US20160215565A1

US20160215565A1 - Look ahead pore pressure prediction

Info

Publication number: US20160215565A1
Application number: US14/607,974
Authority: US
Inventors: Giuseppe DE PRISCO; Ivar BREVIK; Alejandro Bello PALACIOS
Original assignee: Statoil Gulf Services LLC
Current assignee: Statoil Gulf Services LLC
Priority date: 2015-01-28
Filing date: 2015-01-28
Publication date: 2016-07-28
Also published as: WO2016122331A1

Abstract

A method for predicting jumps in pore pressure of a subsurface includes the steps of obtaining a porosity and a resistivity log value while drilling; dividing a cross plot of the porosity and the resistivity log values into two regions where the split of the two regions is based on the following (I): R=0.062/ø^1.5; averaging the obtained porosity and resistivity log values in the subsurface within a set interval to obtain a representative value of resistivity and porosity for the subsurface within the set interval; and giving a first warning of a high overpressure region if the representative value of resistivity at the representative value of porosity is lower than 0.062/ø^1.5. The method may also include giving a second warning that a jump in pore pressure is coming within 100-300 meters if the normalized values of resistivity and porosity has a turning down point in its trajectory.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to look ahead pore pressure prediction. This system provides a warning if a high overpressure regions is being drilled. The system then provides a second warning when a high overpressure sand is approaching within 300 meters from the drill bit.
2. Description of Background Art
FIG. 1 shows an exemplary diagram of a drilling operation. One of ordinary skill in the art will appreciate that the drilling operation shown in FIG. 1 is provided for exemplary purposes only and accordingly should not be construed as limiting the scope of the present invention. For example, the drilling operation shown in FIG. 1 is a seafloor drilling operation, but the drilling operation may alternatively be a land drilling operation.
As shown in FIG. 1, a drilling rig 105 is configured to drill into a formation (e.g., a formation below a seafloor 110) using a drill bit (not shown) coupled to the distal end of a drill string 125. Specifically, the drill bit is used to drill a borehole 130 extending to a target lithology 120. The target lithology 120 may be filled by hydrocarbon or a mineral resource targeted by a drilling operation.
When a sediment is buried or compacted at a relatively high depositional rate, fluid may be trapped in pores within the resulting structure. Typically, compaction and dewatering of silts or clays is much faster than compaction of sand lithology. The compaction of silts or clays lowers the permeability of these sediments and, in turn, delays further the dewatering of sand layers that may be present below the clays. Fluid trapped in a sand formation in this manner exerts pressure (defined as pore pressure) on the surrounding formation. Formations in which pore pressure exceeds hydrostatic pressure at a given depth are referred to as overpressured. This is one example of a mechanism that gives an overpressure sand formation. However, other mechanisms are possible (e.g., cementation, aquathermal pressuring, etc.). The mechanism of overpressure itself does not modify the scope of the present invention. The only relevant factor is whether a sand is in an overpressure condition. It is not important how the sand arrived to this condition.
When drilling in an overpressured formation, the mud weight (i.e., the weight of drilling fluids transmitted to the borehole) must be high enough to prevent either the pore pressure from moving formation fluids into the borehole in case of high enough permeability formation (e.g., sand) or the pore pressure from breaking down the formation and eventually causing borehole-walls collapse in case of low enough permeability formation (e.g., shale). In the worst case of a high enough permeability formation, formation fluids entering a borehole may result in loss of the well and/or injury to personnel operating the drilling rig. Accordingly, for safe and economic drilling, it is essential that the pore pressure be predicted with sufficient accuracy. In particular, it is beneficial to predict upcoming jumps in pore pressure at a location that the drill bit has not yet reached.
Further, when drilling in overpressured formations, the number of required casing strings (i.e., structural supports inserted into the borehole) may be increased. Specifically, if a sufficiently accurate pore pressure prediction is not available, additional casing strings may be inserted prematurely to avoid the possibility of well control problems (e.g., influx of formation fluids, borehole collapse, etc.). Prematurely inserting casing strings may delay the drilling operation and/or reduce the size of the borehole and result in financial loss.
The knowledge of accurate pore pressure is crucial when drilling a well in order to ensure the success of the drilling operation. Pore pressure is also a controlling input parameter in borehole stability modeling, well planning, design, and wellpath optimization.
A problem often encountered when drilling wells in many parts of the world is narrow drilling margins that require great precision in pore pressure prediction in order to prevent any shale instability problem resulting in risk of lost circulation and/or gas kicks/blowouts.
There is a great need in the art for a method that makes it possible to accurately predict pore pressure in real time measurements at the rig site. If such data were available, it would also be useful for identifying high risk shallow water zones, optimizing mud weight, detecting shallow hazard zones, detecting abnormal pressure zones, determining formation strength for wellpath optimization, and, in general, for obtaining the most trouble-free, cost effective drilling.
Conventional technology only addresses and infers pressure magnitude in a shale lithology looking at the data available. As such, conventional technology does not predict ahead of the drill bit if a high overpressure sand is coming or only predicts the presence of a high overpressure sand within tens of meters, see for example US 2007/0127314 A1.
Therefore, there is an industry-wide need for a method and system of more accurately predicting pore pressures.

