Hydrofracture Analysis Essay

Endnotes

1 An excellent summary of the history of mining and the petroleum industry is to be found in Martin, G. L. 2003. Gesner’s Dream: The Trials and Triumphs of Early Mining in New Brunswick. Geological Society of Canada Publication.

2 The Last Billion Years: A Geological History of the Maritime Provinces of Canada, published by the Atlantic Geoscience Society (2001), provides an excellent layperson’s introduction not only to local geology, but also to many key concepts, such as the formation of oil and gas. Also informative is the New Brunswick Department of Natural Resources webpage at www2.gnb.ca/content/gnb/en/departments/natural_resources/Promo?Natural_Gas/CarboniferousGeology.html.

3 Energy Information Administration, U.S. Department of Energy, at www.eia.doe.gov.

4 I took part in a radio discussion with a prominent representative of the local anti–shale gas campaign in fall 2011. The exaggerated figures I subsequently quote come from this radio broadcast or related websites. This is also an example of uncritical alternative media journalism, though by no means the only one. The podcast is available at http://chsrfm.ca/blog/2011/11/08/can-episode-2-what-the-frack-hydraulic-fracturing-in-new-brunswick/.

5 Gasland is a 2010 film directed by Josh Fox that deals with methane leaks and other problems allegedly brought on by shale-gas hydrofracturing in west Pennsylvania, Texas, and Colorado. Be…without Water is a 2012 film directed by Ron Turgeon, narrating the issues some fifty families near Penobsquis, NB, have had with water wells failing over the last decade.

6 See Mooney, C. “The Truth About Fracking.” Scientific American (November 2011): 80–85. Also available at www.ScientificAmerican.com. Mooney’s article addresses all the points usually raised by the anti–shale gas movement but the tone is refreshingly free of the apocalyptic imagery. For an interesting comparison between the methods of anti– shale gas and anti–GM crop activism, see Richard Black’s article “Is shale gas the GM of energy?” at www.bbc.co.uk/news/science-environment-17741416.

7 The issues associated with groundwater are dealt with in more depth and greater expertise at www.unb.ca/initiatives/shalegas/. The authors of this article are a UNB hydrogeologist and geochemist, geophysicist, river scientist and drilling engineer, none of whom are associated with industry or government, or the anti–shale gas lobby.

8 See Osborn, S. G., Vengosh, A., Warner, N. R. and Jackson, R. B. 2011. “Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing.” Proceedings of the National Academy of Sciences of the United States of America. Available at www.pnas.org/content/early/2011/05/02/1100682108.

9 The chemicals considered here are based on the additives list released publicly for the Elgin test well in 2011. Other chemical data regarding formaldehyde, glutaraldehyde, and polyacrylamide come from Google searches for these chemicals.

10 See for example Haszeldine, S. 2012. “Shale gas, NW England earthquakes, and UK regulation Briefing Note” for DECC SAG 21 November 2011, update February 2012 (Sect. 10), update April 2012 (Sect.12) available www.decc.gov.uk/assets/decc/11/about-us/science/5223-shale-gas-nw-england-earthquakes-briefing.pdf. This document also illustrates the mining-related earthquakes occurring in the same region. They are far more common and of the same magnitude as the Blackpool earthquakes beneath the northern part of the Greater Manchester area, producing no damage or threat to life.

11 See www.bgs.ac.uk/research/earthquakes/blackpoolApril2011.html; www.bgs.ac.uk/research/earthquakes/blackpoolMay2011.html; also http://maps.bgs.ac.uk/GeoIndex/default.aspx. The Richter magnitude scale for earthquakes is logarithmic, such that magnitude 2.0 is ten times magnitude 1.0, and 3.0 ten times 2.0. Unless the earthquake rupture is especially shallow (< 4 km depth), earthquakes of less than magnitude 3.0 are not felt at the surface. It is often difficult to detect earthquakes less than magnitude 2.0 in urban areas because they are masked by background noise, such as that produced by heavy traffic.

12 Geological Survey of Canada earthquake archive at http://www.earthquakescanada.nrcan.gc.ca/cite-eng.php.

13 British Geological Survey 1:50,000 Solid Geology maps 1977 Blackpool sheet.

14 U.S. Geological Survey earthquake archives at http://earthquake.usgs.gov/earthquakes/.

15 The UK advisory report prepared by the Royal Society and Royal Academy of Engineers appeared in early July 2012. At the time of writing the complete document was not available online, but a summary can be seen at www.raeng.org.uk/news/releases/shownews.html?NewsID+711.

