Simulated core sample estimated from composite borehole measurement
Methods, systems, and devices for evaluating an earth formation intersected by a borehole using information from standard resolution measurements. Methods include generating an image representative of the formation over an interval of borehole depth, the image having a second resolution greater than the first resolution. Generating the image may be carried out by identifying layers corresponding to lithotype facies within the interval, the layers defined by boundaries having boundary locations along the borehole; and using a unified characterization of the formation within the interval determined from the standard resolution measurements and the boundary locations within the interval to solve for a value for the formation parameter corresponding to each layer consistent with the unified characterization of the interval. The unified characterization may be an average value for the formation parameter within the interval.
1. A method for evaluating an earth formation intersected by a borehole using standard resolution measurements, the standard resolution measurements having a first resolution resulting from a downhole instrument used to obtain the measurements, the method comprising:
generating an image representative of the formation over an interval of borehole depth, the image having a second resolution greater than the first resolution, by:
identifying thinly bedded layers corresponding to lithotype facies within the interval, the layers defined by boundaries having boundary locations along the borehole;
using a unified characterization of the formation within the interval determined from the standard resolution measurements and the boundary locations within the interval to solve for a value for the formation parameter corresponding to each layer of the thinly bedded layers consistent with the unified characterization of the interval.
2. The method of claim 1 wherein solving for the value for the formation parameter corresponding to each layer comprises generating a plurality of solutions, each solution including values for the formation parameter corresponding to each layer; and determining an optimal solution from the plurality of solutions providing a maximum combined likelihood for the values for the formation parameter.
3. The method of claim 1 wherein the unified characterization comprises a fraction of each of a plurality of mineral constituent solids forming a rock matrix within the interval, wherein each mineral constituent solid of the plurality of mineral constituent solids has a defined chemical composition.
4. The method of claim 1 wherein the unified characterization comprises a composition of the interval comprising at least one of: i) a bulk elemental composition; and ii) a bulk mineralogical composition.
5. The method of claim 1 wherein the formation parameter comprises a lithological parameter.
6. The method of claim 5 wherein the formation parameter comprises a gross lithology selected from a set of gross lithologies consisting of: i) sandstone; ii) shale; iii) limestone; iv) dolomite; v) evaporitic sedimentary rocks, vi) salts and vii) igneous and metamorphic rocks.
7. The method of claim 6 further comprising identifying those of the gross lithologies present in the interval.
8. The method of claim 1 comprising estimating dimensions of each layer.
9. The method of claim 1 wherein identifying the layers comprises obtaining a borehole image with a second downhole instrument having a resolution greater than the first resolution.
10. The method of claim 1 wherein solving for the value for the formation parameter corresponding to each layer comprises estimating the most likely volume fraction for each mineral in each layer.
11. The method of claim 10 wherein estimating the most likely volume fraction comprises:
generating at least one proposed lithology for each layer;
representing a probability for particular volume fractions in a range of volume fractions for each mineral in each layer using a corresponding probability distribution function;
generating a solution that has a maximum aggregate probability by using the corresponding probability distribution functions to estimate the most likely volume fraction for each mineral in each layer.
12. The method of claim 11 wherein generating the solution comprises, for each mineral volume fraction:
identifying equality constraints and inequality constraints from the lithotype facies;
defining a solution space for the respective mineral volume fraction based on inequality constraints;
determining for the respective mineral volume fraction a solution surface based on equality constraints;
determining the solution surface within the solution space representing both equality constraints and inequality constraints on possible values; and
performing sampling on the solution surface to determine the solution having the maximum probability for the respective mineral volume fraction.
13. The method of claim 12 wherein performing sampling comprises sampling at uniform intervals from the initial solution on the solution surface.
14. The method of claim 12 wherein performing sampling comprises:
initially determining a starting point on the solution surface satisfying the inequality constraints, the starting point initiated as the initial solution; determining a random direction on the surface; and
estimating the point with the maximum probability for the respective mineral volume fraction by taking samples along the randomly selected direction at intervals from the starting point; calculating a probability for each sampled point; and using the point associated with a new maximum as a new starting point.
15. The method of claim 14 comprising computing maximum step sizes consistent with inequality constraints; estimating the point with the maximum probability for the respective mineral volume fraction by taking samples along the randomly selected direction at varying intervals from the starting point within the maximum step sizes.
16. The method of claim 9 , wherein the second downhole instrument comprises at least one of: i) an acoustic imager; ii) a resistivity imager; iii) an RF imager; and iv) a nuclear imager.
17. The method of claim wherein each layer consists of one of a plurality of lithological components.
18. A method for evaluating an earth formation intersected by a borehole using standard resolution measurements, the standard resolution measurements having a first resolution resulting from a downhole instrument used to obtain the measurements, the method comprising:
generating an image representative of the formation over an interval of borehole depth, the image having a second resolution greater than the first resolution, by:
identifying layers corresponding to lithotype facies within the interval, the layers defined by boundaries having boundary locations along the borehole;
using a unified characterization of the formation within the interval determined from the standard resolution measurements and the boundary locations within the interval to solve for a value for the formation parameter corresponding to each layer consistent with the unified characterization of the interval.
19. The method of claim 18 wherein solving for the value for the formation parameter corresponding to each layer comprises estimating the most likely volume fraction for each mineral in each layer.
20. The method of claim 18 wherein each layer consists of a bed.