Paper: A Technique For Obtaining In Situ Saturations Of Underpressured Reservoirs

Pressure coring offers a method for obtaining In Situ reservoir Saturations. However due to the requirement of pressure balance during coring it has henceforth been limited to use in reservoirs with pressure gradients greater than 0.25 psi per foot. This paper will describe techniques used to obtain the first known successful pressure cores taken using a foam mud system; thereby extending the useful range of pressure coring to under-pressured reservoirs. Stable foam is a compressable Non-Newtonian fluid that requires special design considerations when used in conjunction with pressure coring. Careful well design is necessary to insure Bottom hole pressure during drilling and coring operations does not fall below reservoir pressure. This can easily occur if foam degradation and nonlinear pressure gradients are not considered. A complete technique for using foam to pressure core, including well design, field implementation, and core handling is presented in this paper. This technique includes a well bore design, a pressure analysis method, a method of selecting optimal foam design, a description of logistics, an empirical calibration test, and a description of pressure coring operations and core handling.