Relating To Injection Wells

Description

The present invention relates to injecting fluids into wells and more particularly, though not exclusively, to a method for determining cross-flow and the presence of sand in an injection well in order to take steps to reduce sand production and maintain injectivity.

Current hydrocarbon production is primarily focussed on maximising the recovery factor from a well. This is because we have already exploited all the areas which might contain oil leaving only those that are in remote and environmentally sensitive areas of the world (e.g. the Arctic and the Antarctic). While there are huge volumes of unconventional hydrocarbons, such as the very viscous oils, oil shales, shale gas and gas hydrates, many of the technologies for exploiting these resources are either very energy intensive (e.g. steam injection into heavy oil), or politically/environmentally sensitive (e.g. ‘fraccing’ to recover shale gas).

To improve the recovery factor in a well it is now common to inject fluids, typically water, into the reservoir through injection wells. This form of improved oil recovery uses injected water to increase depleted pressure within the reservoir and also move the oil in place so that it may be recovered. If produced water is re-injected this also provides environmental benefits.

During fluid injection into a multi-layered reservoir, a different pressure gradient is generated across the face of each permeable zone. This pressure gradient generates driving forces in the wellbore during well shut-in that causes the injected fluid to move from higher pressure zones to lower pressure zones, a phenomenon known as cross-flow. If the formation producing during cross-flow is weak, sand may be produced. This sand will progressively accumulate in the rat-hole over time due to gravitational settling. The porosity of the formation near the perforations in the producing interval will increase progressively over time. Over time and a number of shut-ins the porosity of the formation near the perforations in the producing interval will reach a critical limit. Once this limit has been reached, the formation around the perforations in the producing interval can be liquefied by the water hammer pressure wave which occurs on injection. The liquefaction of the formation results in a total collapse and massive sand influx in the well. In some cases, the well may lose all injectivity by being filled with sand. Therefore, the determination of cross-flow and the presence of sand in an injection well is needed in order to take steps to reduce sand production, maintain injectivity and increase well life expectancy.

Current techniques first establish the injectivity index decrease across shut-in, were the injectivity index is a measure of injection rate to the injection pressure (corrected for bottom hole flowing conditions) minus the far-field reservoir pressure. Next an ILT (injection logging tool) log is run to measure cross-flow. This log provides cross-flow rate and direction at a given time after shut-in. A sand bailer or a ring gauge is run to measure the level of sand fill in the rat-hole. Finally a sand strength analysis is performed to establish the risk of sand production. This data is analysed to decide whether or not to isolate one of the injection intervals and/or enforce procedures of progressive shut-in to avoid sand fluidisation whenever possible—i.e. pump tripping may not always allow it.

The first diagnosis indicating a potential sanding problem in a water injector is considered as a loss of injectivity index across shut-in. Unfortunately other reservoir related factors may lead to injectivity index decreases across shut-in. It has been found that cross-flow cannot be modelled accurately sometime after the well's start-up due to changes of the reservoir pressure in various layers, remaining largely unknown. The only way to confirm cross-flow is by running an Injection Logging Tool (ILT) on wireline and measuring flow during the shut-in pass. This is the current technique. Currently, the only way to confirm filling of the rat-hole is by running a sand bailer on wireline. Unfortunately, wireline deployment is expensive in itself: less for dry wellhead and more for subsea wells. More disadvantageously is the fact that well intervention is required. Running wireline interrupts the well injection for hours to days which in turn delays production.

It is also known to monitor the decline in the bottom hole pressure (BHP) in an injection well during shut-in. FIG. 1(a) shows a plot of BHP A and injection rate B against time C. Four points, labelled 1 to 4 are marked to show positions on the decline curve of the BHP. The so-called characteristic curve is a diagnostic plot in which the measurements of the decline of the BHP during shut-in are shown in a log-log chart as:

dBHP d  ln  ( Δ  t )   vs .  Δ  t

This is as illustrated in FIG. 1(b) as curve D. The four points, 1 to 4, can be seen to match different slopes of the curve D. Based on the most used solutions of the pressure diffusion equations in well testing, different flow regimes can be detected according to the slope of the plotted trend D matching the four points as: Slope 1 has a gradient of 1 and can be considered as wellbore storage which is expected to be very short for a water injector as it's 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 with a gradient of zero, is pure radial flow.

