Production forecasting analog methods

Oil and gas development projects need production forecasts for planning purposes and to understand the economic viability of each project. Sometimes, especially in the exploration and appraisal stages, it is necessary to develop a range of production forecasts for a project based on very limited data, often in the absence of flow information for that particular field,. One of the methods often used in this type of scenario is analog studies.

Analogs

Analogs are similar types of reservoirs to the targeted pool, which help to reduce exploitation risk as well as to suggest the type of reservoir development mechanism applicable and what future production performance might be expected. Analogues can be used to estimate likely reservoir parameters, initial rates or production profiles, as well as expected recovery factor and ultimate recovery.

Analogous reservoirs

PRMS defines an analogous reservoirs (as used in resource assessment) as having “similar rock and fluid properties, reservoir conditions (depth, temperature, and pressure) and drive mechanisms, but are typically at a more advanced stage of development than the reservoir of interest and thus may provide concepts to assist in the interpretation of more limited data and estimation of recovery”.

Features and characteristics

Analogous features and characteristics can include approximate depth, pressure, temperature, reservoir drive mechanism, original fluid content, oil gravity, reservoir size, gross thickness, pay thickness, net-to-gross ratio, lithology, level of heterogeneity, porosity and permeability.

Development

The development scheme for a reservoir (e.g. as reflected by well spacing, lift mechanism, fluid injection type) can also be important in establishing the relevance of the analogy and becomes especially important when using analogues for prediction how a production profile will look, as opposed to just overall recovery factor.

Identification

The task of identifying analogous reservoirs in petroleum systems requires active collaboration between geoscience and reservoir engineering. Analogues are not necessarily situated in the same geographical location. While reservoirs in the same geographic area, and of the same age, typically provide better analogs, such proximity alone may not be the primary consideration.

Qualitative approach

The analog method is a more qualitative than quantitative approach which generally results in a wide range of estimates of performance with no clear basis for a most-likely estimate. It is often used as a cross check alongside other more in depth forecasting methodologies.

Quantitative approach

If the analog method is to be the primary forecasting method for an asset then a more rigorous assessment of the analog dataset is warranted, moving the use of analogues into a more empirical methodology based on more quantitative, statistical based relationships between reservoir parameters. This type of analysis requires ranking of similar types of reservoirs based on a large database of geologic and reservoir properties. A master database of the properties of interest must be validated and normalized (normal probability distribution) while potential outliers are identified and eliminated. Other important tasks will include missing value analysis and data standardization. Some specific statistical techniques available to use in ranking analogs include principal component analysis as well as cluster analysis.

Principal component analysis

Principal component analysis is a method that is used to recognize patterns in data to highlight similarities and differences is data sets. The major benefit of using this method is that the data in question can be reduced to a specific number of elements while the data are still kept in a meaningful state.

Cluster analysis

Cluster analysis is a way of analyzing groups of data by using homogeneity of the data as the classification criteria. This method entails grouping data to reflect what is homogeneous and multiple groups can be created to further simplify specific properties.

Analog example

A good example of analog use in oil reservoirs is found in Sidle et al (2010), which considers “a regionally productive formation” which “has multiple closures against updip faults as it dips to the south. Initially, a shallower structure in this formation, Jonathan Field, was discovered and developed as a commercially productive oil reservoir, Later a deeper closure, Macintosh Field, was drilled with oil discovered there also. No production tests were done on the deeper reservoir but log and core data were taken, along with a wireline fluid sample.” The comparative data from Jonathan and Macintosh Fields are contained in Table 1.