16 The geological maps of New Brunswick are all available through the NB Department of Natural Resources website, but a summary is available at the address cited in note 2. For the Duke University report see Jackson, R. B., Pearson, B. R., Osborn, S. G., Warner, N. R. and Vengosh, A. 2011. “Research and Policy Recommendations for Hydraulic Fracturing and Shale-gas Extraction.” Nicholas School of the Environmental and Earth Sciences, Duke University, available at www.nicholas.duke.edu/cgc/HydraulicFracturingWhitePaper2011.pdf.

17 For a fairly readable and rational discussion of this see Leggett, J. 2005. The Empty Tank: Oil, Gas, Hot Air and the Coming Global Financial Catastrophe. New York: Random House. Written before the 2008 financial crisis, it has the benefit of being tested by subsequent events.

What are the important design issues associated with large diameter HDD projects and how can these issues be addressed? You could answer the first part of the question by identifying the issues that can prevent successful completion of an HDD bore. These issues include both technical and non-technical issues. Even the non-technical issues require engineering evaluation and technical solutions.

Permitting and regulatory oversight concerns drive a substantial portion of the technical analyses for large-diameter HDD projects. Credibility must be established with regulatory and permitting staff through honest dialogue and diligent follow-through efforts. The need to protect sensitive environmental resources from inadvertent drilling fluid returns and existing improvements and utilities from strikes and excessive settlement must be balanced against the need for sufficient rig side and pipe layout work space, reasonable setback distances, entry and exit angles, bore lengths, depths and reamed diameter vs. product diameter. Surface topography and ground conditions often complicate the possible solution. Technical issues including pipe pullback, buckling and bending stresses must be balanced against available work space, existing improvements, topography, ground conditions and costs and availability of suitable materials. Appropriate factors of safety must be applied in the analyses to ensure safety without destroying economic feasibility. Sometimes, one or more of these issues cannot be resolved through HDD and a different trenchless solution must be devised.

Good design demands all relevant issues be addressed in a balanced, rational way. To be successful, the design engineer needs the right analytical tools, judgment and experience to select appropriate input parameter values. People skills are critical to listen to the concerns of regulatory and permitting staff and establish credibility with these staff by properly identifying and mitigating the risks. Cost estimating and scheduling expertise are also important.

In Part I of this series, guidance is offered on one of the issues: hydrofracture.

Hydrofracture  

The cavity expansion model is the appropriate model for evaluating hydrofracture risks, specifically maximum allowable drilling fluid pressures, for the majority of HDD projects in soils. The U.S. Army Corps of Engineers Design Manual for Levees (EM-1110-2-1913, “Design and Construction of Levees”) states in Chapter 8 that the Delft (cavity expansion) equation in Appendix A of WES CPAR-GL-98-1 (Staheli et al.,1998) should be used to evaluate maximum allowable drilling fluid pressure.

The tensile strength model is more appropriate for HDD projects in rock, especially those with relatively shallow cover. The total stress model ignores contributions of soil strength and leads to unnecessarily conservative bore depths. Estimation of minimum required drilling fluid pressure to ensure cuttings are transported from the bore is also required. The solution for minimum required pressure derives from well-established fluid mechanics principles.

Discussion of the cavity expansion model, with example calculations from actual HDD case histories can be found elsewhere. This paper was intended to help engineers become comfortable with the approach, the results predicted and their significance. More accurate evaluation of hydrofracture risks, is only one step in reducing hydrofracture risks. Coupled with improved evaluation, improvements in management of drilling fluid properties and drilling methods are needed. Contingency measures such as conductor casing, relief wells and piezometers should also be considered for reducing risks on difficult, challenging bores. The third edition of the HDD Good Practices Guidelines addresses these topics, with updated information and a new chapter on design issues.

Inadvertent fluid returns are often referred to universally as hydrofractures or frac-outs. However, not all of these instances are actually caused by hydrofracture. Other sources of inadvertent fluid returns include existing fissures in the soil, preferential seepage paths along piers, piles or other structures and open-graded, loose gravel or rocks above the bore. Hydraulic fracturing is a specific occurrence in non-fissured cohesive soils when the pressure of the drilling fluid exceeds the strength and confining stress of the surrounding soils and the excess pressure fractures the soil around the bore. Plastic yielding can occur in cohesive and non-cohesive soils and represents the condition where fluid pressures exceed the shear strength and confining stress of the soil. Plastic yielding results in fluid losses to the surrounding formation.