It is an object of the present invention to provide a method for determining cross-flow in an injection well which 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 the rat-hole of an injection well which 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, comprising the steps:

• • (a) injecting a fluid into a well;
  • (b) shutting-in the well;
  • (c) measuring pressure at the well;
  • (d) constructing a characteristic curve; and
  • (e) identifying late wellbore storage.

In the characteristic curve of the prior art, wellbore storage is the first flow regime following shut-in. The inventor has surprisingly determined that late wellbore storage i.e. wellbore storage indicated by a slope with a gradient of one in the characteristic curve occurring sometime after shut-in has begun, indicates an in-flow which is cross-flow.

Preferably, the pressure is measured at the wellhead. In this way, the determination of cross-flow can be an intervention-less process. This makes it more cost effective. More preferably, pressure is measured continuously during shut-in. In this way, cross-flow is immediately identified and preventative measures can be implemented quickly. Preferably the pressure is measured at a frequency of around 1 Hz. In this way, multiple data points are gathered to provide a clear indication of late wellbore storage.

Preferably, duration of cross-flow is measured on the characteristic curve by the time duration of a length of a slope of the late wellbore storage. In this way, alternative information on cross-flow is provided over the prior art ILT log which measures 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 at shut-in. In this way, the water hammer pressure wave can be used to gain further information. Preferably, a last injection rate of fluid into the well at step (a) is calculated to provide sufficient water hammer amplitude without damaging the well. In this way, analysis of the water hammer pressure wave in the well can be undertaken. Preferably, attenuation of the water hammer pressure wave is analysed to identify the presence of fill in the rat-hole. In this way, rapid attenuation of the water hammer pressure wave shows that the rat-hole is filled with debris. Preferably, reflection of the water hammer pressure wave is analysed to identify a clean rat-hole. In this way, if the water hammer pressure wave is reflected, then the rat-hole is free of debris. Thus the present invention will identify the presence of fill in the rat-hole in contrast to the prior art use of a sand bailer which measures the level of sand fill in the rat-hole. More preferably, analysis is done on the measured wellhead pressure. In this way, the wellhead pressure can be used to both detect sand-fill in the rat-hole and determine cross-flow in the well.

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

Preferably, the method includes the further steps of taking action in response to the determination of cross-flow in the well. Such action may take the form of isolating one or more injection intervals. In this way, cross-flow is prevented. Alternatively such action may be to enforce procedures of progressive shut-in. 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. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope languages such as including, comprising, having, containing or involving and variations thereof is intended to be broad and encompass the subject matter listed thereafter, equivalents and additional subject matter not recited and is not intended to exclude other additives, components, integers or steps. Likewise, the term comprising, is considered synonymous with the terms including or containing for applicable legal purposes. Any discussion of documents, acts, materials, devices, articles and 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 form part of the prior art based on a common general knowledge in the field relevant to the present invention. All numerical values in the disclosure are understood as being modified by “about”. All singular forms of elements or any other components described herein are understood to include plural forms thereof and vice versa.

While the specification will refer to up and down along with uppermost and lowermost, these are to be understood as relative terms in relation to a wellbore and that the inclination of the wellbore, although shown vertically in some Figures, may be inclined or even horizontal.

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

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

FIG. 2 is a graph of the characteristic curve illustrating late wellbore storage and consequently cross-flow according to an embodiment of the present invention;

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

FIG. 4 is a graph of cross-flow rate against time illustrating natural cross-flow;

FIG. 5 is a graph of cross-flow rate against time illustrating forced cross-flow; and

FIG. 6 is a graph of pressure against time for two shut-ins illustrating reflection and attenuation of the water pressure hammer wave to qualify the rat-hole according to an embodiment of the present invention.

Reference is initially made to FIG. 2 of the drawings which is a graph of a characteristic curve, generally indicated by reference numeral 10, taken following shut-in of a water injection well 30. Wellhead pressure 12 was monitored and towards the later times 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. The slopes 16a,b indicate wellbore storage and the characteristic curve 10 therefore exhibits late wellbore storage 18, indicating cross-flow 20 in the well 30, according to an embodiment of the present invention.

