Laboratory characterization of the porosity and permeability of gas shales using the crushed shale method: Insights from experiments and numerical modelling

doi:10.1016/j.marpetgeo.2017.05.027

Marine and Petroleum Geology

Volume 86, September 2017, Pages 95-110

https://doi.org/10.1016/j.marpetgeo.2017.05.027 Get rights and content

Highlights

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A round-robin test is reported where 6 shales were analysed by 4 laboratories.
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Permeabilities measured by each laboratory using the crushed shale method (CSM) do not correlate.
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Permeabilities vary by orders of magnitude depending on how the data are interpreted.
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Results from the CSM method do not correlate with measurements made on core plugs.
•
Pressure transient results from CSM experiments were inverted using analytical and numerical models.

Abstract

Gas production from shale resource plays has transformed the USA energy market. Despite the knowledge gained from the analysis of large amounts of shale core, appraisal of shale gas resource plays requires a large number of wells to be drilled and tested. Ideally, core analysis results would provide an indication of both the gas filled porosity and permeability of shale resource plays, which could then be used to reduce the number of wells needed during appraisal. A combination of laboratory experiments, numerical modelling and a round-robin test have been conducted to assess the validity of the crushed shale method (CSM), which has been widely used in industry to assess the porosity and permeability of shale. The results suggest that the CSM can provide reasonably precise estimates of porosity measured at ambient stress if a standard sample cleaning method is adopted; although a reliable method to correct these values to subsurface conditions needs to be developed. The CSM does not, however, appear to provide useful information on shale permeability. A round-robin test shows that differences of up to four orders of magnitude in permeability were provided by different laboratories when analysing the same sample. These huge differences seem to occur due to a combination of errors in calculating permeabilities from pressure transients, differences in the way that permeability is calculated as well as uncertainties regarding the effective size of crushed shale particles. However, even if standardized, the CSM may not be particularly useful for characterizing the flow capacity of shale because it is insensitive to the presence of high permeability zones that would control flow in the subsurface.

Introduction

Shale gas production has revolutionized the energy market in the USA where production reached 40 bcf/d in 2015 and contributed around 50% of the total dry gas produced (IEA, 2015). By 2014, over 50,000 producing wells had been completed in the seven key shale gas plays, Barnett, Haynesville, Woodford, Fayetteville, Marcellus, Eagle Ford, and Bakken (Hughes, 2015). Exploration and appraisal of shale gas resource plays is now active in many other parts of the world including: Australia, Argentina, China, India, Mexico, Poland, Romania and the UK. Appraisal of shale gas resource plays remains difficult and expensive despite the large number of wells that have been drilled, cored and tested. For example, Haskett (2014) suggested that over ten pilot wells may be needed to have 90% confidence that the results are representative of the shale gas play. Also, up to 100 wells may be needed before optimal production efficiency has been reached and production costs minimized (Haskett, 2014).

Appraisal of shale gas resource plays differs significantly from that of conventional reservoirs. In particular, appraisal of conventional reservoirs often involves drilling, coring and testing a small number of wells and then building geological and simulation models based on core analysis data to predict the volume of petroleum present and future production rates as well as to optimize production strategies. Appraisal of shale gas resource plays involves the drilling and testing of far more pilot wells with less emphasis on core analysis and little or no emphasis on production simulation modelling.

The reduced emphasis on core analysis and production simulation modelling in shale gas resource plays is a response to several realities. First, there remains a large uncertainty regarding how gas flows from the shale matrix to hydraulic fractures. For example, the role of natural fractures, sedimentary lamina with higher permeability, and the presence of intragranular vs. organic matter porosity in controlling gas storage and flow rates are still widely debated (e.g. Schieber, 2010, Curtis et al., 2012). Second, shale gas resource plays are heterogeneous in terms of their sedimentology, gas distribution, fracture content and stress magnitude/orientation so many wells are needed to estimate average performance. Third, there are no well-established links between core analysis results and production rates. Indeed, despite a large number of core analysis measurements being conducted, there exists little consensus on how they can be used to estimate flow rates. Forth, industry-standard protocols for the analysis of shale core do not exist. Indeed, comparative studies, often referred to as round-robin tests, indicate laboratories provide very different measurements of key properties, such as porosity and permeability (Passey et al., 2010, Dadmohammadi et al., 2016).

The current paper addresses key issues regarding the analysis of cores obtained from shale gas resource plays. In particular, it attempts to critically appraise the meaning of results from the laboratory method most commonly used by industry to assess the porosity and permeability of shale samples – namely the crushed shale method. In doing so, it attempts to identify causes of discrepancies between laboratories and provide recommendations for future core analysis. The paper begins by providing a review of the laboratory techniques that are commonly used to assess the porosity and permeability of core samples from shale gas resource plays. Results from numerical modelling and laboratory analysis are then used to provide some insight into the meaning of porosity and permeability data obtained from shales. The laboratory experiments conducted by the authors of this paper have been combined with a round-robin test in which the porosity and permeability of six shale samples have been analysed by three of the leading companies providing shale analysis services to industry.

