METHODS AND SYSTEMS FOR DETERMINING BULK DENSITY, POROSITY, AND PORE SIZE DISTRIBUTION OF SUBSURFACE FORMATIONS

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First Claim
1. A system for characterization of a subsurface formation, the system comprising:
 a fluidsaturated sample of a subsurface formation;
a balance configured to receive the fluidsaturated sample and output the inair mass of the fluidsaturated sample;
a computer comprising one or more processors and a nontransitory computer readable medium comprising computer executable instructions that when executed by the one or more processors, trigger the computer to;
receive inair mass of a fluidsaturated sample of the subsurface formation, wherein the inair mass comprises mass of the sample, mass of a fluid surrounding the sample, and mass of the fluid inside the sample, the inair mass of the fluidsaturated sample, m_{s}, given by the formula;
m_{s}=V_{m}ρ
_{m}+(V_{ϕ}+V_{sur})ρ
_{l }where ρ
_{m }is a density of the matrix of the subsurface formation, ρ
_{l }is a density of the fluid inside and surrounding the sample, V_{m }is a volume of the matrix, V_{ϕ }is a volume of the fluid inside the sample, and V_{sur }is a volume of the fluid surrounding the sample;
determine volume of the fluid inside the sample, V_{ϕ}, and volume of the fluid surrounding the sample, V_{sur}, using nuclear magnetic resonance (NMR);
receive mass of the fluidsaturated sample in a weighing fluid;
determine mass of the fluidsaturated sample without the surrounding fluid in the weighing fluid, m_{f}, given by the formula
m_{f}=V_{m}ρ
_{m}+V_{ϕ}ρ
_{l}−
V_{c}ρ
_{f }where ρ
_{f }is the density of the weighing fluid; and
determine a volume of the fluidsaturated sample without the surrounding fluid, V_{c}, using the formula
V_{c}=(m_{s}−
m_{f}−
V_{sur}ρ
_{l})/ρ
_{f}.
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Abstract
Herein methods and systems for determining matrix or grain density of a subsurface formation are described. This includes measuring inair mass of a fluidsaturated sample of the subsurface formation, wherein the inair mass comprises mass of matrix or grains of the sample, mass of a fluid surrounding the sample, and mass of the fluid inside the sample. The volume of the fluid inside the sample, V_{ϕ}, and volume of the fluid surrounding the sample, V_{sur}, are determined using nuclear magnetic resonance (NMR). The fluidsaturated sample can then be submerged in a predetermined volume of a weighing fluid and mass of the fluidsaturated sample without the surrounding fluid in the weighing fluid, m_{f }is measured. Using the measured and determined values one can determine the volume of the sample without the surrounding fluid, V_{c}, the bulk density of the fluidsaturated sample without the surrounding fluid, ρ_{b}, the volume of the matrix, V_{m}, and the matrix or grain density of the subsurface formation, ρ_{m}.
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4 Claims
 1. A system for characterization of a subsurface formation, the system comprising:
a fluidsaturated sample of a subsurface formation; a balance configured to receive the fluidsaturated sample and output the inair mass of the fluidsaturated sample; a computer comprising one or more processors and a nontransitory computer readable medium comprising computer executable instructions that when executed by the one or more processors, trigger the computer to; receive inair mass of a fluidsaturated sample of the subsurface formation, wherein the inair mass comprises mass of the sample, mass of a fluid surrounding the sample, and mass of the fluid inside the sample, the inair mass of the fluidsaturated sample, m_{s}, given by the formula;
m_{s}=V_{m}ρ
_{m}+(V_{ϕ}+V_{sur})ρ
_{l }where ρ
_{m }is a density of the matrix of the subsurface formation, ρ
_{l }is a density of the fluid inside and surrounding the sample, V_{m }is a volume of the matrix, V_{ϕ }is a volume of the fluid inside the sample, and V_{sur }is a volume of the fluid surrounding the sample;determine volume of the fluid inside the sample, V_{ϕ}, and volume of the fluid surrounding the sample, V_{sur}, using nuclear magnetic resonance (NMR); receive mass of the fluidsaturated sample in a weighing fluid; determine mass of the fluidsaturated sample without the surrounding fluid in the weighing fluid, m_{f}, given by the formula
m_{f}=V_{m}ρ
_{m}+V_{ϕ}ρ
_{l}−
V_{c}ρ
_{f }where ρ
_{f }is the density of the weighing fluid; anddetermine a volume of the fluidsaturated sample without the surrounding fluid, V_{c}, using the formula
V_{c}=(m_{s}−
m_{f}−
V_{sur}ρ
_{l})/ρ
_{f}. View Dependent Claims (2, 3, 4)
1 Specification
This application is a continuation of and claims priority to U.S. patent application Ser. No. 15/673,996 filed on Aug. 10, 2017 titled “METHODS AND SYSTEMS FOR DETERMINING BULK DENSITY, POROSITY, AND PORE SIZE DISTRIBUTION OF SUBSURFACE FORMATIONS,” which is hereby incorporated herein by reference in its entirety.