Proper use of the cavity expansion model requires judgment and accurate geotechnical data. Assumptions are required regarding contractor practices, mud properties, pilot and reamed diameters. When assumptions regarding geotechnical conditions and drilling practices are invalid, results are likewise unsatisfactory. The cavity expansion model provides a mechanism for predicting maximum allowable pressures. Minimum required pressures for drilling and reaming must also be calculated and compared against maximum allowable pressure to assess hydrofracture risks.

When drilling fluid pressures exceed maximum allowable pressure of the surrounding soil, localized plastic yielding of the soil surrounding the annulus occurs. The limiting radius of yielding occurs at the point where pressure is equal to the pressure required to cause plastic yielding. Beyond this zone, hydrofracture does not occur. Figure 1 shows this concept.

Localized hydrofractures — those that do not reach the surface — are typically sealed by excess groundwater and earth pressures soon after the HDD bore is completed. This healing phenomenon is supported by experience in large scale HDD tests conducted by the Corps of Engineers.

Maximum allowable calculated pressure at any point is the pressure required to create a plastic (failed) zone equal to the depth of soil above the pipeline at that point. That is, the factor of safety against the plastic zone reaching the ground surface is 1.0 for any location along the maximum allowable pressure graph. The actual factor of safety applied to a project must account for potential consequences and reliability of input parameter values used.

Minimum drilling fluid pressure required to return the soil cuttings back through the HDD bore to the surface is a critical factor in evaluating hydrofracture risk. Minimum pressure depends on the length, depth and diameter of the bore, the weight of the drilling fluid and the flow rate. Minimum required pressure is a combination of the drilling fluid head pressure that must be overcome and the frictional resistance to flow from the bore wall.

Drilling fluid pressures are often highest during the pilot bore, because of the smaller annulus and one-way flow path. During reaming, drilling fluid can flow out through the entry or exit; the annulus is larger, so pressures are usually lower. But pressures during pullback can be high, because the larger diameter of the product pipe reduces the annular flow path.

Actual drilling fluid weights vary. However, good practice dictates that drilling fluid properties, including weight, be properly managed to achieve satisfactory results. Mud weights should be maintained below 9.5 lb/gal.

Once the maximum allowable and minimum required pressures have been calculated, it is important to compare the two numbers at critical features, such as locations of low earth cover, crossings beneath utilities, beneath rivers and environmentally sensitive areas, near embankment toes or at large distances from the entry point, to get a comprehensive view of hydrofracture risk.

Drilling fluid pressures can vary greatly with the contractor’s methods and changes in ground conditions. The minimum required pressure is exactly that: a minimum. Although calculations may indicate there is little risk of hydrofracture, an inexperienced operator or unforeseen soil conditions can greatly affect that risk. Selection of an experienced, qualified contractor is an important step in preventing hydrofracture. A thorough and accurate geotechnical investigation is a pre-requisite for success. Since the results of a hydrofracture risk evaluation are so dependent on the assumptions regarding ground conditions, groundwater levels, contractor means and methods and drilling fluid properties, the evaluation should be conducted using a range of values and assumptions to bracket the limits.

If comparison of the maximum allowable pressure and the minimum required pressure indicate an unacceptable risk of hydrofracture, the design should be altered to reduce that risk. Hydrofracture risk can be reduced by altering the geometry of the bore — shortening the bore, deepening the bore or selecting better soil strata for the alignment. Conductor casings can be used to help maintain bore continuity through rocky, gravelly or loose soils near the surface so that drilling fluids don’t escape the annulus. Once the bore has passed beyond the end of the conductor casing, increased thickness and typically higher strengths of the soils help reduce hydrofracture risk. Additional risk mitigation techniques include relief wells, piezometers and monitoring.

Relief wells can be installed at locations where excessive drilling fluid pressures may exceed the soil’s capability to resist hydrofracture. Locations should be selected that are accessible for containment and cleanup equipment, making it easier to maintain a clean worksite, while avoiding damage to sensitive features.

Regardless of the preventative measures used, any project with a significant risk of hydrofracture should have a contingency plan. This plan should include a procedure for containing and cleaning up any inadvertent fluid returns and describe materials that the contractor should have on hand such as sand bags, hay bales or wattles to contain the fluid, a vac-truck or trailer, shovels, brooms, barrels to contain the fluid and submersible pumps to remove the liquid.

Hydrofracture risks can be evaluated and managed using the tools described above, coupled with experience and judgment, to allow a successful, safe project.

Dr. David Bennett, Ph.D., P.E., is president of Bennett Trenchless Engineers, Folsom, Calif.

One thought on “Hydrofracture Analysis Essay

Leave a Reply

Your email address will not be published. Required fields are marked *