METHOD FOR THE IDENTIFICATION OF OPERATIONAL PROBLEMS IN WELLS THAT PRODUCE BY GAS-LIFT

Abstract

The present invention addresses to a method for the identification of operational problems based on the hydrodynamic characterization of the gas-lift and production well/lines set, by means of the injection of an alcohol, preferably ethanol, in the gas-lift line. The detection of the passage and the return of the alcohol is carried out by the direct analysis of information about the process variables (pressure and temperature at the bottom of the well, at the Christmas tree and at the surface, gas or liquid flowmeters, etc.). Preferably, this detection should also be confirmed by measuring the alcohol concentration at the return, either by sequential collection of samples of the liquid produced by the well and subsequent physicochemical analysis in the laboratory, or by an online meter specially installed for this purpose. A practical sample collection procedure has been developed for application together with this invention.

Chemical analysis methods for the determination of small amounts of water in alcohol are known from the State of the Art, but the reverse is not found. Thus, procedures for analyzing small amounts of alcohol in water have been specially developed for application together with this invention.

Claims

1. A METHOD FOR THE IDENTIFICATION OF OPERATIONAL PROBLEMS IN WELLS THAT PRODUCE BY GAS-LIFT, comprising: allowing the hydrodynamic evaluation of the well/lines set by injection of at least one alcohol in the gas-lift line, collecting samples at the well sampling point, and detecting alcohol passage and return by direct analysis of information about process variables provided by the well instrumentation.

2. THE METHOD FOR THE IDENTIFICATION OF OPERATIONAL PROBLEMS IN WELLS THAT PRODUCE BY GAS-LIFT according to claim 1, wherein the injected alcohol is ethanol.

3. THE METHOD FOR THE IDENTIFICATION OF OPERATIONAL PROBLEMS IN WELLS THAT PRODUCE BY GAS-LIFT according to claim 1, wherein the detection of injection and gas leak points is confirmed by measuring the alcohol concentration at the return, either by the sequential collection of samples of the liquid produced by the well and subsequent physicochemical analysis in the laboratory, or by an online meter.

4. THE METHOD FOR THE IDENTIFICATION OF OPERATIONAL PROBLEMS IN WELLS THAT PRODUCE BY GAS-LIFT according to claim 1, wherein the collection of samples is done directly in plastic bags by adding one or more drops of demulsifier, and after a waiting time, piercing each bag at its base with a needle, the first jet being discarded and, next, a sample flask being filled, leaving an amount of supernatant oil to avoid ethanol evaporation.

5. THE METHOD FOR THE IDENTIFICATION OF OPERATIONAL PROBLEMS IN WELLS THAT PRODUCE BY GAS-LIFT according to claim 1, further comprising: analyzing water samples produced with alcohol by NMR or based on physicochemical properties.

6. THE METHOD FOR THE IDENTIFICATION OF OPERATIONAL PROBLEMS IN WELLS THAT PRODUCE BY GAS-LIFT according to claim 4, wherein the plastic bags comprise 2 L bags.

7. THE METHOD FOR THE IDENTIFICATION OF OPERATIONAL PROBLEMS IN WELLS THAT PRODUCE BY GAS-LIFT according to claim 5, wherein the physicochemical properties include at least one of specific mass, refractive index, and electrical conductivity.

Description

[0031] This invention will be described in more detail below, with reference to the attached figures which, in a schematic form and not limiting the inventive scope, represent examples of its embodiment. In the drawings, there are:

[0032] FIG. 1 illustrating a schematic of a well equipped for a gas-lift;

[0033] FIG. 2 illustrating the temperature profile at the arrival on the platform at for a wet completion offshore well and the alcohol concentration profile obtained by the specific mass method;

[0034] FIG. 3 illustrating profiles of some process variables for a wet completion offshore well, where an ethanol injection operation was carried out in the gas-lift;

[0035] FIG. 4 illustrating the variation of the ethanol mass fraction in relation to the specific mass of the ethanol mixture in distilled water and in water produced by the well, in a range of low concentration of ethanol, at 20° C.

