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EP-4308978-B1 - METHOD TO AUTOMATICALLY PICK FORMATION TOPS USING OPTIMIZATION ALGORITHM

EP4308978B1EP 4308978 B1EP4308978 B1EP 4308978B1EP-4308978-B1

Inventors

  • AL ISMAIL, MARWAH
  • MEZGHANI, MOKHLES M.
  • AL-MASRAHY, Mohammed Ali

Dates

Publication Date
20260506
Application Date
20220316

Claims (18)

  1. A method comprising: obtaining, by a computer processor (705), at least one key log in each of a set of training wells (102) located, at least partially, within a hydrocarbon reservoir (100, 614), where the key log comprises a resistivity log, a gamma ray log, a density log or a sonic slowness log; identifying an intersection of a target formation bounding surface (202) with each of the set of training wells (102); generating, by a computer processor (705), an initial depth surface for the target formation bounding surface (202) from the intersections; determining, by a computer processor (705), an initial depth estimate of the target formation bounding surface (202) at a location of a current well (106) from the initial depth surface for the target formation bounding surface (202); forming, by a computer processor (705), an objective function based, at least in part on a correlation between each key log in each of the set of training wells (102), and each corresponding key log in the current well (106); wherein forming the objective function, further comprises: selecting, for each key log in each of the set of training wells (102), a portion of the key log defined by a cross-correlation window (316); depth-shifting the portion of each key log by a difference between the depth of the initial depth surface at the location of each of the training well (102) and the depth of the initial depth surface at the location of the current well (106); selecting, for the current well (106), a portion of the key log defined by a cross-correlation window (316); calculating a cross-correlation value between the key log in the training well (102) and the key log in the current well (106) for a depth shift, wherein the depth shift lies within a maximum depth shift window (314); and summing the cross-correlation value for each key log in each of the set of training wells (102) to determine a formation bounding surface correlation value; the method further comprising: optimizing, by a computer processor (705), the objective function by: varying a depth shift between each of the set of training wells (102) and the current well (106), to determine an optimum depth shift that produces an extremum of the objective function; and producing, by a computer processor (705), a final depth estimate of the target formation bounding surface (202) at the location of the current well (106) based on combining the initial depth estimate of the target formation bounding surface (202) at the location of the current well (106) with the optimum depth shift, wherein the objective function is defined as a top correlation score that comprises a summation of a plurality of well correlation scores determined for each training well (102); wherein each well correlation score of the plurality of well correlation scores is determined as a summation of a plurality of Pearson correlation coefficients, calculated between each of the set of training wells and the current well, over the key well log types, and divided by the total number of key well log types that each of the set of training wells (102) and the current well (106) have in common; and wherein the Pearson correlation coefficient is a function of depth shift.
  2. The method of claim 1, further comprising planning and drilling a new well within the hydrocarbon reservoir (100, 614).
  3. The method of claims 1 or 2, further comprising designing and constructing surface production facilities to process hydrocarbons produced from the hydrocarbon reservoir (100, 614).
  4. The method of any one of claims 1 to 3, wherein generating the initial depth surface for the target formation bounding surface (202) comprises using an interpolation method selected from a group consisting of simple kriging, ordinary kriging, universal kriging, indicator kriging, probability kriging, disjunctive kriging, inverse distance weighted interpolation, natural neighbour inverse distance weighted interpolation, natural cubic spline interpolation, B-spline interpolation, and Hermite spline interpolation.
  5. The method of any one of claims 1 to 4, wherein determining an initial depth estimate of the target formation bounding surface (202) at a location of a current well, further comprises determining a depth of the initial depth surface at the location of the current well (106).
  6. The method of any one of claims 1 to 5, wherein optimizing the objective function, further comprises: selecting, a new depth shift within a maximum depth shift window (314) based, at least in part, on a previous determination of the formation bounding surface correlation value; evaluating the objective function based, at least in part, on the new depth shift; determining whether a stopping criterion has been met; and executing an action selected from a group consisting of updating the new depth shift and terminating the optimization, based at least, on the determination of whether the stopping criterion has been met.
  7. The method of claim 6, wherein the stopping criterion of the optimization is selected from a group consisting of determining a local extremum of the objective function, determining an asymptote of the objective function, and evaluating the objective function a predetermined number of times.
  8. The method of any one of claims 1 to 7, wherein the extremum of the objective function is selected from a group consisting of a maximum and a minimum.
  9. The method of any one of claims 1 to 8, wherein producing a final depth estimate of the target formation bounding surface (202) at the location of the current well (106), further comprises adding the optimum depth shift to the initial depth estimate of the target formation bounding surface to obtain the final depth estimate of the target formation bounding surface at the location of the current well (106).
