Technology Focus: Formation Evaluation (February 2023)
Authors: Mukherjee Sandeep 
Published: March 2023

The domain of formation evaluation explores how varied a reservoir could be. In the life cycle of a field, from exploration and production to development, as we hone our knowledge to understand the reservoir, we often explore various subdomains of this discipline at various scales. In this issue, we explore three such subdomains at three different scales.
Heterogeneity of reservoir properties, as well as the associated uncertainty, from measured to predicted, is best captured when disciplines are studied at different scales, from concepts to models and, finally, to experiences.
Paper URTeC 2021-5434 attempts to understand the dynamics of a basin. This research studies the Wolfcamp formation of the Delaware Basin, where quality of the source rock and thermal maturity is deeply governed by the basin tectonics. The western half of the basin, which was relatively stable through geologic time, shows higher maturity in the source rocks than the relatively active eastern half. This known and observed phenomenon has been reported in various works, from vitrinite reflectance studies to the higher gas/oil ratio in producing wells. The authors explore the possibility of higher resident heat flux from igneous activities in the western part of the basin during the Cenozoic that generated these conditions. This, in my opinion, is the absolute big picture. Knowledge like this helps comprehension of why we see what we see. This is not just an academic exercise; it helps address practical problems in constructing facilities, pipelines, and compressor stations when the field enters the development stage.
Once basin-scale regional concepts are established, local reservoir-scale models provide the intellectual infrastructure for the perception of reservoir-scale heterogeneities. Models such as these seek answers from various spectra of measurements. From well logs to cores and from structural maps to production histories, the various subdisciplines all play a crucial role in subsequent hypotheses tests. Applying these conceptual models, then, to modern-day analogs provides justification or critiques of the process for the present and has time and again been the key to the past.
In paper IPTC 21225, the authors explore the validity of constructing a variable-scale reservoir model of a submarine fluvial/deltaic system from multidisciplinary data sets. The authors integrate these measurements of various resolutions and granularity, enabling the analyses to pan and zoom on the scales of the heterogeneity. After 20-plus years of production in this field, the knowledge here is still invaluable for understanding the reservoir compartmentalization and, thus, determining infill locations. I am intrigued by the complex nature of these depositional systems and the efforts the authors make to understand them.
Once the big picture has been established, it becomes time to put the figurative boots on the ground, or the knowledge to work, for drilling wells and producing fluids. The third choice for this issue, paper URTeC 2021-5410, discusses one of those aspects.
Pore-pressure prediction is a crucial exercise in this respect, with a very tight margin for error. In early phases of exploration, when well data are limited if not scarce, safe drilling and well construction often rely on the precision of pore-pressure expectancy and prediction models. Subsequently, as the well matures, understanding changes in pore pressure helps design artificial lift systems, if necessary, to optimize production and maximize the life of the field.
In this third paper, the authors challenge some of the practiced notions and empirical parameters that are widely used in pore-pressure-prediction models. One of those, the Biot’s coefficient, is hard to measure, rarely available from experiments, and grossly overgeneralized. The authors discuss the implications of these parameters and the resulting excursion from the true value that overassumptions could make. In my opinion, this is a must-read for any geomechanics enthusiast.
I hope the readers appreciate the scales of heterogeneity with which formation evaluation is concerned. The purposes here are beyond purely academic, and research often is burdened by a lack of measured data. Uncertainties can be minimized, however, when knowledge is sought by cross-referencing complementary domains and exploring heterogeneity in variable scales.