The present invention relates to a fluid density monitor, and a method of monitoring fluid density.
In order to produce efficiently from a hydrocarbon reservoir, it is desirable to monitor and control the movement of fluids in the borehole and in the reservoir.
Borehole fluid monitoring is usually performed by permanent devices such as downhole flowmeters and gradiomanometers. However, these require calibration, power supplies and involve complex equipment.
Reservoir fluid monitoring may be performed by electrical resistivity measurements on the bulk formation. These measurements can detect the replacement of hydrocarbon by conductive brine in formation porosity and so e.g. monitor the approach towards the borehole fluid inlet points of the water front (i.e. the boundary in the reservoir between the predominantly oil and predominantly water phases). However, because the electrical conductivities of oil and gas are similar, the technique is less useful for detecting the movement of the oil/gas front. Also the equipment for measuring electrical resistivity extends outside the borehole casing, which is inconvenient and increases cost and complexity.
Wireline radioactive logging is also used to evaluate hydrocarbon wells. Naturally occurring radioactivity is used to identify shale and non-shale zones by total gamma-ray count rates, and to determine mass concentrations of formation radioisotopes (mainly Th, U and K). Information obtained in this way can also be used e.g. to correlate zones in different wells and control depth when running other logging tools.
We have found that it is possible to monitor fluid densities by analysing natural background radiation traversing the fluid, the proportion of natural background radiation able to traverse the fluid being related to the density of the fluid. In general terms the present invention relates to a method and apparatus for monitoring subterranean (e.g. in formation or in a borehole) fluid densities by analysing natural background radiation traversing the fluid. In the following, by xe2x80x9crelative densityxe2x80x9d we mean the density of a fluid calculated from one measurement relative to the density of the fluid calculated from other measurements.
In a first aspect, the present invention provides a method of monitoring the density of a subterranean fluid, comprising the steps of:
(a) using a subterranean radiation detector to obtain measurements over a plurality of time intervals of the amount of natural background radiation (typically gamma-ray radiation) traversing the fluid, and
(b) calculating the relative density of the fluid at each of the time intervals from the measurements.
By monitoring fluid density in this way it is possible to detect changes in fluid density in the pores of a subterranean formation. Such changes may be associated with, e.g. the passage of a fluid front in a formation (such as a water front or an oil/gas front) in which the fluids to either side of the front have different densities. A particular advantage over electrical resistivity measurements is the ability to detect the presence of an oil/gas front, oil and gas generally having different densities.
Also, unlike the equipment for electrical resistivity measurements, which extends outside the wellbore casing, the detector of the present invention can be located entirely within a well bore and is therefore much less disruptive and invasive. Preferably the detector is located between the production tubing and the wellbore casing (e.g. mounted to a mandrel), where it does not interfere with hydrocarbon production, but other locations are also possible, such as within the production tubing or outside the casing (which generally involves cementing the detector to the casing).
Preferably the detector is held stationary so that the density is monitored at a specific location.
Preferably the detector is permanently or semi-permanently installed below ground (i.e. rather than being operated as part of a wireline tool), so that interruptions to hydrocarbon production caused by moving the detector into position are avoided.
The duration of each time interval may be at least ten minutes. However, the duration may be e.g. at least one hour or at least one day. Longer durations provide increased detection sensitivity, but clearly make density monitoring more time-consuming.
Gamma-ray detectors can be relatively simple and robust devices, comprising essentially a scintillator (e.g. a crystal of NaI(Tl)) and a photomultiplier in a suitable housing. They are, therefore, particularly suitable for permanent or semi-permanent downhole installation.
The method can also be used to monitor fluid density within a borehole e.g. so that water/oil ratios can be monitored.
Shielding may be provided around parts of the detector so that the natural background radiation arrives principally from only certain directions. For example, the detector may be shielded from the production tubing. This reduces the amount of detected radiation e.g. (i) received from tubing scale which contains naturally occurring radioactive material (NORM) and (ii) affected by changes in fluid density within the production tubing. In this way, the influence of formation pore fluid density changes on the measurements can be substantially isolated so that such changes can be better monitored.
Alternatively the detector may be shielded so that to a significant extent it only receives background radiation which has traversed the production tubing. This allows the detector principally to monitor fluid densities within the production tubing rather than in the formation.
A particular advantage of the present invention is that the detector does not need to be provided with a dedicated source of radiation, such as an artificial gamma-ray source or an X-ray generator. This reduces equipment cost, complexity and power consumption.
Preferably the method further comprises the step of performing a spectroscopic analysis of the radiation arriving at the detector to distinguish naturally occurring formation radioisotopes from other sources of activity, such as NORM in production tubing scale.
In a second aspect, the present invention provides a method of locating the position of a subterranean fluid front, comprising the steps of:
(a) using a plurality of spaced subterranean radiation detectors to obtain respective measurements of the amount of natural background radiation traversing said fluid during a time interval,
(b) calculating a relative density of said fluid from each of said measurements, and
(c) correlating the position of a change in the relative density of said fluid with the position of said fluid front. The advantages, optional preferred features and alternative embodiments discussed above in relation to the first aspect of the invention apply also to this aspect of the invention.
Preferably the method further comprises the step of:
(d) repeating steps (a) to (c) for subsequent time intervals to track the progress of said fluid front.
So by deploying an array of detectors it is possible not only to detect and locate a fluid front, but also to track the movement of that front, e.g. as the front moves sequentially past some or all of the detectors.
Indeed, sequential detection of fluid density changes by some or all of the detectors is characteristic of the movement of a fluid front. This allows natural background radiation arriving at the detectors to be distinguished from other (static) sources such as NORM in scale.
In a third aspect, the present invention provides an apparatus for monitoring the density of a subterranean fluid, comprising a radiation detector and a signal processor. The radiation detector is adapted to obtain measurements over a plurality of time intervals of the amount of natural background radiation traversing said fluid, and is further adapted to send corresponding measurement signals to the signal processor. The signal processor is adapted to receive the measurement signals and configured to calculate the relative density of the fluid at each of the time intervals from the measurement signals.
The signal processor may be a programmable computer suitably programmed to calculate the relative density of the fluid. Alternatively the processor may comprise dedicated hardware, e.g. based on ROM, for performing the calculation.
The detector may have protective shielding which is adapted, in use, substantially to prevent radiation arriving at the detector from certain directions.
The signal processor may also be configured to perform a spectroscopic analysis of the measurement signals to distinguish the natural background activity from radiation traversing the fluid from other sources of radiation, such as NORM in scale.
Preferably the detector is adapted to be held stationary at a specific location in a hydrocarbon well. More preferably the detector is adapted to be permanently or semi-permanently installed below ground.
The detector may be adapted to be e.g. mounted to a hydrocarbon well production tubing, or cemented to a hydrocarbon well borehole casing.
The apparatus may further comprise a plurality of the radiation detectors.
The apparatus is, therefore, suitable for performing the method of the first or second aspect of the invention.