Effect of Natural-Fracture Density on Production Variability of Individual Wells in the Nikanassin Tight Gas Formation

Abstract

Summary A statistical model with a strong correlation has been developed to determine average fracture density (FD) on the basis of production variability of 271 individual wells producing exclusively from the Nikanassin and equivalent formations in a large area of more than 15 000 km2 in the western Canada sedimentary basin (WCSB) in Alberta and British Columbia. Fractional production-variability plots (FPVPs) published by Nelson (2001) have been used successfully in the past in many naturally fractured reservoirs around the world. Up to now, the generation of such graphs has been based on empirical observations from field-production data. The variability of the graphs is interpreted qualitatively as a measure of reservoir heterogeneities. This paper presents a sequential methodology to reproduce empirical fractional variability plots of the Nikanassin tight gas formation using real data, an empirical variability-distribution model (VDM), and dual-porosity numerical simulation. Different simulation approaches and multiple sensitivities were generated from the simplest to the most-detailed cases to understand what causes the curvature of the FPVP in Nikanassin wells. The base simulation case is a homogeneous dual-porosity model, where porosity and permeability are set constant for matrix and fractures. A second case accounts for a heterogeneous dual-porosity model generated through statistical distributions of porosity and permeability. Finally, discrete-fracture-network (DFN) methodology is used to generate multiple fracture models from which stochastic fracture properties are generated for the simulation model. It is concluded that curvature of the variability plot is affected mainly by the occurrence of natural-fracture density. This finding permits estimating FD approximately parallel to the northwest/southeast-trending thrust belt of the Canadian Rocky Mountains in both the west and the east side of the deformation wedge. An unexpected result is that for a significant change in fracture permeability and porosity with constant FD, the fractional production-variability curve is not affected significantly in the case of gas reservoirs. Although the method is applied specifically to the Nikanassin tight gas formation, the theory is developed in detail in such a way that the methodology can be applied in other tight gas reservoirs around the world. Findings from this work are in good agreement with the geological description of the Nikanassin formation and with a production analysis performed in six Nikanassin study areas based on the cumulative number of wells vs. yearly cumulative gas production.

Integrated Reservoir Characterization and 3D Modeling of the Monteith Formation: A Case Study of Tight Gas Sandstones in the Western Canada Sedimentary Basin, Alberta, Canada

“…Calculate fracture density from production data following the procedure proposed by Gonzalez and Aguilera (2013). Fracture density is used as the main variable because a sensitivity study carried out by those authors showed that the greatest impact on cumulative production in naturally fractured reservoirs is caused by variations in fracture density.…”

“…Relative permeability curves, fluid properties, and completion data were provided by previous studies carried out by members of the GFREE [an integrated research program that includes Geoscience (G), Formation evaluation (F), Reservoir drilling, completion and stimulation (R), Reservoir Engineering (RE), and Economics and externalities (EE)] research group at the University of Calgary (Deng 2011;Deng et al 2011;Gonzalez 2012;Ramirez Vargas and Aguilera 2013;Gonzalez et al 2014). The static model includes effective matrix and fracture properties.…”

“…Recently, a study conducted by Gonzalez and Aguilera (2013) determined that there is a strong correlation between fracture intrinsic properties and production variability from well to well, with fracture density having the largest effect. Gonzalez and Aguilera (2013) related fracture density with production variability in the study area through Eq.…”

“…Gonzalez and Aguilera (2013) related fracture density with production variability in the study area through Eq. 4:…”

“…The economic potential of the very-low-permeability Monteith reservoirs involves preferentially a commingled completion that often includes the overlying Cadomin Formation. Production data available in the public domain within the study area were gathered, and the methodology proposed by Gonzalez and Aguilera (2013) was applied to obtain fracture-density points for the well locations and the different stratigraphic units. These points were fitted with the VSD model.…”