  10. A non-transitory computer readable medium storing instructions which, when executed by a computer processor (705), cause the computer processor (705) to carry out the steps of: obtaining at least one key log in each of a set of training wells (102) located, at least partially, within a hydrocarbon reservoir (100, 614), where the key log comprises a resistivity log, a gamma ray log, a density log or a sonic slowness log; identifying an intersection of a target formation bounding surface (202) with each of the set of training wells (102); generating an initial depth surface for the target formation bounding surface (202) from the intersections; determining from the initial depth surface for the target formation bounding surface (202) an initial depth estimate of the target formation bounding surface (202) at a location of a current well (106); forming an objective function based, at least in part, on a correlation between each key log in each of the set of training wells (102) and each corresponding key log in the current well (106); wherein forming the objective function, further comprises: selecting, for each key log in each of the set of training wells (102), a portion of the key log defined by a cross-correlation window (316); depth-shifting the portion of each key log by a difference between the depth of the initial depth surface at the location of each of the training well (102) and the depth of the initial depth surface at the location of the current well (106); selecting, for the current well (106), a portion of the key log defined by a cross-correlation window (316); calculating a cross-correlation value between the key log in the training well (102) and the key log in the current well (106) for a depth shift, wherein the depth shift lies within a maximum depth shift window (314); and summing the cross-correlation value for each key log in each of the set of training wells (102) to determine a formation bounding surface correlation value; wherein the instructions when executed by the computer processor, further cause the computer processor to carry out the steps of: optimizing the objective function by: varying a depth shift between each of the set of training wells (102) and the current well (106) to determine an optimum depth shift that produces an extremum of the objective function; and producing a final depth estimate of the target formation bounding surface (202) at the location of the current well (106) based on combining the initial depth estimate of the target formation bounding surface (202) at the location of the current well (106) with the optimum depth shift, wherein the objective function is defined as a top correlation score that comprises a summation of a plurality of well correlation scores determined for each training well (102); and wherein each well correlation score of the plurality of well correlation scores is determined as a summation of a plurality of Pearson correlation coefficients calculated between each of the set of training wells and the current well, over the key well log types, and divided by the total number of key well log types that each of the set of training wells (102) and the current well (106) have in common; and wherein the Pearson correlation coefficient is a function of depth shift.
  11. The non-transitory computer readable medium of claim 10, wherein the instructions, when executed by the computer processor (705), cause the computer processor (705) to carry out the step of planning a new well within the hydrocarbon reservoir (100, 614).
  12. The non-transitory computer readable medium of claims 10 or 11, the instructions, when executed by the computer processor (705), cause the computer processor (705) to carry out the step of designing a surface production facility to process hydrocarbons produced form the hydrocarbon reservoir (100, 614).
  13. The non-transitory computer readable medium of any one of claims 10 to 12, wherein generating the initial depth surface for the target formation bounding surface (202), further comprises using an interpolation method selected from a group consisting of simple kriging, ordinary kriging, universal kriging, indicator kriging, probability kriging, disjunctive kriging, inverse distance weighted interpolation, natural neighbor inverse distance weighted interpolation, natural cubic spline interpolation, B-spline interpolation, and Hermite spline interpolation.
  14. The non-transitory computer readable medium of any one of claims 10 to 13, wherein determining an initial depth estimate of the target formation bounding surface (202) at a location of a current well, further comprises determining a depth of the initial depth surface at the location of the current well (106).
  15. The non-transitory computer readable medium of any one of claims 10 to 14, wherein optimizing the objective function, further comprises: selecting a new depth shift within a maximum depth shift window (314) based, at least in part, on a previous determination of the formation bounding surface correlation value; evaluating the objective function based, at least in part, on the new depth shift; determining whether a stopping criterion has been met; and executing an action selected from a group consisting of updating the new depth shift and terminating the optimization, based at least, on the determination of whether the stopping criterion has been met.
  16. The non-transitory computer readable medium of claim 15, wherein the stopping criterion of the optimization is selected from a group consisting of determining a local extremum of the objective function, determining an asymptote of the objective function, and evaluating the objective function a predetermined number of times.
  17. The non-transitory computer readable medium of any one of claims 10 to 16, wherein the extremum of the objective function is selected from a group consisting of a maximum and a minimum.
  18. The non-transitory computer readable medium of any one of claims 10 to 17, wherein producing a final depth estimate of the target formation bounding surface (202) at the location of the current well (106), further comprises adding the optimum depth shift to the initial depth estimate of the target formation bounding surface to obtain the final depth estimate of the target formation bounding surface at the location of the current well (106).

