteration in the ratios between light and heavy hydrocarbons, technical solutions were also developed in the field which do not provide for any sampling of mud outside of the return piping. These techniques propose the extraction of the gaseous components dissolved in the mud directly inside the flow line. For this purpose probes are used, provided with semi-permeable membranes which allow the passage through diffusion only of some molecules. Thanks to these probes, inserted directly inside the pressurised piping, the gases dissolved in the mud are extracted. These gases are then usually analysed with chromatographic analysis procedures. The measurements supplied by the devices based on the use of membranes are, however, strongly subject to the changes in temperature, pressure and volumetric flow rate of the mud lapping these membranes. The diffusive capacity of said membranes depends, in fact, not only on the permeability of the membrane to each solute but also on the temperature, on the pressure and on the flow rate of the solvents on both sides of the membrane. In order to seek to limit the consequences of these problems the devices based on the use of membranes are provided with means for measuring some of the parameters mentioned above (temperature, pressure and flow rate of the solvents) and with a software which corrects the results of the analysis on the basis of the estimates of the effect of these parameters on the measurements, said estimates being performed by means of appropriate mathematical algorithm. This software correction is naturally subject to errors since it is based on an abstract model of a physical reality and is much different from a real chemical analysis which measures the effective quantity of gases dissolved in the mud.

Moreover in the case wherein the measured values of gas are too low or even zero this software prediction cannot be operated.

SUMMARY

The object of the present invention is, therefore, to provide a method and relative system for the extraction of gases contained in drilling mud, preferably, in a managed pressure drilling system, which allows quantitative and qualitative measurements of these gases to be obtained which are independent of the conditions of temperature, of pressure and of volumetric flow of the mud itself inside the return piping.

This object is achieved by means of a process of extraction of the gases contained in the drilling mud which provides for the use of a degasser and the monitoring and the regulation of the quantity of mud taken from the pressurised piping, before the degasser, said regulation being suitable for ensuring that a constant volumetric flow rate of mud arrives at the degasser.

The monitoring of the quantity of mud sampled takes place by means of the use of direct or indirect means of measurement of this quantity. The regulation of the volumetric flow of mud takes place by means of an automatic valve with proportional opening, said opening being regulated on the basis of the information supplied by said measurement means. Thanks to the combined use of said monitoring means and of said regulation means, it is possible to send to the degasser a constant volumetric flow of mud so as to guarantee the precision and the accuracy of the successive analyses.

These and further features of the present invention will be made clearer by reading the following detailed description, relating to a preferred embodiment of the present invention to be considered by way of a non-limiting example of the more general concepts claimed.

DRAWINGS

The following description refers to the accompanying drawings, in which:

FIG. 1 is a block diagram explaining of the process which is the object of the present invention and of the relative system, as a whole;

FIG. 2 is a simplified representation of the devices which allow the implementation of the process which is the object of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the process which forms the object of the present invention consists of extracting the drilling mud from the return piping (1), regulating its quantity by means of appropriate regulation means (2). In FIG. 1, the mud's path is indicated by an unbroken line. The regulation means (2) are placed downstream of the point (4) of extraction of the mud from said return piping (1) and upstream of the monitoring means (3). The monitoring means (3) are, in turn, placed upstream of a degasser (5), which has the function of extracting therefrom the gaseous sample dissolved in the mud, after having preferably brought it and maintained it constantly at a high temperature (approximately 50° C./60° C.), according to the practice in the field.

The regulation means (2) operate on the basis of information sent by said monitoring means (3), said monitoring means (3) having the function of performing a continuous control of the quantity of mud taken from the piping (1). More particularly, said regulation means (2) are able to receive electronic signals coming from said monitoring means (3), and operate in such a way that the quantity of mud extracted from said return piping (1) varies according to the information transmitted by said signals. In FIG. 1, the path of the signals sent by the monitoring means (3) to the regulation means (2) is indicated by a dotted line, which goes from the monitoring means (3) to the regulation means (2).

Referring to FIGS. 1 and 2, said monitoring means (3) comprise a container (6) and a level sensor (7). Said container (6) can be, preferably, a can (6) and has the function of collecting the drilling mud extracted from the return piping (1). Inside said can (6) the pressure is equal to the environmental pressure. The level sensor (7) measures the level reached by the drilling mud inside said can (6).

The regulation means (2) comprise an automatic valve with proportional opening (8) which receives, in input, the drilling mud extracted from the return piping (1) and regulates, in output, the quantity of mud which enters the can (6). The extraction of the mud takes place, preferably, by means of a self-cleaning filtration probe installed in the sampling point (4). The quantity of mud which enters the can (6) will depend, in particular, on the pressure upstream of the valve (8) and on a signal of opening sent to the valve (8). In particular the valve (8) can be a pneumatic valve controlled electronically by signals coming from the level sensor (7). The latter can be, for example, a microwave sensor. The valve (8), the can (6) and the level sensor (7) form a feedback circuit which allows the level of the mud in the can (6) to be maintained constant. In this way, the flow from said valve (8) in the can (6), the pressure inside said can (6) being equal to the ambient pressure, will be the same aspirated by the degasser (5) which, consequently, will be constant in time. The functioning of this circuit is the following: if the level L of the mud inside the can (6), indicated by the signal sent by the sensor (7), is higher than a certain threshold, the opening of the valve (8) is reduced, returning the level of the mud in the can (6) to the pre-established value. If instead the level L of the mud inside the can (6), indicated by the signal sent by the sensor (7), is lower than a certain threshold, the valve (8) tends to open in order to reach this level. In both cases the opening or closure of the valve (8) will be proportional to the difference between the current level L of the mud and the threshold value In the embodiment of the present invention described here, it was chosen to maintain this flow rate of mud equal to approximately 3 litres a minute, a value which on the basis of experiments performed was found to be optimal.

The process described here, although being, preferably, developed for managed pressure drilling systems, can also be used with the so-called underbalanced systems or in other drilling systems where it is necessary or preferable to sample the gas from pressurised mud.

The invention described above achieves the objects set, overcoming the disadvantages of the prior art.

Thanks to the introduction of a feedback circuit comprising means of regulation and means of monitoring of the quantity of mud, taken from the pressurised return piping, a method and relative system are produced which allow a control of the volumetric flow rate of mud which arrives at the degasser and the maintaining of this flow rate at a constant value. In this way, it is possible to obtain successively accurate and repeatable qualitative and quantitative measurements on the gaseous mixture as well as independent of the values of temperature, pressure and flow rate of the mud inside the return piping.