Insufficient borehole pressure integrity (BHPI) is a significant drilling challenge in deep, high-temperature, high-pressure (HTHP) wells in south Texas, as it is in many wells. Shales and/ or sands weakened by depletion, leaking faults, or unfavorable rock properties result in lost returns when mud weights are close to pore pressures. In one field, short (~50 ft) transitions from normal (11 lb/gal) to overpressured (17.5 to 18.0 lb/gal) Frio formations compound the severity of this challenge. Setting casing to isolate normal-pressure from high-pressure zones can be problematic if faults exist at the casing shoe and/or the cement job does not provide a good hydraulic seal. In one case, the intermediate casing shoe failed to test, and conventional cement squeezes were unable to correct the problem. In the productive portion of the well, preventing skin and or formation damage in an interval that had a wide range of pore pressures (8.5 to 17.8 lbm/gal), was a major concern with any treatment option to increase borehole integrity.
This paper describes successful applications of new BHPI treatment materials and methods for increasing borehole integrity. BHPI treatments have allowed higher drilling and cementing circulation rates. This has helped optimize drilling performance and improve well conditions during cementing operations, which has resulted in improved primary cementing success. It has been suggested that skin damage in the zones of interest can be minimized since BHPI treatments can be designed and targeted to only enter areas with low BHPI. In one case, a BHPI treatment entered a low-pressure productive interval, which, after a planned stimulation program, did not seem to affect production performance. In another case, after BHPI treatments helped increase wellbore integrity, the productive interval in one well was successfully cemented without requiring a drilling liner, which would have limited completion flexibility.
A theoretical rock mechanics model is discussed to help explain how the new BHPI treatments can rapidly and substantially increases the pressure integrity of holes located across both sand and shale formations. Minor BHPI filtrate invasions during tests in high and low-permeability sandstone cores should explain why the new BHPI system also limits formation damage.