This paper develops two methodologies to determine the pore pressure regime vertically through the Spraberry, Dean, & Wolfcamp intervals as well as aerially across the Midland Basin. Pore pressure is difficult to determine in unconventional reservoirs like the Wolfcamp especially because of its low permeability and the commingled completions used to produce the Spraberry/Dean/Wolfcamp intervals.
The first technique uses a Pad ISIP methodology to obtain calculated pore pressure values across all 10 completion stages in each vertical well using 10 bbl injection tests at the beginning of each frac stage. The ISIP at the end of the injection test is used as the approximate closure stress, and the poroelastic closure stress equation is rearranged so that pore pressure is the dependent variable. A regional geomechanical model based on evaluated dipole sonic logs distributed according to the regional stratigraphy is used to determine the Poisson's Ratio for each interval. The results of these calculations are reasonable and repeatable, and they identify a pore pressure regime that varies significantly (and predictably) throughout the Spraberry/Dean/Wolfcamp column.
Next, this data is used to interpret vintage well completion data so that a pseudo-pore pressure can be characterized with additional resolution in the Wolfcamp intervals across the Midland Basin. A relationship exists between the Pad ISIP pore pressure and the end of job frac gradient in the Wolfcamp B, C, and D intervals, and this relationship is used to calculate a pseudo-pore pressure value for 5,398 frac stages for which this data was recorded over the last decade.
The Spraberry, Dean, and Wolfcamp formations in the Midland Basin (Fig. 1) have been popular targets for vertical wells for many years. Recently horizontal wells have been shown to have significant economic profitability in several intervals throughout the column. Despite the high levels of activity in the Midland Basin there is little information in literature regarding the pore pressure of the producing formations.