Abstract:
A gravity measuring apparatus is provided comprising a gravitational force detector including a test mass and which produces measurements related to gravitational force exerted on the test mass, characterized in that the gravitational force detector is adapted for use downhole and includes a compensator to compensate for errors in measurements made whilst downhole. The compensator may comprise a guide rail along which the gravitational force detector is moveable over a calibrated distance. Where the test mass is biased toward an equilibrium position about which measurements are made.

Description:
FIELD OF THE INVENTION 
     This invention relates to gravity measuring apparatus for use in a borehole, and in particular gravity measuring apparatus which is able to compensate for long term drift in measurements made by the apparatus. 
     BACKGROUND TO THE INVENTION 
     Gravity measurements within boreholes are used to monitor fluid movements within a reservoir, such as oil displacement by water or liquid displacement by gas. Such fluid movements occur normally in oil and gas reservoirs as fluids are produced from or injected into wells. In many cases, it is desirable to control the movement of fluids within the reservoir by controlling production or injection rates to maximise producible reserves or to minimise production of water. However, to intelligently control the movement of reservoir fluids, it is necessary to measure parameters such as bulk formation density which are sensitive to the movement of these fluids. 
     However once the gravity measuring apparatus, or gravity meter, is downhole, it is inaccessible unless withdrawn from the borehole. Long term drift occurs in measurements made by gravity meters and whilst drift can be readily compensated for in the case of surface gravity meters, compensation is a particular problem when gravity meters are placed downhole. This is particularly so as to withstand conditions downhole, stability is often sacrificed for apparatus robustness which increases problems with long term drift. 
     SUMMARY OF THE INVENTION 
     It is one aim of the present invention to provide gravity measuring apparatus which can compensate for drift in measurements made when downhole, and which is sensitive to the movement of fluids within a reservoir. 
     In accordance with one aspect of the present invention there is provided a gravity measuring apparatus comprising gravitational force detection means including a test mass and which produces measurements related to gravitational force exerted on the test mass, characterised in that the gravitational force detection means is adapted for use downhole and includes compensation means to compensate for errors in measurements made whilst downhole. 
     By providing compensation means, measurements relating to the gravitational force can be corrected downhole for offset errors occurring within the apparatus. 
     By measuring the spatial gradient of gravity, offset errors can be compensated for. Thus preferably the compensation means may enable movement of the gravitational force detection means between at least two spaced apart positions to produce at least two measurements, and the compensation means may offset the two measurements against each other to compensate for errors. 
     Thus the compensation means may comprise a guide rail along which the gravitational force detection means is moveable over a calibrated distance. This is of particular advantage where the gravity measuring apparatus is removably positioned downhole on a wireline. 
     Where the test mass is biased by a biasing means, such as a spring, toward an equilibrium position about which measurements are made, compensation may occur by attaching the biasing means to the compensation means, with the compensation means being responsive to and compensating for changes in biasing force associated with the biasing means. 
     Preferably the test mass is attached to a first lever and is suspended between first electromagnetic field generating elements and the compensation means is attached to the first lever by the biasing means so as to compensate for changes in biasing force. 
     One form of compensation means may comprise a second lever and second electromagnetic field generating elements, wherein at least part of the second lever is positioned at an equilibrium position between the second electromagnetic field generating elements, and movements about the equilibrium position are used to compensate for changes in biasing force. 
     The first and second electromagnetic field generating elements desirably communicate with feedback means, with the feedback means altering the electromagnetic field generated by the first and second electromagnetic field generating elements so as to maintain the first and the second levers at their respective equilibrium positions, with signals produced by the second electromagnetic field generating elements being used by the feedback means to compensate for changes in biasing force. 
     Where the apparatus to be used permanently downhole, preferably the biasing means is made of non-magnetic steel for improved stability. 
     In accordance with another aspect of the invention, a further gravity measuring apparatus is provided, the apparatus comprising gravitational force detection means including a test mass and which produces measurements related to gravitational force exerted on a test mass, characterised in that the test mass is biased by a biasing means toward an equilibrium position about which measurements are made, and the biasing means is attached to compensation means to compensate for changes in biasing force associated with the biasing means. 
     The gravity measuring apparatus as aforesaid may be adapted for use permanently downhole, and may be used in a horizontal borehole. 
     The invention also lies in a method of installing a gravity measuring apparatus within a borehole, comprising the steps of incorporating a gravity measuring apparatus in drill pipe and placing the drill pipe down a borehole to complete a well for fluid production. 
    
