Interpretation of reservoir pressure in low-permeability reservoirs

UDK: 622.76.1/.4
DOI: 10.24887/0028-2448-2021-3-66-70
Key words: well-test, reservoir pressure, low permeability reservoirs, build-up curves interpretation
Authors: E.V. Yudin (Gazpromneft LLC, RF, Saint-Petersburg), O.A. Kolyuk (Gazpromneft LLC, RF, Saint-Petersburg; Peter the Great St. Petersburg Polytechnic University, RF, Saint-Petersburg), S.V. Zamakhov (Peter the Great St. Petersburg Polytechnic University, RF, Saint-Petersburg; Gazpromneft-Digital Solutions, RF, Saint-Petersburg)

Recording of the build-up curve (BC) is one of the main and most common types of well testing. However, the implementation and interpretation of the BC in low permeability reservoirs faces a number of difficulties, primarily related to the insufficient duration of well shutting-in, as well as requirement for a stable rate before well test. This paper is devoted to the development of an approach to the interpretation of reservoir pressure based on BC in low permeability reservoirs, devoid of mentioned disadvantages. The proposed method is based on the general form of solving the single-phase flow equation (diffusion equation) in a heterogeneous reservoir. The desired pressure distribution in the reservoir can be described through a series of the diffusion equation eigenfunctions. The eigen functions can change depending on the distribution of reservoir pressure at the time of well shutting-in, but the eigenvalues are constant and characterize the reservoir properties only. Earlier works have already described some possibilities for the restoration of the BC based on this principle, but our calculations shows that this approach is poorly applicable to low permeability reservoirs due to the need for a long well shutting time to calculate the eigenvalues. In this paper, the authors modified the method so that after evaluation the eigen functions and eigen values in one long-term study, it was possible to use them for short-term BCs interpretation. This allows us to carry out the so-called "accumulated interpretation", improving reservoir pressure estimation in each study. Paper provides an example of application of the proposed approach to improve reservoir pressure estimation in low permeability reservoirs during short well stops using the information from long-term well shutting-in. In contrast to traditional approaches of build-up interpretation, the proposed technique is applicable to a well of arbitrary completion in a heterogeneous reservoir. The proposed approach is verified both for synthetic examples and for field cases. The use of the method in real wells is demonstrated by the low-permeability reservoirs in the Orenburg region and Yamalo-Nenets autonomous district. It is shown that the proposed approaches allow to carry out short BCs for monitoring reservoir pressure.

References

1. Horner D.R., Pressure build-up in wells, WPC 4135.

2. Coats K.H., Rapoport L.A., McCord J.R., Drews W.P., Determination of aquifer influence function from field data, SPE-897-PA, 1964.

3. Gavalas G.R., Seinfeld J.H., Reservoirs with spatially varying properties: estimation of volume from late transient pressure data, SPE 4169-PA, 1973.

4. Barenblatt G.I., Entov V.M., Ryzhik V.M., Dvizhenie zhidkostey i gazov v prirodnykh plastakh (Movement of liquids and gases in natural reservoirs), Moscow: Nedra Publ., 1982, 211 p.

5. Crump J.G., Hite R.H., A new method for estimating average reservoir pressure: The muskat plot revisited, SPE-1027330-PA, 2008, DOI: 10.2118/102730-PA.

6. Yudin E., Galyautdinov I., Piotrovskiy G. et al., Approach to determining the optimal parameters of well performance in fractured reservoirs with a gas cap: Orenburg GCF case study, SPE-196852-RU, 2019, DOI: 10.2118/196852-MS.

Recording of the build-up curve (BC) is one of the main and most common types of well testing. However, the implementation and interpretation of the BC in low permeability reservoirs faces a number of difficulties, primarily related to the insufficient duration of well shutting-in, as well as requirement for a stable rate before well test. This paper is devoted to the development of an approach to the interpretation of reservoir pressure based on BC in low permeability reservoirs, devoid of mentioned disadvantages. The proposed method is based on the general form of solving the single-phase flow equation (diffusion equation) in a heterogeneous reservoir. The desired pressure distribution in the reservoir can be described through a series of the diffusion equation eigenfunctions. The eigen functions can change depending on the distribution of reservoir pressure at the time of well shutting-in, but the eigenvalues are constant and characterize the reservoir properties only. Earlier works have already described some possibilities for the restoration of the BC based on this principle, but our calculations shows that this approach is poorly applicable to low permeability reservoirs due to the need for a long well shutting time to calculate the eigenvalues. In this paper, the authors modified the method so that after evaluation the eigen functions and eigen values in one long-term study, it was possible to use them for short-term BCs interpretation. This allows us to carry out the so-called "accumulated interpretation", improving reservoir pressure estimation in each study. Paper provides an example of application of the proposed approach to improve reservoir pressure estimation in low permeability reservoirs during short well stops using the information from long-term well shutting-in. In contrast to traditional approaches of build-up interpretation, the proposed technique is applicable to a well of arbitrary completion in a heterogeneous reservoir. The proposed approach is verified both for synthetic examples and for field cases. The use of the method in real wells is demonstrated by the low-permeability reservoirs in the Orenburg region and Yamalo-Nenets autonomous district. It is shown that the proposed approaches allow to carry out short BCs for monitoring reservoir pressure.

References

1. Horner D.R., Pressure build-up in wells, WPC 4135.

2. Coats K.H., Rapoport L.A., McCord J.R., Drews W.P., Determination of aquifer influence function from field data, SPE-897-PA, 1964.

3. Gavalas G.R., Seinfeld J.H., Reservoirs with spatially varying properties: estimation of volume from late transient pressure data, SPE 4169-PA, 1973.

4. Barenblatt G.I., Entov V.M., Ryzhik V.M., Dvizhenie zhidkostey i gazov v prirodnykh plastakh (Movement of liquids and gases in natural reservoirs), Moscow: Nedra Publ., 1982, 211 p.

5. Crump J.G., Hite R.H., A new method for estimating average reservoir pressure: The muskat plot revisited, SPE-1027330-PA, 2008, DOI: 10.2118/102730-PA.

6. Yudin E., Galyautdinov I., Piotrovskiy G. et al., Approach to determining the optimal parameters of well performance in fractured reservoirs with a gas cap: Orenburg GCF case study, SPE-196852-RU, 2019, DOI: 10.2118/196852-MS.



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