Patent Application
20080149332
1. Field of the Invention
The invention relates generally to the field of exploration and production of hydrocarbons from wellbores. More specifically, the present invention relates to an apparatus and method for sampling connate fluid from a subterranean formation.
2. Description of Related Art
Sampling fluids entrained within a subterranean formation is one way of obtaining hydrocarbon producing potential of the formation. These fluids, also referred to as connate fluid, are typically analyzed in a laboratory. This sampling generally causes a minimum amount of damage to the tested formations. Additionally, a continuous record of the control and sequence of events during the test is made at the surface. From this record, valuable formation pressure and permeability data as well as data determinative of fluid compressibility, density and relative viscosity can be obtained for formation reservoir analysis.
Generally connate fluid sampling involves disposing a sonde 10 into a wellbore 5 via a wireline 8. Oppositely located on the outer portion of the sonde 10 usually are a sample port 14 and an urging means 12. When the sample port 14 is proximate to a formation of interest 6, the urging means 12 is extended against the inner surface of the wellbore 5 thereby engaging the sample port 14 into the formation 6. The engagement of the sample port 14 pierces the outer diameter of the wellbore 5 and enables fluid communication between the connate fluid in the formation 6 and the sample port 14. After inserting the sample port 14 into the formation 6, the connate fluid can be siphoned through the sample port 14 into the sonde 10 with a pumping means disposed therein.
Downhole multi-tester instruments have been developed with extendable sampling probes that engage the borehole wall and withdraw fluid samples from a formation of interest as well as measure pressure of the fluid within the formation. Traditionally these downhole instruments comprise an internal draw-down piston that is reciprocated hydraulically or electrically for drawing connate fluid from the formation to the instrument.
When exposed to an open hole, the fluid characteristics of formation fluid can change rapidly, thus it is important that the formation fluid be removed at least at the same flow rate as the formation fluid natural flow rate. However, it is important that the formation flow rate be regulated in order to prevent dropping the fluid pressure below its “bubble-point”. To be a representative sample the connate fluid must be in its original state without any portion of it allowed to bubble off. Should some of the lighter ends of the sample evaporate, these evaporated components cannot be easily recombined with the remaining portion of the sample. Thus even a sample with a small amount of evaporation results in an unrepresentative sample having altered fluid properties.
Connate fluid pressure in the formation can also be measured during the sampling process. One goal of measuring fluid pressure is to obtain a measurement of the pressure gradient within the formation 6. The pressure gradient represents the change in pressure per depth, this value is typically measured in psi/ft. The pressure increases generally in a linear fashion with respect to the depth. Pressure gradients may be obtained by first taking multiple pressure readings along the wellbore. The pressure readings are then correlated to the corresponding wellbore depths where each measurement was taken. Thus in order to obtain an accurate pressure gradient, the wellbore depths where each measurement was taken should be accurate. Often times however due to inherent uncertainties in wellbore measurements, the wellbore depth used in the pressure gradient calculation does not match the true vertical depth of where the measurements were obtained. These uncertainties are not limited to inaccurate true vertical depth readings, but extend also to incorrect differences in depth between adjacent measurement points, thereby resulting in an incorrect pressure gradient.
The present invention includes a method of evaluating a formation gradient comprising, sampling the formation pressure at locations a set distance apart, and calculating the formation gradient based on the sampled formation pressure and the set distance. The step of sampling may be conducted with probes.
An optional method of sampling subterranean formation fluid is provided that comprises drawing a first amount of connate fluid from the formation, thereby creating a clean region in the formation and drawing a second amount of connate fluid from the clean region of the formation. The step of drawing amounts of connate fluid may be performed with probes. The probes may optionally be adjacently disposed.
Also disclosed herein is a fluid sampling system comprising probes disposed apart a set distance and an analyzer configured to calculate subterranean formation gradient based on a fluid pressure measurement of the probes and the set distance. The system may further comprise a body on which the probes are located, the probes may be configured to pierce the wall of a wellbore and sample fluid from a formation adjacent the wellbore.