Description

BACKGROUND Identifying the shape of rock formations is an important task in developing and producing a hydrocarbon reservoir and one routinely undertaken in the oil and gas industry. The shape of a formation includes its lateral extent and the topography of the bounding surfaces above and below the reservoir. This information is important for calculating the volume of the reservoir, which influences the quantity of hydrocarbon the reservoir may contain, and for planning future well locations and production strategies. One element of determining the shape of a formation is identifying the depth at which a plurality of wells intersect it. This activity, often referred to as "formation top picking", may be supplemented by reference to seismic maps and images to interpolate and extrapolate the bounding surfaces between wells. Manual formation top picking involves two major steps. First, one or more suitable well logs, measuring a quantity such as resistivity, gamma ray, density, or sonic slowness, must be identified. A suitable well log should show a clear difference between the section of the well log recorded within the formation of interest and the sections recorded in adjacent formations above or below it. Such a difference is often referred to as a signature. A clear signature may be a sharp change between the value of the log in the formation of interest and the value in the formation immediately above or below. Other more subtle signature bases are also used. Second, a geoscientist manually inspects the suitable well log(s) in each well in the field to find the depth in the well at which the signature is visible and identifies that position in the well as a point on the bounding surface of the formation. The point has a depth and two horizontal coordinates. This process of manual inspection and identification is prone to error and the individual biases of the geoscientist performing the process. Patent document US 6012017 A describes a method for analyzing, by a computer processor, well logs. Patent document US 2015/0088424 A describes a method including: analyzing, by a computer processor, a plurality of training well logs of a plurality of training wells in a field to generate a plurality of training well markers, wherein the plurality of training well markers identify where the plurality of training wells intercept a plurality of geologic interval boundaries in the subterranean formation of the field; propagating, by the computer processor and onto a target well log of a target well in the field, the plurality of training well markers to generate a plurality of target well markers, wherein the plurality of target well markers identify where the target well intercepts the plurality of geologic interval boundaries; and performing a field operation based at least on identifying where the target well intercepts the plurality of geologic interval boundaries. SUMMARY This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. A method and a non-transitory computer readable medium according to the invention are defined in the appended independent claims. Specific embodiments are defined in the dependent claims. In general, in one aspect, embodiments relate to a method including obtaining, by a computer processor, at least one key log in each of a set of training wells located, at least partially, within a hydrocarbon reservoir, identifying a target formation bounding surface in each of the set of training wells, and generating an initial depth surface for the target formation bounding surface from the target formation bounding surface in each of the set of training wells. The method further including, determining from the initial depth surface an initial depth estimate of the target formation bounding surface at a location of a current well, forming an objective function based, at least in part on a correlation between each key log in each of the set of training wells, and each corresponding key log in the current well, and optimizing the objective function by varying a depth shift between each of the set of training wells and the current well, to determine an optimum depth shift that produces an extremum of the objective function. The method still further including combining the initial depth estimate of the target formation bounding surface at the location of the current well with the optimum depth shift to produce a final depth estimate of the target formation bounding surface at the location of the current well. In general, in one aspect, embodiments relate to a non-transitory computer readable medium storing instructions executable by a computer processor, the instructions including functionality for obtaining at least one key log in each of a set of tr