Fracture attribute scaling and connectivity in the Devonian Orcadian Basin with implications for geologically equivalent sub-surface fractured reservoirs

Abstract

Abstract. Fracture attribute scaling and connectivity datasets from analogue systems are widely used to inform sub-surface fractured reservoir models in a range of geological settings. However, significant uncertainties are associated with the determination of reliable scaling parameters in surface exposures, particularly for fault widths and fracture aperture. This has limited our ability to upscale key parameters that control fluid-flow at reservoir to basin scales. In this study, we present nine 1D transect (scanline) fault and fracture attribute datasets from Middle Devonian sandstones in Caithness (Scotland) that are widely used as an onshore analogue for nearby sub-surface reservoirs such as the Clair Field, West of Shetland. Our multiscale analysis confirms power-law behaviours for both length over 8 orders of magnitude (10−4 to 104) and fracture aperture and fault width (including fills) over 4 orders of magnitude (10−6 to 10−2). We also present a 2D fault and fracture topology analysis which allows assessment of the heterogeneity of connectivity and self similarity. This multiscale approach provides a new basis for upscaling micro- to meso-scale fracture attributes collected in outcrop analogues for use in static and dynamic reservoir models at reservoir to basin scales.

Basement reservoir plumbing: fracture aperture, length and topology analysis of the Lewisian Complex, NW Scotland

“…Length distributions are known to be particularly prone to censoring and truncation (Odling et al 1999). However, Odling et al (1999) and Dichiarante et al (2020) have shown that a multi-scale analysis can help to confirm that power-law scaling is an appropriate choice to model the fracture length distributions. In the present study, the basement fracture lengths sampled in 1D show a scaling relationship across 8 orders of magnitude and the 2D sample windows show consistent and comparable length distributions between onshore and offshore datasets.…”

“…For instance, when large fractures are incompletely sampled in a power-law population, the resulting plot can resemble a log-normal distribution. Following Ortega et al (2006), Dichiarante et al (2020) have shown how a multi-scale approach can be used to better constrain the scaling laws for fracture size attributes. As pointed out by Clauset et al (2009), use of the maximum likelihood estimator (MLE) is preferred over a least square regression analyses (R 2 ) for the fitting of power-law distributions.…”

“…In this study we used MLE scripts developed by Rizzo et al (2017) as used in the FracPaQ toolbox (Healy et al 2017). In addition, we followed the Dichiarante et al (2020) modification in which the MLE for power-law, exponential and log-normal fits are calculated on systematically truncated and censored datasets to find the optimum distribution parameters (see S1).…”

Comment to "Fracture attribute scaling and connectivity in the Devonian Orcadian Basin with implications for geologically equivalent sub-surface fractured reservoirs"

“…In this regards, a possible explanation to the finding that "upper cut has a greater influence over the sample fit". This outcome is a direct result of the current method used in Rizzo et al (2017a) and Healy et al (2017) to find the best fitting law (which follows Clauset et al, 2009): this method looks for that minimum value (xmin) for which the theoretical distribution law will hold (either Power-law, Lognormal, or Exponential). Large values therefore, do not influence the fitting.…”