SUMMARY OF THE INVENTION

The first embodiment of the present invention is directed to a method for predicting jumps in pore pressure of a subsurface, comprising obtaining at least two logs value while drilling (e.g., density and resistivity, density and shear velocity, resistivity and shear velocity, etc.), or a combination of multiple variables like resistivity, density, velocities, etc. in, for example, a multivariate way. The following description looks at the values of porosity (from bulk density) and resistivity logs, just two variables, in order to explain in a simplistic way the idea of the present invention. However, any two or multiple values described above could be used. A cross plot of porosity—resistivity is divided in two regions where the split of the two regions is based on the following (I):
R=0.062/ø^1.5 (I)
wherein R is the resistivity log value and is the porosity log value. The measured log values of resistivity and porosity are averaged in the shale formation (rejecting values from any other formation) within 1 [m], or 5 [m], or 10 [m], or 15 [m], or 20 [m] or any other interval to obtain a representative value of resistivity and porosity for that formation within the specified interval. When a sand formation that is not water saturated up to 85% of its volume or any other water saturation that does not make the sand properly saturated with water is present in the selected interval, the interval should be discarded from the analysis. The presence of hydrocarbon in that section could compromise the resistivity measurement within the neighbor shale. The next step is giving a first warning of a high overpressure region if the measured averaged resistivity value at a specific measured averaged porosity value is lower than 0.062/ø¹⁵. In this case, the expected overpressure value in the specific region will be higher than 5000 PSI. In order to avoid a false alarm due to noise of the measured averaged resistivity and porosity, different thresholds are defined to quantify the degree of risk for a high overpressure region. For example, measured and averaged resistivity value (at a specific measured and averaged porosity value):
lower than 0.08/ø¹⁵but higher than 0.075/ø^1.5, it is not a dangerous situation (green);
lower than 0.075/ø¹⁵but higher than 0.07/ø^1.5, it is a low dangerous situation (light green);
lower than 0.07/ø^1.5but higher than 0.066/ø^1.5, it is a mild dangerous situation (orange);
lower than 0.066/ø^1.5but higher than 0.06/ø^1.5, it is a dangerous situation (dark orange);
lower than 0.06/ø^1.5but higher than 0.055/ø^1.5, it is a high dangerous situation (red); and
lower than 0.055/ø^1.5, it is a very high dangerous situation (dark red).
For the very dangerous situation, the expected overpressure is higher than 5000 [PSI]
The method may also include the detection of an upcoming sudden pressure increase (e.g., overpressure sand). In order to do this, resistivity and porosity are normalized by the resistivity and porosity value obtained in the previous step when the measurements cross the threshold of a dangerous situation. From that point, a new non-dimensional cross plot is made that starts from the value on the normalized resistivity and porosity (the value being one and one). All the subsequent measured values of resistivity and porosity are normalized and followed while drilling: the normalized values while drilling are going to make a trajectory that is followed in the new non-dimensional crossplot. The trajectory of the subsequent normalized resistivity and porosity is expected to go to values larger than the resistivity reference and smaller than the porosity reference with increasing depth. Thus, in the normalized plot, the value of normalized resistivity is expected to be larger than 1 with increasing depth, and the value of normalized porosity is expected to be lower than 1 with increasing depth. The resistivity value is larger than the reference at a shallow point because compaction is acting with increasing depth. Porosity is expected to be smaller than the reference for the same reason. The normalization eliminates any other effects, like salinity, that could push the resistivity lower than the reference. In fact, salinity is expected to increase with depth. As such, the normalization is going to make salinity a second order effect within the analyzed interval deeper than the reference, if the surrounding pore pressure is high enough. In fact, the overpressure is high at this point in depth because a first warning was sent (previous point) that the region is characterized by high overpressure. Note that in this context, normalization has also the meaning of a zoom in the porosity resistivity cross plot. A threshold should be set for the measured averaged porosity to be small enough: a value of porosity lower than 24%, preferably lower than 20%, more preferably lower than 17% has given good results in all the cases analyzed. If the porosity is larger than this value, other mechanisms could be driving the porosity resistivity cross plot (for example salinity) even with a large enough pore pressure so that a low value of resistivity with depth could be obtained due to mechanisms other than high pore pressure. When an upcoming high over pressure sand is approaching, the resistivity and porosity measurements are going to be affected by the large pressure in the sand that is communicated in the shale above. In this case, resistivity should decrease compared with the reference point, and porosity should slightly increase. This trend is reflected in the normalized plot with a turning point of the trajectory resistivity versus porosity followed with a normalized resistivity decrease and normalized porosity increase, which means that an upcoming pore pressure jump must be preset in the deepest section. The change in the direction of the trajectory should persist for some depth (10 [m], 20 [m], 50 [m] or any other interval) long enough to make sure that the turning point is not due to noise in the data. Note that the use of two variables (resistivity and porosity) also has the purpose of enhancing the signal to noise ratio in order to extract better information than with only one variable. An improvement, which is easy to understand by one of ordinary skill in the art, is to use multiple variables (for example resistivity, density, and velocity) in