Optionally, a connate sampling system is disclosed that comprises a sample probe and a cleaning probe. A pump in fluid communication with at least one of the probes may be included with the optional connate sampling system. The sampling system may further comprise a second cleaning probe as well as a body. The cleaning probe may be configured to selectively draw contaminated connate fluid and the sample probe may be configured to selectively draw clean connate fluid. The sampling systems may also comprise an information handling system.
a and 4b are a detailed view of a portion of a downhole tool.
a-6c represent pressure plots of formation compartments.
The apparatus and method described herein involves sampling subterranean connate fluid from within a formation that has been pierced by a wellbore. As is known, drilling subterranean wellbores produces a wall of mud cake is formed on the outer periphery of the wellbore. This wall generally extends along the length of the wellbore. Additionally, due to the drilling operations as well as drilling fluids introduced during drilling, large amounts of particulate matter may be contained within the connate fluid within the formation.
With reference now to
As is known, the pads are disposed within or just adjacent to the tool housing when running the tool 16 within the wellbore 18. Recesses (not shown) may be formed onto the body 17 of the sampling tool 16 for receiving the probes while being moved in and out of the wellbore 18. Thus in order to sample the connate fluid the probes must be urged out and away from the body 17. This can be accomplished by any number of motivational means, such as by hydraulic, electrical, or spring loaded, to name but a few. It is within the capabilities of skilled artisans to develop an extension and retraction means for proper pad deployment and stowage.
In one mode of operation of the embodiment of
As shown, urging means 21 are provided on the downhole tool 16 opposite the side of the sample probes. Thus engagement of the sample probes with the wellbore wall 19 can optionally occur by outwardly extending the urging means 21 against the wellbore wall 19 to provide an opposing force sufficient to pierce the wellbore wall 19. Moreover, the sample probe engagement shown in
By inserting the probe tips (26, 30, 34, 38, and 42) into the formation 20, connate fluid within the formation can be drawn from the formation into a tank or reservoir (not shown) in the downhole tool 16. One method of drawing connate fluid into the tool 16 comprises creating a differential pressure between the formation and the tool reservoir (or some other fluid receptacle) thereby forcing the fluid into the downhole tool 16. One or more pumps (not shown) can be provided within the downhole tool 16 configured to create such a differential pressure. The one or more pumps could selectively be in fluid communication with each probe tip such that only one, or more than one pump, drives the pressure differential. Optionally, a dedicated pump for each particular sample probe could be provided for the pumping action. Alternatively, the reservoir could be introduced into the wellbore 18 with an internal pressure sufficiently below the formation pressure to induce connate fluid flow therein.
Fluid communication between the formation 20 and the downhole tool 16 enables pressure measurement of the formation 20 by the downhole tool 16. The downhole tool 16 may be equipped with a pressure gauge or sensor dedicated for each sample probe. The gauge and sensor is used to measure the pressure. Optionally, the downhole tool 16 may have a single pressure gauge in communication with each sample probe. Other embodiments exist where a pressure gauge serves two or more sample probes in combination with one or more additional pressure gauges for the remaining probes.
The multiple sample probes arranged along the length of the downhole tool 16 provide multiple locations for discrete pressure measurement where each probe tip samples the connate fluid. The vertical distance between each pressure measurement location is equal to the vertical distance between the probe tips, thus the vertical distance between each pressure measurement location is known with accuracy and precision. The distance between the probe tips is known to a set distance before the tool 16 is disposed in the wellbore 18, moreover this set distance remains substantially the same during fluid sampling of the tool 16 in the wellbore 18. Thus while the probes are disposed in the wellbore 18, the distance between the probes is known with reliable accuracy.
As discussed above, a formation pressure gradient can be obtained by correlating the measured formation pressures with the corresponding measured relative depths. The correlation comprises taking the ratio of the changes in measured pressure along the set distances within the wellbore. A formation pressure gradient can therefore be accurately measured with the downhole tool 16 of
It should be pointed out that the sample probes (24, 28, 32, 36, and 40) are not limited to being situated equidistance apart along the length of the downhole tool 16, but can be arranged such that the set distance can be any number of distance combinations on the tool. For example, the sample probe 24 and the sample probe 28 could be arranged such that their respective tips (26, 30) are at 1 vertical foot apart, whereas the sample probe 28 and the sample probe 32 could be arranged such that their respective tips (30, 34) are at 2 vertical feet apart. The remaining sample probes could be vertically spaced apart at the same distance, or a varying distance.
