--> --> Seeing the Forest for the Trees: A Simplified Workflow for Target Optimization in Heterogeneous Reservoirs, an Example from the Middle Bakken, North Dakota

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Seeing the Forest for the Trees: A Simplified Workflow for Target Optimization in Heterogeneous Reservoirs, an Example from the Middle Bakken, North Dakota

Abstract

Over the past few years, the Middle Bakken play in the Williston Basin has evolved from delineation and step-out drilling, to significant infill development. As wells are being drilled at a faster pace, the need to understand the target zone is of greater importance. This study presents a workflow for evaluating the effect that drilling ‘in-zone’ has on well performance through the integration of log facies modelling, reverse-geosteering, and correlation matrices.

In this study, we created a regional petrophysical log facies scheme by employing a Principle Component Analysis (PCA) utilizing standard wireline log suites and core analyses. We then ‘reverse geosteered’ 164 Middle Bakken horizontal wells in an Area of Interest (AOI) in the basin center to quantitatively determine which log facies the wells transected. The percentage of feet drilled in each facies and the net thickness of each interval were compiled for each well in a database along with other variables including: gross thickness of the reservoir, hydrocarbon porosity feet (SoPhiH), the amount of hydrocarbons generated from the Bakken Shales, 6 months cumulative oil, and stabilized producing water cut. Following compilation of the variables, a correlation matrix was utilized to identify key geologic production drivers. The geologic drivers were utilized to normalize the horizontal wells, creating subsets of horizontal wells drilled in areas of ‘similar’ geology.

Results from the initial study show a very poor relationship between drilling in-zone and well results in our AOI. The poor correlation is likely a combination of uncertainty in geosteering interpretations, variations in well completion methods, lateral variations in reservoir quality, and/or homogenous reservoir (minimal vertical variations in porosity and permeability). Evaluation of mercury injection capillary pressure data for the Middle Bakken show that the reservoir in the AOI is relatively homogenous with regards to pore-throat size, and therefore permeability. In the second phase of the study, the same methodology for evaluating percent in-zone was applied to another portion of the basin where capillary pressure data showed the Middle Bakken to have greater pore-throat heterogeneity. Results from the second phase of the study show that in regions of the Williston Basin where pore-throat heterogeneity in the Middle Bakken is greater, in-zone drilling greatly improves well results. While this study specifically analyzes reservoir heterogeneity related to pore throat variations and well results in the Middle Bakken, this novel approach can be applied to others reservoirs to determine where in-zone drilling may improve well results.