CN111094697A

CN111094697A - Improvements in or relating to injection wells

Info

Publication number: CN111094697A
Application number: CN201880041300.0A
Authority: CN
Inventors: F·J·桑塔蕾丽
Original assignee: Geological Engineering Co
Current assignee: Geological Engineering Co
Priority date: 2017-06-22
Filing date: 2018-06-21
Publication date: 2020-05-01
Also published as: CA3066970A1; GB201709960D0; AU2018288016A1; WO2018234806A1; GB2563643B; EP3642452A1; MX2019015446A; GB2563643A; US20210148227A1; EA201891497A1

Abstract

A method for determining cross-flow in an injection well in a multi-layered reservoir. A wellhead pressure measurement is made and the characteristic curve is analyzed to identify late wellbore storage indicative of cross-flow. The water hammer pressure wave can also be analyzed at shut-in locations to identify if a rat hole is filled with sand. By measuring the wellhead pressure, the injection well can be continuously monitored with less intervention so that action can be taken to reduce sand production and maintain injectability.

Description

Improvements in or relating to injection wells

Technical Field

The present invention relates to the injection of fluids into wells and more particularly, but not exclusively, to a method for determining cross-flow and presence of sand in an injection well such that steps are taken to reduce sand production and maintain injectivity.

Background

Current hydrocarbon production is primarily focused on maximizing the recovery factor from the well. This is because we have mined all areas that may contain oil, leaving only those areas of the world that are in remote and environmentally sensitive areas (e.g., north and south). Despite the large number of unconventional hydrocarbons, such as very viscous oils, oil shale, shale gas, and natural gas hydrates, many of the technologies that utilize these resources are either very energy intensive (e.g., steam injection into heavy oil) or politically/environmentally sensitive (e.g., "cracking" to recover shale gas).

To increase the recovery factor in a well, it is now common to inject a fluid (usually water) into the reservoir through an injection well. This form of enhanced oil recovery uses injection of water to increase the depletion pressure within the reservoir and also moves the oil into position so that it can be recovered. This also results in environmental benefits if the production water is re-injected.

During injection of fluids into a multi-layered reservoir, different pressure gradients are created across the surface of each permeable zone. This pressure gradient generates a driving force in the wellbore during shut-in, causing the injected fluid to move from a high pressure zone to a low pressure zone, a phenomenon known as cross-flow. If the formation created during the cross flow is weak, sand may be created. As a result of gravity settling, this sand will gradually accumulate in the rat hole over time. The porosity of the formation near the perforations in the production interval will gradually increase over time. Over time and the number of shut-in wells, the porosity of the formation near the perforations in the production interval will reach a critical limit. Once this limit has been reached, the formation surrounding the perforations in the production interval may be liquefied by the water hammer pressure wave that occurs at the time of injection. Formation liquefaction results in complete collapse and a large influx of sand into the well. In some cases, the well may lose all of its injectivity due to being sand filled. Thus, in order to take steps to reduce sand production, maintain injectivity, and increase well life expectancy, cross-flow and the presence or absence of sand needs to be determined in the injection well.

Current techniques first determine a drop in injectability index across the shut-in well, which is a measure of the injection rate of the injection pressure (corrected for bottom hole flow conditions) minus the far-field reservoir pressure. Next, an ILT (injection logging instrument) log is run to measure cross-flow. This log provides the cross-flow rate and direction at a given time after shut-in. A bailer barrel or ring gauge is operated to measure the sand fill level in the mousehole. Finally, a sand strength analysis is performed to establish a sand risk. This data is analyzed to determine whether to isolate an injection interval and/or to force a progressive shut-in procedure, which may not always be allowed, to avoid sand fluidization whenever possible, i.e., pump tripping.

The first diagnosis indicated a potential sanding problem in the injector, which was considered to be a loss of injectability index across the shut-in well. Unfortunately, other reservoir related factors may lead to an exponential decline in injectability across shut-in wells. It has been found that cross-flow cannot be accurately simulated at some time after well initiation due to changes in reservoir pressure in the layers, the remainder being largely unknown. The only way to confirm cross-flow is to run an Injection Logging Tool (ILT) on the cable and measure the flow during shut-in. This is the current technology. Currently, the only way to confirm mousehole filling is by running a bailer on the cable. Unfortunately, cable deployment is inherently expensive: less for dry wellheads and more for subsea wells. A further disadvantage is the fact that well intervention is required. Running the cable can interrupt the well injection for hours to days, which in turn can delay production.