    
     The invention will now be described by way of example, with reference to the accompanying drawings in which: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic diagram of a gravity measuring apparatus according to the present invention when in position downhole; 
     FIG. 2 shows a prior art gravity measuring apparatus; and 
     FIG. 3 shows a schematic diagram of the gravity measuring apparatus in accordance with the invention. 
    
    
     DESCRIPTION 
     In FIG. 1, a gravity meter  10  in accordance with the present invention is shown within a horizontal borehole  12  in an oil-producing formation  14 , although the gravity meter may also be placed in a vertical borehole. A gas zone  18 , oil-bearing zone  20  and water-bearing zone  22  occur within the formation  14 . The gravity meter  10  detects the position of an oil/water contact line  24  between the oil-bearing zone  20  and the water-bearing zone  22  by monitoring changes in the gravitational force,        F   =     GMm     r   2                              
     where F is the gravitational force resulting from two masses m and M attracting each other, M is the effective mass of the Earth, m is a test mass within the gravity meter  10 , r is the distance between the centres of the two masses and G is the gravitational constant. 
     The vertical (z) component of gravity g of sensed at a point (x,y,z) is given by          g        (     x   ,   y   ,   z     )       =     G        ∫     ∫     ∫            x   ′                 y   ′                 z   ′                (       z   ′     -   z     )          ρ        (       x   ′     ,     y   ′     ,     z   ′       )             [         (       x   ′     -   x     )     2     +       (       y   ′     -   y     )     2     +       (       z   ′     -   z     )     2       ]       3   /   2                                          
     where ρ(x′,y′,z′) is the density at the source point (x′,y′,z′) inside the formation and G is the universal constant of gravitation, 6.67×10 −8  cm 3 g −1  sec −2 . 
     A borehole gravity survey is sensitive to the bulk density ρ, which changes when oil or gas are produced. The density depends on the volumes and densities of the solid and fluid components of the reservoir: 
     
