Challenges and Surprises of Abnormal Pore Pressures in Shale Gas Formations

Zhang, Jincai; Wieseneck, James

doi:10.2118/145964-ms

Cited by 41 publications

(9 citation statements)

References 5 publications

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“…Lucier et al (2011) evaluated the gas saturation on acoustic log data from the Haynesville shale gas play in NW Louisiana, USA. Zhang and Wieseneck (2011) identified and verified the compressional velocity slowed down by the gas presencein theBossier and Haynesville shale gas formations. They also demonstrated that gas-corrected in-situ velocity data are consistent with established mudrock sonic compressional velocity (Vp) and sonic shear velocity (Vs) trends.…”

Section: Seismic Interval Velocitymentioning

confidence: 65%

“…Drilling and completion in shale gas formations of the Bossier and Haynesville present many challenges, where pore pressure can increase from normal pressure to an abnormally high pressure over a comparatively short depth interval (Zhang and Wieseneck 2011). The prediction of abnormal pore pressure is generally requiredfor avoiding or mitigating drilling risks.…”

Section: Introductionmentioning

confidence: 99%

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Pore Pressure and Fracture Gradient Prediction in Shale Gas Formations: Accounting for Complex Rock Properties and Anisotropies

Purdy

2012

All Days

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The prediction of abnormal pore pressure is necessary for avoiding, or at least mitigating, drilling risks but is even more critical in shale gas formations where pore pressure can increase from normal pressure to an abnormally high pressure over a comparatively short depth interval. Arguably, the pore pressure prediction in shale gas plays offers even greater challenges than conventional reservoirs. There are numerous reasons why shale gas presents additional challenges. First, anisotropic rock properties in shale formations thataffect log responses will also affect pore pressure prediction. Second, the presence of gas in the shale causes the compressional interval travel time to read higher (slower), which affects derived rock properties, negatively impacting the accuracy of conventional pressure prediction techniques. Third, the geologically-old formations have higher resistivities with complex compaction and uplift histories. The same properties that create challenges with pore pressure prediction also cause uncertainties in fracture gradient prediction.New pore pressure prediction methods have been developed to address the unique characteristics of unconventional gas plays. The new methods incorporate gas corrections in the calculation of pore pressure and fracture gradient. Becausethe shear wave velocity has a much smaller gas effect than the compressional wave, it is used to correct the compressional data. Based on the corrected compressional log data, a better pore pressure prediction can be achieved. For resistivity-based pore pressure prediction, a new depth-dependent compaction, which accounts for complex compaction and reduces the gas effect, is used for improved pore pressure prediction. Additional improvements in the prediction of pore pressures, fracture gradient, and minimum stress are achieved by characterizing the effects of shale property anisotropies (e.g., Poisson's Ratio). The improvements achieved with the new methods are demonstrated through comparative analyses with conventional methods in the Bossier and Haynesville shales. IntroductionDrilling and completion in shale gas formations of the Bossier and Haynesville present many challenges, where pore pressure can increase from normal pressure to an abnormally high pressure over a comparatively short depth interval (Zhang and Wieseneck 2011). The prediction of abnormal pore pressure is generally requiredfor avoiding or mitigating drilling risks. However, pore pressure prediction becomes even more critical in shale gas formations wheregreater challenges exist versus conventional reservoirs. There are many reasons why drilling in shale gas formations presents additional challenges (e.g., formation anisotropy, gas-bearing shale, and gas effect on well logging data). Anisotropic rock properties (e.g., Poisson's Ratio and Young's Modulus) are more dominated in shale gas formations than in conventional formations. In-situ stresses andnear-wellbore stress redistributions, which are critically important in drilling (Li et al. 2010), are also affect...

