Geomechanics has become a tool for engineers and geologists, and plays an pronounced role in various aspects of hydrocarbon exploitation. During waterflooding, water is injected into the reservoir formation to displace residual oil. In light of the economic benefits of water injection, operators try to maximize the injection pressure and, consequently, oil recovery. However, a number of geomechanical related issues can arise. Although the subterranean assets are not limited to hydrocarbons such as oil, throughout this document, the terms “oilfield” and “oilfield operation” may be used interchangeably with the terms “field” and “field operation” to refer to a site where any types of valuable fluids can be found and the activities required for extracting them. The terms may also refer to sites where substances are deposited or stored by injecting them into the surface using boreholes and the operations associated with this process. Further, the term “field operation” refers to a field operation associated with a field, including activities related to field planning, wellbore drilling, wellbore completion, and/or production using the wellbore.
In general, in one aspect, embodiments relate to a method, system, and computer readable medium for waterflooding operation in a subterranean formation. A first maximum injection pressure is determined based on an analytical model to avoid out-of-zone fracture propagation. A second maximum injection pressure is determined based on the analytical model to avoid fracture reactivation. The waterflooding operation is performed based at least on the first maximum injection pressure and the second maximum injection pressure.
Other aspects will be apparent from the following description and the appended claims.
The appended drawings illustrate several embodiments of screening tool for geomechanical risks during waterflooding and are not to be considered limiting of its scope, for screening tool for geomechanical risks during waterflooding may admit to other equally effective embodiments.
FIGS. 1.2-1.11 show diagrams for modeling geomechanical risks during waterflooding in accordance with one or more embodiments.
FIGS. 4.1-4.3 depict an example of screening tool for geomechanical risks during waterflooding in accordance with one or more embodiments.
Aspects of the present disclosure are shown in the above-identified drawings and described below. In the description, like or identical reference numerals are used to identify common or similar elements. The drawings are not necessarily to scale and certain features may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.
Aspects of the present disclosure include a method, system, and computer readable medium of screening tool for geomechanical risks during waterflooding. As noted above, operators try to maximize the injection pressure and, consequently, oil recovery during waterflooding operation. However, a number of geomechanical related issues can arise. Analytical methods for early screening of the potential geomechanical risks are described herein. The potential problems associated with waterflood techniques include fault reactivation and out-of-zone hydraulic fracture propagation. Generally, these risks may lead to the following undesired outcomes:
Fracture the cap rock;
Does not maximize oil recovery;
Does not displace residual oil;
No reservoir pressure maintenance;
Overcharge other permeable formations;
Associated drilling risks;
Contamination/Environment risks; and/or
Reduce reservoir model predictability.
As shown in
As shown in
In one or more embodiments, the surface unit (202) is operatively coupled to the wellsite system (204). In one or more embodiments, surface unit (202) may be located at the wellsite system (204) and/or remote locations. The surface unit (202) may be provided with computer facilities for receiving, storing, processing, and/or analyzing data from data acquisition tools (not shown) disposed in the wellbore (103) or other part of the field (104). The surface unit (202) may also be provided with or functionally for actuating mechanisms at the field (100) such as the downhole equipment (109). In one or more embodiments, the maximum pressure may be controlled by the drilling fluid density and surface pressure in an application while drilling where the pump is used to drill. The surface unit (202) may then send command signals to the field (100) in response to data received, for example to control and/or optimize various field operations described above, in particular the waterflooding operation.
As noted above, the surface unit (202) is configured to communicate with data acquisition tools (not shown) disposed throughout the field (104) and to receive data therefrom. In one or more embodiments, the data received by the surface unit (202) represents characteristics of the subterranean formation (104) and may include information related to porosity, saturation, permeability, natural fractures, stress magnitude and orientations, elastic properties, etc. during a drilling, fracturing, logging, or production operation of the wellbore (103) at the wellsite system (204). For example, data plot (108-3) may be a wireline log, which is a measurement of a formation property as a function of depth taken by an electrically powered instrument to infer properties and make decisions about drilling and production operations.
In one or more embodiments, the surface unit (202) is operatively coupled to the downhole equipment (109) to send commands to the downhole equipment (109) and to receive data therefrom. For example, the downhole equipment (109) may be adapted for injecting water (or other types of fluids) at a controlled temperature and pressure through one or more perforations in the wellbore (103). An expanded view of the subterranean formation (104) and the downhole equipment (109) is depicted in
Further, an expanded view of the formation (104) near the fault (107) and near the waterflooding zone (111) is depicted in
Returning to the discussion of
Due to the complexity of the problems and coupled interactions between production, injection and stress change, a comprehensive analysis of the waterflooding geomechanical risks traditionally uses numerical modeling involving coupling of geomechanics with porous media fluid flow, injection and fault behavior. However, the analytical equations are very useful and present many advantages when compared to numerical models. Analytical methods for early screening the potential geomechanical risks are used by the waterflooding geomechanical risks screening system (208) to model the out-of-zone hydraulic fracture propagation and fault reactivation and to determine the maximum injection pressure before these geomechanical risks take place.