       
         ρ=(1−φ)ρ ma   +φS   w ρ w   +φS   o ρ o   +φS   g ρ g   
       
     
     where φ is the formation porosity; ρ ma , ρ w , ρ o , and ρ g  are the densities of matrix, water, oil, and gas respectively; and S w , S o , and S g  are water, oil, and gas saturations, respectively. As production continues, saturations and formation density change. 
     Since density depends on fluid saturations, borehole gravity is able to monitor fluid movements. Because the volume of investigation is very large, gravity measurements are significantly affected by distant large-scale structures and by basin geometry. Thus a borehole survey is preferably made when a well is first drilled. This initial gravity profile is sensitive to both the geological setting and fluid saturations. The differences between the initial and subsequent surveys depend only on fluid saturation changes, assuming there is no subsidence or other structural change. Thus time-lapse gravity measurements can detect gas, oil, or water movement within the formation. 
     When the oil/water contact line  24  moves upward as the result of removal of oil from the formation  14  or reservoir, the force of gravity on the test mass m within the gravity meter increases. This is because water has a higher density than oil, and as water moves upward within the formation the centre of M moves so that r between the centres of the respective masses is reduced. Similarly, if the gas cap moves downwards, the gas is lower in density than the oil it replaces and again there is a change in the mass distribution of M with the centre of mass M moving so that r is reduced again. Thus the gravitational force on the test mass m increases. By monitoring changes in the gravitational force with time, the movement of fluids within the reservoir  14  can be tracked. 
     When completion of a well occurs, gravity meters and other sensors within the wellbore are used to measure properties of the oil-bearing formation  14  and borehole fluids. Analysis of these properties allows the manipulation of valves associated with the completed well to control flow rates and pressures so as to optimise recovery from an oil-bearing formation. The sensors, power source, method of telemetry, method of deployment of the sensors and type of completion are all highly interrelated. By use of a gravity meter within the borehole  12 , and not external to the borehole casing, a method for monitoring fluid fronts is provided where power, telemetry and deployment is simplified by providing a wireline contained within the completion string itself. Since gravity is a deep reading, any gravity sensor does not have to be in direct contact with the formation and may be installed within the completion string along with flow/temperature/pressure sensors, all connected to the surface via wireline. 
     Changes in fluid density distribution within the reservoir only produce very small changes in gravitational force, typically of the order of 10 ng, and thus accurate compensation for instrumental error is a necessity to achieve measurements downhole over long periods of time. 
     A prior art gravity meter  28  is shown in FIG.  2 . This gravity meter is known as a Lacoste-Romberg meter and relies on a null measurement technique. Other prior art gravity meters are described in W. Torge, “Gravimetry”, Berlin: Walter de Gruyter (1989) and D. Chapin, “Gravity Instruments: Past, Present, Future,” The Leading Edge, January 1998, pg. 100-112. The gravity meter  28  comprises a test mass  30  attached to one end of a lever  32 . The lever  32  comprises a beam  34  and a fulcrum  38 , with the fulcrum  38  provided at the end of the beam  34  distal from the test mass  30 . The test mass  30  is centred electrostatically between two capacitor plates  40 ,  42  and is held in this equilibrium central position as a result of an upwards biasing force provided by a spring  44  attached to an upper support  48 . The capacitor plates  40 ,  42  are connected to an electrical supply, such as a battery, via a feedback system and generate an electrostatic field. 
     When a change in gravitational force occurs due to a change in the mass distribution of M, this change in force will act to move the test mass  30  away from the centred position and so extend the spring  44 . However any change in position of the test mass  30  is immediately detected by the feedback system which alters the electrical field applied to the capacitor plates  40 ,  42  to keep the test mass  30  electrostatically centred between the plates  40 ,  42  and so prevent movement of the test mass  30  away from the equilibrium position. The amount of change in electrical field required to maintain the test mass  30  in the central equilibrium position provides a measure of the change in gravitational force. 
     Suitable electrostatic position sensing and force feedback systems for use in such meters are described in B. Block and R. D. Moore, “Measurements in the Earth Mode Frequency Range by Electrostatic Sensing and Feedback Gravimeter”, Journal of Geophysical Research 71, 4361-4375 (1966), and R. D. Moore and W. E. Farrell, “Linearization and Calibration of Electrostatically Fedback Gravity Meters,” Journal of Geophysical Research 75, 928-932 (1970). 
     A major cause of instrumental error in such gravity meters is the suspension system and, in particular, the spring  44  which balances the gravitational force on the test mass  30 . Slow changes in spring strength are indistinguishable from changes in gravity and produce errors such as scale factor drift and bias drift in the measurements made by the conventional gravity meter. 
     Scale factor drifts alter the meter&#39;s sensitivity to gravity changes. Scale factor drifts are amplification errors and for a permanently emplaced gravity meter can be monitored by comparing the semidiurnal variation of meter output to the tidal variation of gravity, which can be calculated with great precision at any point on earth [I. M. Longman, “Formulas for Computing the Tidal Acceleration Due to the Moon and the Sun”, Journal of Geophysical Research 64, 2351-2355 (1959); R. J. Warburton and J. M. Goodkind, “Detailed Gravity-Tide Spectrum Between One and Four Cycles per Day”, Geophysical Journal of the Royal Astronomical Society 52, 117-136 (1978)]. 
     However the bias drift is an offset error which is more difficult to monitor, because it mimics the slow variations in gravity associated with reservoir fluid movements. 
     One way to compensate for bias drift is to measure the spatial gradient of gravity, rather than the absolute value. This is accomplished by using a relative gravity meter held on a wireline within a borehole. The relative gravity meter is similar to the prior art gravity meter relying on a test mass held between centring capacitor plates with the assistance of a biasing means such as a spring. However the relative gravity meter has a rail of calibrated distance on which a test mass moves between two spaced apart positions, or stations. This gravity meter thus measures the difference in gravitational force between the two spaced apart positions along the borehole, the two positions being either vertically or horizontally spaced. As the relative gravity meter needs to be rugged to survive travel downhole on a wireline, a quartz spring is used to support the test mass as quartz springs are more robust, although less stable, than steel springs. 
     The gradient in gravitational force at spaced apart positions along the borehole  12  will change as the reservoir fluid fronts  24  move and provide information similar to the absolute value of gravity. Drift correction errors will be small if the two consecutive measurements along the rail are made effectively at the same time and applying linear and non-linear drift correction terms to the measurements obtains a differential gravity reading. Thus when the gravity meter  10  is shuttled back and forth between two stations, bias drifts can be measured and removed from the differential measurement. 
     Another way to compensate for bias drift, and which permits temporal measurements of gravitational force to be made, is to compensate for changes in spring strength with the gravity meter  50  shown in FIG.  3 . The gravity meter  50  comprises two levers  52 ,  54  interconnected by a non-magnetic steel, or other non-magnetic metal, spring  58 . A test mass m  60  is attached to one end of the first lever  52 , which comprises a beam  56  and a fulcrum  58 , with the fulcrum  58  provided at the end of the beam  56  distal from the test mass  60 . The test mass  60  is electrostatically centred between two capacitor plates  62 ,  64  and is held in this central position as a result of an upwards biasing force provided by the non-magnetic steel spring  58  attached to the second lever  54 . The second lever similarly comprises a fulcrum  68  and beam  70 , with the spring  58  attached partway along the beam  70  and the end of the beam distal from the fulcrum  68  positioned centrally between another pair of capacitor plates  72 ,  74 . Beam  70  has a relatively small mass ml compared to lower beam  60  which has a much larger mass m 2 , i.e. m 1 &gt;&gt;m 2 . The spring  58  is held immovable by the second pair of capacitor plates  72 ,  74  and a feedback system  78 . The lever  54  can be made to have very low mass and is assumed to have zero mass. Each pair of capacitor plates  62 ,  64 ,  72 ,  74  is connected to an electrical supply, such as a battery, via one common feedback system  78 . 
     Assuming the spring is prestressed so that it tends to pull the masses m 1  and m 2  towards each other, the spring being assumed to be massless, the force on the upper beam  70  is given by 
     