DETAILED DESCRIPTION OF THE INVENTION

[0036] The present invention discloses a method for the identification of operational problems in wells that produce by gas-lift, by means of the injection of an alcohol, preferably ethanol, in the gas-lift line. The detection of the injection and the return of alcohol is performed by the direct analysis of information about the process variables (pressure and temperature, mainly) provided by the well instrumentation. This detection must also be confirmed by measuring the concentration of alcohol in the liquid produced at the return, either by sequential sample collection and subsequent physicochemical analysis in the laboratory, or by an online meter specially installed for this purpose. Procedures for analyzing small amounts of alcohol in water have been specially developed for application together with this invention.

[0037] As can be seen schematically in FIG. 1, the system of a well equipped for gas-lift consists of: reservoir (1), casing string (2), production string (3), packer (4), chamber below the packer (5), annular space (6), perforated area (7), collection point (8), gas source (9), well (10), Christmas tree (11), gas-lift valve (12). In a common configuration for this method, high pressure natural gas from a source, usually a compressor (9), is conducted to the Christmas tree (11), usually through a pipe or gas-lift line, and injected into the annular space (6) formed between the casing string (2) and the pipeline or production string (3) through which the production of fluids from the reservoir (1) flows. In some positions along the production pipeline, valves, known as gas-lift valves (12), are placed, which control the flow of gas from the annular space (6) into the production string (3). The expansion of the pressurized gas and the consequent reduction in the apparent density of the multiphase mixture allow the flow of fluids from the reservoir (1) to be possible at a determined flow rate. The mixture of fluids from the reservoir with the gas from the gas-lift arrives at the collection point (8), which is a platform or production station, generally through a pipeline or production line.

[0038] There are several types of gas-lift valves (12). In general, some of the valves along the pipeline are used (i.e., they only open) in case the well is unloaded after intervention with a rig or in the eventual need to resume production, after an accidental shutdown or a preventive stop of the production of the well. These are called dump (or start-up) valves. Normally, in the daily operation of the well, the injection takes place through only one gas-lift valve (12), usually the deepest one, which is then called the operator valve. There are situations, as seen in FIG. 1, in which only a single gas-lift valve (12) is used, which plays the role of operator and discharge valve.

[0039] The objective of the method of this invention is to diagnose if the gas is being injected only in the operator valve (12), or if there are other improper injection points. The method also seeks to investigate whether there is gas escaping from the annulus (6) to the rock formations crossed by the well or not.

[0040] The method developed in this invention for the identification of operational problems in gas-lift wells comprises the following steps for application in a specific well:

Before the Diagnostic Test

[0041] a) Define the objectives of the diagnostic test with the client; [0042] b) Collect relevant system information (geometry, fluid properties, fluid flow rates, etc.). Analyze the files and reports of routine well simulations in computer simulators at the client's choice; [0043] c) Carry out additional simulations, as necessary, and estimate the passage times of alcohol along the different sections of the system; [0044] d) Define the alcohol flow rate (and the maximum concentration at the return, therefore), taking into account the objective of the diagnostic test, the detection levels of the technique used for alcohol analysis, and the operational limitations of the platform in question; [0045] e) Set the alcohol injection time. Also analyze whether there is interference from other chemicals and their solvents that are eventually regularly injected into the well or not; [0046] f) Define practical aspects of the diagnostic test with the various actors involved, in particular those who work directly in the operation of the platform. Define logistics aspects, sample collection points or online meter coupling point, onboard resources, necessary maneuvers, etc.