FIGS. 1.2-1.11 show diagrams for modeling geomechanical risks during waterflooding in accordance with one or more embodiments.
The analytical equations for modeling out-of-zone fracture propagation are discussed below in reference to
In one or more embodiments, some simplifications are used to derive the analytical equations: minimum horizontal stress is considered as the minimum principal stress; the fracture energy for propagation is not considered; and friction loss during injection is neglected (pressure loss during water flow inside the fracture). Consequently, the developed formulation is designed to be a conservative solution, convenient to screen initial risk. In one or more embodiments, the temperature difference between injection fluid and formation is included.
The maximum injection pressure to avoid fracture propagation across the barrier is given by:
Where ΔPmax is the injection pressure increment with respect to the reservoir pressure (Pp), σh is the minimum horizontal stress at the barrier and ΔT is the temperature difference between injected fluid and formation barrier. The elastic properties at the impermeable barrier are the Young's Modulus (E), fluid thermal expansion coefficient (αT) and Poisson's Ratio (ν).
The analytical equations for modeling fault reactivation are discussed below in reference to FIGS. 1.4-1.11. Fault reactivation modeled in these analytical equations is the fault slip produced when the injected fluid locally increases the pore pressure into the fault. The slip tendency analysis based on frictional constraints is used to assess the likelihood of waterflooding induced fault reactivation that may enhance leakage pathways. Fault reactivation may cause undesired connection between different reservoirs, or connection between the reservoir and the surface causing oil and gas seeps.
The normal and shear stresses applied in the fault depends on the orientation of the fault, related to the in situ stresses. Therefore, the analysis on fault reactivation will start from general context, which is any fault orientation with respect to the far field stress. After that, the critical fault orientation, where the injection pressure without slip tendency is reduced, will be identified.
In order to develop a general scheme that can take into consideration any orientations of in situ stresses and fault orientation, it is convenient to introduce a particular system of coordinate system (150) shown in the
Deriving the analytical equations for modeling the fault reactivation is to rotate the in situ stresses (σh,σv,σH) to the general system (NEZ) is presented starting in the
E is the in situ stress tensor on the stress coordinate system (150) (σh,σv,σH), and is given by:
S corresponds to the stress tensor in the general coordinate system, and is given by:
The stresses with respect to the fault plan are then calculated.
n=[sin(δ)cos(αd)sin(δ)sin(αd)−cos(δ)] (6)
The normal stress (σn) on the fault would be a scalar given by:
σn=n·S·NT (7)
The total stress (σt) over the fault would be given by a vector with coordinates NEZ. This vector is obtained by:
αv=n·S (8)
Finally the shear stress (τ) over the fault can be obtained by:
The obtained stresses will be verified against the Mohr-Coulomb criterion along the fault plane (113) as:
Where Co is the fault cohesion and c is the fault friction angle. Equation 11 determines the maximum injection pressure (Pp) for a general fault orientation to avoid shear failure and resultant slippage, i.e., the fault reactivation. Next, the critical fault orientation is calculated.
Based on
(i) The fault is oriented in the critical direction, where Dip angle equals to the Beta angle (Equation 12), and Dip Azimuth equals to the Azimuth of σh.
(ii) The pore pressure in the cap rock varies only into the fault, but constant in the impermeable formation.
(iii) The total stresses in the cap rock vary due to thermal effects.
According to the Mohr-Columb criterion, the β critical stress relationship generating shear failure along the fault can be written as:
σ′v≦UCS+σ′h tan2 β (13)
where:
Considering that water injection increases the fault pore pressure and consequently reduces the fault effective stresses, the critical variation on pore pressure (ΔP) that induces shear failure can be expressed as:
σ′v+Δσ′v≦UCS+(σ′h+Δσ′h)tan2 β (14)
where
Δσ′v Variation in effective vertical stress
Δσ′h Variation in effective minimum horizontal stress
Considering the assumption (iii), the stress variation as a function of the temperature difference between the injected fluid and cap rock formation can be obtained:
where
ν Poisson's ratio (barrier)
α Biot's poroelastic coefficient (barrier)
αT Fluid thermal expansion coefficient
Substituting Eqs. (15) and (16) into (14) produces
Substituting the Biot's effective stress and rearranging the equation (17), the maximum injection pressure for the critical fault orientation is given by:
The relationship between the minimum horizontal stress (σh) and the effective minimum horizontal stress (σ′h) is given by the Biot's effective stress as:
σ′h=σh−α
The pore pressure is changing along reactivated faults according to the equation (18).