a multivariate manner in order to improve resolutions. Thus, a second warning is sent that a jump in pore pressure is coming within 100-300 meters. After the second warning, and so after the drill bit goes through the pore pressure jump, a new reference has to be defined for resistivity and porosity so that a new normalized plot can be obtained. Changing the reference value is also useful when the drill bit goes from one lithology to another that, for example, were deposited with different depositional attributes (different initial porosity, depositional rate and so on). The method may also include adjusting, using the processor, a drilling operation associated with the drilling location based on the predicted jump in pore pressure. The step of adjusting the drilling operation may include at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole. The drilling location may include a location below an operating drill bit in a borehole. The first and second warning may be displayed on a graphical user interface.
The second embodiment of the present invention is directed to a non-transitory computer readable medium comprising instructions to perform a method for predicting jumps in pore pressure, the instructions executable on a processor and comprising functionality for obtaining a porosity and a resistivity log value while drilling; dividing a cross plot of the porosity and the resistivity log values into two regions where the split of the two regions is based on the following (I):
R=0.062/ø^1.5
wherein R is the resistivity log value and ø is the porosity log value; averaging the obtained porosity and resistivity log values in the subsurface within a set interval to obtain a representative value of resistivity and porosity for the subsurface within the set interval; and rejecting intervals where sand formations are not water saturated and porosity values that are larger than a specified threshold; and giving a first warning of a high overpressure region if the representative value of resistivity at the representative value of porosity is lower than 0.062/ø^1.5. The instructions may also include functionality for normalizing the first representative values of resistivity and porosity after the first warning is given to obtain a resistivity reference and a porosity reference; creating a new cross plot of normalized values of resistivity and porosity and following their normalized path or trajectory; and giving a second warning that a jump in pore pressure is coming within 100-300 meters if the normalized values of resistivity and porosity has a turning point of the trajectory of resistivity versus porosity followed by a normalized resistivity decrease and a normalized porosity increase. After the second warning, and so after the drill bit goes through the pore pressure jump, a new reference has to be defined for resistivity and porosity so that a new normalized plot can be obtained. Changing the reference value is also useful when the drill bit goes from one lithology to another that, for example, were deposited with different depositional attributes (different initial porosity, depositional rate and so on). The instructions may also include functionality for adjusting a drilling operation associated with the drilling location based on the predicted jump in pore pressure. The step of adjusting the drilling operation may include at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole. The drilling location may include a location below an operating drill bit in a borehole. The first and second warning may be displayed on a graphical user interface.
The third embodiment of the present invention is directed to a downhole tool configured to perform a method for predicting jumps in pore pressure, the downhole tool comprising a processor; a memory comprising software instructions for enabling the downhole tool under control of the processor to obtain a porosity and a resistivity log value while drilling; divide a cross plot of the porosity and the resistivity log values into two regions where the split of the two regions is based on the following (I):
R=0.062/ø^1.5
wherein R is the resistivity log value and ø is the porosity log value; average the obtained porosity and resistivity log values in the subsurface within a set interval to obtain a representative value of resistivity and porosity for the subsurface within the set interval; and rejecting intervals where sand formations are not water saturated, and porosity values that are larger than a specified threshold; and give a first warning of a high overpressure region if the representative value of resistivity at the representative value of porosity is lower than 0.062/ø¹⁵. The memory may also enable the downhole tool to normalize the first representative values of resistivity and porosity after the first warning is given to obtain a resistivity reference and a porosity reference; create a new cross plot of normalized values of resistivity and porosity to form a trajectory; and give a second warning that a jump in pore pressure is coming within 100-300 meters if the normalized values of resistivity and porosity has a turning point of the trajectory resistivity versus porosity followed with a normalized resistivity decrease and normalized porosity increase, so it differs from the normal compaction trajectory. After the second warning, and so after the drill bit goes through the pore pressure jump, a new reference has to be defined for resistivity and porosity so that a new normalized plot can be obtained. Changing the reference value is also useful when the drill bit goes from one lithology to another that, for example, were deposited with different depositional attributes (different initial porosity, depositional rate and so on). The memory may also enable the downhole tool to adjust a drilling operation associated with the drilling location based on the predicted jump in pore pressure. The step of adjusting the drilling operation may include at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole. The drilling location may include a location below an operating drill bit in a borehole. The first and second warning may be displayed on a graphical user interface.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to one of ordinary skill in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary diagram of a drilling operation.