Now making reference to FIG. 3 of the drawings there is illustrated the injection well 30. Injection well 30 is as known in the art. Downhole components and the completion are not shown to aid clarity and dimensions are also greatly altered to highlight the significant areas of interest. Well 30 is in a multi-layered reservoir 22. Porous zones 24,26 have been identified and are isolated from each other by a non-permeable zone 28. The tubing 32 is perforated 34,36 in each of the permeable zones 24,26, respectively.

At surface 38, there is a standard wellhead 40. Wellhead 40 provides a conduit (not shown) for the passage of fluids into the well 30. Wellhead 40 also provides a conduit 42 for the injection of fluids from pumps 44. Wellhead sensors 46 are located on the wellhead 40 and are controlled from the data acquisition unit 48 which also collects the data from the wellhead sensors 46. The data acquisition unit 48 can analyse and/or transmit data to a remote location. Wellhead sensors 46 include a temperature sensor, a pressure sensor and a flow rate sensor. The sensors 46 have a sampling frequency of between 0.2 Hz 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 changes in the pressure when shut-in occurs. All of these surface components are standard at a wellhead 40.

Following injection the well 30 is shut-in by known techniques and provides the injection and pressure profile as illustrated in FIG. 1(a). During injection a different pressure gradient is generated across each permeable zone 24,26. This pressure gradient generates driving forces in the wellbore during well shut-in that causes the injected fluid to move from higher pressure zones to lower pressure zones, a phenomenon known as cross-flow. If the formation producing during cross-flow is weak, sand may be produced. This sand will progressively accumulate in the rat-hole over time due to gravitational settling. Shown in FIG. 3 is the production of sand 38 from the upper zone 24 as cross-flow occurs from the upper zone 24 to the lower zone 26. Additionally, sand 38 has fallen out and is settling in the base of the well in the rat-hole 50.

Cross-flow and the consequential sand production has been considered in F. J. Santarelli, E. Skomedal, P. Markestad, H. I. Berge and H. Nasvik (1998). Sand production on water injectors: Just how bad can it get? Paper SPE/ISRM 47329, Proc. EUROCK'98 Conf., Vol 2. pp 107-115, and F. J. Santarelli, F. Sanfilippo, J. M. Embry, M. White and J. Turnbull (2011). The sanding mechanisms of water injectors and their quantification in terms of sand production—Example of the Buzzard field. Paper SPE 146551, Proc. 2011 SPE ATCE, incorporated herein by reference. These found:

• • When a water injector is completed in multiple zones, it may cross-flow during shut in;
  • If the formation producing during cross-flow is weak, sand may be produced;
  • This sand will progressively accumulate in the rat-hole over time due to gravitational settling;
  • The porosity of the formation near the perforations in the producing interval will increase progressively over time;
  • Over time and a number of shut-ins the porosity of the formation near the perforations in the producing interval will reach a critical limit;
  • Once this limit has been reached, the formation around the perforations in the producing interval can be liquefied by the water hammer pressure wave;
  • The liquefaction of the formation results in a total collapse and massive sand influx in the well; and
  • In some cases, the well may lose all injectivity by being filled with sand.

Cross-flow in multi-layered reservoirs has been analysed in M. Jalali, J. M. Embry, F. Sanfilippo, F. J. Santarelli and M. B. Dusseault (2016). Cross-flow analysis of injection wells in a multi-layered reservoir, Petroleum 2, 273-281, incorporated herein by reference. This paper modelled cross-flow behaviour, being dependent on the initial pressure in the permeable layers and may be referred to as natural cross-flow (identical or natural initial pressures) and forced cross-flow (different initial pressures because of exploitation).

FIG. 4 illustrates natural cross-flow as modelled on four layers 52a-d in an injection well. This considered a 48 hour injection period at a rate of 35,000 bpd followed by a shut-in of another 48 hours. Cross-flow rate (bls/day) 54 is plotted against time 14 after shut-in.