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Section: Seismic Interval Velocitymentioning

confidence: 65%

Section: Introductionmentioning

confidence: 99%

Pore Pressure and Fracture Gradient Prediction in Shale Gas Formations: Accounting for Complex Rock Properties and Anisotropies

Purdy

2012

All Days

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“…The two most common methods for pore pressure prediction in the offshore environment are the Eaton Ratio method and the Equivalent Depth Method (EDM). The Bowers technique is also common but requires a larger dataset to calibrate successfully; it was a Bowers-type approach used by Zhang and Wieseneck (2011) and Couzens-Shultz et al, (2013). In both the Eaton and EDM processes a normal compaction trend (NCT) is required.…”

Section: Theory And/or Methodsmentioning

confidence: 99%

Using Traditional Methods to Predict Pore Pressure in Devonian Black Shale Basins of North East British Columbia

Green

Loeffler

Clarke

et al. 2018

Proceedings of the 6th Unconventional Resources Technology Conference

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In unconventional resource plays, pore pressure plays a critical role in the ability to predict fracture behaviour, and hence in the exploitation of these plays. Yet it is a parameter that is poorly understood, and little work has been done to understand whether it can be predicted in an unconventional setting. The case study presented here shows how the traditional methods (Eaton Ratio and Equivalent Depth) can be used to predict pore pressure using a Pressure Reference Trend (PRT) in-lieu of a Normal Compaction Trend (NCT) that would be used in the offshore environment. The PRT is not linked to the expected compaction behaviour of the rock (as inferred from an NCT) but it is simply an empirical depth trend from which the pore pressure can be predicted using industry standard formulae.

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“…This abnormally high pore pressure gradient has caused many drilling incidents (eg, kicks and blowouts). However, not many cases were reported on pore pressure prediction and overpressure mechanism studies in shale plays (eg, []).…”

Section: Introductionmentioning

confidence: 99%

Geological characteristics and abnormal pore pressure prediction in shale oil formations of the Dongying depression, China

Zhang

Yong

et al. 2020

Energy Science & Engineering

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Pore pressures in shale oil formations in the Dongying depression of the Bohai BayBasin are highly overpressured. Understanding overpressure principles not only can reveal the mechanism of shale oil generation to provide prospects for shale oil exploration and production, but also can reduce potential drilling risks and provide insights for sweet spot identification. Overpressure generation mechanisms are analyzed based on petrophysical data, core analysis, sedimentation rate, oil saturation, and smectite and illite data. We find that undercompaction, oil generation, and smectite and illite transformation are primary causes of overpressures. Undercompaction is the major cause of overpressures in the Es3 formations because of their rapid deposition rate of 300-700 m/m.y. For the overpressures primarily generated by undercompaction, sonic transit time can be used to predict shale pore pressure with correctly selected depth-dependent normal compaction trend. In the Es3 and Es4 formations, overpressure data are consistent with the increase of vitrinite reflectance and oil saturation, indicators of oil generation. Additionally, in shallow formations, starting at about 2000 m, the smectite to illite transformation occurs, which makes rock property changes with depth. A composite normal compaction trendline is proposed to account for the smectite to illite transformation. Using well log data, the shale oil formations can be identified and there are two shale oil formations in the studied area, and a shale oil geo-marker is found. At this geo-marker, the pore pressure gradient reaches the highest value. This high pore pressure behavior is beneficial for shale oil production because of a higher pore pressure corresponds to a higher production. K E Y W O R D Soil generation, petroleum geology, pore pressure prediction, shale oil, sonic transit time, undercompaction

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Challenges and Surprises of Abnormal Pore Pressures in Shale Gas Formations

Cited by 41 publications

References 5 publications

Pore Pressure and Fracture Gradient Prediction in Shale Gas Formations: Accounting for Complex Rock Properties and Anisotropies

Pore Pressure and Fracture Gradient Prediction in Shale Gas Formations: Accounting for Complex Rock Properties and Anisotropies

Using Traditional Methods to Predict Pore Pressure in Devonian Black Shale Basins of North East British Columbia

Geological characteristics and abnormal pore pressure prediction in shale oil formations of the Dongying depression, China

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