Borehole stability/hydrofracture in rock

[LRJ]

I was wondering if anyone knew of a calculation procedure to predict the pressure at which borehole stability can be ensured and/or hydraulic fracture would be initiated in a borehole in rock? The aim would be to determine the allowable pressure range during the drilling construction stage of a drilled and grouted pile: borehole stability analyses would provide the lower pressure bound and hydraulic fracture analyses would provide the upper pressure bound.

There are a variety of methods for calculating this for soils, but the mechanism in rock is likely different (i.e. reliant on the shear strength of rock discontinuities). I'm not looking to do any testing to confirm these pressures - I am looking to provide a quantitative basis to inform recommendations.

 

[oldestguy]

My question is: Do you know of problems on real jobs where pressure is found to be insufficient? From what I have experienced the only problem noted is withdrawing the auger too rapidly causing a partial vacuum and a resulting cave-in of soil not rock. No way to measure that negative pressure down there that I know of.

 

[miningman]

What is your definition of rock?. In all my experience with rock , I have found that the hydrostatic pressure within a borehole can be approximated at 40psi per 100 feet of depth. However this is rock with a compressive strength say 12-40,000 psi. I think some better definitions are required here

 

[LRJ]

The purpose of the borehole stability analysis is to assess whether to use a casing (and what length it should be) or to use a drilling fluid of a specified density. The borehole stability assessment is therefore more relevant for the surficial soils, so I'm not so worried about that - there are existing methods to predict this for soil.

However, I am more concerned by the risk of hydrofracture along rock discontinuities. The method for predicting borehole stability is going to be the same as for hydrofracture (just looking at different bounds to the problem), and I'm not aware of a method to predict hydrofracture specifically for rock. I expect to do this correctly you'd have to consider the discontinuities explicitly and possibly also the rock mass strength.

The UCS ranges from approximately 1 MPa to 60 MPa, though the discontinuities will have a significantly smaller shear strength.

 

[sraesttam]

I know this is an old thread... How deep of a borehole are you talking about? 20 feet or 20,000 feet? In oil and gas drilling regional charts are developed using average pore pressure gradient vs fracture pressure gradients to determine what depth to set the next casing string. The website onepetro.org has paper "SPE-21607-PA" titled "Pore-Pressure and Fracture-Gradient Predictions" and many other papers on pore pressure and fracture gradients. I would tend to think that the same theory applies to shallower holes as oil and gas wells, but I don't know anything about the geotechnical side.

The SPE "Petroleum Engineering Handbook" says that fracture gradients of 1.0 PSI/FT or higher commonly occur at depths less than 2000 foot and 0.7 PSI/FT is common below 4000 foot. It also says that very few cases have been documented with fracture gradients between 0.7-1.0 PSI/FT. Additionally, salt or swelling clays would also cause borehole washout / instability due to the mud itself if your additives are not correct. I have seen dump truck loads of clay/shale chunks coming out of wells with the wrong mud properties.

 

[LRJ]

Thanks for the information. I'll check out that article.

I've since found a number of sources by searching the term 'wellbore stability'. The methods for rock and soil are quite similar, though have some notable differences due to Mohr-Coulomb not being appropriate for rock (most soil methods assume Mohr-Coulomb).