It is also known to monitor the drop in Bottom Hole Pressure (BHP) in an injection well during shut-in. Fig. 1(a) shows a plot of BHP a and injection rate B versus time C. Four points, labeled 1 to 4, are marked to show the position on the BHP descending curve. The so-called signature curve is a diagnostic plot in which the measure of the decline in BHP during shut-in is shown in the log curve as:

this is illustrated in fig. 1(b) as curve D. It can be seen that the four points 1 to 4 are matched to the different slopes of the curve D. Based on the most commonly used solution of the pressure diffusion equation in the test well, different flow states can be detected from the slope of the plotted trend D matching these four points as: slope 1 has a gradient 1, which can be considered wellbore storage, which is expected to be very short for a water injector that is a low compressibility fluid; slope 2 has a gradient of 0.5 indicating linear flow; slope 3 has a gradient of 0.25 representing bilinear flow; and slope 4 has a gradient of zero which is a pure radial flow.

Disclosure of Invention

It is an object of the present invention to provide a method for determining cross-flow in an injection well, which method obviates or mitigates at least some of the disadvantages of the prior art.

It is a further object of at least one embodiment of the present invention to provide a method for determining the presence of sand in a rat hole of an injection well that obviates or mitigates at least some of the disadvantages of the prior art.

According to a first aspect of the present invention, there is provided a method for determining cross-flow in an injection well, the method comprising the steps of:

(a) injecting a fluid into the well;

(b) closing the well;

(c) measuring pressure at the well;

(d) constructing a characteristic curve; and

(e) identifying later wellbore storage.

In the prior art profile, wellbore storage is the first flow condition after shut-in. The inventors have surprisingly determined that late wellbore storage, i.e. wellbore storage indicated by a slope of one gradient in the characteristic curve occurring at some time after shut-in was initiated, indicates that it is a cross-flow inflow.

Preferably, the pressure is measured at the wellhead. Thus, the determination of cross-flow may be a less intrusive process. Which makes it more cost effective. More preferably, the pressure is measured continuously during shut-in. In this way, cross-flows can be identified immediately and preventive measures can be implemented quickly. Preferably, the pressure is measured at a frequency of about 1 Hz. In this manner, multiple data points are aggregated to provide a clear indication for later wellbore storage.

Preferably, the duration of the cross-flow is measured on the characteristic curve by the duration of the slope length stored in the later wellbore. In this way, alternative information about cross-flow is provided on the over-the-prior-art ILT log that measures the cross-flow rate and direction at a given time.

Preferably, step (b) is a hard shut-in. In this way, the shut-in is instantaneous. More preferably, a water hammer pressure wave is created upon shut-in. In this way, the water hammer pressure wave can be used to obtain further information. Preferably, the rate at which fluid is last injected into the well is calculated at step (a) to provide sufficient water hammer amplitude without damaging the well. In this manner, analysis of the water hammer pressure wave in the well may begin. Preferably, the attenuation of the water hammer pressure wave is analyzed to identify the presence of filler in the rat hole. In this way, the rapid attenuation of the water hammer pressure wave shows that the rat hole is filled with debris. Preferably, the reflection of the water hammer pressure wave is analyzed to identify clean rat holes. In this way, if the water hammer pressure wave is reflected, the rat hole is debris free. Thus, the present invention will identify the presence of sand filling in a rat hole, as compared to prior art bailing canister that measures sand filling levels in a rat hole. More preferably, the analysis of the wellhead pressure with respect to the measurements is completed. In this way, wellhead pressure can be used both in detecting packed sand in a mousehole and determining cross-flow in a well.

Preferably, the method comprises the initial step of establishing an exponential decline in injectivity across shut-in wells. Preferably, the method comprises the further step of performing a sand strength analysis to establish a risk of sand production. In this way, the present invention is incorporated into the prior art steps by replacing the intervening steps of running the ILT log and the bailer.