FIG. 2 shows a diagram of a system in accordance with one or more embodiments of the present invention.

FIG. 3 shows a flowchart in accordance with one or more embodiments of the present invention.

FIG. 4 shows a diagram of a computer system in accordance with one or more embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described with reference to the accompanying drawings.
The present invention is directed to a predictable relationship between resistivity and porosity (derived from density) that occurs up to 300 meters above an overpressured sandstone in the overlying shales. Because high overpressure from a sand body is communicated to the shale above, a variation from the normal trend of the resistivity/porosity (or any other combination of variables, like resistivity and velocity) field is expected when approaching the high overpressured sand.
The present invention is also directed to a two-warning system based on this information. First, an early warning alerts that there is an overpressured region below. Then, a second warning occurs when the overpressured layer is approaching in order to better locate it.
As a drill bit drills, porosity and resistivity are measured and plotted on a graph with porosity on the x-axis and resistivity on the y-axis. The graph also includes a line that corresponds to the equation 0.062/(porosity)^1.5. From this plot, the first warning is given if the drill bit will enter a high overpressure region. In this regard, if the porosity-resistivity value is to the left of the line, the first warning is given because the drill is in a high overpressure region. If the porosity-resistivity value is to the right of the line, a warning is not given because the drill is not in a high overpressure region.
In other words, porosity and resistivity are measured as the drill bit drills in order to obtain a resistivity-porosity value that is above or below the following (I):
R=0.062/ø^1.5 (I)
wherein R is the resistivity log value and o is the porosity log value. If the actual measured log value of resistivity is lower than the resistivity obtained according to (I), then the drill is in a high overpressure region, and the first warning is given. If the actual measured value of resistivity is the same as or higher than the resistivity obtained according to (I), then the drill is not in a high overpressure region, so no warning is given.
A region is considered a high overpressure region if the overpressure is higher than 5,000 psi. A region is considered a medium overpressure region if the overpressure is between 1,400 and 5,000 psi. A region is considered a low overpressure region if the overpressure is less than 1,400 psi.
For the second warning, the first representative values of resistivity and porosity after the first warning is given are normalized to obtain a resistivity reference and a porosity reference. Then, a new cross plot of normalized values of resistivity and porosity is created to form a trajectory while drilling. The trajectory is supposed to increase while drilling (normalized resistivity larger than the normalized reference (1) and normalized porosity smaller than the normalized reference (1)) with increasing depth. If the normalized values of resistivity and porosity differ from the trajectory, so that a turning point is detected followed by a persisting decreasing trajectory, then a jump in pressure is coming within 100-300 meters, and the second warning is given. As the ratio of pore pressure to effective stress increases, the trajectory is expected to decrease and a jump in pore pressure can be expected. Accordingly, the second warning is given. Effective stress is the stress on the matrix frame.
After the second warning, and so after the drill bit goes through the pore pressure jump, a new reference has to be defined for resistivity and porosity so that a new normalized plot can be obtained. Changing the reference value is also useful when the drill bit goes from one lithology to another that, for example, were deposited with different depositional attributes (different initial porosity, depositional rate and so on).
With the pore pressure calculations explained, the different embodiments of the present invention can be further explained. In general, embodiments of the present invention provide a method and system for pore pressure prediction according to (I) discussed above. Then, a drilling operation associated with the drilling location is adjusted based on the predicted pore pressure.