During the Diagnostic Test

[0047] g) Apply regularly used alcohol from the platform tank; [0048] h) Adjust and calibrate the flow rate of the platform metering pump(s) to the required value. Maintain the alcohol injection for the required injection time; [0049] i) Taking into account the expected return time of the alcohol front and the injection time, define the total time of the diagnostic test, adding a safety margin; [0050] j) Preferably, align the well during the diagnostic test period to the test separator with fiscal measurement (that is, put the well in production test), for correct flow measurement. The injection of alcohol must only be performed after the stabilization of the well in the test separator. This improves the accuracy of interpretation of diagnostic test results, particularly with respect to mass balance. However, if this is not possible, the operation can be carried out with the well aligned normally for the production separator, by scheduling a production test of the well as soon as possible. Also check if the information of the process variables of interest are being acquired normally in the supervisory system; [0051] k) The point of collection of oil and water samples must be the same manifold that already exists for collection of samples, due to the ease of access. Even with the use of an online meter, or using only the signals of the process variables for detection, samples must be collected at adequate time intervals. Without the online meter or the variable signals, or even for a more thorough test, the sampling interval can be reduced to the practical minimum possible. Also collect samples of alcohol and other chemicals that may be eventually used; [0052] l) Use the most appropriate sample collection procedure for a specific case. For example, production fluids (water and oil) can be collected directly in plastic bags, preferably with a volumetric capacity of 2 L. To speed up the operation, one or more drops of demulsifier can be added. After a certain waiting time, each bag is punctured at its base with a special needle, the first jet is discarded, and, next, a sample flask is filled with water. Allow the sample to contain a certain amount of oil, because it promotes the formation of an oil seal on the surface, which helps to preserve the alcohol against evaporation, particularly if the samples cannot be analyzed on the platform itself, having to be transported to the shore. Cap the flask and label it correctly for future reference. Forward all collected samples to the laboratory on the platform or on shore for chemical analysis.

After the Diagnostic Test

[0053] m) Analyze the produced water samples using the appropriate method. In addition to NMR, an analysis method based on physicochemical properties, particularly specific mass, can be used. In this case, a calibration curve must be constructed, preferably using a sample of the water produced by the well. Analyze the other collected samples (from the injected alcohol, for example); [0054] n) With the results of alcohol concentration at the return (concentration profile), of the online meter (if any), and of the signals of the process variables, define the passage times of the alcohol in the system and the mass balance, in order to determine the positions of the gas injection points in the system and the eventual occurrence of a gas leak. Employ computer simulation of the transient flow of alcohol in the system to improve the quality of diagnosis; [0055] o) Discuss the results with the client and issue a specific diagnostic test report.

[0056] FIG. 2 shows the temperature profile at the arrival on the platform for a wet completion offshore well and the alcohol concentration profile obtained by the specific mass method. The coincidence between the profiles confirms the return of ethanol to the platform.

[0057] FIG. 3 presents the profiles of some process variables for a wet completion offshore well, in which an ethanol injection operation was performed in the gas-lift. From top to bottom, there are shown the profiles of the temperature of the produced fluid at the arrival on the platform (“T PLAT”), the temperature of this fluid in the wet Christmas tree (“T WCT”), the pressure downstream the gas-lift choke valve (“GL P”), and gas-lift flow rate (“GL FR”). The vertical bars with dotted lines define the observed or expected time interval for the passage of the ethanol cushion at that specific point (Christmas tree or platform) and the vertical bars with a dash-dot line indicate the time interval of ethanol injection. Therefore, it can be verified that the passage of ethanol is recorded by the process signals.

[0058] In FIG. 4, there is shown the variation of the mass fraction of ethanol in relation to the specific mass of the ethanol mixture in distilled water and in produced water, in a range of low concentration of ethanol at 20° C. It is observed that, for the concentration range of interest of ethanol, the relationship is linear and very precise. However, the calibration curve must be made with the water produced by the well. Other physicochemical parameters can be used, such as viscosity, refractive index, electrical conductivity, etc.

EXAMPLES

[0059] So far, five applications of the method have been carried out in offshore gas-lift wells, the first in a dry completion well and the others in wet completion wells:

Example 1

[0060] In this application, the method was used to determine the gas injection point in a dry completion well. The return of alcohol was recorded and a conclusion regarding the well status was issued, indicating simultaneous injection through two gas-lift valves.

