A central theme of all conventional and unconventional oil and gas wells is well integrity. The concept of “well integrity failure“ in connection with shale gas production (see SHIP News) is based on the assumption that either the borehole between the gas-bearing formations (shale, or porous formation) and the above-ground facility is not tight, or that gases from by-passed formations that are not intended for commercial exploitation, are able to migrate to higher levels (e.g. groundwater) through paths due to an insufficient seal or escape to the surface.

Aside from the sealing devices at the wellhead, casing strings and cement are the fundamental safety barriers that are meant to seal wells and isolate the formations (with different media and different pressures). The paper by Prohaska and Thonhauser (2012) deals with the main failure mechanisms and best practices for casing and cementing.

Since permanent measurements in the borehole and the continuous transmission of these measurements are not available, data and observations must be collected at the surface (e.g. pressure in the annular space) in order to provide an initial indication for irregularities in addition to measurements and tests performed during the well production phase. In the wake of the scientific discussion on the environmental risks of shale gas production, however, different research teams have carried out mainly statistical analyses of these indicators and observations. The results of their analysis indicated high failure rates, presumably even rising in the future. It is therefore worth taking a closer look at the current findings as an addition to the SHIP article “New numbers on well integrity failures“:

1. The latest study by Darrah et al (2014) showed that there is no evidence of a connection between hydraulic fracturing of the shale and the groundwater formations, neither in the Marcellus shale, nor in the Barnett shale. This was investigated as a primary hypothesis for several years (see Davies 2011 and Jackson et al 2011).
2. However, Darrah et al (2014) found several groups of wells where stray gas (between groundwater and shale) that escaped through leaking annuli or defective casings was identified as the cause of groundwater contamination. The latter has not been proven in all cases, as has been shown in studies by Texas RRC (2014).
3. Contrary to the high number of cases of well barrier leakage quoted in the SHIP News, competent authors mention much lower numbers: The US Groundwater Protection Council (GWPC 2011) reports well failure rates (well integrity) of 0.03% for Ohio and 0.01% for Texas as well as 0.33% for Pennsylvania (based on PA DEP data). King & King (2013) mention very low failure rates between 0.005 and 0.03% for all oil, gas and injection wells.
4. The main cause, and also the most plausible, is a “cheap“ casing string design with surface casing and production casing only, but no intermediate casing. This problem has since been eliminated thanks to stricter US directives for well design and cementing that were put in place since 2012 (e.g. Ohio 2014). I believe that similar new directives are not necessary for Europe, where stricter guidelines are already in place. The UK guidelines, for example (Oil & Gas UK 2012) are very clear and stringent. In her Well Integrity Management Concept, Cuadrilla (2014) demonstrates a good example of Best Practice Well Designs.
5. In my opinion, classifying the numbers provided by Vidic, Ingraffea and Davies as “well failure” or “incidents” is not plausible. They are, first of all, symptoms which cannot be equated with the “failure“ of wells. Fig. 6 in Davies (2014) clearly shows categories the meaning of which is defined with different degrees of vagueness: gas migration 0.5%, blowouts & venting 0.9%; on the other hand: cement & casing 8.7% (?).

This might initially be defined as “barrier failure“, but not as well failure in general. The analysis of these “symptoms” is complex, because of their ambiguity:

• Sustained Casing Pressure (SCP), that is continuous annulus pressure, could, for example, also have been caused by rising temperatures during the production process. Moreover, as long as just one of several barriers has been breached, this does not constitute well failure.
• In addition, the problems in Pennsylvania frequently occurred in the same regions in the Northeast, where shallow gas (“stray gas“) is found throughout (including in many water wells from before the beginning of gas production). 
• Statistical analyses refer only to history and the valid guidelines and techniques in use at that time. The new guidelines for casing design and cementation (see 4.) aim directly at solving this known problem. Ingraffea et al (last in 2014) however, neglect to include the changes in the design and construction of the wells that have been made since. King & King (2013) comment on this matter: “Failure rates of well barriers and well-integrity failures, measured on wells constructed in a specific time period, are artifacts of that era; they are not identical to failure rates of wells designed later“.

The plausibility of clearly increased problems with unconventional wells as compared to conventional wells can only be regional (gas-bearing formation) and temporary (before: easy casing procedure) features.

On the author:

Prof. Reichetseder was professor for natural gas supply and petroleum engineering from 2001 – 2003 at the Institute of Petroleum Engineering at Clausthal University of Technology. Before and after that he was head of the Exploration and Production division and Managing Director of different Oil & Gas companies. Due to his many years of experience in the fields of research and industry, Mr. Reichetseder has acquired an in-depth knowledge and considerable experience with regard to the technologies applied in this field.

Literature:

Cuadrilla 2014: Well Integrity – Cuadrilla Land Based Wells. www.cuadrillaresources.com/wp-content/uploads/2012/02/DrillingPage-Well-Integrity-Document.pdf

Darrah T. H., Vengosh A., Jackson R. B., Warner N. R., Poreda R. J. 2014: Noble gases identify the mechanisms of fugitive gas contamination in drinking-water wells overlying the Marcellus and Barnett Shales. Proceedings of the National Academy of Sciences, September 15, 2014, doi: 10.1073/pnas.1322107111

Davies, R.J., 2011. Methane contamination of drinking water caused by hydraulic fracturing remains unproven. Proceedings of the National Academy of Sciences 108, E871.

Davies, R. J., Almond, S., Ward, R. S., Jackson, R. B., Adams, Ch., Worrall, F., Herringshaw, L. G., Gluyas, J. G., Whitehead, M. A. 2014: Oil and gas wells and their integrity: Implications for shale and unconventional resource exploitation. Marine and Petroleum Geology (2014)

GWPC 2011: State Oil and Gas Agency Groundwater Investigations And Their Role in Advancing Regulatory Reforms. Ground Water Protection Council, Ohio and Texas, Aug. 2011

Ingraffea A. R., Wells M. T., Santoro R. L., Shonkoff S. B. C. 2014: Assessment and risk analysis of casing and cement impairment in oil and gas wells in Pennsylvania, 2000 – 2012. Proceedings of the National Academy of Sciences, Vol. 111 (30), 2014, 10955-10960

Jackson, R.B., Osborn, S.G., Vengosh, A., Warner, N.R., 2011. Reply to Davies: Hydraulic fracturing remains a possible mechanism for observed methane contamination of drinking water. Proceedings of the National Academy of Sciences 108, E872.

King, G.E., King, D.E., 2013. Environmental Risk Arising from Well-construction Failure e Differences between Barrier and Well Failure and Estimates of Failure Frequency across Common Well Types, Locations and Well Age. SPE 166142.

Ohio 2014: Comparison of well construction standards. Ohio Drilling Regulations, www.capitolintegrity.com/documents/OhioDrillingRegulations.pdf

Oil & Gas UK 2012: Well Integrity Guidelines. Issue 1, July 2012. Oil and Gas UK 2012b, OP069, Available for purchase online, www.oilandgasuk.co.uk/cmsfiles/modules/publications/pdfs/OP069.pdf

Prohaska, M., Thonhauser, G. 2012: The Importance of Wellbore Integrity for Groundwater Protection in Shale Gas Well Construction. June 22, 2012

RRC 2014: Water Well Complaint Investigation Report, Silverado on the Brazos Neighborhood, Parker County. Railroad Commission of Texas, May 23, 2014