Preferably, the method comprises the further step of taking action in response to a determination of cross-flow in the well. Such action may take the form of isolating one or more injection segment layers. In this way, cross-flow is prevented. Alternatively, such an action may be to force a gradual shut-in procedure. In this way, sand fluidization can be avoided.

Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. Also, the terms and phrases used herein are for descriptive purposes only and are not to be construed as limiting the scope of the language, such as including, having, containing or relating to and variations thereof, which is intended to be broad and encompass the subject matter listed thereafter, equivalents thereof and additional subject matter not listed, and is not intended to exclude other additives, components, integers or steps. Also, for the purposes of applicable law, the term comprising is considered synonymous with the term comprising or containing. Any discussion of documents, acts, materials, devices, articles or the like is included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base on the common general knowledge in the field relevant to the present invention. All numerical values in this disclosure are understood to be modified by the word "about". All singular forms of elements or any other components described herein should be understood to include the plural forms thereof and vice versa.

Although the description will refer to up and down as well as uppermost and lowermost, these are to be understood as relative terms with respect to the wellbore, and the inclination of the wellbore, although illustrated vertically in some figures, may be inclined or even horizontal.

Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIGS. 1(a) and 1(b) show prior art graphs of BHP and injection rate versus time, respectively, and a characteristic curve;

FIG. 2 is a graph depicting a characteristic curve of late wellbore storage and thus cross-flow, in accordance with an embodiment of the present invention;

FIG. 3 is a schematic illustration of an injection well including wellhead pressure monitoring used in a method according to an embodiment of the invention;

fig. 4 is a graph showing cross-flow rate versus time for natural cross-flow;

FIG. 5 is a graph depicting cross-flow rate versus time for forced cross-flow; and

FIG. 6 is a graph of pressure versus time for two shut-in wells showing reflection and attenuation of hydraulic hammer waves to define a rat hole, in accordance with an embodiment of the present invention.

Detailed Description

Reference is first made to figure 2 of the drawings, which is a graph of a characteristic curve, generally indicated by reference numeral 10, obtained following shut-in of an injection well 30. The wellhead pressure 12 is monitored and at a later time 14 after shut-in, it can be seen that the curve 10 shows a trend towards slopes 16a, b. The gradient of each slope 16a, b is approximately one. According to an embodiment of the invention, the slopes 16a, b are indicative of wellbore storage, and thus the characteristic curve 10 exhibits a late wellbore storage 18 indicative of cross-flow 20 in the well 30.

Reference is now made to figure 3 of the drawings, which shows an injection well 30. Injection wells 30 are known in the art. Downhole components and completions are not shown to aid clarity, and dimensions are also greatly varied to highlight important areas of interest. The well 30 is in a multi-layered reservoir 22.

Porous regions

24, 26 have been identified and are separated from each other by impermeable regions 28. The tubes 32 are perforated at 34, 36 in each of the

permeable zones

24, 26, respectively.

At the surface 38, there is a standard wellhead 40. The wellhead 40 provides a conduit (not shown) for the passage of fluids into the well 30. The wellhead 40 also provides a conduit 42 for injecting fluid from a pump 44. Wellhead sensor 46 is located on wellhead 40 and is controlled by data acquisition unit 48, which also collects data from wellhead sensor 46. The data acquisition unit 48 may analyze the data and/or transmit the data to a remote location. The wellhead sensors 46 include temperature sensors, pressure sensors, and flow rate sensors. The sensor 46 has a sampling frequency between 0.2Hz and 1 Hz. Preferably, the pressure sensor samples at a rate of 1 Hz. Other sampling frequencies may be used, but they must be sufficient to measure the change in pressure when shut-off occurs. All of these surface components are standard at the wellhead 40.

Following injection, the well 30 is shut in by known techniques and an injection and pressure profile as shown in fig. 1(a) is provided. During injection, a different pressure gradient is generated across each

permeable zone

24, 26. This pressure gradient generates a driving force in the wellbore during shut-in, causing the injected fluid to move from a high pressure zone to a low pressure zone, a phenomenon known as cross-flow. If the formation created during the cross flow is weak, sand may be created. As a result of gravity settling, this sand will gradually accumulate in the rat hole over time. In fig. 3, it is shown that sand 38 is produced from the upper region 24 when cross flow occurs from the upper region 24 to the lower region 26. Additionally, sand 38 has fallen out and settled at the bottom of the well in the rat hole 50.