Embodiments relate to reservoir evaluation. More specifically, example embodiments relate to methods and systems for determining bulk density, porosity, and pore size distribution of subsurface formations. These methods and systems utilize a combination of (Nuclear Magnetic Resonance) NMR and gravimetric techniques.
Bulk density is one of the most important parameters in reservoir evaluation. It is widely used for estimation of reserves of hydrocarbons in reservoirs. Traditionally, well logs and core measurements are the two approaches to obtain key petrophysical parameters for reservoir evaluation and description. These measurements are expensive and many times they require extra rig time, which is also very expensive.
For example, bulk density can be measured in real time with logging while drilling (LWD) density log or can be measured using wireline (WL) density log. Both use a gamma ray source and measure the attenuated gamma ray coming to the detector after interacting with the formation. Generally speaking, the LWD density measurement represents the bulk density of the rock with the formation fluids in the pore space, whereas the WL density measures the bulk density of the rock with invaded fluids; for low permeable unconventional rocks, the difference should be minimal. Bulk density can be precisely measured using core plugs when they are available.
Obtaining accurate petrophysical parameters from drill cuttings is beneficial and desirable for at least two reasons. First, drill cuttings are readily available from any drilled well and thus does not add extra rig time or extra cost to the operation. Second, measurement can be done at the wellsite and offers data for realtime operational decisions, such as drilling and the succeeding hydraulic fracturing.
However, it is a challenge to measure the volume of the cutting accurately as it is hard to remove the fluid on the surface of the cutting. The traditional sample preparation method uses a damp paper towel to remove the excess fluid from the surface, and due to the irregular shape of the surface features, the validity of the total removal of the surface fluid is always questionable. Moreover, if the paper towel is too dry, the fluid within the cutting sample can be lost due to capillary force.
Example embodiments disclosed herein relate to improved methods and systems for determining bulk density, porosity, and pore size distribution of subsurface formations.
One example embodiment is a method for determining matrix or grain density of a subsurface formation. The method includes measuring an inair mass of a fluidsaturated sample of the subsurface formation, wherein the inair mass includes mass of the matrix or grains of the sample, mass of a fluid surrounding the sample, and mass of the fluid inside the sample. The inair mass of the fluidsaturated sample, ms, may be given by the formula
m_{s}=V_{m}ρ_{m}+(V_{ϕ}+V_{sur})ρ_{l }
where ρ_{m }is a density of the matrix of the subsurface formation, ρ_{l }is a density of the fluid inside and surrounding the sample, V_{m }is a volume of the matrix, V_{ϕ }is a volume of the fluid inside the sample, and V_{sur }is a volume of the fluid surrounding the sample. The method also includes separately determining the volume of the fluid inside the sample, V_{ϕ}, and the volume of the fluid surrounding the sample, V_{sur}, using nuclear magnetic resonance (NMR). The method may further include placing the sample in a predetermined volume of a weighing fluid, and measuring mass of the fluidsaturated sample in the weighing fluid. The mass of the fluidsaturated sample without the surrounding fluid in the weighing fluid, m_{f}, may be given by the formula
m_{f}=V_{m}ρ_{m}+V_{ϕ}ρ_{l}−V_{c}ρ_{f }
where ρ_{f }is the density of the weighing fluid. The method may further include determining a volume of the fluidsaturated sample without the surrounding fluid, V_{c}, using the formula
V_{c}=(m_{s}−m_{f}−V_{sur}ρ_{l})/ρ_{f}.
The method may also include determining a bulk density of the fluidsaturated sample without the surrounding fluid, ρ_{b}, using the formula
The method may further include determining the volume of the matrix, V_{m}, using the formula
V_{m}=(m_{s}−m_{f}−V_{sur}ρ_{f})/ρ_{f}−V_{ϕ}.