Data from Apple formation	At Jonathan Field	At Macintosh Field
Porosity (φ), %	30	29
Permeability (k), md	930	890
Permeability distribution	D-P variance = 0.15	D-P variance = 0.15
Net Thickness (h), ft	150	150
Continuty	Continuous over board area without faults, etc.	Continuous over board area without faults, etc.
HC saturation, %	70	70
Oil viscosity (at reservoir temperture_ (µ), cp	1.5	0.35
Oil gravity, APIo	24	30
Gas-oil ration, scf/STB	800	1500
Average reservoir depth psi	6000	8500
Average reservoir pressure psi	2650	3750
Reservoir Transmissivity, (k*h/µ), md-ft/cp	93X103	381X103
Table 1 – Reservoir Data from Jonathan and Macintosh Fields

This example emphasizes two fields with slightly different parameters. A comparison of each of these parameters shows that specific properties, such as porosity or permeability, are lower in Macintosh Field. Similar comparisons of other parameters, such as average reservoir pressure and reservoir depth, indicate lower values in Jonathan Field. A consideration of individual analogs can lead to the wrong conclusions regarding suitability of analogs. Analysis of aggregates, such as productivity, makes a more compelling evidence of analogy. Other important rate-dependent parameters include reservoir pressure and gas oil ratios, which can also be used to compare potential analogs in most petroleum systems.

Enhanced oil recovery (EOR) schemes

 Enhanced oil recovery (EOR) schemes require analogs that satisfy most conditions in addition to type of fluid injected, injected rates and injection pressures. There must be sufficient production history to reflect scheme performance and it must be repeatable to be classified as an analog.

Steam injection

A steam injection example could use an aggregate such as φ(So-Sor)hi/ht. A comparison of a target reservoir with its analogs must show that these estimates are similar even if individual properties are slightly different. Other aggregates of properties such as mobility ratio [(Kw/μw)/(Ko/μo)] can be used to determine how successful other miscible flood process, such as waterflood, will potentially perform.

Multiple analogs

In the case of analyzing multiple analogs where the results appear different; the more conservative results will be beneficial to the overall objective of identifying suitable analogs. It also becomes imperative to evaluate how different the aggregated parameters from the analogs are to the target reservoir. If there is a significant difference between analogs and target reservoir, a better understanding of the contributing parameter(s) will be necessary.

Analogs EUR

An example of the use of analogs can be found in shallow natural gas systems where probabilistic distribution of analog parameters can be used to generate initial gas rates and EUR. Cumulative probabilities of parameters, such as initial rates and original gas in place can be plotted against EUR for different analogs on a log-log basis. The distribution shown in Fig 1 represents a fairly good number of analogs. The EURs for the individual gas wells were generated using material balance methods and the respective cumulative probabilities of analogs were determined. The P50 EUR of 100 MMcf is based on as many analogs available in the same gas pool classification, with similar reservoir properties available in the gas pool. The chosen analogs were picked from close proximity to the target location; the search expanded outward from the location of the target reservoir up to a few sections in the field. A different field contained fewer wells but a candidate for analog comparison was one township lateral distance away from target reservoir. The outlier was a high productivity well that had recovered many times the EUR when compared with wells closer to the target reservoir. It was estimated that the well was producing from a different pool and it was dropped because the EUR significantly inflated the EUR for the target reservoir. It must also be noted that the target field had a lot fewer analogs with significantly lower EUR in close proximity.

Figure 1 –Cumulative Probabilities of Analogs vs EUR (Pending permission approval)

To achieve a good process of evaluating analogs, a consistent procedure must be put in place. All expected comparisons and analysis of analogs must satisfy PRMS’s definition of analogs as well as reserves where and when applicable. Table 2 shows a comparison of SPE and SEC analog criteria and guidelines put together by Hodgin et al (2006). Also, it is fair to note that there are different petroleum recovery mechanisms and they continue to evolve. These sets of guidelines will require technical judgment and must be used on a case by case basis.