Cross-flow and consequent sand production have been considered in f.j.santarelli, e.skomedal, p.markestad, h.i.berge and h.nasvik (1998). Sand production on a water injector: how can it be obtained? Paper SPE/ISRM47329, proc. eurock'8conf. volume 2, page 107-115, and f.j.santarelli, f.sanfilippo, j.m.embry, m.white and j.turnbull (2011). Sanding mechanisms for water jet and their dosing for sanding-an example of the Buzzard field. The article SPE 146551, proc.2011spe ATCE, incorporated herein by reference. These findings are:

when one injector is completed in multiple zones, it can cross-flow during shut-in;

sand production may occur if the formation production during cross-flow is weak;

this sand will build up in the rat hole over time due to gravity settling;

the porosity of the formation in the vicinity of the perforations in the production interval will gradually increase over time;

over time and the number of shut-in wells, the porosity of the formation near the perforations in the production interval will reach a critical limit;

once this limit has been reached, the formation surrounding the perforations in the production interval may be liquefied by the water hammer pressure wave;

liquefaction of the formation results in total collapse and a large influx of sand in the well; and

in some cases, the well may lose all of its injectivity due to being sand filled.

Cross-flow in multi-layered reservoirs has been analyzed in m.jalali, j.m.embry, f.sanfilippo, f.j.santarelli and m.b.dusseault (2016). Cross-flow analysis of injection wells in multi-layered reservoirs, Petroleum2, 273- "281, is incorporated herein by reference. This paper simulates cross-flow behaviour, which depends on the initial pressure in the permeable layer and can be referred to as natural cross-flow (same or natural initial pressure) and forced cross-flow (different initial pressure due to production).

Fig. 4 shows a simulated natural cross-flow over four layers 52a-d in an injection well. This allows for a 48 hour injection period at a rate of 35,000bpd followed by a 48 hour shut-in. Cross-flow rate (bls/day) 54 is plotted against time 14 after shut-in. Negative cross flow indicates inflow, while positive cross flow indicates outflow. It can be seen that the maximum cross-flow rate occurs about 2 hours after shut-in, with negligible cross-flow after about 36 hours. This demonstrates that the period of cross-flow is short for natural cross-flow when the layers are in pressure equilibrium.

Fig. 5 shows the forced cross-flow simulated for five layers 56a-e under the same injection and shut-in conditions as in fig. 4, but in this case the five layers 56a-e are at non-pressure equilibrium, so the pressure difference between the layers 56a-e becomes the main driving force after the cross-flow. Cross-flow rate (bls/day) 54 is plotted against time 14 after shut-in. Here it can be seen that there is an initial period of natural cross-flow which can be considered as "in-between" cross-flow, wherein the direction of cross-flow can change over time, followed by pure forced cross-flow, resulting in a mixed inflow and outflow across these layers 56 a-e. The forced cross-flow lasts as long as the shut-in period.

Returning now to fig. 2, the effect of this forced cross-flow is seen in the measurement of wellhead pressure, as demonstrated on characteristic curve 10. The data for this plot was taken from an injection well, with two different intervals perforated. The lower interval was first tested alone and the results showed that it was not connected to the main reservoir (pressurized after injection). Then, the second interval is perforated and the two intervals are tested together. The wellhead pressure is monitored and the resulting curve 10 of fig. 2 is provided. The results show a linear flow (gradient 0.5) slope 17 indicating the fracture prior to late wellbore storage indicated by the slope with the gradient. The ILT log is also taken and this shows a 1000bpd cross-flow from isolated to connected intervals at time 58, thus validating the cross-flow identified by the wellhead pressure measurement and showing the profile 10 of the late wellbore storage 18 (cross-flow) of the well 30.

Thus, in an injection well, the step of running the ILT log to measure cross-flow, i.e. the cross-flow rate and direction at a given time after shut-in, can now be replaced by analyzing only the wellhead pressure. Analysis of the later wellbore stored profile determines the duration of the cross-flow (i.e., natural versus forced). By measuring the wellhead pressure, the method can be performed continuously, and thus certain cross-flows can be identified, and immediate steps taken to reduce detrimental effects, i.e. by isolating intervals and changing shut-in procedures to avoid sand fluidization.