With regard to the term “vertically spaced” it could refer to the vertical distance along the tool itself, or the true vertical distance within the wellbore. In situations when the tool is disposed in a deviated section of a wellbore, a spacing of “X” (between adjacent sample probes) along the tool axis would translate into a vertical distance in the wellbore of some value less than “X”.
The supply header 83 is in fluid communication with a pump 57 for drawing connate fluid into the downhole tool 52. Optionally, each sample probe can be in communication with a dedicated pump for sampling connate fluid through the specific sample probe. The supply header 74 is in fluid communication with a pump 59 that is also configured to draw fluid from the formation 58 and into the downhole tool 52 for sampling purposes. A tank 53 is included shown in fluid communication with the supply header 83 via the pump 57.
Each sample probe (60, 68, 76, 84, and 92) on the downhole tool 52 of
a and 4b illustrate an optional mode of operation of the downhole tool 52, in this mode sample probes (68, 76, and 84) are shown engaged with the wellbore wall 90 with their respective tips (70, 78, and 86) penetrating through the mudcake 56 and into the formation. With reference now to
b illustrates a mode where sample probes (68, 84) adjacent the middle probe 76 are activated while no flow is induced through the middle probe 76 at that time. In doing so, the particulate matter 51 entrained within the connate fluid in the formation 58 is drawn towards the adjacent probes (68, 84). Pulling the particulate matter 51 from the formation in the formation fluid create a clean region within the formation 58, thus these probes (68, 84) are also referred to herein as “cleaning” probes. Subsequent activation of the probe 76 allows for connate fluid to flow from the clean region with no or little particulate matter to flow into and through the sample probe 76. Once drawn into the downhole tool 52, the valves can be selectively activated to divert the connate fluid from the cleaning probes into one of the supply headers and direct the fluid from the sample probe 76 into the other supply header. For example, the valve 73 and valve 90 could be in the open position whereas valve 72 and valve 88 could be in the closed position. This would send the connate fluid obtained through the sample probe 68 and the sample probe 84 to the supply header 74 thereby sending the connate fluid potentially carrying particulate matter to the pump 59 and sampling device 67. In this mode, the valve 80 should be in the open position and the valve 81 should be in the closed position, thereby directing “clean” sampled connate fluid into the supply header 83 for delivery to the sampling device 55 via the pump 57. This also allows the clean fluid to be directed to the tank 53 for storage and possible subsequent fluid analysis. It should be pointed out that the cleaning/sampling mode is not limited to the arrangement of
However the method of creating a clean region within the formation is not limited to the device of
With reference now to
The recorded pressure measurement(s) may be stored within the downhole tool 16 for later analysis or can be transmitted to the surface, such as via wireline 5, for real-time analysis. The analysis includes determining the pressure gradient based on the measured pressures and the true vertical distance between adjacent measured pressures. An analyzer may be employed that is specifically configured to perform this analysis. The analyzer may be disposed with the downhole tool 16 or otherwise disposable within the wellbore 5. Optionally, the analyzer may be solely at the surface.
One specific example of an analyzer is an information handling system (IHS). An IHS may be employed for controlling the steps of measuring the pressure and upward and downward movement of the downhole tool 16 in the wellbore 5. Moreover, the IHS may also be used to store recorded measurements as well as processing the measurements into a readable format. The IHS may be disposed at the surface, in the wellbore, or partially above and below the surface. The IHS may include a processor, memory accessible by the processor, nonvolatile storage area accessible by the processor, and logics for performing each of the steps above described.
The present method described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. For example, drawing or collecting connate fluid from a formation can be accomplished by any number of ways and is not limited to the use of the probes described herein. The methods and apparatus described herein could be accomplished by any manner of sampling fluid from within a subterranean formation. Additionally, the tools herein described can be combined with surface equipment for lowering/raising and controlling the tools within a wellbore. Examples of surface equipment includes surface trucks, information handling systems, as well as means for raising and lowering devices within the wellbore. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.