Comparison of SPE and SEC Analog Criteria and Guidelines
	SPE Criteria	SEC Criteria
Matching Criteria	Same or Similar	Same or better
Location/Proximity	Analog to Validate Producibility - Subject reservoir is analogous to reservoirs in the same area that are producing or have demonstrated the ability to produce on formation tests.	Analog to Validate Producibility - Subject reservoir are analogous to similar reservoirs on the same field which are producing or have demonstrated the ability to produce on a formation test.
	Analog to Support Enhanced Recovery- Successful testing by a pilot project or favorable response of an installed program in the same or an analogous reservoir.	Analog to Support Enhanced Recovery- Reserves cannot be classified as proved undeveloped reserves based on improved recovery techniques until such time that they have been proved effective in that reservoir or an analogous reservoir in the same geological formation in the immediate area.
	Analog to Establish Future Rates and Producing Trends - If production trends have not been established, future rates of production may be estimated by analogy to the respective rates of production of reservoirs in the same geographic area.	N/A
	Current Glossary of Terms - An analogous reservoir is one in the same geographic area.	N/A
	Proposed Glossary of Terms - While reservoirs in the same geographic area typically provide better analogs, such proximity alone may not be the promary consideration.	N/A
	Current Glossary Terms for Field - An area consisting of a single reservoir or multiple reservoirs all grouped on, or related to, the same individual geological structural feature and/or stratigraphic condition. There may be two or more reservoirs in a field that are separated vertically by intervening impermeable rock, laterally by local geologic barriers, or both. It could be defined differently bu regulatory authorities.	Regulation S-X Part 210 Defines Field - An area consisting of a single reservoir or multiple reservoirs all grouped on, or related to, the same individual geological structural feature and/or stratigraphic condition. There may be two of more reservoirs in a field that are separated vertically by intervening impermeable rock, laterally by local geologic barriers, or both. Reservoirs that are associated by being in overlapping or adjacent fields may be treated as a single or common operational field. The geological terms "structural feature" and "stratigraphic column" are intended to identify localized geological features as opposed to the broader terms of basins, trends, provinces, plays, areas-of-interest, etc.
	Current Glossary Term for Reservoir - A subsurface rock formation containing one or more individual and separate natural accumulations of moveable petroleum that is confined by impermeable rock and is characterized by a single-pressure system.	Regulation S-X Part 210 Defines Reservoir - A porous and permeable underground formation containing a natural accumulation of producible oil and/or gasd that is confined by impermeable rock or water barriers and is individual and separate from other reservoirs.
Geologic Processes	Currrent Glossary Term for Analogous Reservoir - Analogous reservoirs are formed by the same, or very similar, processes as regards sedimentation, diagenesis, pressure, temperature, chemical and mechanical history, and structural deformation.	N/A
Petrophysical Parameters	Analog to Validate Producibility - In certain cases, proved reserves may be assigned on the basis of well logs and/or core analysis that indicate that the subject reservoir is hydrocarbon bearing and is analogous to reservoirs in the same area that are producing or have demonstrated the ability to produce on formation tests.	Analog to Validate Producibility - In certain instances, proved reserves may be assigned to reservoirs on the basis of a combination of electrical and other type logs and core analyses which indicate the reservoirs are analogous (if the combination of data is overwhelmingly in support of economic producibility) to similar reservoirs in the same field which are producing or have demonstrated the ability to produce on a formation test. This would probably be a rare event in a exploratory situation... in most cases there must be a conclusive formation test in a new reservoir before ant reserves can be considered to be proved.
	Analog to Support Enhanced Recovery - An analogous reservoir should have similar rock properties.	Analog to Support Enhanced Recovery - An analogous reservoir is one having at least the same values or better for porosity, permeability, permeability distribution, thickness, continuity and hydrocarbon saturation.
	Analog to Establish Future Rates and Producing Trends - If production trends have not been established, future rates of production may be estimated by analogy to the respective rates of production of reservoirs having similar rock characteristics.	N/A
	Current Glossary of Terms - A analogous reservoir has the same of similar reservoir rock properties. Analogous features amd characteristics can include reservoir size, gross thickness, pay thickness, net-to-gross ration, lithology, heterogeneity. porosity and permeability.	N/A
Engineering Parameters	Analog to Validate Producibility - In certain cases, proved reserves may be assigned to the subject reservoir if it is analogous to reservoirs in the same area that are producing or have demonstrated the ability to produce on formation tests.	Analog to Validate Producibility - In certain cases, proved reserves may be assigned to the subject reservoirs that are analogous to similar reservoirs in the same field that are producing or have demonstrated the ability to produce on a formation test. This would probably be a rare event in an exploratory situation... in most cases there must be a conclusive formation test in a new reservoir before ant reserves can be considered to be proved.
	Analog to Support Enhanced Recovery - Successful testing by a pilot project or favorable response of an installed program in the same or an analogous reservoir with similar fluid properties.	Analog to Support Enhanced Recovery - Successful testing by a pilot project, or the operation of an installed program in the reservoir (or according to the 2001 website guidance an analogous reservoir in the same geologic formation in the immediate area). If an improved recovery technique which has not been verifies by routine commercial use in the area is to be applied, the hydrocarbon volumes estimated to be recoverable cannot be classified as proved reserves unless the technique has been demonstrated to the technically and economically successful by a pilot project or installed program in that specific rock volume. Such demonstration should validate the feasibility study leading to the project.
	Analog to Establish Future Rates and Producing Trends - If production trends have not been established, future rates of production may be estimated by analogy to the respective rates of production of reservoirs having similar fluid characteristics.	N/A
	Glossary Term for Analogous Reservoir - An analogous reservoir has the (current-same or similar; proposed-similar) fluid properties. Analogous features and characteristics can include depth, pressure, temperature, reservoir drive mechanism, original fluid content, and oil gravity.	N/A
Development Scheme	Glossary Term for Analogous Reservoir - The development scheme for a reservoir (e.g. as reflected by well spacing) can also be important in establishing the relevance of the analogy. (Both current and proposed contain identical wording)	N/A
Further Considerations	Glossary Term for Analogous Reservoir - An analogous reservoir has the (current-same or similar; proposed-same) geologic age.	N/A
Table 2 – Comparison of SPE and SEC analog criteria and guidelines