Negative cross-flow indicates an in-flow whereas positive cross-flow indicates out-flow. As can be seen the maximum cross-flow rate occurs around 2 hours after shut-in with negligible cross-flow after around 36 hours. This demonstrates that for natural cross-flow when the layers are at pressure equilibrium, the period of cross-flow is short.

FIG. 5 illustrates forced cross-flow as modelled for five layers 56a-e, under the same injection and shut-in conditions as for FIG. 4, but in this instance the five layers 56a-e are not at pressure equilibrium so the pressure difference between the layers 56a-e becomes the main driving force behind cross-flow. Cross-flow rate (bls/day) 54 is plotted against time 14 after shut-in. Here it is seen that there is an initial period of natural cross-flow, a period which can be considered as ‘in-between’ cross-flow where the direction of cross-flow can change over time, and then pure forced cross-flow resulting in mixed in-flow and out-flow across the layers 56a-e. Forced cross-flow lasts as long as the shut-in period.

Now returning to FIG. 2, the effect of this forced cross-flow is seen in the measurement of wellhead pressure as demonstrated on the characteristic curve 10. The data for this Figure was taken from an injection well in which two distinct intervals were perforated. The lower interval was tested alone first and the results showed that it was not connected to the main reservoir (pressuring after injection). The second interval was then perforated and both intervals tested together. Wellhead pressure was monitored and the resultant plot 10 of FIG. 2 provided. Results show a linear flow (gradient 0.5) slope 17 indicating a fracture, before the late wellbore storage indicated by the slope having a gradient of one. An ILT log was also taken and this showed a 1000 bpd cross-flow at time 58, from the isolated to the connected interval, thus verifying the cross-flow identified by the wellhead pressure measurement and the characteristic curve 10 showing the late wellbore storage 18 (cross-flow) of the well 30.

Consequently, in an injection well the step of running an ILT log to measure cross-flow i.e. cross-flow rate and direction at a given time after shut-in can now be replaced with merely analysing the wellhead pressure. Analysing the characteristic curve for late wellbore storage identifies the duration of the cross-flow i.e. natural vs. forced. By measuring wellhead pressure the method can be performed continuously and thus the determination of cross-flow can be identified and immediate steps taken to reduce the deleterious effects i.e. by isolating intervals and changing shut-in procedures to avoid sand fluidisation.

Reference is now made to FIG. 6 of the drawings which illustrates pressure 12, measured at the wellhead 40, against time 14. The graph shows two shut-ins 60a,b and thus first 62 and second 64 pressure responses to shut-in. In each case the well 30 is submitted to a hard shut-in i.e. as instantaneous as possible. It is known that such a shut-in will cause a water hammer pressure wave to be formed. The last injection rate is calculated to have enough water hammer amplitude but not too much to avoid damaging the well. On shut-in, the frequency of pressure recordings is kept high, typically 1 Hz, so that the hammer effect can be seen in time. In the first shut-in 60a, the pressure response 62 shows that the pressure wave is reflected indicating that the rat-hole 50 is free of debris i.e. sand. In contrast, on the second shut-in 60b, the pressure response shows that the pressure wave is attenuated rapidly indicating that the rat-hole 50 is filled with debris.

Consequently, in an injection well the step of running a sand bailer or a ring gauge to measure the level of sand fill in the rat-hole can now be replaced with merely analysing the wellhead pressure. By analysing the attenuation of the water hammer pressure wave at shut-in the presence of fill in the rat-hole can be identified.

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

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

A still further advantage of the present invention is that it provides a method for determining cross-flow in an injection well which uses wellhead pressure measurements. These are already being recorded and thus minimal changes to current measurements are required.

A yet further advantage of an embodiment of the present invention is that it provides a method for determining sand fill in the rat-hole of an injection well which uses wellhead pressure measurements.

Those skilled in the art will recognise that modifications may be made to the invention herein described without departing from the scope thereof. For example, were an injection well has downhole pressure gauges, these may be used. The frequency of pressure measurement recordal may be varied to reduce the amount of collected data. Data may be analysed in real-time or stored for later analysis. A water injection well has been described but the invention could be extended to wells being injected with other fluids.