The advent of Unconventional Resources has created a breadth of challenges for all subsurface disciplines, and technical understanding has generally fallen behind the pace of development. Petrophysicists have been repeatedly challenged and pressured to do more with less. In this extended abstract, the authors detail and demonstrate successful applications of petrophysical modeling that have generated predictive power and the ability to quickly assess rock quality and pay zones within the Permian Basin.

A multi-tiered petrophysical model is presented which shows a practical way to evaluate wells and stitch together a more accurate subsurface picture of the field. The Permian Basin -- and many other unconventional plays -- benefits from a significant legacy dataset that can be leveraged to infill data gaps; however, this is regularly underutilized due to quality and vintage concerns. Through targeted pilots of advanced log and core acquisition (including FMI, wellsite NMR, MDT, and pressurized sidewall coring), a calibrated petrophysical model was created for the lowest common denominator -- the vintage triple combo -- allowing the practicing petrophysicist to gain more granularity in subsurface evaluation and optimize field development.

Unconventional Challenges

As lateral length increases and completion intensity grows, the petrophysicist is challenged to characterize an increasing amount of rock types and reservoir zones that contribute to well production. Data acquisition, however, has generally not grown proportionally. Figure 1, below, illustrates a simple wellbore schematic. To be predictive, the petrophysicist needs to be able to characterize the "Stimulated Vertical Height" – which itself can be difficult to define – typically using a log dataset that could be several miles away. This challenge has been discussed at length, notably by Malik et al., 2013.

Additionally, it is important to recognize that the Permian Basin has significant heterogeneity that further complicates the task of the practicing petrophysicist. The relative abundance and distribution of reservoir facies can vary dramatically over the course of a single wellbore (previously discussed in Blount, 2017), and there is extremely limited horizontal data acquisition to accurately map the erratic deposition. As a further challenge, the Permian Basin has a strong reliance on an abundant legacy dataset (of varying vintage and quality) that is compared and combined with targeted, modern wireline log and core acquisition.

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