The method may also include determining the matrix or grain density of the subsurface formation, ρ_{m}, using the formula
Another example embodiment relates to computer programs stored in computer readable media. The nontransitory computerreadable media may have, for example, computer executable instructions that trigger the computer to perform the operation of receiving inair mass of a fluidsaturated sample of the subsurface formation, wherein the inair mass includes mass of the matrix or grains of the sample, mass of a fluid surrounding the sample, and mass of the fluid inside the sample. The inair mass of the fluidsaturated sample, m_{s}, may be given by the formula
m_{s}=V_{m}ρ_{m}+(V_{ϕ}+V_{sur})ρ_{l }
where ρ_{m }is a density of the matrix of the subsurface formation, ρ_{l }is a density of the fluid inside and surrounding the sample, V_{m }is a volume of the matrix, V_{ϕ }is a volume of the fluid inside the sample, and V_{sur }is a volume of the fluid surrounding the sample. The computer executable instructions may also trigger the computer to determine the volume of the fluid inside the sample, V_{ϕ}, and volume of the fluid surrounding the sample, V_{sur}, from NMR measurements. The computer executable instructions may also trigger the computer to receive the mass of the fluidsaturated sample in a weighing fluid. The mass of the fluidsaturated sample without the surrounding fluid in the weighing fluid, m_{f}, may be given by the formula
m_{f}=V_{m}ρ_{m}+V_{ϕ}ρ_{l}−V_{c}ρ_{f }
where ρ_{f }is the density of the weighing fluid. The computer executable instructions may also trigger the computer to calculate a volume of the fluidsaturated sample without the surrounding fluid, V_{c}, using the formula
V_{c}=(m_{s}−m_{f}−V_{sur}ρ_{l})/ρ_{f}.
The computer executable instructions may further trigger the computer to calculate a bulk density of the fluidsaturated sample without the surrounding fluid, ρ_{b}, using the formula
The computer executable instructions may further trigger the computer to calculate the volume of the matrix, V_{m}, using the formula
V_{m}=(m_{s}−m_{f}−V_{sur}ρ_{f})/ρ_{f}−V_{ϕ}.
The computer executable instructions may further trigger the computer to calculate the matrix or grain density of the subsurface formation, ρ_{m}, using the formula
Another example embodiment is a system for determining matrix or grain density of a subsurface formation. The system may include a fluidsaturated sample of the subsurface formation, and a weighing balance, which may be configured to receive the fluidsaturated sample and output the inair mass and influid mass of the sample. The system may also include a computer having one or more processors and a nontransitory computer readable medium, which may include computer executable instructions that when executed by the one or more processors, trigger the computer to fetch inair mass of the fluidsaturated sample of the subsurface formation from the weighing scale. The inair mass may include mass of the matrix or grains of the sample, mass of a fluid surrounding the sample, and mass of the fluid inside the sample. The inair mass of the fluidsaturated sample, m_{s}, may be given by the formula
m_{s}=V_{m}ρ_{m}+(V_{ϕ}+V_{sur})ρ_{l }
where ρ_{m }is a density of the matrix of the subsurface formation, ρ_{l }is a density of the fluid inside and surrounding the sample, V_{m }is a volume of the matrix, V_{ϕ }is a volume of the fluid inside the sample, and V_{sur }is a volume of the fluid surrounding the sample. The system may also include an NMR, which may be operably connected to the computer and configured to determine the volume of the fluid inside the sample, V_{ϕ}, and volume of the fluid surrounding the sample, V_{sur}, using NMR. The computer may be configured to receive the volume of the fluid inside the sample, V_{ϕ}, and volume of the fluid surrounding the sample, V_{sur}, from the NMR, and the mass of the fluidsaturated sample in a weighing fluid from the weighing scale. The mass of the fluidsaturated sample without the surrounding fluid in the weighing fluid, m_{f}, may be given by the formula
m_{f}=V_{m}ρ_{m}+V_{ϕ}ρ_{l}−V_{c}ρ_{f }
where ρ_{f }is the density of the weighing fluid. The computer executable instructions may also trigger the computer to determine a volume of the fluidsaturated sample without the surrounding fluid, V_{c}, using the formula
V_{c}=(m_{s}−m_{f}−V_{sur}ρ_{l})/ρ_{f}.
The computer executable instructions may further trigger the computer to determine a bulk density of the fluidsaturated sample without the surrounding fluid, ρ_{b}, using the formula
The computer executable instructions may further trigger the computer to determine the volume of the matrix, V_{m}, using the formula
V_{m}=(m_{s}−m_{f}−V_{sur}ρ_{f})/ρ_{f}−V_{ϕ}.
The computer executable instructions may further trigger the computer to determine the matrix or grain density of the subsurface formation, ρ_{m}, using the formula
Example embodiments disclosed propose a method to measure and analyze drill cuttings using a combination of nuclear magnetic resonance (NMR) measurements and mass measurements inair and influid to obtain multiple key petrophysical parameters accurately with little sample preparation. Example embodiments present a new and accurate method to measure the bulk density using saturated drill cuttings, which are readily available for any drilled hydrocarbon well. The method combines NMR and gravimetric techniques, and the results include bulk density, grain density, porosity, and poresize distribution of the drill cuttings.