       
           F   U   =−F   SU   −k (z U   −z   L )− m   1   g+E   U   
       
     
     where F U  is the upward force on the mass m 1 , k is the spring constant of the spring  58 , z U  is the upward displacement of the upper mass m 1  from its null position between position-sensing plates  72  and  74 , z L  is the upward displacement of the lower mass from its null position between position-sensing plates  62  and  64 , F SU  is the force exerted by the spring  58  when the masses are in their null positions, i.e. z U =z L =0, g is the local acceleration of gravity, and E U  is the upward force exerted by the electrostatic feedback system to maintain the mass ml at z U =0. 
     The force on the lower beam  56  is given by 
     
       
           F   L   =+F   SL   −k ( z   L   −z   U )− m   2   g+E   L   
       
     
     where F L  is the upward force on the test mass m 2 , k is the spring constant of the spring  58 , z L  is the upward displacement of the lower mass from its null position between position-sensing plates  62  and  64 , z U  is the upward displacement of the upper mass from its null position between position-sensing plates  74  and  72 , F SL  is the force exerted by the spring when the mass is in its null position, i.e. z L =z U =0, g is the local acceleration of gravity, and E L  is the upward force exerted by the electrostatic feedback system to maintain the mass m 2  at z L =0. 
     When the two beams are at their null positions and stationary, the forces exerted by the spring on the two masses m 1  and m 2  must be equal in magnitude 
     
       
         | F   SL   |=|F   SU   |≡F   S , 
       
     
     where F s  is the bias force. 
     Operation of the meter is now considered for the following three cases: 
     (1) Where the bias force F S  changes, the electrostatic forces required to maintain the masses at their null positions change by an equal amount, ΔE U =ΔE L . 
     (2) Where the spring constant k changes, there is no effect, because the masses are maintained at their null positions, z U =z L . 
     (3) Where the gravitational acceleration g changes, for m 2 &gt;&gt;m 1 , the electrostatic restoring forces required to renull the beams  56 ,  70  will be much greater for the more massive element than for the less massive element, i.e. ΔE L &gt;&gt;ΔE U . This is clearly distinguished from cases (1) and (2). Thus the practice of this invention eliminates the possibility that bias and scale factor changes in the spring can mimic changes of the local gravitational acceleration. 
     Thus when a change in gravitational force occurs, the feedback system  78  detects the effect of this on the test mass  60  and alters the electrical field applied to the capacitor plates  64 ,  62  to ensure the test mass  60  remains electrostatically centred between the plates and that negligible movement of the test mass occurs. The amount of change in electrical field required to maintain the test mass  60  in the central position of electrostatic equilibrium provides a measure of the change in gravitational force. Any change in elasticity of the spring  58  will be reflected in the feedback signal produced by the second pair of capacitor plates  72 ,  74 , which again will act to ensure negligible movement of the second lever  54 . When change in gravitational force on the test mass  60  occurs, the change in force is reflected in the feedback signal supplied to the first pair of centring capacitor plates  62 ,  64 . However, since the test mass does not move (and therefore the spring does not move), there is no effect on the second lever  54  and no feedback signal is generated in the second pair of centring plates  72 ,  74 . 
     Where a change in the elasticity of the spring  58  occurs, which would produce a bias drift in measurements, the change in spring strength will affect both the test mass  60  and the second lever  54 , generating feedback signals from both pairs of capacitor plates. A proportion of the feedback signal from the second pair of centring plates  72 ,  74  is then used to compensate the test mass feedback signal for drift in spring strength, so providing automatic compensation for bias drift when in situ downhole. 
     In this way, measurements of gravitational force are possible over time without being affected by bias drift and the permanent installation of a gravity meter in an oil well is enabled. The time lapse monitoring of gravity will provide information about the movement of fluids within the neighbouring reservoir.