Section snippets

Porosity and permeability analysis of conventional reservoirs

To place the analysis of gas shale into context, the methods that are commonly used to measure storage and flow capacity of conventional reservoirs are briefly described. Porosity is the primary measure of the total storage capacity of gases and liquids in conventional petroleum reservoirs. Permeability is used as the main measure of the flow potential of conventional reservoirs. Porosity and permeability are usually measured on 1 or 1.5 inch diameter core plugs, which have been cleaned and

Experimental methods

A round-robin test has been conducted to better understand the reasons for the discrepancies between different laboratories using the CSM. Six 10–15 cm long, 10 cm diameter cores, here referred to as samples SH1-6, were cut perpendicular to bedding to obtain four identical subsamples. The samples were chosen because they appeared homogenous during visual inspection and from the analysis of CT scans generated from a medical-type CT scanner. Three of these subsamples were sent to different core

Numerical modelling of gas flow in shale

A range of analytical and numerical models were used to both invert the pressure transient results obtained from the crushed shale experiments and also to explore key controls on the experiments; these are described below.

Bulk density

The round-robin test showed that there was a reasonable agreement between the bulk density calculated by the four laboratories (Fig. 3). In particular, standard deviation of bulk densities were 0.015 g/cm³, which is around 0.5%.

Grain density

The round-robin test showed that there was a reasonable agreement between the grain density calculated by the four laboratories (Fig. 4). In particular, standard deviation of grain densities was 0.02 g/cm³, which is around 0.8%. It should, however, be noted that LabB

Crushed shale method

The permeability results obtained by the service companies using the CSM are provided in Table 1; the raw pressure data was not provided so it was not possible to conduct further analysis of the results. A far more detailed analysis could be conducted on the data collected by the authors. In all cases, the pressure in the reference volume rapidly falls and then starts to rise reaching a maximum at around 1s (Fig. 6). The pressure then falls until it reaches the final equilibrium pressure, P_eq.

Porosity

The results from the round-robin test conducted during the current study show far more agreement between the various laboratories than suggested by Passey et al. (2010). All laboratories provided very similar results for bulk density. Leeds, LabA and LabC produced very similar results for grain density (average stdev = 0.015 g/cm³) and porosity (stdev = 0.3 p.u.). This precision is quite similar to the accuracy that API suggest is typical for measurements conducted on conventional sandstones.

Causes for differences between laboratories

The lack of correlation between the permeability measurements supplied by the different laboratories using the CSM is consistent with the round-robin test reported by Passey et al. (2010). Overall, the permeability values obtained by Leeds were several orders of magnitude lower than those provided by the commercial laboratories. Peng and Loucks (2016) reinterpreted pressure transient data from a turn-key CSM instrument supplied by one of the service companies. Their calculations produced

Conclusions

The crushed shale method has been widely used by industry to measure the porosity and permeability of shale. Theoretically, the method offers many advantages over traditional laboratory techniques. The current study has conducted experimental measurements, numerical modelling and sent six samples to leading service companies for a round-robin test. The results suggest that the CSM test can provide precise estimates of porosity measured at ambient stress if standard sample cleaning techniques

Acknowledgements

We are extremely grateful for the sponsorship of this work provided by Chevron, EBN and Nexen. Schlumberger and Emerson are thanked for granting us licences to use Eclipse™ and Enable™.

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Research paperLaboratory characterization of the porosity and permeability of gas shales using the crushed shale method: Insights from experiments and numerical modelling

Highlights

Abstract

Introduction

Section snippets

Porosity and permeability analysis of conventional reservoirs

Experimental methods

Numerical modelling of gas flow in shale

Bulk density

Grain density

Crushed shale method

Porosity

Causes for differences between laboratories

Conclusions

Acknowledgements

Int. J. Coal Geol.

Mar. Pet. Geol.

Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

Mar. Pet. Geol.

Geomechanical Simulation of Core Microfractures while Pulling Out of Hole

Permeability of granite under high pressure

J. Geophys. Res.

Comparison of shale permeability to gas determined by pressure-pulse transmission testing of core plugs and crushed samples

Shale-gas permeability and diffusivity inferred by improved formulation of relevant retention and transport mechanisms

Transp. Porous Media

Shale permeability determined by simultaneous analysis of multiple pressure-pulse measurements obtained under different conditions

Meas. Gas Permeab. Diffus. Tight Reserv. Rocks Differ. Approaches Appl. Geofluids

Simultaneous Estimation of Intrinsic Permeability, Effective Porosity, Pore Volume Compressibility, and Klinkenberg-slip Facture of Ultra-tight Rock Samples Based on Laboratory Pressure-step Decay Method

A practical approach for determining permeability from laboratory pressure-pulse decay measurements

A numerical study of microscale flow behavior in tight gas and shale gas reservoir systems

Transp. Porous Media

Improved petrophysical core measurements on tight shale reservoirs using retort and crushed samples

The myth of sweet spot exploration

Experimental investigation of matrix permeability of gas shales

AAPG Bull.

Research paper
Laboratory characterization of the porosity and permeability of gas shales using the crushed shale method: Insights from experiments and numerical modelling