FIG. 2 shows a diagram of a system in accordance with one or more embodiments of the present invention. Specifically, FIG. 2 shows a diagram of a computing environment 205 in accordance with one or more embodiments of the present invention.
In one or more embodiments of the present invention, the computing environment 205 may include one or more computer systems (e.g., computer system A 210, computer system N 215) configured to perform drilling-related tasks. In one or more embodiments of the present invention, the computer system(s) (e.g., 210, 215) may be web servers, embedded systems (e.g., a computer located in a downhole tool), desktop computers, laptop computers, personal digital assistants, any other similar type of computer system, or any combination thereof.
Specifically, in one or more embodiments of the present invention, one or more of the computer systems (e.g., 210, 215) may include a pore pressure calculator 235. In one or more embodiments of the present invention, the pore pressure calculator 235 may be located in a single computer system (e.g., 210, 215), distributed across multiple computer systems (e.g., 210, 215), or any combination thereof In one or more embodiments of the present invention, the pore pressure calculator 235 may include one or more software modules, one or more hardware modules, or any combination thereof Further, in one or more embodiments of the present invention, the pore pressure calculator may be configured to communicate with each other via function calls, application program interfaces (APIs), a network protocol (i.e., a wired or wireless network protocol), electronic circuitry, any other similar type of communication and/or communication protocol, or any combination thereof.
In one or more embodiments of the invention, the pore pressure calculator 235 may be configured to predict a jump in pore pressure in 200-300 meters and in 100-300 meters as set forth above.
FIG. 3 shows a flowchart in accordance with one or more embodiments of the present invention. Specifically, FIG. 3 shows a flowchart of a method for predicting a jump in pore pressure in accordance with one or more embodiments of the present invention.
In one embodiment of the present invention, a drilling location corresponds to a location that has not yet been drilled. In other words, the drill bit has not reached the drilling location. However, the drilling location is in the intended path of the drill bit and, unless the trajectory of the borehole changes, the drill bit will eventually reach the drilling location. In one embodiment of the present invention, the method described in FIG. 3 may be performed while drilling.
Turning to FIG. 3, porosity and resistivity log values at reference depths are obtained (Step 305) for a depth interval of specified length (e.g., 1 [m], 5[m]. 10 [m], 20 [m] or any other length) long enough to obtain a representative averaged value for such interval.
In Step 320, a cross plot of the porosity and the resistivity log values is divided into two regions where the split of the two regions is based on (I). In Step 322, the obtained porosity and resistivity log values in the subsurface within a set interval are averaged to obtain a representative value of resistivity and porosity for the subsurface within the set interval. An averaged value of resistivity and porosity belonging to an interval with the presence of a sand that is not water saturated would be rejected. All values of porosity lower than a specific threshold, for example 20%, would also be rejected. If the representative value of resistivity at the representative value of porosity is not lower than 0.062/ø¹⁵(via Step 330), then porosity and resistivity log values are measured again at the new depth in Step 305, and the process begins again. If the representative value of resistivity at the representative value of porosity is lower than 0.062/ø^1.5(via Step 330), then a first warning is given that the drill bit is in a high overpressure region in Step 335. Then, in Step 340, the drilling operation is adjusted based on the predicted high pore pressure. Specifically, in one or more embodiments of the present invention, adjusting the drilling operation may involve adjusting a drilling fluid density (i.e., increasing or decreasing the drilling fluid density as appropriate), adjusting a drilling trajectory (e.g., to avoid an overpressured area, to pass through a low-pressure area, etc.), optimizing the number of casing strings in the borehole (i.e., adding a casing string, delaying addition of a casing string, etc.), or any other similar type of adjustment.