Reference is now made to fig. 6 of the drawings, which shows the pressure 12 measured at the wellhead 40, versus time 14. Thus, the figure shows two shut-in wells 60a, b and a first pressure response 62 and a second pressure response 64 in the shut-in wells. In each case, the well 30 is shut in hard, i.e., as instantaneously as possible. It is known that such shut-in will result in the formation of a water hammer pressure wave. The final injection rate was calculated to have sufficient water hammer amplitude (but not too great) to avoid damaging the well. The frequency of the pressure recordings remains high (typically 1Hz) when the well is shut in, so that the water hammer effect can be seen in time. In the first shut-in well 60a, a pressure response 62 shows that the pressure wave is reflected, indicating that the rat hole 50 is free of debris (i.e., sand). Conversely, on the second shut-in well 60b, the pressure response shows that the pressure wave is rapidly attenuated, indicating that the rat hole 50 is filled with debris.

Thus, in an injection well, it is now possible to replace the step of running a bailer barrel or ring gauge to measure the sand fill level in the mousehole with only an analysis of the wellhead pressure. By analyzing the attenuation of the water hammer pressure wave at the shut-in, the presence of a filling in the rat hole can be identified.

The principle advantage of the present invention is that it provides a less intrusive method for determining cross-flow in an injection well.

A further advantage of an embodiment of the present invention is that it provides a less intrusive method for determining sand filling in a rat hole of an injection well.

Yet another advantage of the present invention is that it provides a method of determining cross-flow in an injection well using wellhead pressure measurements. These have already been recorded and therefore require minimal changes to the current measurements.

Yet another advantage of an embodiment of the present invention is that it provides a method for determining sand filling in a mousehole of an injection well, which uses wellhead pressure measurements.

Those skilled in the art will recognize that modifications may be made to the present invention without departing from the scope thereof. For example, if the injection well has downhole pressure gauges, these pressure gauges may be used. The frequency of pressure measurement recordings may be varied to reduce the amount of data collected. The data may be analyzed in real time or stored for later analysis. Injection wells have been described, but the invention extends to wells injected with other fluids.

Claims

1. A method for determining cross-flow in an injection well, the method comprising the steps of:

(a) injecting a fluid into the well;

(b) closing the well;

(c) measuring pressure at the well;

(d) constructing a characteristic curve; and

(e) identifying later wellbore storage.

2. The method of claim 1, wherein the pressure is measured at a wellhead.

3. The method of claim 2, wherein the pressure is measured continuously during shut-in.

4. The method of claim 2 or claim 3, wherein the pressure is measured at a frequency of about 1 Hz.

5. The method of any preceding claim, wherein the duration of cross-flow is measured on the characteristic curve by the duration of the slope of the late wellbore storage.

6. The method of any preceding claim, wherein step (b) is hard shut-in.

7. A method according to any preceding claim, wherein a water hammer pressure wave is generated upon shut-in of the well.

8. The method of claim 7, wherein the final injection rate of fluid into the well at step (a) is calculated to provide sufficient water hammer amplitude without damaging the well.

9. The method of claim 7 or claim 8, wherein the attenuation of the water hammer pressure wave is analyzed to identify the presence of a filler in a rat hole.

10. The method of claim 9, wherein the rapid attenuation of the water hammer pressure wave shows the rat hole filled with debris.

11. The method of claim 9, wherein the reflection of the water hammer pressure wave exhibits a mousehole free of debris.

12. The method of any one of claims 9 to 11, wherein measurements of wellhead pressure are used both to detect sand fill in a mousehole and to determine cross-flow in the well.

13. A method according to any preceding claim, wherein the method comprises the following initial step of establishing a reduction in injectability index across a shut-in well.

14. A method according to any preceding claim, wherein the method comprises the further step of performing a sand strength analysis to determine the risk of sand production.

15. The method of any preceding claim, wherein the method comprises the further step of isolating one or more injection intervals and/or forcing a progressive shut-in procedure.