Turning now to the figures,
Additionally, the collected cuttings may be washed using sufficient fluid such that it minimizes the impact of small particles from drilling mud that stick to the cutting surface or in the surrounding fluid which can impact both mass measurements and NMR measurements. Washing may also benefit other subsequent measurements, such as gammaray measurement, on the drill cuttings because the effect of the small particles on the gamma ray measurements can be significant.
The figure on the left in
The next step of the method is to measure the inair mass of the collected drill cutting 10.
m_{s}=V_{m}ρ_{m}+(V_{ϕ}+V_{sur})ρ_{l }
where ρ_{m }is a density of the matrix of the subsurface formation, ρ_{l }is a density of the fluid inside and surrounding the sample, V_{m }is a volume of the matrix, V_{ϕ }is a volume of the fluid inside the sample, and V_{sur }is a volume of the fluid surrounding the sample.
The next step is to separately determine volume of the fluid inside the sample, V_{ϕ}, and volume of the fluid surrounding the sample, V_{sur}, using nuclear magnetic resonance (NMR). To clearly separate the NMR signals for liquid inside and surrounding the cuttings, a sufficient amount of surrounding fluid may be used one time or in a stepwise fashion. Due to the clay sensitivity issues, many wells in unconventional plays are drilled using oil based mud (OBM). The example embodiments disclosed propose a new method to separate the NMR signal of the fluid on the cuttings surfaces and the fluids from the interior pores of the cutting samples based on two assumptions: (1) fluids inside the shale cuttings have short relaxation time, and (2) fluid from OBM has a longer T_{2}, even in the presence of cuttings.
A series of NMR experiments with cuttings demonstrate that the mode position of the T_{2 }signal of the OBM outside the cuttings does move to longer relaxation times as more fluid is gradually added (
No additional fluid is added in this variation of the method. A cut off 51 is selected from the incremental T_{2 }distribution line (a vertical dotted line drawn at the trough on the incremental curve in
In case where excess fluid is present a plot can be graphed as seen in
The next step is to measure the sample mass in a weighing fluid.
The mass of the sample in the weighing fluid, m_{f}, may be given by the formula
m_{f}=V_{m}ρ_{m}+V_{ϕ}ρ_{l}−V_{c}ρ_{f }
where ρ_{f }is the density of the weighing fluid. From the combination of two mass measurements and NMR measurement, multiple key parameters can be obtained as outlined in the following sections for reservoir characterization. These parameters include porosity, cutting total volume, bulk density, and matrix/grain density. For example, the method may further include determining a volume of the fluidsaturated sample without the surrounding fluid, V_{c}, using the formula
V_{c}=(m_{s}−m_{f}−V_{sur}ρ_{l})/ρ_{f}.
In the next step, the method may also include determining a bulk density of the fluidsaturated sample without the surrounding fluid, ρ_{b}, using the formula
In the next step, the method may further include determining the volume of the matrix, V_{m}, using the formula
V_{m}=(m_{s}−m_{f}−V_{sur}ρ_{f})/ρ_{f}−V_{ϕ}.
As a last step, the method may include determining the matrix or grain density of the subsurface formation, ρ_{m}, using the formula
These measurements can be performed on the cutting samples along the entirety of the drilled well and, thus, data can be obtained to evaluate the heterogeneity of the vertical or horizontal wells. This could potentially be used in real time to optimize the number and placement of frac stages for unconventional reservoirs.
Here, the contribution of the sample support device (12 in
m_{s}=V_{m}ρ_{m}+(V_{ϕ}+V_{sur})ρ_{l }
where ρ_{m }is a density of the matrix of the subsurface formation, ρ_{l }is a density of the fluid inside and surrounding the sample, V_{m }is a volume of the matrix, V_{ϕ }is a volume of the fluid inside the sample, and V_{sur }is a volume of the fluid surrounding the sample. The method also includes separately determining volume of the fluid inside the sample, V_{ϕ}, and volume of the fluid surrounding the sample, V_{sur}, using nuclear magnetic resonance (NMR), at step 104. The method may further include placing the sample in a predetermined volume of a weighing fluid at step 106, and measuring the mass of the fluidsaturated sample in the weighing fluid, at step 108. The mass of the fluidsaturated sample without the surrounding fluid in the weighing fluid, m_{f}, may be given by the formula
m_{f}=V_{m}ρ_{m}+V_{ϕ}ρ_{l}−V_{c}ρ_{f }
where ρ_{f }is the density of the weighing fluid. At step 110, the method may further include determining a volume of the fluidsaturated sample without the surrounding fluid, V_{c}, using the formula
V_{c}=(m_{s}−m_{f}−V_{sur}ρ_{l})/ρ_{f}.