In Step 310, the first representative values of resistivity and porosity after the first warning is given is used as reference to normalize all the upcoming deeper resistivity and a porosity measurements. In Step 312, a new cross plot of normalized values of resistivity and porosity is created to follow their path or trajectory while drilling. If the normalized values of resistivity and porosity make a monotonically increasing trajectory, filtering out the noise, (via Step 315), then the cross plot of normalized values of resistivity and porosity continues to be created at the new depth in Step 312, and the process begins again. If the normalized values of resistivity and porosity has a turning down point followed by a decreasing path or trajectory (via Step 315), then a second warning is given that a jump in pore pressure is coming within 100-300 meters in Step 325. Then, in Step 340, the drilling operation is adjusted based on the predicted jump in pore pressure. Specifically, in one or more embodiments of the present invention, adjusting the drilling operation may involve adjusting a drilling fluid density (i.e., increasing or decreasing the drilling fluid density as appropriate), adjusting a drilling trajectory (e.g., to avoid an overpressured area, to pass through a low-pressure area, etc.), optimizing the number of casing strings in the borehole (i.e., adding a casing string, delaying addition of a casing string, etc.), or any other similar type of adjustment.
After the second warning, and so after the drill bit goes through the pore pressure jump, a new reference is defined for resistivity and porosity in Step 345 so that a new normalized cross plot can be obtained in Step 312 to determine whether another jump in pore pressure is coming. Changing the reference value is also useful when the drill bit goes from one lithology to another that, for example, were deposited with different depositional attributes (different initial porosity, depositional rate and so on).
The processes of the first warning system and the second warning system can occur simultaneously or the process of the second warning system can start after the first warning is given.
One or more embodiments of the present invention provide a means for accurately predicting a jump in pore pressure. Accordingly, one or more embodiments of the present invention may prevent formation fluids from entering a borehole, thereby preventing damage to the well and/or personnel operating a drilling rig. Further, one or more embodiments of the present invention may prevent the financial overhead of prematurely inserting casing strings.
The present invention may be implemented on virtually any type of computer regardless of the platform being used. For example, as shown in FIG. 4, a computer system 400 includes a processor 402, associated memory 404, a storage device 406, and numerous other elements and functionalities typical of today's computers (not shown). The computer 400 may also include input means, such as a keyboard 408 and a mouse 410, and output means, such as a monitor 412. The computer system 400 may be connected to a network 414 (e.g., a local area network (LAN), a wide area network (WAN) such as the Internet, or any other similar type of network) via a network interface connection (not shown). One of ordinary skill in the art will appreciate that these input and output means may take other forms.
Furthermore, one of ordinary skill in the art will appreciate that one or more elements of the aforementioned computer system 400 may be located at a remote location and connected to the other elements over a network. Further, software instructions to perform embodiments of the present invention may be stored on a non-transitory computer readable medium such as a compact disc (CD), a diskette, a tape, a file, or any other non-transitory computer readable storage device. In addition, in one embodiment of the present invention, the predicted jump in pore pressure (including all the calculations using the method described in FIG. 3) may be displayed to a user via a graphical user interface (e.g., a display device).
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. A method for predicting jumps in pore pressure of a subsurface, comprising:

