Abstract
Most Horizontal Directional Drilling (HDD) projects in North America continuously monitor annular pressure during construction for mitigating risk of inadvertent returns (“frac out”). Downhole tooling automatically records “realtime” pressure data throughout the length of the HDD crossing. This data has been collected and compared to the calculated theoretical drilling fluid pressure within the annulus of the borehole. Results of this analysis show the accepted drilling fluid models used to calculate the annular pressure during drilling generally compare well to the data obtained. Although the accepted models accurately predict actual pressure downhole, the accurate prediction of containment pressure (ie hydraulic fracture) is a different story. In this paper, as-drilled annular pressure data are used to determine the date, time, and pressure magnitude of actual hydraulic fracture occurrences in HDD crossings. Through careful evaluation, these occurrences have been isolated within the as-drilled data and compared to the predicted hydraulic fracture pressure calculated by the “Delft” equation using site specific geotechnical parameters. Literature suggests factors of 1/2 and 2/3 multiplied by the overburden height be placed on the value of Rp,max, for clay and sand, respectively. Results of the analysis of as-drilled data show that in most, if not all cases the “Delft” solution overpredicts the actual hydraulic fracture pressure. Previously, assumptions in the value of Rp,max have not been investigated and compared to actual hydraulic fracture data. Contained within this research, the Rp,max value is modified to determine at what value the Delft equation best predicts the actual hydraulic fracture pressure.
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