The method may also include determining a bulk density of the fluidsaturated sample without the surrounding fluid, ρ_{b}, using the formula
At step 112, the method may further include determining the volume of the matrix, V_{m}, using the formula
V_{m}=(m_{s}−m_{f}−V_{sur}ρ_{f})/ρ_{f}−V_{ϕ}.
Finally, at step 114, the method may include determining the matrix or grain density of the subsurface formation, ρ_{m}, using the formula
Computer Readable Medium
Another example embodiment relates to computer programs stored in computer readable media. Referring to
m_{s}=V_{m}ρ_{m}+(V_{ϕ}+V_{sur})ρ_{l }
where ρ_{m }is a density of the matrix of the subsurface formation, ρ_{l }is a density of the fluid inside and surrounding the sample, V_{m }is a volume of the matrix, V_{ϕ }is a volume of the fluid inside the sample, and V_{sur }is a volume of the fluid surrounding the sample. The computer executable instructions may also trigger the computer to determine volume of the fluid inside the sample, V_{ϕ}, and volume of the fluid surrounding the sample, V_{sur}, using nuclear magnetic resonance (NMR). The computer executable instructions may also trigger the computer to receive mass of the fluidsaturated sample in a weighing fluid. The mass of the fluidsaturated sample without the surrounding fluid in the weighing fluid, m_{f}, may be given by the formula
m_{f}=V_{m}ρ_{m}+V_{ϕ}ρ_{l}−V_{c}ρ_{f }
where ρ_{f }is the density of the weighing fluid. The computer executable instructions may also trigger the computer to determine a volume of the sample without the surrounding fluid, V_{c}, using the formula
V_{c}=(m_{s}−m_{f}−V_{sur}ρ_{l})/ρ_{f}.
The computer executable instructions may further trigger the computer to determine a bulk density of the fluidsaturated sample without the surrounding fluid, ρ_{b}, using the formula
The computer executable instructions may further trigger the computer to determine the volume of the matrix, V_{m}, using the formula
V_{m}=(m_{s}−m_{f}−V_{sur}ρ_{f})/ρ_{f}−V_{ϕ}.
The computer executable instructions may further trigger the computer to determine the matrix or grain density of the subsurface formation, ρ_{m}, using the formula
Example System
Another example embodiment is a system 1200 for determining matrix or grain density of a subsurface formation. The system 1200 may include a fluidsaturated sample 10 of the subsurface formation, as illustrated in
m_{s}=V_{m}ρ_{m}+(V_{ϕ}+V_{sur})ρ_{l }
where ρ_{m }is a density of the matrix of the subsurface formation, ρ_{l }is a density of the fluid inside and surrounding the sample, V_{m }is a volume of the matrix, V_{ϕ }is a volume of the fluid inside the sample, and V_{sur }is a volume of the fluid surrounding the sample. The system 1200 may also include a NMR device 500, which may be operably connected to computer 200 and configured to determine the volume of the fluid inside the sample, V_{ϕ}, and volume of the fluid surrounding the sample, V_{sur}, using nuclear magnetic resonance (NMR). The computer 200 may be configured to receive the volume of the fluid inside the sample, V_{ϕ}, and volume of the fluid surrounding the sample, V_{sur}, from the NMR device 500, and the mass of the fluidsaturated sample in a weighing fluid from the weighing scale 25. The mass of the fluidsaturated sample without the surrounding fluid in the weighing fluid, m_{f}, may be given by the formula
m_{f}=V_{m}ρ_{m}+V_{ϕ}ρ_{l}−V_{c}ρ_{f }
where ρ_{f }is the density of the weighing fluid. The computer executable instructions may also trigger the computer to determine a volume of the fluidsaturated sample without the surrounding fluid, V_{c}, using the formula
V_{c}=(m_{s}−m_{f}−V_{sur}ρ_{l})/ρ_{f}.
The computer executable instructions may further trigger the computer to determine a bulk density of the fluidsaturated sample without the surrounding fluid, ρ_{b}, using the formula
The computer executable instructions may further trigger the computer to determine the volume of the matrix, V_{m}, using the formula
V_{m}=(m_{s}−m_{f}−V_{sur}ρ_{f})/ρ_{f}−V_{ϕ}.
The computer executable instructions may further trigger the computer to determine the matrix or grain density of the subsurface formation, ρ_{m}, using the formula
While the invention 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 can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.