Noteworthy papers in OnePetro

Martin Rodriguez, H., Escobar, E., Embid, S., Rodriguez, N., Hegazy, M., & Lake, L. W. 2013. New Approach to Identify Analogue Reservoirs. Society of Petroleum Engineers. http://dx.doi.org/10.2118/166449-MS.

"Petroleum Resources Management System." 2011. Petroleum Reserves and Resources Definitions. Society of Petroleum Engineers. http://www.spe.org/industry/reserves.php

Sharma, A., Srinivasan, S., & Lake, L. W. 2010. Classification of Oil and Gas Reservoirs Based on Recovery Factor: A Data-Mining Approach. Society of Petroleum Engineers. http://dx.doi.org/10.2118/130257-MS.

Noteworthy books

Society of Petroleum Engineers (U.S.). 2011. Production forecasting. Richardson, Tex: Society of Petroleum Engineers. WorldCat or SPE Bookstore

See also

Production forecasting glossary

Aggregation of forecasts

Challenging the current barriers to forecast improvement

Commercial and economic assumptions in production forecasting

Controllable verses non controllable forecast factors

Discounting and risking in production forecasting

Documentation and reporting in production forecasting

Empirical methods in production forecasting

Establishing input for production forecasting

Integrated asset modelling in production forecasting

Long term verses short term production forecast

Look backs and forecast verification

Material balance models in production forecasting

Probabilistic verses deterministic in production forecasting

Production forecasting activity scheduling

Production forecasting analog methods

Production forecasting building blocks

Production forecasting decline curve analysis

Production forecasting expectations

Production forecasting flowchart

Production forecasting frequently asked questions and examples

Production forecasting in the financial markets

Production forecasting principles and definition

Production forecasting purpose

Production forecasting system constraints

Quality assurance in forecast

Reservoir simulation models in production forecasting

Types of decline analysis in production forecasting

Uncertainty analysis in creating production forecast

Uncertainty range in production forecasting

Using multiple methodologies in production forecasting