The example in
τ=0.85σ (19)
Equation (20) corresponds to the Mohr-Coulomb properties of:
C
=0, φ=40.36°
Replacing these values in equation (18), and assigning the Biot coefficient as 1, the maximum injection pressure ΔPmax to avoid fault reactivation can be derived as:
where ΔPmax is the maximum injection pressure increment in the fault and ΔT=TFluid−TFormation. Analyzing ΔT in Equation (20) can be seen that the lower the temperature of the fluid injected the lower ΔPmax will be allowed.
In one or more embodiments, the waterflooding geomechanical risks screening system (208) includes the fracture propagation analyzer (221) that is configured to determine a first maximum injection pressure based on an analytical model to avoid out-of-zone fracture propagation. The out-of-zone fracture propagation is described in reference to
In one or more embodiments, the fracture propagation analyzer (221) is a software module.
In one or more embodiments, the waterflooding geomechanical risks screening system (208) includes the fracture reactivation analyzer (224) that is configured to determine a second maximum injection pressure based on the analytical model to avoid fracture reactivation. The fracture reactivation is described in reference to
In one or more embodiments, the fracture reactivation analyzer (224) is a software module.
In one or more embodiments, the waterflooding geomechanical risks screening system (208) includes the data repository (234) that is configured to store the analytical model and any input, output and intermediate working data used by the analytical model. In one or more embodiments, the data repository (234) may be a disk storage device, a semi-conductor memory device, or any other suitable device for data storage.
In one or more embodiments, the waterflooding geomechanical risks screening system (208) includes the display (233) that is configured to display the result of the analytical model and any input, output and intermediate working data used by the analytical model. For example, information described in reference to FIGS. 4.1-4.3 below may be displayed using the display (233). In one or more embodiments, the display (233) may be a two dimensional display device, a three dimensional display device, a flat panel display device, a CRT based display device, or any other suitable information display device.
In one or more embodiments, the surface unit (202) of
Initially in block 301, a first maximum injection pressure is determined based on an analytical model to avoid out-of-zone fracture propagation. As noted above, the out-of-zone fracture propagation is described in reference to
In block 302, a second maximum injection pressure is determined based on an analytical model to avoid fracture reactivation. As noted above, the fracture reactivation is described in reference to
In block 303, the waterflooding operation is performed based at least on the first maximum injection pressure and the second maximum injection pressure. In one or more embodiments, the first maximum injection pressure and the second maximum injection pressure are compared to determine the lower of the two as the maximum limit for the water injection pressure during the waterflooding operation.
FIGS. 4.1-4.3 depict an example of screening tool for geomechanical risks during waterflooding in accordance with one or more embodiments.
The mechanical earth model (MEM) is a numerical representation of the state of stress and rock mechanical properties for a specific stratigraphic section in a field or basin.
The workflow (420) for the analysis to derive the maximum waterflooding pressure in the reservoir area (401) is illustrated in
Characterizing the geomechanics risks based on the screening analysis is useful to plan for mitigations. Mitigations may include reducing injection pressure to acceptable risk; developing a more detailed comprehensive analysis; and monitoring fracture propagation during the waterflooding operation. Understanding the various potential processes and ability to predict the field behavior is useful for the optimal management of the reservoir for maximum productivity and recovery using the waterflooding operation.
Embodiments of screening tool for geomechanical risks during waterflooding may be implemented on virtually any type of computer regardless of the platform being used. For instance, as shown in
Further, one or more elements of the aforementioned computer system (500) may be located at a remote location and connected to the other elements over a network. Further, one or more embodiments may be implemented on a distributed system having a plurality of nodes, where each portion of the implementation may be located on a different node within the distributed system. In one or more embodiments, the node corresponds to a computer system. In one or more embodiments, the node may correspond to a processor with associated physical memory. In one or more embodiments, the node may correspond to a processor with shared memory and/or resources. Further, software instructions to perform one or more embodiments may be stored on a computer readable medium such as a compact disc (CD), a diskette, a tape, or any other computer readable storage device.
While screening tool for geomechanical risks during waterflooding has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of screening tool for geomechanical risks during waterflooding as disclosed herein. Accordingly, the scope of screening tool for geomechanical risks during waterflooding should be limited only by the attached claims.
This application claims benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/610,946, filed on Mar. 14, 2012, and entitled, “Screening Potential Geomechanical Risks During Waterflooding.” U.S. Provisional Patent Application Ser. No. 61/610,946 is incorporated herein by reference in its entirety. This application also claims benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/637,635, filed on Apr. 24, 2012, and entitled, “Screening Potential Geomechanical Risks During Waterflooding.” U.S. Provisional Patent Application Ser. No. 61/637,635 is incorporated herein by reference in its entirety.
Number | Date | Country | |
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61610946 | Mar 2012 | US | |
61637635 | Apr 2012 | US |