obtaining a porosity and a resistivity log value while drilling;

dividing, using a processor, a cross plot of the porosity and the resistivity log values into two regions where the split of the two regions is based on the following (I):

R=0.062/ø^1.5 (I)

wherein R is the resistivity log value and ø is the porosity log value;

averaging the obtained porosity and resistivity log values in the subsurface within a set interval to obtain a representative value of resistivity and porosity for the subsurface within the set interval;

rejecting intervals where the subsurface is a sand formation that is not water saturated and rejecting porosity values that are larger than a specified threshold; and

giving a first warning of a high overpressure region if the representative value of resistivity at the representative value of porosity is lower than 0.062/ø^1.5.

2. The method of claim 1, further comprising:

normalizing the first representative values of resistivity and porosity after the first warning is given to obtain a resistivity reference and a porosity reference;

creating a new cross plot of normalized values of resistivity and porosity to form a trajectory;

giving a second warning that a jump in pore pressure is coming within 100-300 meters if the normalized values of resistivity and porosity has a turning point in their normalized trajectory; and

defining a new reference for resistivity and porosity to obtain a new normalized plot.

3. The method of claim 1, further comprising:

adjusting, using the processor, a drilling operation associated with the drilling location based on the predicted jump in pore pressure.

4. The method of claim 2, further comprising:

5. The method of claim 3, wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole.

6. The method of claim 4, wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole.

7. The method of claim 1, wherein the drilling location comprises a location below an operating drill bit in a borehole.

8. The method of claim 2, wherein the drilling location comprises a location below an operating drill bit in a borehole.

9. The method of claim 1, wherein the first warning is displayed on a graphical user interface.

10. The method of claim 2, wherein the second warning is displayed on a graphical user interface.

11. A non-transitory computer readable medium comprising instructions to perform a method for predicting jumps in pore pressure of a subsurface, the instructions executable on a processor and comprising functionality for:

obtaining a porosity and a resistivity log value while drilling;

dividing a cross plot of the porosity and the resistivity log values into two regions where the split of the two regions is based on the following (I):

R=0.062/ø^1.5 (I)

wherein R is the resistivity log value and o is the porosity log value;

12. The non-transitory computer readable medium of claim 11, wherein the instructions further comprise functionality for adjusting a drilling operation associated with the drilling location based on the predicted jump in pore pressure.

13. The non-transitory computer readable medium of claim 12, wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole.

14. The non-transitory computer readable medium of claim 11, wherein the drilling location comprises a location below an operating drill bit in a borehole.

15. The non-transitory computer readable medium of claim 11, wherein the first warning is displayed on a graphical user interface.

16. The non-transitory computer readable medium of claim 11, wherein the instructions further comprise functionality for:

giving a second warning that a jump in pore pressure is coming within 100-300 meters if the normalized values of resistivity and porosity has a turning point in the trajectory; and

17. The non-transitory computer readable medium of claim 16, wherein the instructions further comprise functionality for adjusting a drilling operation associated with the drilling location based on the predicted jump in pore pressure.

18. The non-transitory computer readable medium of claim 17, wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole.

19. The non-transitory computer readable medium of claim 16, wherein the drilling location comprises a location below an operating drill bit in a borehole.

20. The non-transitory computer readable medium of claim 16, wherein the second warning is displayed on a graphical user interface.

21. A downhole tool configured to perform a method for predicting jumps in pore pressure of a subsurface, the downhole tool comprising:

a processor;

a memory comprising software instructions for enabling the downhole tool under control of the processor to:

obtain a porosity and a resistivity log value while drilling;

divide a cross plot of the porosity and the resistivity log values into two regions where the split of the two regions is based on the following (I):

R=0.062/ø ^1.5 (I)

wherein R is the resistivity log value and ø is the porosity log value;

average the obtained porosity and resistivity log values in the subsurface within a set interval to obtain a representative value of resistivity and porosity for the subsurface within the set interval;

reject intervals where the subsurface is a sand formation that is not water saturated and reject porosity values that are larger than a specified threshold; and

give a first warning of a high overpressure region if the representative value of resistivity at the representative value of porosity is lower than 0.062/ø^1.5.

22. The downhole tool of claim 21, wherein the memory also enables the downhole tool to adjust a drilling operation associated with the drilling location based on the predicted jump in pore pressure.

23. The downhole tool of claim 22, wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole.

24. The downhole tool of claim 21, wherein the drilling location comprises a location below an operating drill bit in a borehole.

25. The downhole tool of claim 21, wherein the first warning is displayed on a graphical user interface.

26. The downhole tool of claim 21, wherein the memory also enables the downhole tool to:

normalize the first representative values of resistivity and porosity to obtain a resistivity reference and a porosity reference;

create a new cross plot of normalized values of resistivity and porosity to form a trajectory;

give a second warning that a jump in pore pressure is coming within 100-300 meters if the normalized values of resistivity and porosity has a turning point in the trajectory; and

27. The downhole tool of claim 26, wherein the memory also enables the downhole tool to adjust a drilling operation associated with the drilling location based on the predicted jump in pore pressure.

28. The downhole tool of claim 27, wherein adjusting the drilling operation comprises at least one selected from the group consisting of adjusting a drilling fluid density, adjusting a drilling trajectory, and optimizing a number of casing strings in a borehole.

29. The downhole tool of claim 26, wherein the drilling location comprises a location below an operating drill bit in a borehole.

30. The downhole tool of claim 26, wherein the second warning is displayed on a graphical user interface.