Example 2

[0061] In this case, the method was applied in a well with gas-lift and production lines of 4 km in length and with gas injection by the WCT (X-O). The objective was to calibrate and improve the method. The return of alcohol was very clear, complete, and compatible with the time estimated by simulation. The signals of the process variables confirmed the passage of alcohol.

Example 3

[0062] The third application was made in a well with very long lines (12 km), being an extreme test for said invention. No return of alcohol was recorded during the monitored time period. This well normally receives gas from a manifold and was specially aligned for a service line that was used by another well. There is no evidence whether the reason for not having recorded the return of alcohol is related to this or not.

Examples 4 and 5

[0063] These applications were carried out in two wet completion wells that produce by gas-lift with 2 km lines, to calibrate the method in one case and identify a possible hole in the string in the other case. In the first case, the injection in the deepest gas-lift valve was identified, with no evidence of a hole or gas leak. In the second case, a probable hole was identified in the production string in the shallowest third of the well.

[0064] Although ethanol is the preferred alcohol for application in this invention, another type of alcohol can be used, such as methanol and MEG, without significant changes in the procedures. If the chosen alcohol is not available on board, it can be boarded. Also, more than one type of alcohol can be injected, simultaneously or in sequence, to improve operational diagnosis.

[0065] If any process variable relevant to the interpretation of the alcohol injection operation is not being regularly monitored, due to a sensor failure, for example, digital portable equipment for the measurement and acquisition of data can be boarded to be used during the operation.

[0066] Although an operational diagnostic procedure, such as alcohol injection, is normally performed only when a problem is suspected in the well, it is recommended to perform at least one alcohol injection operation during the normal operation of the well to serve as a reference, which helps in the differential diagnosis of the operation performed with the well already presenting an operational problem.

[0067] The detection of holes in the string or casing is of utmost importance to guarantee the integrity of the wells. Periodic investigation of wells can lead to early diagnosis of problems, before they become more serious, requiring immediate intervention.

[0068] Ethanol (the alcohol most suitable for use in this invention), at its return, when diluted, is not toxic to humans or the environment, presenting no known risks to people, the environment or the process.

[0069] Alcohol is a product routinely used on platforms for eventual or permanent injection in the gas-lift. There are already facilities prepared for its application, established procedures, and the operators are already familiar with this type of injection. There is, therefore, no safety risk in addition to that normally encountered in an industrial installation of this type.

[0070] The conventional test, with a run of pressure and temperature profiles, has operational limitations, since there is an insertion of a tool inside the well and, in the case of wet completion wells, it requires the coupling of an intervention vessel to the well. This test also has considerable risks of oil leakage to the platform or the sea. Such risks do not exist in the present invention.

[0071] Regarding the reliability of the production control, the method of the present invention can confirm the indirect analyses performed in the simulations, increasing confidence in the actions to be implemented to solve the problems and optimize production.

[0072] The method of the present invention is conceptually simple and its operational application is also relatively simple, which, with a little more systematization and training, can be performed, in whole or in part, by the platform own operational team, whenever necessary, streamlining greatly in the process of diagnosing operational problems.

[0073] A test by the conventional method of direct diagnosis in gas-lift offshore wells costs from a few hundred to a few million reais, depending on whether the well is of dry or wet completion. In turn, testing by the method of this invention has a much lower cost. The diagnostic result can lead to more accurate operational decisions, saving resources, reducing costs and increasing the revenue, with values that can even be much more expressive than the simple savings previously reported in relation to the cost of conventional testing.

[0074] It should be noted that, although the present invention has been described in relation to the procedure referring to a test in an offshore well of dry or wet completion, operating with gas-lift and with dedicated lines, it may undergo modifications and adaptations for other situations of onshore or offshore wells in different arrangements, such as configurations with gas injection or production with manifolds or rings, depending on the specific situation, but provided that within the inventive scope defined herein.